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  #1  
Old February 1st 06, 06:28 AM posted to sci.astro,sci.astro.seti,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro,sci.astro.seti] Welcome! - read this first

Archive-name: astronomy/sci-astro-intro
Posting-Frequency: weekly
Last-modified: $Date: 2000/05/17 23:02:30 $
Version: $Revision: 4.1 $
URL: http://sciastro.astronomy.net/

------------------------------

Subject: Introduction

sci.astro and groups in the sci.astro.* hierarchy are newsgroups for
the discussion of astronomical topics. This post documents the topics
generally accepted as appropriate as well as guidelines for posting in
these groups. New readers (as well as more experienced ones!) are
encouraged to review this material with the hope that it will maximize
their use and enjoyment of the astronomy newsgroups.

This post is an extract of the material found in the sci.astro FAQ.
The FAQ is posted on a regular basis to the newsgroup sci.astro. It
is available via anonymous ftp from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/, and
it is on the World Wide Web at
URL:http://sciastro.astronomy.net/sci.astro.html and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

The material in this document was contributed by
Philippe Brieu ,
Walter I. Nissen, Jr. CDP , and
Steven Willner , with editing by
Joseph Lazio .

------------------------------

Subject: What are the astro newsgroups about?

There are eight groups in the sci.astro hierarchy:

sci.astro Astronomy discussions and information.
sci.astro.seti The Search for ExtraTerrestrial Intelligence (SETI)
sci.astro.amateur Amateur astronomy equipment, techniques, info, etc.
sci.astro.fits Issues related to the Flexible Image Transport System.
sci.astro.hubble Processing Hubble Space Telescope data. (Moderated)
sci.astro.planetarium Discussion of planetariums.
sci.astro.research Forum in astronomy/astrophysics research. (Moderated)
sci.astro.satellites.visual-observe
Visual observing of artificial satellites

By default, everything that is related to astronomy/astrophysics and
is NOT covered by one of the other sci.astro.* groups is acceptable
for posting in sci.astro. If something belongs in one of those
groups, then it does NOT belong in sci.astro and should NOT be
(cross)posted there. In particular, this includes all amateur
observations, hardware, software, and trade (see sci.astro.amateur).

The sci.astro hierarchy is NOT the appropriate forum for

* metaphysical discussions (try alt.paranet.metaphysics);
* astrology (alt.astrology); or
* creationism (talk.origins for that).

These are science groups, not religion, sociology, or philosophy (even
of science) groups.

In addition, a number of topics related to astrophysics are better
suited for other groups. For instance, elementary particle physics
should be discussed in sci.physics.particle (but discussions of
astronomical consequences are welcome in sci.astro). Likewise for
photons and the speed of light (sci.physics). Finally, all space
related issues (e.g. spacecraft and faster than light/time travel)
have a home in the sci.space.* hierarchy (but astronomical results
from space missions are welcome).

------------------------------

Subject: What are the guidelines for posting on astro newsgroups?

Ask yourself: Is this post about the science of astronomy? Will many
of the thousands and thousands of readers here, people interested in
the science of astronomy, find it of personal benefit? Has somebody
else recently posted a similar article? If your query or comment is
unique and concerns astronomy, post; otherwise, either there is
probably a better newsgroup for your post or your question has already
been answered.

If you will follow this group for a month or so before posting here,
you will greatly reduce the likelihood that you will participate in
making the newsgroup less productive and friendly and then end up
regretting it. If you are new here, it is likely that any question
you have has already been asked. If so, its answer is probably in one
of the FAQ files. Check out the newsgroups news.answers, sci.answers,
and news.announce.newusers, or ask your local help file or
administrator to point you toward the FAQs. Alternately, it may be in
a Usenet archive such as DejaNews, URL:http://www.dejanews.com/. If
you become really frustrated, pick on one of the more helpful posters
here and send e-mail (not a post) politely asking for some help.
Conversely, if your question is novel and not in a FAQ, readers will
likely be intensely interested in considering it.

Certain topics repeatedly come up and lead to lengthy, loud-mouthed
discussions that never lead anywhere interesting. Often these topics
have extremely little to do with the science of astronomy. Experience
also shows that when messages are cross-posted to other groups,
followups very seldom are appropriate in sci.astro.

If you do ask a question, please consider writing up the answer for a
FAQ file. New entries to the FAQ are always welcome!

Moreover, there are a number of common rules for all newsgroups. If
you are a new Usenaut, please visit the newsgroup
news.announce.newusers for an introduction to the Usenet.

------------------------------

Subject: How do I subscribe to sci.astro*?

(This question has been answered offline enough times that I thought
it would be worthwhile to include it here. The FAQ is distributed
widely enough that people may happen upon it through non-Usenet
channels.)

In order to access sci.astro (or other astronomy newsgroups), you need
an internet service provider (ISP). This could be a large commercial
provider, like AOL or Prodigy in the U.S., or a more local one (check
your phonebook under "Computer Networks" or "Internet"). If you're
enrolled at a college or university in the U.S. (or overseas?), talk
to your computer center; many colleges and universities are now
providing free Internet access to students. If you don't have an ISP,
you'll have to choose one. If you're interested in reading the
sci.astro* groups, as you search for an ISP, you'll want to ask the
various contenders if they provide access to Usenet and specifically
to the sci. hierarchy. If they don't, or can't tell you, that's a bad
sign.

If you already have an ISP, you'll have to read their documentation or
talk to their technical help. Some ISPs provide Usenet access through
a Web browser (like Mosaic, Netscape, or Internet Explorer), others
provide access through a dedicated news reading program like tin, rn,
or GNUS. There are many different possibilities.
  #2  
Old February 2nd 06, 02:34 AM posted to sci.astro,sci.astro.seti,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro,sci.astro.seti] Resources (Frequently AskedQuestions) (1/9)

Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part1
Version: $Revision: 4.5$
Last-modified: $Date: 2001/02/06 23:49:24$
URL: http://sciastro.astronomy.net/

------------------------------

Subject: Introduction


*sci.astro* is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This document
attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each month to
the newsgroup *sci.astro*. It is available via _anonymous ftp_,
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq, and it is
on the World Wide Web at the _sci.astro FAQ site_,
URL:http://sciastro.astronomy.net/sci.astro.html, and _Internet FAQ
Archives_, URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list
of worldwide _mirrors_, URL:http://sciastro.astronomy.net/mirrors.html,
(both ftp and Web) is also available. (As a general note, many other FAQs
are also available from _rtfm.mit.edu_,
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer, Joseph
Lazio .

------------------------------

Subject: The Internet and other information sources


[Dates in brackets are last edit.]

1. What are the sci.astro* newsgroups about? [1999-11-04]

2. How do I subscribe to sci.astro*? [1998-02-28]

3. What are the guidelines for posting on astronomy (sci.astro*) newsgroups?
[1996-12-1]

4. What should I do if I see an article that doesn't follow these guidelines?
(What about cranks?) [1997-02-04]

5. Can I get help on my homework from the Net? [1995-07-26]

6. What are good Net sites for astronomy info and images? [2003-01-17]

7. How can I find contact addresses for astronomers/observatories?
[2003-01-17]

8. Which observatories offer tours or public viewing? [1995-09-17]

9. Is there a list of astro jokes? [1999-12-15]

10. What are good books on astronomy (especially for beginners)? [1995-06-27]

11. Are there other sources of information? [1996-01-24]

12. How can I find an astronomy club? [1996-01-24]

13. Where can I find out about public lectures or star parties? [1995-09-17]

------------------------------

Subject: A.01 What are the sci.astro* newsgroups about?
Author: Philippe Brieu philippeumich.edu,
Walter I. Nissen Jr. CDP ,
Steven Willner

There are eight groups in the *sci.astro* hierarchy:

*sci.astro*
Astronomy discussions and information

*sci.astro.amateur*
Amateur astronomy equipment, techniques, info, etc.

*sci.astro.seti*
The Search for ExtraTerrestrial Intelligence (SETI)

*sci.astro.fits*
Issues related to the Flexible Image Transport System

*sci.astro.hubble*
Processing Hubble Space Telescope data. (Moderated)

*sci.astro.planetarium*
Discussion of planetariums

*sci.astro.research*
Forum in astronomy/astrophysics research. (Moderated)

*sci.astro.satellites.visual-observe*
Visual observing of artificial satellites

Each group except *sci.astro* has a charter that defines appropriate
postings. You can get the full charters via _anonymous ftp_,
URL:ftp://ftp.uu.net/usenet/news.announce.newgroups/sci/.

By default, everything that is related to astronomy/astrophysics and is
NOT covered by one of the other *sci.astro.** groups is acceptable for
posting in *sci.astro*. If something belongs in one of those groups, then
it does NOT belong in *sci.astro* and should NOT be (cross)posted there.
In particular, this includes all amateur observations, hardware, software,
and trade (see *sci.astro.amateur*).

The *sci.astro* hierarchy is NOT the appropriate forum for metaphysical
discussions. There are other groups for that (e.g.
*alt.paranet.metaphysics*). Neither is it the right group to discuss
astrology (*alt.astrology* is), which has nothing to do with astronomy, or
topics such as creationism (*talk.origins* for that). This is a science
group, not one for religion, sociology, or philosophy (even of science).

In addition, a number of topics related to astrophysics are better suited
for other groups. For instance, elementary particle physics should be
discussed in *sci.physics.particle* (but discussions of astronomical
consequences are welcome in astro groups). Likewise for photons and the
speed of light (*sci.physics*). Finally, all space related issues (e.g.
spacecraft and faster than light/time travel) have a home in the
*sci.space.** hierarchy (but astronomical results from space missions are
welcome).

------------------------------

Subject: A.02 How do I subscribe to *sci.astro**?
Author: Joseph Lazio

(I've answered this question offline enough times that I thought it would
be worthwhile to include it here. The FAQ is distributed widely enough
that people may happen upon it through non-Usenet channels.)

In order to access *sci.astro*, you need an internet service provider
(ISP). This could be a large commercial provider, like AOL or Prodigy in
the U.S., or a more local one (check your phonebook under "Computer
Networks" or "Internet"). If you're enrolled at a college or university in
the U.S. (or overseas?), talk to your computer center; many colleges and
universities are now providing free Internet access to students. If you
don't have an ISP, you'll have to choose one. If you're interested in
reading the *sci.astro* groups, as you search for an ISP, you'll want to
ask the various contenders if they provide access to Usenet and
specifically to the sci. hierarchy. If they don't, or can't tell you,
that's a bad sign.

If you already have an ISP, you'll have to read their documentation or
talk to their tech help. Some ISPs provide Usenet access through a Web
browser (like Mosaic, Netscape, or Internet Explorer), others provide
access through a dedicated news reading program like tin, rn, or GNUS.
There are many different possibilities.

------------------------------

Subject: A.03 What are the guidelines for posting on astronomy (sci.astro*)
newsgroups?
Author: Philippe Brieu philippeumich.edu,
Walter I. Nissen Jr. CDP ,
Steven Willner

If you will follow this group for a month or so before posting here, you
will greatly reduce the likelihood that you will participate in making the
newsgroup less productive and friendly and then end up regretting it. If
you are new here, it is likely that any question you have has already been
asked. If so, its answer is probably in one of the FAQ files. Check out
the newsgroups *news.answers*, *sci.answers*, and *news.announce.newusers*,
or ask your local help file or administrator to point you toward the FAQs.
Also, please check an Usenet archive like _Google_,
URL:http://groups.google.com/, to see if somebody has posted a comment or
query similar to yours recently. If you become really frustrated, pick on
one of the more helpful posters here and send e-mail (not a post) politely
asking for some help. Conversely, if your question is novel and not in a
FAQ, readers will likely be intensely interested in considering it.

Certain topics repeatedly come up and lead to lengthy, loud-mouthed
discussions that never lead anywhere interesting. Often these topics have
extremely little to do with the science of astronomy. Experience also
shows that when messages are cross-posted to other groups, followups very
seldom are appropriate in *sci.astro*. It would also help if you would ask
yourself a few simple questions before posting:

If you do ask a question, please consider writing up the answer for a FAQ
file. New entries to the FAQ are always welcome!

There are also a number of common rules for all newsgroups. The following
types of posts are NOT acceptable (see the newsgroup
*news.announce.newusers* and its FAQs at _rtfm.mit.edu_,
URL:ftp://rtfm.mit.edu/pub/usenet/news.announce.newusers/, for more
details):

* advertising (other than announcement of availability of products of
direct use to people interested in astronomy without any kind of hype);

* late breaking news (e.g., "CNN just announced that..."), although
questions about recent announcements are acceptable;

* questions answered in the FAQ: always check the appropriate FAQ before
asking a question;

* answers to questions covered by these or other FAQs or posts saying that
the answer is in the FAQ. Instead send email to the poster with a pointer
to the relevant FAQ. If you have a better answer to a FAQ, by all means
contact the maintainer!

* personal messages (e.g. "Looking for..."), especially if it is because
you cannot reach your party by e-mail;

* test messages (there are dedicated groups for that);

* corrections to your own posts (if they are minor and likely to be evident
to the reader), especially if it is just a missing signature;

* "me too" messages: if someone posts a request for something you would
like to get and asks for a reply by e-mail, do NOT post an article to say
you want it too (instead send e-mail to the person who posted the request
and ask to have the information forwarded to you by e-mail).

Also, please try to follow the following USENET guidelines when posting:

* keep your text under 72 columns wide and make sure lines have a newline
character at the end; do not insert any control character; do not use all
upper or all lower cases (mix them);

* post the same message only ONCE (it may not appear immediately on your
news server, but that does not mean that the rest of the world has not
received it yet)---only if your news software tells you it could not post
the article should you try to post it again (but make sure you cancel
previous posts);

* unless you have something to say that is of interest to all/most readers,
reply to the poster by e-mail instead of following up on the group (think
carefully about this);

* keep in mind that private e-mail is copyrighted by law, and that you may
not post it (in whole or in part) without the author's permission;

* before following up, check all other articles in the group for potential
followups that might make what you were going to say useless to say;

* when following up, check the headers (especially newsgroups) and edit
appropriately (especially the subject line if you are changing topics);

* do not quote the entire post you are following up (trim to the minimum
amount of text needed to make your message understood, and eliminate
signatures and useless headers);

* avoid posting the same message to more than one group; crosspost ONLY if
the subject is CLEARLY of EQUAL interest to several groups (check the FAQs
and charters for all groups in the hierarchy to decide where to post);

* never "spam."

------------------------------

Subject: A.04 What should I do if I see an article that doesn't follow
these guidelines? (What about cranks?)
Author: Philippe Brieu philippeumich.edu,
Walter I. Nissen Jr. CDP ,
Steven Willner

You may come to this newsgroup in search of information and productive
discussion. Others may have different motives. Their posts are often
pretty sophisticated in that they have been designed and tested to be
effective in pushing your hot buttons. And please bear in mind that some
of these people will come into possession of new identities and will post
something that sure looks like it comes from a hapless newbie.

DO *NOT* POST A FOLLOWUP UNDER ANY CIRCUMSTANCES!

To reemphasize, you should NOT post anything in response to an
inappropriate post in *sci.astro*. Other readers are probably as annoyed
as you are by that post, and the last thing they/you want is to waste
their/your time/disk space by adding more useless articles and fueling a
useless discussion.

What should you do then? Ignore people you consider crackpots (sometimes
a.k.a. cranks) altogether: do not send them e-mail, do not refer to their
posts or even name in your messages. Just pretend they do not exist and
they will go away! Why? Because attention and an opportunity to argue is
all they are looking for. Ignoring them is the ONLY way to deal with them.

One particularly easy way to ignore people is to use a KILL file. KILL
files allow you to specify that you do not want to see any articles on a
certain topic or by a certain person. If used, they can increase your
enjoyment of sci.astro considerably. The creation and maintenance depends
upon the particular newsreader you use, but you may want to consult the
_KILL file FAQ_, URL:http://www.faqs.org/faqs/killfile-faq/, (also
available via _anonymous ftp_,
URL:ftp://rtfm.mit.edu/pub/usenet-by-group/news.answers/killfile-faq,).

What about *spams*? Spams are the posting of a totally irrelevant (often
commercial) message to several (often many) groups by people who are just
trying to reach as many USENET readers as possible, indiscriminately. They
do not target you personally, but rather all of USENET. The ONLY
appropriate action is to send a message to their news administrator (
usenet or ) complaining about it and asking for their
account to be closed (be sure to include the full spam message with all
headers). You can send a copy of your message to the posters so that they
end up being "mailbombed" by readers (but do NOT mailbomb them by
yourself!). There is no point in posting to the group because the spammers
do NOT read it anyway!

If the post you read is inappropriate in another way, chances are it is
not intentional. The poster was probably unaware of netiquette or rules
for this particular hierarchy/group. Be understanding: do not flame them
on the group. Instead, tell them politely what to do by private e-mail,
and refer them to this FAQ. Of course, if it is a repeat offender, feel
free to flame, but only by e-mail.

------------------------------

Subject: A.05 Can I get help on my homework from the Net?
Author: Philippe Brieu philippeumich.edu,
Steven Willner

A recurring subject of discussion is the posting of homework problems.
Students should NOT ask readers to solve their homework problems in detail
in this group because they are supposed to do it themselves in the first
place, and readers are unlikely to be sympathetic to a lazy attitude. More
importantly, answers are not guaranteed to be correct (far from that!), and
instead of getting an answer, you may initiate a long and useless
discussion on factors of two. Do not try to disguise homework: long time
readers (there are many) will detect it and you will get flamed!

However, if there is a concept you do not understand in a problem and
would like some guidance or some help getting started (not the solution),
then feel free to ask. Or if you find conflicting sources, it's fine to
ask about that. Basically, think of the net as a group of friends. You
wouldn't ask your friends to do your homework for you, but you might well
ask for help in the circumstances described. Of course it's up to you to
evaluate the answers you get!

Please keep in mind that articles take anywhere from one hour to several
days to propagate to other sites. Therefore, it is hopeless to get an
answer for an assignment you have to turn in the next day, or after the
weekend... USENET is NOT a last minute solution!

------------------------------

Subject: A.06 What are good Net sites for astronomy info and images?
Author: many

This list is an attempt to compile the locations of the biggest sites and
those with extensive cross-references. Please let me know other sites
*that fall into these categories* or *categories not included*. The FAQ
can't list everybody's favorite site, but it should list sites that
cross-reference most people's favorites.

Indices
* AstroWeb: Astronomy/Astrophysics on the Internet [multiple mirror sites]

- _NRAO, US East_, URL:http://fits.cv.nrao.edu/www/astronomy.html,

- _STScI, US East_, URL:http://www.stsci.edu/net-resources.html,

- _CDS, France_, URL:http://cdsweb.u-strasbg.fr/astroweb.html,

- _ESO_, URL:http://ecf.hq.eso.org/astroweb/yp_astro_resources.html,

- _ESA, Spain_, URL:http://www.vilspa.esa.es/astroweb/astronomy.html,

- _ANU, Australia_, URL:http://www.mso.anu.edu.au/%7Eanton/astroweb/,

* _Students for the Exploration and Development of Space_,
URL:http://www.seds.org/, (SEDS) (Images, Info, and Software Archive)

- _anonymous ftp_, URL:ftp://seds.lpl.arizona.edu/pub/,

- _astroftp (text)_,
URL:ftp://seds.lpl.arizona.edu/pub/faq/astroftp.txt, list

- _astroftp (HTML)_,
URL:ftp://seds.lpl.arizona.edu/pub/faq/astroftp.html, list

* _Galaxy_, URL:http://galaxy.einet.net/galaxy/Science/Astronomy.html,

* _Google Groups_, URL:http://groups.google.com/,

Data Archives and Catalogs
* _JPL Solar System Dynamics_, URL:http://ssd.jpl.nasa.gov/,
("information [about] all known bodies in orbit around the Sun.")

* _Centre de Donnees astronomiques de Strasbourg_,
URL:http://cdsarc.u-strasbg.fr/, (in English and includes SIMBAD)

* _NSSDC Astrophysics Data_, URL:http://nssdc.gsfc.nasa.gov/astro/,
(space missions and catalog data)

* _Astronomical Data System_, URL:http://adswww.harvard.edu/,
(professional journals, conference proceedings, data)

* APS _Catalog of the Palomar Observatory Sky Survey_,
URL:http://isis.spa.umn.edu/,

Images & Simulations
* _Astronomy Picture of the Day_,
URL:http://antwrp.gsfc.nasa.gov/apod/astropix.html,

* _SkyView_, URL:http://skyview.gsfc.nasa.gov, (digitized images of any
sky coordinates, multi-wavelength)

* _Jet Propulsion Laboratory_, URL:http://www.jpl.nasa.gov/, (JPL)

* The Nine Planet _Planetary Picture List_,
URL:http://seds.lpl.arizona.edu/billa/tnp/picturelist.html,

* _NASA JSC Digital Image Collection_, URL:http://images.jsc.nasa.gov/,
(mostly Earth and spacecraft)

* _U.S. Geological Survey_,
URL:http://www-pdsimage.wr.usgs.gov/PDS/public/mapmaker/mapmkr.htm,

* _The Web Nebulae_, URL:http://seds.lpl.arizona.edu/billa/twn/,

* _Messier Database_, URL:http://www.seds.org/messier/,

* _Solar System Live_, URL:http://www.fourmilab.ch/solar/solar.html,

Societies, Institutions, Publishers
* _American Astronomical Society_, URL:http://www.aas.org/,

* _Royal Astronomical Society_, URL:http://www.ras.org.uk/,

* _American Association of Variable Star Observers_,
URL:http://www.aavso.org/,

* _NASA_, URL:http://www.nasa.gov/,

* _Space Telescope Electronic Information Service_,
URL:http://www.stsci.edu/resources/,

* _Sky and Telescope_, URL:http://www.skyandtelescope.com/,

* USGS _Astrogeology Research Program_,
URL:http://astrogeology.usgs.gov/,

Related Usenet newsgroups (see also A.01)
* _*sci.physics*_, URL:news:sci.physics,: Physical laws, properties, etc.

* _*sci.physics.particle*_, URL:news:sci.physics.particle,: Particle
physics discussions

* *sci.space.**: Discussions of space policy, travel, technology, etc.

* *talk.origins*: Discussions of creationism vs. evolution, the Big Bang,
and other science topics

Related FAQs
Many related newsgroups have FAQ's. Most can be obtained by anonymous ftp
from _rtfm.mit.edu_, URL:ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/.

FAQ for *sci.physics*
available via _anonymous ftp_,
URL:ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/physics, and on the
Web from various mirrors including _US West Coast mirror_,
URL:http://math.ucr.edu/home/baez/physics/, _European mirror_,
URL:http://www.desy.de/user/projects/Physics/, and _Australia mirror_,
URL:http://hermes.physics.adelaide.edu.au/%7Edkoks/Faq/.

FAQ for *sci.space*
available via _anonymous ftp_,
URL:ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/space/science,

Astro/Space Frequently Seen Acronyms
available via _anonymous ftp_,
URL:ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/space/science,

FAQ for *sci.astro.planetarium*
available via the _Web_,
URL:http://www.lochness.com/pltref/sapfaq.html,

FAQ for *sci.skeptic*
available via _anonymous ftp_,
URL:ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/skeptic,

FAQ for *talk.origins*
available from the _talkorigins_,
URL:http://www.talkorigins.org/origins/faqs-qa.html, Web site

FAQ for relativity
_Usenet Relativity FAQ_,
URL:http://www.weburbia.com/physics/relativity.html,

FAQ for black holes
_Black Holes FAQ_,
URL:http://cosmology.berkeley.edu/Education/BHfaq.html,

FAQ for calendars
_Calendar FAQ_, URL:http://www.tondering.dk/claus/calendar.html,

FAQ for supernovae and supernova remnants
_Supernovae and Supernova Remnants FAQ_,
URL:http://www.talkorigins.org/faqs/supernova/,

Lecture notes, essays, compilations, etc.
* _The Nine Planets_, URL:http://seds.lpl.arizona.edu/billa/tnp/,

* Nick Strobel's _Astronomy Notes_, URL:http://www.astronomynotes.com/,

* _The Constellations and Their Stars_,
URL:http://www.astro.wisc.edu/%7Edolan/constellations/,

* John Baez's _General Relativity Tutorial_,
URL:http://math.ucr.edu/home/baez/gr/gr.html,

* _The Astronomy Cafe_, URL:http://www.astronomycafe.net/,

* _Virtual Trips_, URL:http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html,
to black holes and neutron stars

* _Bad Astronomy_, URL:http://www.badastronomy.com/,

* _Powers of 10_,
URL:http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/,---The
Size of the Universe (interactive Java tutorial)

------------------------------

Subject: A.07 How can I find contact addresses for
astronomers/observatories?
Author: Ralph Martin ,
Luisa Rebull ,
Joseph Lazio

The first thing to try would be to visit your favorite search engine and
enter the astronomer or observatory's name. Most astronomers and
observatories today have Web sites. Although they may not be
comprehensive, there are two astronomy-oriented Web sites with astronomer
and/or observatory information. They are the _Astronomy Search Engine
(English site)_, URL:http://star-www.rl.ac.uk/astrolist/astrosearch.html,
or _Astronomy Search Engine (German site)_,
URL:http://www.astro.uni-jena.de/Often_used/astrosearch.html, and the
_Star*s Family of Astronomy_,
URL:http://vizier.u-strasbg.fr/starsfamily.html, resources. The latter
is part of the AstroWeb Consortium (see A.06).

------------------------------

Subject: A.08 Which observatories offer tours or public viewing?
Author: Joseph Lazio

Many larger observatories do offer tours. If the observatory of interest
has a Web page, that should provide a way to contact somebody at the
observatory, see the FAQ "What are good Net sites for astronomy info and
images?" and How can I find contact addresses for
astronomers/observatories?.

------------------------------

Subject: A.09 Is there a list of astro jokes?
Of course! Astronomy is not an entirely sirius subject. Rather than try
to explain how many astronomers change light bulbs, please see the _Science
Jokes Collection_, URL:http://www.xs4all.nl/%7Ejcdverha/scijokes/, and
Yahoo!'s _science humor_,
URL:http://dir.yahoo.com/Entertainment/Humor/Science/, entries.

------------------------------

Subject: A.10 What are good books on astronomy (especially for beginners)?
Author: Hartmut Frommert

Observing guides and images
* Kenneth Glyn Jones. _Messier's Nebulae and Star Clusters_. Sky
Publishing 1968, 2nd ed 19XX. 427p. A great handbook and resource!
Contains introduction to historical and astronomical background together
with data, historic and newer descriptions with a finder chart, drawing,
and photo (in appendix) for each object, plus biographical and historical
material on Messier and the other discoverers and early researchers of the
Messier objects.

* John Mallas & Everitt Kreimer. _The Messier Album_. Sky Publishing
1978, 248p. Messier biography (by Owen Gingerich), reprint of Messier's
original catalog (in French), descriptions for each object (but M102) with
finder chart, drawing (from 4") and b/w photo (12 1/2"). Messier object
chart of the Heavens, check list, color photos of some, 248 p.

* Hans Vehrenberg. _Atlas of Deep Sky Splendors_. Vehrenberg+Sky
Publishing 1st ed. 196X, 4th edition 198X, 242p. Original title: _Mein
Messier-Buch (My Messier Book)_. Schmidt photo charts of all Messier and
many other Deep Sky objects, partially color, descriptions, some with
photos from observatories.

* Don Machholz. _Messier Marathon Observer's Guide -- Handbook and Atlas_.
Make Wood Products, P.O.Box 1716, Colfax, CA 95713 (USA). Interesting
stuff on Charles Messier, his comets, his catalog including discussion of
"nebulous" (missing, stellar, and the star cloud) and "add-on" objects, a
catalog, finder charts, plus proposed Marathon.

------------------------------

Subject: A.11 Are there other sources of information?
Author: Hartmut Frommert ,
Joseph Lazio

In general, do not underestimate your local library. It likely contains
encyclopediae and other reference sources to answer many questions.

Pictures and/or other astronomical information
*
The Armagh Planetarium
College Hill
Armagh BT61 9DB, Northern Ireland, U.K.
Fax: +44 (0)861 52 6187


*
Astronomical Society of the Pacific
390 Ashton Avenue
San Francisco, CA 94112, USA
e-mail:
(customer correspondence)
Phone: +1 (415) 337 2624, Toll free (U.S. only): 800 335 2624
Fax: +1 (415) 337 5205


*
The Hansen Planetarium
1845 South 300 West,#A
Salt Lake City, Utah 84115, USA
Phone: +1 (801) 483 5400, Toll free (USA only): 800 321 2369
Fax: +1 (801) 483 5484


*
Holiday Film Corporation
P.O.Box 619
12607 E. Philadelphia St.
Whittier, CA 90608, USA


*
List of Great Observatories making Astronomical Photographs publicly
available
Hartmut Frommert
University of Constance
Dept. of Physics
P.O.Box 5560 M 678
D-78464 Konstanz, Germany
Phone: +49 7531-88-3789
E-Mail:

_http://www.seds.org/%7Espider/obs-ims.txt_, URL:http://www.seds.org/%7Espider/obs-ims.txt,_anonymous ftp list_, URL:ftp://www.seds.org/pub/info/obs-ims.txt,

------------------------------

Subject: A.12 How can I find an astronomy club?
Author: Joseph Lazio ,
Steve Willner

There are a few different ways to find astronomy clubs (listed in no
particular order):

* Check Sky & Telescope's annual listing of astronomy clubs and societies.

* Contact a local university or college (if there is one near you). Often
times if there's a department of physics and/or astronomy, somebody within
it may know of a local club.

* Contact local science museums, planetaria, or other similar
organizations.

* Check the AstroWeb listing, see the FAQ "What are good Net sites for
astronomy info and images?"

------------------------------

Subject: A.13 Where can I find out about public lectures or star parties?
Author: Joseph Lazio

Very often public lectures and star parties are hosted by astronomy clubs.
The list of ways to find astronomy clubs, given in the FAQ "How can I find
an astronomy club?", can be exploited to find lectures and parties as well.

------------------------------

Subject: Copyright


This document, as a collection, is Copyright 2002 by T. Joseph W. Lazio
. The individual articles are copyright by the
individual authors listed. All rights are reserved. Permission to use,
copy and distribute this unmodified document by any means and for any
purpose EXCEPT PROFIT PURPOSES is hereby granted, provided that both the
above Copyright notice and this permission notice appear in all copies of
the FAQ itself. Reproducing this FAQ by any means, included, but not
limited to, printing, copying existing prints, publishing by electronic or
other means, implies full agreement to the above non-profit-use clause,
unless upon prior written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults. Any
express or implied warranties, including, but not limited to, any implied
warranties of merchantability, accuracy, or fitness for any particular
purpose, are disclaimed. If you use the information in this document, in
any way, you do so at your own risk.





--
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sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html
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Old February 2nd 06, 02:35 AM posted to sci.astro,sci.answers,news.answers
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Posts: n/a
Default [sci.astro] General (Astronomy Frequently Asked Questions) (2/9)


Last-modified: $Date: 2004/01/27 00:00:01 $
Version: $Revision: 4.10 $
URL: http://sciastro.astronomy.net/
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part2

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/sci.astro.html and
URL:http://www.faqs.org/faqs/astronomy/faq. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: B.00 General

[Dates in brackets are last edit.]

B.01 What good is astronomy? [1997-08-06]
B.02 What are the largest telescopes? [2000-04-04]
B.03 What new telescopes/instruments are being built? [2000-01-01]
B.04 What is the resolution of a telescope? [1995-08-23]
B.05 What's the difference between astronomy and
astrology? [1995-08-23]
B.06 Is there scientific evidence for/against
astrology? [1995-08-23]
B.07 What about God and the creation? [1995-08-27]
B.08 What kind of telescope should I buy? [2001-01-17]
B.09 What are the possessive adjectives for the
planets? [1995-12-05]
B.10 Are the planets associated with days of the week?
[2000-11-12]
B.11 Why does the Moon look so big when it's near the
horizon? [1997-01-21]
B.12 Is it O.K. to look at the Sun or solar eclipses using
exposed film? CDs? [1996-11-20]
B.13 Can stars be seen in the daytime from the bottom of a tall
chimney, a deep well, or deep mine shaft? [1996-06-14]
B.14 Why do eggs balance on the equinox? [1996-06-14]
B.15 Is the Earth's sky blue because its atmosphere is nitrogen
and oxygen? Or could other planets also have blue
skies? [1998-02-06]
B.16 What are the Lagrange (L) points? [2003-10-18]
B.17 Are humans affected psychologically and/or physically by
lunar cycles? [2000-06-03]
B.18 How do I become an astronomer? What school should I
attend? [1996-07-03]
B.19 What was the Star of Bethlehem? [2002-05-04]
B.20 Is it possible to see the Moon landing sites? [2003-09-18]

------------------------------

Subject: B.01 What good is astronomy anyway? What has it contributed
to society?
Author: many

This question typically arises during debates regarding whether a
government should spend money on astronomy. There are both pratical
and philosophical reasons that the study of astronomy is important.

On the practical side...

Astronomical theories and observations test our fundamental theories,
on which our technology is based. Astronomy makes it possible for us
to study phenomena at scales of size, mass, distance, density,
temperature, etc., and especially on TIME scales that are not possible
to reproduce in the laboratory. Sometimes the most stringent tests of
those theories can only come from astronomical phenomena. It must be
understood that these theories influence us even if they don't tell us
that we can invent new things, because they can tell us that we can't
do certain things. Effort spent on astronomy can prevent effort
wasted trying to come up with antigravity, for instance.

Astronomy provided the fundamental standard of time until it was
superseded by atomic clocks in 1967. Even today, astronomical
techniques are needed to determine the orientation of the Earth in
space, e.g., URL:http://www.usno.navy.mil/. This has military
applications but is also needed by anyone who uses the Global
Positioning System (GPS). Furthermore, it may be that millisecond
pulsars can provide an even more stable clock over longer time scales
than can atomic clocks.

Closely related is navigation. Until relatively recently (post-WW II)
celestial navigation was the ONLY way in which ships and aircraft
could determine their position at sea. Indeed, the existence of
navigation satellite systems today depends heavily on the lessons
learned from aspects of astronomy such as celestial mechanics and
geodesy. Even today, in the UK, RAF crews and RN officers need to
learn the rudiments of celestial navigation for emergency purposes;
until the late 1990s so did US Naval officers.

Astronomical phenomena have been important in Earth's history.
Asteroid impacts have had major effects on the history of life, in
particular contributing to the extinction of the dinosaurs and setting
the stage for mammals. The Tunguska impact in 1908 would have had a
far greater effect if it had occurred over London or Paris as opposed
to Siberia.

The debate over the magnitude, effect, and cost of greenhouse warming
is motivated, in part, by research on Venus. Astronomy has prompted
study of the Earth's climate in other ways as well. The study of the
atmospheres of other planets has helped to test and refine models of
the Earth's atmosphere. The Sun was fainter in the past, an important
constraint on the history of the climate and life. Understanding how
the Earth's climate responded to a fainter Sun is important for
evolution and for the progress of climate modelling. More generally,
there is weak evidence that solar activity influences climate changes
(e.g., variations in sunspot cycle, the Maunder minimum, and the
Little Ice Age) and therefore is important in the greenhouse warming
debate. (This is by no means proven by current evidence but *may*
prove to be important.)

The element helium was discovered (in a real sense) and named, not by
chemists, but by astronomers. In addition to making many birthday
parties more festive, liquid helium is useful for many low-temperature
applications.

Solar activity affects power-grids and communications (and
space travel). Prediction is therefore important, indeed is
funded by the U.S. Air Force.

Many advances in medical imaging are due to astronomy. Even the
simple technique that astronomers used for decades, of baking or
otherwise sensitizing photographic materials, was slow to catch on in
medical circles until astronomers pointed out that it could reduce the
required x-ray dose by more than a factor of 2. Many of those now
involved in some of the most advanced developments of medical imaging
and imaging in forensics were trained as astronomers where they
learned the basic techniques and saw ways to apply them. More
recently, image reconstruction of the flawwed Hubble images led to
earlier detection of tumors in mammograms (see back issues of Physics
Today).

While we don't yet have a good method for predicting earthquakes, the
techniques of Very Long Baseline Interferometry are used routinely to
measure ground motion.

Interferometry has also led to the development of Synthetic Aperture
Radar. Today SAR is used for earth remote sensing. Applications
include mapping sea ice (safety of ships, weather forecasting) and
ocean waves (ditto), resource location, agricultural development and
status checks.

Jules Verne would never have written "From the Earth to the Moon"
without astronomy. Astronomy helped spawn science fiction, now an
important component of many publishing houses and film studio
productions.

There has been a complex interplay between scientific, military, and
civil users, but astronomy has played an important role in the
development of such things as security X-ray systems (like those at
airports), electro-optics sensors (security cameras, consumer video
cameras, CCDs, etc.), and military surveillance technology (like spy
satellites).


On the philosophical side...

Perhaps the most important aspect of being human is our ability to
acquire knowledge about the Universe. Astronomy provides the best
measure of our place in the Universe.

In this century, the ability of astronomy to test General Relativity
led directly to Karl Popper's distinction between science and
pseudo-science and from there to the way intellectuals (at least) look
at science. Astronomy's support of modern physics (such as quantum
mechanics) in this century had have important influences on general
philosphical and intellectual trends. The "Earthrise" photo, of the
Earth rising over the Moon's horizon, from an Apollo mission is often
credited as being partially responsible for driving environmental and
"save the planet" impulses.

In previous centuries, astronomy led to Copernicanism and subsequent
"Principle of Mediocrity" developments---that the Earth, and by
extension, humans, is not at the center of the Universe. Eliminating
geo- and human-centred perspectives was a major philosophical leap.
Astronomy's support of a mechanistic universe in the 19th century had
important influences on general philosphical and intellectual trends.

In general, but certainly more vaguely, the last century of astronomy
has provided many supports to the view that the scientific method is
capable of answering many questions and that naturalistic thinking can
explain the world. Thus, scientists can answer many creation
questions (e.g., where metals come from, why the Sun shines, why there
are planets).

------------------------------

Subject: B.02 What are the largest telescopes?
Author: Bill Arnett ,
William Keel ,
Joseph Lazio ,
Steve Willner , Jennifer Imamura

The "largest" telescope is a bit difficult to determine. One can
obtain many different answers, depending upon the adjectives placed in
front of "largest." Nonetheless, what follows is one such list.

A list of astronomical instruments is also at
URL:http://www.futureframe.de/astro/instr/index.html, and a list of
large optical telescopes is at
URL:http://www.seds.org/billa/bigeyes.html.

A list of space-based observatories is at
URL:http://www.seds.org/~spider/oaos/oaos.html.


(Optical/Infrared telescopes, nighttime)

The list below gives the largest optical telescopes operating today.
For complicated pupil shapes, the effective aperture diameter is
given. Location is geographic; we omit most organizational details,
amusing and intricate as they may be. The list has been truncated at
3 m because there are so many telescopes of that size or smaller.
URL's are given where known.

Aperture Name Location
10.0 Keck I Mauna Kea, Hawaii
(mirror composed of 36 segments)
URL:http://astro.caltech.edu/mirror/keck/index.html
6.5 Multiple Mirror Mt. Hopkins, Arizona
(6 mirrors, 1.8 m each; see also B.03)
URL:http://sculptor.as.arizona.edu/foltz/www/mmt.html
6.0 BTA Nizhny Arkhyz, Russia
(Bolshoi Teleskop Azimutalnyi = Large Altazimuth Telescope)
URL:http://www.sao.ru/
5.0 Hale Palomar Mountain, California
URL:http://astro.caltech.edu/observatories/palomar/public/index.html
4.2 William Herschel La Palma, Canary Islands
URL:http://ing.iac.es/WHT.html
4.0 Victor Blanco Cerro Tololo, Chile
URL:http://www.ctio.noao.edu/4m/base4m.html
4.0 Mayall Kitt Peak, Arizona
URL:http://www.noao.edu/kpno/kpno.html
3.9 Anglo-Australian Siding Spring, Australia
URL:http://www.aao.gov.au/
3.8 UK Infrared Mauna Kea, Hawaii
URL:http://www.jach.hawaii.edu/UKIRT/
3.6 ESO Cerro La Silla, Chile
URL:http://www.ls.eso.org/
3.6 Canada-France-Hawaii Mauna Kea, Hawaii
URL:http://www.cfht.hawaii.edu/
3.5 New Technology Cerro La Silla, Chile
URL:http://www.eso.org/NTT/
3.5 MPI-CAHA Calar Alto, Spain
URL:http://www.mpia-hd.mpg.de/CAHA/
3.5 ARC Apache Point, New Mexico (mostly remote control)
URL:http://www.apo.nmsu.edu/
3.5 WIYN Kitt Peak, Arizona
URL:http://www.noao.edu/wiyn/
3.5 Starfire Kirtland AFB, New Mexico
URL:http://www.sor.plk.af.mil/default.html
3.0 Shane Mount Hamilton, California
URL:
http://cgi.irving.org/cgi-bin/irving...k+shnentry+A+M

3.0 NASA IRTF Mauna Kea, Hawaii
URL:http://irtf.ifa.hawaii.edu/

Other telescopes of note:

Solar Telescope:

Global Oscillation Network Group (GONG), six sites around the world
for velocity imaging
http://helios.tuc.noao.edu/gonghome.html

Largest single dish radio telescope: Arecibo Observatory
(Nat. Astron. & Ionosphere Center, Cornell U.) 305-m, Puerto Rico
URL:http://www.naic.edu/

Largest fully-steerable single dish radio telescope: Max Planck
Institut fuer Radioastronomie, 100 m, Effelsburg, Germany
URL:http://www.mpifr-bonn.mpg.de/effberg.html

Largest millimeter wave radio telescope: Nobeyama Radio Observatory,
45m, Japan
URL:http://radio.utsunomiya-u.ac.jp/NAO/nobeyama.html

Largest sub-millimeter radio telescope: James Clerk Maxwell Telescope
(Joint Astron. Center = UK, Canada, Netherlands), Mauna Kea, 15 m
URL:http://www.jach.hawaii.edu/JCMT/

Largest (connected-element) radio interferometric arrays:
Very Large Array (NRAO, New Mexico),
27 dishes, each 26.4 m effective diameter
The maximum separation between antennas is ~35 km.
URL:http://www.aoc.nrao.edu/vla/html/VLAhome.shtml

MERLIN (NRAL, University of Manchester, UK)
up to 8 dishes, various specifications.
The maximum separation between antennae is 217 km (between the
Cambridge and Knockin dishes).
URL:http://www.jb.man.ac.uk/merlin/
[MERLIN actually uses radio links between the antenna elements, so
maybe it should go into a separate category.]

Longest-baseline (dedicated) radio interferometric array: Very Long
Baseline Array (NRAO), 10 dishes, each 26.4 m effective diameter,
United States. The maximum separation between antennas is ~8600 km,
between the islands of St. Croix and Hawaii.
URL:http://www.aoc.nrao.edu/vlba/html/VLBA.html

HALCA (ISAS), 8 m dish, in Earth orbit
URL:http://www.vsop.isas.ac.jp/

Infrared:
Infrared Space Observatory (ISO) (ESA)
URL:http://isowww.estec.esa.nl/

Ultraviolet:

Extreme Ultraviolet Explorer (EUVE) (NASA)
URL:http://www.cea.berkeley.edu/

International Ultraviolet Explorer (IUE) [defunct] (NASA, PPARC and ESA)
URL:http://www.vilspa.esa.es/iue/iue.html

X-ray:

Chandra, the Advanced X-ray Astrophysics Facility (NASA)
URL:http://asc.harvard.edu/

X-Ray Astronomy Satellite (SAX) (ESA)
URL:http://www.sdc.asi.it/

X-Ray Timing Explorer (XTE) (NASA), 2 instruments: PCA & HEXTE
URL:http://heasarc.gsfc.nasa.gov/docs/xte/XTE.html

ASCA/ASTRO-D (ISAS)
URL:http://www.astro.isas.ac.jp/xray/mission/asca/ascaE.html

Roentgen Satellite (ROSAT) (MPE)
URL:http://wave.xray.mpe.mpg.de/rosat/

Einstein, the second High Energy Astronomy Observatory (HEAO-B) [defunct]
(NASA), 5 instruments: IPC, HRI, SSS, FPCS, & OGS
URL:http://heasarc.gsfc.nasa.gov/docs/einstein.html

Gamma-ray:

Fred Lawrence Whipple Gamma-Ray Observatory (SAO), a 10 m and 11 m
instrument
URL:http://linmax.sao.arizona.edu/help/FLWO/whipple.html

CANGAROO (U. Adelaide & Nippon), 4 4-m cameras
URL:http://www.physics.adelaide.edu.au/astrophysics/cangaroo.html

Compton Gamma-Ray Observatory (NASA) [space-based],
4 instruments: OSSE, EGRET, COMPTEL, & BATSE
URL:http://cossc.gsfc.nasa.gov/cossc/cgro.html

Cosmic ray:

The High Resolution Fly's Eye Cosmic Ray Detector HiRes
URL:http://www.physics.adelaide.edu.au/astrophysics/FlysEye.html

------------------------------

Subject: B.03 What new telescopes/instruments are being built?
Author: Bill Arnett ,
William Keel ,
Steve Willner ,
Joseph Lazio ,
Jennifer Imamura
with corrections and additions by many others

(These lists are undoubtedly incomplete. Additions and corrections
welcome!)

A list of astronomical instruments is also at
URL:http://www.futureframe.de/astro/instr/index.html.

Optical/Infrared Telescopes (nighttime):

Now actually under construction:
16.4 Very Large Telescope Cerro Paranal, Chile
(quartet of 8.2-m telescopes)
URL:http://www.hq.eso.org/projects/vlt/
11.0 Hobby-Eberly Telescope, Mt. Fowlkes, Texas
(spectroscopy only)
URL:http://www.as.utexas.edu/mcdonald/het/het.html
URL:http://www.astro.psu.edu/het/
8.0 Gemini North Mauna Kea, Hawaii
8.0 Gemini South Cerro Pachon, Chile
URL:http://www.gemini.edu/
8.2 Subaru (JNLT) Mauna Kea, Hawaii
URL:http://www.naoj.org/
6.5 MMT Mt. Hopkins, Arizona
(replace current six mirrors with single one; see B.01)
URL:http://sculptor.as.arizona.edu/foltz/www/mmt.html
2.2 SOFIA NASA
(included because it will be an airborne observatory)
URL:http://sofia.arc.nasa.gov/

Others likely to start soon:
Large Binocular Telescope, (Italy; U. Arizona), pair of 8-m
telescopes, Mt. Graham, Arizona
URL:http://lbtwww.arcetri.astro.it/

Canary Islands Large Telescope Canary Islands, Spain, 10 m segmented mirror
URL:http//www.iac.es/10m/uk10m.html

Magellan (Carnegie Institution Observatories), 6.5 m, Las Campanas
URL:http//medusa.as.arizona.edu/mlab/mag.html

Radio telescopes under construction in design stages:

Submillimeter Array, (Smithsonian Astrophysical Observatory), six 8-m
dishes at Mauna Kea
URL:http//sma2.harvard.edu/index.html

Millimeter Array (MMA) (NRAO)
URL:http//www.mma.nrao.edu/

Green Bank Telescope (NRAO)
URL:http//www.gb.nrao.edu/GBT/GBT.html

X-ray:

Astro-E (ISAS)
URL:http//www.astro.isas.ac.jp/xray/mission/astroe/

High-Throughput X-Ray Spectroscopy Mission (ESA)
URL:http//astro.estec.esa.nl/XMM/xmm.html

Gamma-ray:

INTEGRAL (ESA)
URL:
http://astro.estec.esa.nl/SA-general.../integral.html


Neutrino:

Antarctic Muon and Neutrino Detector Array (AMANDA)
URL:http//amanda.berkeley.edu/

Deep Undersea Muon and Neutrino Detection (DUMAND)
URL:http//www.phys.washington.edu/~dumand/

Gravitational Waves:

LIGO, (US), 4 km path
URL:http//www.ligo.caltech.edu/

Virgo, (Italy), 3 km path
URL:http//www.pi.infn.it/virgo/

------------------------------

Subject: B.04 What is the resolution of a telescope?
Author: Steve Willner

The _limiting_ resolution of a telescope can be no better than a size
set by its aperture, but there are many things that can degrade the
resolution below the theoretical limit. Obvious examples are
manufacturing defects and the Earth's atmosphere. Another interesting
one is the addition of a central obstruction (e.g., secondary mirror)
which degrades the resolution for most practical purposes even though
it _shrinks_ the size of the central diffraction disk. The problem is
that even though the disk diameter decreases, the central disk
contains a smaller fraction of the incident light (and the rings
contain more). This is why modest sized refractors often outperform
reflectors of the same size.

Giving a precise value for the resolution of an optical system depends
on having a precise definition for the term "resolution." That isn't
so easily done; the most general definition must be based on something
called "modulation transfer function." If you don't want to be
bothered with that, it's enough to note that in all but pathological
cases, the diameter (full width at half maximum in radians) of the
central diffraction disk will be very close to the wavelength in use
divided by the diameter of the entrance pupil. (The often seen factor
of 1.22 refers to the radius to the first null for an _unobstructed_
aperture, but a different factor will be needed if there is a central
obstruction.) In practical units, if the wavelength (w) is given in
microns and the aperture diameter (D) in meters, the resolution in
arcseconds will be:
R = 0.21 w/D .

------------------------------

Subject: B.05 What's the difference between astronomy and astrology?
Author: Phillippe Brieu

Although astronomy and astrology are historically related and many
individuals were interested in both, there is today no connection
between the two. Hence two different USENET newsgroups exist:
sci.astro (for the former) and alt.astrology (for the latter). DO NOT
CONFUSE THEM.

Astronomy is based on the laws of physics (and therefore mathematics)
and aims at describing what is happening to the universe based on what
we observe today. Because the laws of physics are constant (as far as
we can tell), astronomy can also explain how the universe behaved in
the past and can propose a limited number of possible scenarios for
its future (see FAQ entry about Big Bang). Everyday life applications
of astronomy include calculations/predictions of sunrise/sunset times,
moon phases, tides, eclipse locations, comet visibility, encounters
between various celestial bodies (e.g., SL9 comet crash onto Jupiter
in 1994), spacecraft trajectories, etc.

Astrology on the other hand claims it can predict what will happen to
individuals (or guess what is happening to them), or to mankind, based
on such things as solar system configurations and birth dates. Common
applications include horoscopes and such. Regardless of whether there
is scientific support for astrology, its goal and methods are clearly
distinct from those of astronomy.

------------------------------

Subject: B.06 Is there scientific evidence for/against astrology?

Yes, but this question should be discussed in alt.astrology and/or
sci.skeptic, not in sci.astro.

------------------------------

Subject: B.07 What about God and the creation?
Author: Joseph Lazio

Astronomy is silent on the matter of God and the creation.

Astronomy is based on applying the laws of physics to the Universe.
These laws of physics attempt to describe the natural world and are
based on experiments here on Earth and our observations of the rest of
the Universe. The key words are "natural world." It is obvious that
the existence of a supernatural being(s) is outside the realm of the
natural laws.

It should be noted that people do use the results of astronomy to
attempt to deduce the existence of God (or gods). Unfortunately, one
can reach two, equally valid conclusions:

* Many atheists (including some astronomers) argue that the
regularity of the natural world, combined with our apparent lack
of distinction in it (the Earth is just one planet, around one
star, in one galaxy, etc.), are compelling reasons not to believe
in any god.

* Many theists (including ordained ministers and priests who are
also astronomers) find the study of the natural world another
means of understanding God. The beauty, order, and sheer scope of
the natural world are profound clues to the power and intelligence
which created it all.

Since sci.astro is devoted to science of astronomy (i.e., the natural
world), sci.astro is not the appropriate forum for such a religious
debate. If you would like to discuss such things, you should go to
talk.origins, talk.religion.*, or maybe soc.religion.*

------------------------------

Subject: B.08 What kind of telescope should I buy?

See the Purchasing Amateur Telescopes FAQ, posted regularly to
sci.astro.amateur, or at your favorite FAQ location.

------------------------------

Subject: B.09 What are the possessive adjectives for the planets?
Author: Steve Willner ,
Andrew Christy

Mercury Mercurian mercurial
Venus Venerian venereal
Venusian
Cytherean
Earth Terrestrial
Telluric
Mars Martian martial
Arean
Jupiter Jovian jovial
Saturn Saturnian saturnine
Uranus Uranian
Neptune Neptunian
Pluto Plutonian

The first form(s) refers to the planet as an object (e.g., "Saturnian
rings"). The second form refers to human characteristics historically
associated with the planet's astrological influence or with the god or
goddess represented by the planet (e.g., "a jovial individual").

------------------------------

Subject: B.10 Are the planets associated with days of the week?
Author: many

Surprisingly, yes. This comes from the historical association of the
"planets" with gods and goddesses. In ancient times, the word
"planets" was from the Greek for "wanderers" and referred to objects
in the sky that were not fixed like the stars. Some of these
associations are clearer in English, especially if we compare with
names of Norse or Old English gods/goddesses, while others are clearer
from comparing French/Spanish with the Roman gods and goddesses. We
have:

Sun Moon Mars Mercury Jupiter Venus Saturn

Roman Luna Mars Mercury Jupiter Venus Saturn
Norse Tiw Woden Thor Freya

French dimanche lundi mardi mercredi jeudi vendredi samedi
Spanish domingo lunes martes miercoles jueves viernes sabado
Italian Domenica Lunedi Martedi Mercoledi Giovedi Venerdi Sabato
English Sunday Monday Tuesday Wednesday Thursday Friday Saturday
German Sonntag Montag Dienstag Mittwoch Donnerstag Freitag Samstag

Notes:
1. Sun: Dimanche and domingo are from the Latin for "Day of the Lord."
2. Saturn: Sabado is from "Sabbath."
3. German and English use Teutonic, not Scandinavian forms of the God
names, e.g., "Woden" in "Wednesday," not "Odin," which is the Norse
equivalent. The God of Tuesday was Tiw.
4. Russian numbers three days (Tuesday = 2nd, Thursday = 4th, and
Friday= 5th) and does not use God/Planet names for the rest.

In Sanskrit (an Indo-European language), we also find ("vaar" means day)

Sun Ravivaar Ravi Sunday
Moon Somvaar Som Monday
Mars Mangalvaar Mangal Tuesday
Mercury Budhvaar Budh Wednesday
Jupiter Brihaspativaar Brihaspati Thursday
Venus Shukravaar Shukr Friday
Saturn Shanivaar Shani Saturday

This association between planets and days of the week holds in at
least some non-European languages as well.

In Japanese the days Tuesday through Saturday (and the associated
planets) are named after the five Asian elements, rather than gods.

Japanese
days planets

Sun nichiyoubi hi (same kanji as nichi)
Moon getsuyoubi tsuki (same kanji as getsu)
Mars kayoubi kasei
Mercury suiyoubi suisei
Jupiter mokuyoubi mokusei
Venus kinyoubi kinsei
Saturn doyoubi dosei

For additional reading, particularly about Eastern day naming, see
URL:http://www.cjvlang.com/Dow/.

------------------------------

Subject: B.11 Why does the Moon look so big when it's near the horizion?
Author: Carl J. Wenning ,
Steve Willner

The effect is an optical illusion. You can verify this for yourself
by comparing the size of the Moon when it's on the horizon to that of
a coin held at arm's length. Repeat the measurement when the Moon is
overhead. You will find the angular size unchanged within the
accuracy of the measurement.

In fact two effects contribute to making the Moon slightly *smaller*
on the horizon than overhead. Atmospheric refraction compresses the
apparent vertical diameter of the Moon slightly. A really precise
measurement will reveal that the horizontal diameter is about 1.7%
smaller when the Moon is on the horizon because you are farther from
it by approximately one Earth radius.

The Sun, incidentally, shows the much same effects as the Moon, though
it's a *really* BAD idea to look directly at the Sun without proper
eye protection (NOT ordinary sunglasses). The change in apparent
angular diameter is, of course, less than 0.01% instead of 1.7%
because the Sun is farther away. (See the next entry.)

The probable explanation for this illusion is that the "background"
influences our perception of "foreground" objects. If you've seen the
"Railroad Track Illusion"---in which two blocks of the same size
placed between parallel lines will appear to be different
sizes---you're familiar with the effect. The Moon illusion is simply
the railroad track illusion upside-down. For some reason, the sky
nearer the horizon appears much more distant than the point directly
overhead. The explanation for this apparent difference in distance is
not known, but an informal survey by one of the authors (CJW)
indicates that all people see this distance difference. The
explanation for the Moon illusion is then that when we see the moon
"against" a more "distant" horizon it appears larger than when we see
it "against" a much "closer" one.

Additional evidence in support of this idea is the behavior of
"afterimages." An afterimage of a constant size can be impressed upon
the human eye by staring at a light bulb for a few minutes. By
projecting the afterimage on a sheet of white paper, the size of the
afterimage can be varied by changing the eye-to-paper distance. A
similar effect is seen with the night sky---an afterimage projected
toward the horizon appears larger than one projected toward the
zenith.

Much more extensive discussions are available in

* The Planetarian, Vol. 14, #4, December 1985, also available
at URL:http://www.griffithobs.org/IPSMoonIllus.html; and
* Quarterly Journal of the Royal Astronomical Society, vol. 27,
p. 205, 1986.

------------------------------

Subject: B.12 Is it O.K. to look at the Sun or solar eclipses using
exposed film? CDs?
Author: Joseph Lazio ,
Steve Willner

This question appears most frequently near the time of solar eclipses.

The short answer is no! The unobscured surface of the sun is as
bright as ever during a partial eclipse and just as capable of causing
injury. The injured area on the retina may be a bit smaller, of
course, but that's no reason to risk damage. Moreover, there are no
nerve endings in the retina, so one can do permanent damage without
being aware of it.

People have proposed a host of methods for viewing the Sun, including
exposed film and CDs. These home-grown methods typically suffer from
two flaws. First, they do not cut out enough visible light. Second,
they provide little protection against ultraviolet or infrared light.

The only safe method for viewing the Sun directly is using No. 14
arc-welder filter or a metallicized glass or Mylar filter. A local
hardware store or construction supply store should carry or know where
to obtain arc-welder filters. Many astronomy magazines carry ads for
solar filters.

Whatever filter you use, inspect it to make sure it has not been
damaged. Even a pinhole can let through enough light to cause injury.
If you use a filter over a telescope or binocular, make sure the
filter is firmly attached and cannot come off accidentally! Never use
an eyepiece filter, which can overheat and crack. Any filter should
cover the entire entrance aperture (or more precisely, any part of the
entrance aperture that isn't covered by something completely opaque).
If using only one side of a binocular, cover the other side.

An alternative way to view the sun is in projection. You can use a
pinhole camera or a telescope, eyepiece, and screen. Many observing
handbooks illustrate suitable arrangements. This method is not only
safe, it can give a magnified image and make it easier to see details.

If you are lucky enough (or put in the advance planning) to see a
total solar eclipse, the total phase can be enjoyed with no eye
protection whatsoever. In fact, experienced eclipse-goers often cover
one eye with a patch for several minutes before totality so the eye
will be dark-adapted during totality. Just be sure to look away (or
through your filter again) the instant totality is over.

Additional information on the safe viewing of solar eclipses is at the
Eclipse Home Page, URL:http://sunearth.gsfc.nasa.gov/eclipse/.

------------------------------

Subject: B.13 Can stars be seen in the daytime from the bottom of a
tall chimney, a deep well, or deep mine shaft?
Author: Michael Dworetsky

The short answer is no (well, almost no). The long answer is given by
David Hughes in the Quarterly Journal of the Royal Astron. Soc., 1983,
vol. 24, pp 246-257.

This mistaken notion was first mentioned by Aristotle and other
ancient sources, and was widely assumed to be correct by many literary
sources of the 19th century, and even believed by some astronomers.
But every astronomer who has ever tested this by experiment came away
convinced it was impossible.

If you want to try an interesting experiment to see why it is believed
that whatever people see up chimneys cannot be stars, try the
experiment at night, as I have done, using a cardboard tube centre
from a paper towel roll (mine had an opening of 25 square degrees).
You will see that, at random, you will seldom include one visible
star, rarely two, and virtually never more than two, in the field.

Separate experiments to attempt to see Vega and Pollux through tall
chimneys were performed by J. A. Hynek and A. N. Winsor. They were
unable to detect the stars under near perfect conditions, even with
binoculars.

The daytime sky is simply too bright to allow us to see even the
brightest stars (although Sirius can sometimes be glimpsed just after
the Sun rises if you know exactly where to look.) Venus can be seen
as a tiny white speck but again, you have to be looking exactly at the
right spot.

The most likely explanation for the old legend is that stray bits of
rubbish get caught in the updraft and catch the sunlight as they
emerge from the chimney. It is possible to see stars in the daytime
with a good telescope, as long as it has been prefocused and can be
accurately pointed at a target.

------------------------------

Subject: B.14 Why do eggs balance on the equinox?
Author: Bob Riddle

Luck. In short, there's no validity to the idea that eggs can only be
balanced on the equinox.

This question often arises during March and September, when it is not
unusual to hear, see, or read news reports about the equinox occurring
during that month. It is also not unusual to hear news reports being
able to balance an egg on the equinox day. In fact many times these
reports will highlight a classroom wherein the students are shown
trying to balance eggs. Naturally some eggs will balance and others
will not---one time, then perhaps do differently the next time.

The focus in these reports, however, seems to be on the eggs that do
balance rather than the observations from the experiment that not all
eggs balanced the first time tried, nor did all eggs always balance,
or perform the same way every time.

There are a number of problems with the idea of balancing an egg:

1. Typically, explanations about the balancing act involve gravity.
One explanation that I've heard suggested that gravity is "balanced"
when the sun is over the earth's equator. Another gravity-based
explanation is that the sun exerts a greater gravitational attraction
on the earth on these two days. If gravity is involved in balancing
the egg shouldn't other objects balance as well? Or is gravity
selective such that only an egg is affected on this particular day?

2. The equinox is a certain day, while the sun is actually at the
equinox point for an instant (0 degrees on the celestial equator and
12 hours within the constellation Virgo). Therefore, shouldn't the egg
only be balanced at the specific time that the sun reaches that
position?

3. If the Sun's gravity is involved, shouldn't latitude have an
effect? For example I live at 40 degrees north. Shouldn't the egg
lean at an angle pointing towards the sun where I live---and if so,
then it should only be standing straight up at the equator?

You can of course conduct your own experiment. Issues to consider
when designing your experiment include, Would the same egg balance on
any other day(s) during the year? What would be the results of
standing the same egg under the same physical conditions and at the
same time each day throughout the year?


------------------------------

Subject: B.15 Is the Earth's sky blue because its atmosphere is
nitrogen and oxygen? Or could other planets also have blue
skies?
Author: Paul Schlyter

The Earth's sky is blue because the air molecules (largely nitrogen
and oxygen) are much smaller than the wavelength of light. When light
encounters particles much smaller than its wavelength, the scattered
intensity is inversely proportional to the 4'th power of the
wavelength. This is called "Rayleigh scattering," and it means that
half the wavelength is scattered with 2**4 = 16 times more intensity.
That's why the sky appears blue: the blue light is scattered some 16
times more strongly than the red light. Rayleigh scattering is also
the reason why the setting Sun appears red: the blue light has been
scattered away from the direct sunlight.

Thus, if the atmosphere of another planet is composed of a transparent
gas or gases whose molecules are much smaller than the wavelength of
light, we would, in general, also expect the sky on that planet to
have a blue color.

If you want another color of the sky, you need bigger particles in the
air. You need something bigger than molecules in the air---dust.

Dust particles can be many times larger than air molecules but still
small enough to not fall out to the ground. If the dust particles are
much larger than the wavelength of light, the scattered light will be
neutral in color (i.e., white or gray)---this also happens in clouds
here on Earth, which consist of water droplets. If the dust particles
are of approximately the same size as the wavelength of light, the
situation gets complex, and all sorts of interesting scattering
phenomena may happen. This happens here on Earth from time to time,
particularly in desert areas, where the sky may appear white, brown,
or some other color. Dust is also responsible for the pinkish sky on
Mars, as seen in the photographs returned from the Viking landers.

If the atmosphere contains lots of dust, the direct light from the Sun
or Moon may occasionally get some quite unusual color. Sometimes,
green and blue moons have been reported. These phenomena are quite
rare though---they happen only "once in a blue moon...." The dust
responsible for these unusual color phenomena is most often volcanic
in origin. When El Chicon erupted in 1982, this caused unusually
strongly colored sunsets in equatorial areas for more than one year.
The much bigger volcanic explosion at Krakatoa, some 110 years ago,
caused green and blue moons worldwide for a few years. (See also
Section 3 of the FAQ, Question C.08, on the meaning of the term "blue
moon.")

One possible exception to the above discussion is if the clouds on the
planet are composed of a strongly colored chemical. This might occur
on Jupiter, where the clouds are thought to contain sulfur, phosphorus,
and/or various organic chemicals.

It's also worth pointing out that the light of the planet's primary is
quite insignificant. Our eyes are highly adaptable to the dominating
illumination and perceive it as "white," within a quite wide range of
possible colors. During daytime, we perceive the light from the Sun
(6000 K) as white, and at night we perceive the light from our
incandescent lamps (2800 K, like a late, cool M star) as white. Only
if we put these two lights side-by-side, at comparable intensities,
will we perceive a clear color difference.

If the Sun was a hot star (say of spectral type B), it's likely we
still would perceive its light as "white" and the sky's color as blue.

Additional discussion of the color of the sky on planets and moons in
the solar system is in Chapter 10 of _Pale Blue Dot_ by Carl Sagan.

------------------------------

Subject: B.16 What are the Lagrange (L) points?
Author: Joseph Lazio ,
John Stockton

The Lagrange points occur in a three-body system. Take a system
consisting of a large mass M, orbited by a smaller mass m, and a third
mass u, where M m u. There are five points where u can be and
have the same orbital period as m.

Three lie on the line connecting M and m. One (L1) lies between M and
m, one (L2) lies outside the orbit of m, and one (L3) lies on the
other side of M from m.

Two are in the orbit of m, 60 degrees ahead (L4) and 60 degrees behind
it (L5).

Pictorially, we have something like this (not too scale!), with the
direction of revolution indicated for m:

L4
\
\ orbit of m ^
\ |
L3 M L1 m L2 |
/ |
/
/
L5

The Lagrangian points are often considered as places where objects,
such as satellites can be "parked" for long periods. For instance,
the SOHO satellite sits at the Sun-Earth L1 point in order to have a
continuous, unobstructed view of the Sun, and the Wilkinson Microwave
Anisotropy Probe observed from the L2 point. There is a group of
asteroids, known as Trojans, which occupy the Sun-Jupiter L4 and L5
points. There are also various groups advocating human colonization
of space which support putting a colony at the Earth-Moon L5 point.

In fact, the L1, L2, and L3 points are "unstable equilibria." That
is, an object placed there will slowly drift away if there are any
other gravitational tugs on it (which there always will be due to
other objects in the solar system). Thus, placing a spacecraft at the
Sun-Earth L1 or L2 point requires regular "course corrections" so that
it doesn't move too far from the L1 or L2 point. The L4 and L5 points
are generally stable so that one should be able to remain at them
indefinitely.

Additional diagrams for the L points is at the WMAP site,
URL:http://map.gsfc.nasa.gov/m_mm/ob_techorbit1.html.

------------------------------

Subject: B.17 Are humans affected psychologically and/or physically by
lunar cycles?
Author: Joseph Lazio

I contend that the answer is yes and no.

Some people will travel hundreds, even thousands of kilometers to
watch a total solar eclipse in which the Moon passes in front of the
Sun. Professional astronomers routinely ask for "dark time," i.e.,
time during the new Moon, for their observations. (The reason is that
the light from the Moon can make it more difficult to see faint
objects. Compare the difference in the brightness of the sky between
new and full Moon some month.) Clearly these are examples in which the
phase of the Moon affects people's behavior.

However, when people talk about the effect of the Moon, they are
typically referring to the idea that X increases during the full Moon,
where X is "crime," "births," or some other aspect of human behavior.
(The word "lunacy" is derived from "luna," the Latin word for Moon.) I
am aware of almost no evidence to support this belief, despite ardent
support for it from police officers and emergency room and OB/GYN
nurses. For instance, the late astronomer George Abell examined the
birth records from LA hospitals for over 10,000 natural births (i.e.,
no C-sections). He could find no correlation between the number of
births and the phase of the Moon.

The accepted explanation for this perceived effect is a human tendency
to find order where there is none. After a particularly busy shift one
night, a police officer or nurse will notice a full or nearly full
Moon. The full Moon can be such a brilliant sight that it is easy to
see how one might think there would be an association. Humans also
have a tendency to forget contrary evidence. Thus, the police officer
or nurse will not remember the last busy night that was during a new
Moon (after all it is difficult to see the new Moon!). From this
start, it doesn't take long for one to become convinced that the full
Moon might have an effect on humans. This belief might also become
self-fulfilling. For instance, a police officer might become less
tolerant of minor offenses during the full Moon (and the additional
light provided by the full Moon might help him/her see more). Another
contributing factor might be people's inability to tell when the full
Moon actually occurs. When I was teaching astronomy, I had a student
tell me that the first-quarter Moon was "full."

I've also been told by a futures trader that recommended practice is
to buy during one phase and sell during another. Although he thought
it was a result of the phase of the Moon influencing the buying and
selling, I think a more simple explanation is that this practice is
apparently what they are taught (perhaps resulting from the same kind
of misconception that produces the crime and birth myths). (I'm not
picking on police officers or nurses. I've just heard this belief
expressed most strongly from them, and their professions can require
them to be up late at night, when the full Moon is most likely to be
noticed.)

Another common belief is that the human female's menstrual cycle is
influenced by the phase of the Moon. There are two problems with this
belief. First, the average woman's menstrual cycle is 28 days, which
is close to the orbital period of the Moon, but is not exactly equal
to it. The range of menstrual cycle lengths, though, is quite large.
I've heard of women having cycles as short as 21 days and as long as
52 days. If the Moon controlled or influenced the length of the cycle,
it is not clear why the range would be so large. Second, other major
mammals do not have a cycle close to 28 days. In particular, the
length of the cycle for chimpanzees, our closest relative species, is
35 days.

------------------------------

Subject: B.17 How do I become an astronomer? What school should I
attend?
Author: Suzanne H. Jacoby

This material is extracted from the National Optical Astronomy
Observatories' Being an Astronomer FAQ,
URL:http://www.noao.edu/education/astfaq.html.

Astronomers are typically good at math, very analytical, logical, and
capable of sound reasoning (about science, anyway). Computer literacy
is a necessity. While not all astronomers are skilled computer
programmers, all should be comfortable using a computer for editing
files, transferring data across networks, and analyzing their
astronomical data and images. Other valuable traits are patience and
determination for sticking to a difficult problem or theory until
you've seen it through---which can take years. The final product of
scientific research is the dissemination of the knowledge gained, so
don't overlook the importance of communication skills like effective
public speaking at professional meetings and the ability to publish
well written articles in scientific journals.

Many of these skills are developed during one's education and
training. In the U.S., a typical astronomer will obtain a Bachelor of
Science (B.S.) degree in a physical science or mathematics, then
attend graduate school for 5--7 years to obtain a Ph.D. After earning
a Ph.D., it is common to take a postdoctoral position, a temporary
appointment which allows an astronomer to concentrate on his or her
own research for about two to three years. These days, most people
take a second postdoc or even a third before they are able to land a
faculty or scientific staff position.

If you want to become an astronomer, a general principle is to obtain
as broad and versatile an education as possible while concentrating in
mathematics, physics, and computing. It is not critical that your
Bachelor's degree be in astronomy. Students with a strong core of
physics classes in addition to some astronomy research experience are
most likely to be accepted to graduate programs in astronomy.

Additional information on astronomy as a career can be obtained from
the American Astronomical Society,
URL:http://www.aas.org/education/career.html, and the
Harvard-Smithsonian Center for Astrophysics (contact their
Publications Department, MS-28, 60 Garden Street, Cambridge, MA 01238,
USA, or call 617-495-7461, ask for the brochure "Space for Women").

------------------------------

Subject: B.19 What was the Star of Bethlehem?
Author: Mike Dworetsky

[This question is most popular around Christmas time.]

It is first and most important to stress that the Bible is a religious
book. The Star of Bethlehem is mentioned only briefly in the book of
Matthew. As such Matthew's description of it may have been religious
rather than scientific. Indeed, it has also been pointed out that the
Star story is similar to a Jewish Midrash, or moral tale illustrating
a religious point, which does not necessarily have to have any basis
in fact. Furthermore, at the time the Bible was written the word
"star" could be used to indicate essentially anything in the sky. The
Star of Bethlehem was almost certainly not what we understand today a
star to be (namely a ball of gas shining by interior thermonuclear
fusion).

Nearly any spectacular sky phenomenon (comet, supernova, nova, etc.)
has been identified as the Star of Bethlehem at one time or another,
but recent interest has focussed on conjunctions of various planets,
possibly in auspicious constellations. Two examples are the
following:

Michael Molnar has found that there was an double occultation of
Jupiter in March and April of 6 BC in Aries that would have been
calculable even by the means available to astrologers (which the Magi
were) and that would have been of high significance in magian
astrology (which differed somewhat from astrology of the modern era).
However it would have been invisible, taking place in daylight. Thus
there is a perfectly good explanation as to why Herod's courtiers had
not seen it, but "wise men from the East" knew all about it. The
occultation also provided a neat explanation of why the star was seen
over Bethlehem---from Jerusalem, the second occultation's azimuth was
close to the direction of the town. Molnar also points out that the
Romans regarded the horoscope of Jesus as a royal one.

And for a small commentary on one of Molnar's points, see my paper
with Steve Fossey in The Observatory in 1998 or at
URL:http://www.star.ucl.ac.uk/~mmd/star.html.

On 3 May 19 BC, the planets Saturn and Mercury were in close
conjunction, within 40 minutes of arc of each other. Then Saturn moved
eastward to meet with Venus on 3 June 12 BC. During this conjunction
the two were only 7.2 minutes of arc apart. Following this
conjunction, on 3 August 12 BC, Jupiter and Venus came into close
conjunction just before sunrise, coming within 4.2 minutes of arc from
each other as viewed from earth, and appearing as a very bright
morning star. This conjunction took place in the constellation Cancer,
the "end" sign of the Zodiac. Ten months later, on 2 June 17 BC, Venus
and Jupiter joined again, this time in the constellation Leo. The two
planets were at best 6 seconds of arc apart; some calculations
indicate that they actually overlapped each other. This conjunction
occurred during the evening and would have appeared as one very bright
star. Even if they were 6 seconds of arc apart, it would have required
the sharpest of eyes to split the two, because of their brightness.

(Some of this information is adapted from a longer article at
URL:http://sciastro.net/portia/articles/thestar.htm. There is also
other pertinent information at this site regarding the astronomy
during that time.)

------------------------------

Subject: B.20 Is it possible to see the Moon landing sites?
Author: David W. Knisely

It is possible to locate and observe the Apollo landing "sites," but
it is *not* possible with current equipment to see the hardware left
there, since their sizes are far too small to be resolved
successfully. For example, a common backyard 6 inch aperture
telescope can only resolve craters on the moon which are about 1.5
miles or so across. Even telescopes with a resolution comparable to
that of the Hubble Space Telescope can only resolve details about 100
meters across (the size of a football or soccer field). Lasers fired
from Earth are bounced off special retro-reflectors left at these
sites by the astronauts, and the faint return pulse is then detected
by Earth-based telescopes equipped with special instruments to measure
the Earth-moon distance, but otherwise, we can't see any man-made
equipment left at the landing sites. If you wish to see the sites
through a telescope for yourself, here are the approximate locations
of the Apollo landing sites (see the Project Apollo Web site,
URL:http://www.ksc.nasa.gov/history/apollo/apollo.html, for more
exact locations and descriptions or
URL:http://www.boulder.swri.edu/%7Edurda/Apollo/landing_sites.html for
set of images of the landing sites at increasingly higher resolution):

APOLLO 11: 0.67 deg. N, 23.49 deg. E, near southwest edge of Mare
Tranquillatis a little northwest of the 6-mile wide crater Moltke.

APOLLO 12: 3.20 deg. S, 23.38 deg. W, in Oceanus Procellarum southeast
of the crater Lansberg (also the landing site of Surveyor 3).

APOLLO 14: 3.67 deg. S, 17.47 deg. W., in Fra Mauro highlands just north
of northwestern rim of large shallow Fra Mauro crater.

APOLLO 15: 26.10 deg.N., 3.65 deg. E., Next to Hadley Rille and
southwest of Mt. Hadley in the lunar Apennine Mountains.

APOLLO 16: 8.99 deg. S., 15.52 deg. E., higlands north of the ruined
crater Descartes and southeast of the double crater Dolland B/C.

APOLLO 17: 20.16 deg. N., 30.77 deg. E., in the southwestern Taurus
Mountains roughly between the craters Littrow and Vitruvius.

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2000 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
  #4  
Old February 2nd 06, 02:36 AM posted to sci.astro,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro] Time (Astronomy Frequently Asked Questions) (3/9)


Last-modified: $Date: 2003/07/16 00:00:01 $
Version: $Revision: 4.5 $
URL: http://sciastro.astronomy.net/
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part3

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/ and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: C.00 Time, Calendars, and Terrestrial Phenomena

[Dates in brackets are last edit.]

C.01 When is 02/01/04? or is there a standard way of writing
dates? [2001-12-14]
C.02 What are all those different kinds of time? [2002-05-07]
C.03 How do I compute astronomical phenomena for my location?
[2002-05-04]
C.04 What's a Julian date? modified Julian date? [1998-05-06]
C.05 Was 2000 a leap year? [2000-03-17]
C.06 When will the new millennium start? [2001-01-01]
C.07 Easter:
07.1 When is Easter? [1996-05-01]
07.2 Can I calculate the date of Easter? [1996-12-11]
C.08 What is a "blue moon?" [2001-10-02]
C.09 What is the Green Flash (or Green Ray)? [1999-01-01]
C.10 Why isn't the earliest Sunrise (and latest Sunset) on the
longest day of the year? [2002-01-30]
C.11 How do I calculate the phase of the moon? [1996-10-08]
C.12 What is the time delivered by a GPS receiver? [2002-05-07]
C.13 Why are there two tides a day and not just one? [1999-12-15]

There is also a calendar FAQ maintained by Claus Tondering
,
URL:http://www.tondering.dk/claus/calendar.html.

------------------------------

Subject: C.01 When is 02/01/04? or is there a standard way of writing dates?
Author: Markus Kuhn

The international standard date notation is: YYYY-MM-DD

For example, February 4, 1995 is written as 1995-02-04. This notation
is standardized in International Standard ISO 8601. For more details
regarding this standard, please
URL:http://www.cl.cam.ac.uk/~mgk25/iso-time.html.

Other commonly used notations are e.g., 2/4/95, 4/2/95, 4.2.1995,
04-FEB-1995, 4-February-1995, and many more. Especially the first two
examples are dangerous, because as both are used quite often and can
not be distinguished, it is unclear whether 2/4/95 means 1995-04-02 or
1995-02-04.

Advantages of the ISO standard date notation a

- easily parsed by software (no 'JAN', 'FEB', ... table necessary)
- easily sortable with a trivial string compare
- language independent
- can not be confused with other popular date notations
- consistent with 24h time notation hh:mm:ss which comes also
with the most significant component first and is consequently
also easily sortable (e.g., write 1999-12-31 23:59:59).
- short and has constant length (makes keyboard data entry easier)
- identical to the Chinese date notation, so the largest cultural
group (25%) on this planet is already familiar with it.
- 4-digit year representation avoids overflow problems after
1999-12-31.

In shell scripts, use

date "+%Y-%m-%d %H:%M:%S"

in order to print the date and time in ISO format. In C, use the
string "%Y-%m-%d %H:%M:%S" as the format specifier for strftime().

Other useful information on the ISO standard is at URL:
http://dmoz.org/Science/Reference/St...ards/ISO_8601/
.


------------------------------

Subject: C.02 What are all those different kinds of time?
Author: Paul Schlyter ,
Markus Kuhn ,
Paul Eggert

In the beginning there were only solar days: sunset was considered to
be the end of the day and the beginning of the next day. The Jewish
and Moslem calendars, which nowadays are used only for religious
purposes, still start a new date at sunset instead of midnight.

Later, the solar days were divided into hours: 12 hours for the day and
12 hours for the night. The different lengths of day/night were ignored,
therefore the daylight hours were longer in summer than in winter.

APPARENT (or TRUE) SOLAR TIME: Still later, the hours were made
equally long: the day+night was 24 hours. The "day" now started at
midnight, not at sunset, which was marked as 00:00 (or 12:00 midnight
in English time format). Noon was at 12:00 (or 12:00 noon in English
time format). This is what we now refer to as "true solar time"---it
is the time shown by a properly setup sundial. This time is local, it
is different for different longitudes. (In strict English
construction, 12:00 cannot be given either an A.M. = ante meridiem or
P.M. = post meridiem designation, but it has become common to use
12 A.M. to mean midnight and 12 P.M. to mean noon. In traditional
English, 12 M. = meridies means _noon_; nowadays one is just as likely
to see 12 M. = midnight and 12 N. = noon.)

(In general, the old English A.M./P.M. notation is extremely
problematic. A shorter and more obvious time notation is the modern
24h notation in which the hours in the day range from 00:00 to 23:59.
This notation even allows one to distinguish midnight at the start of
the day [00:00] from midnight at the end of the day [24:00], while the
old English notation requires kludges like starting a contract at
12:01 A.M. in order to make clear which of the two midnights
associated with a date had been intended. The 24h notation is the
official international standard time notation (ISO 8601) and displayed
by almost all digital clocks outside the U.S.A. The 24h notation is
also recommended by the U.S. Naval Observatory in Washington, which
defines official time in the U.S.)

MEAN SOLAR TIME: True Solar Time isn't a uniform time. The time
difference between one noon and the next noon varies through the year,
due to two causes: 1. The earth's orbit is elliptical, not perfectly
circular, and the Earth's speed in its orbit is greater when closer to
the sun. This makes the solar days shorter in July and longer in
January. 2. The Earth's axis of rotation does not point in the same
direction as the axis of the Earth's orbit round the Sun. (The angle
between these two is called the "obliquity of the ecliptic" and is
about 23.45 degrees.) This makes the solar days shorter in March and
September and longer in June and December. To account for these
effects, a fictitious sun, "The Mean Sun," was invented: it moves with
uniform velocity in the plane of the Earth's equator, with the same
average speed as the true Sun. This Mean Sun defines Mean Solar Time:
When the Mean Sun is due south (for northern hemisphere observers), it
is noon Mean Solar Time. Now the time difference between two
consecutive local noons is always the same (ignoring small
irregularities in the Earth's rotation---more about that later).

SIDEREAL TIME: Closely connected with the Mean Solar Time is the
Sidereal Time, which is defined as the RA (Right Ascension) of the
Local Meridian: when the Vernal Point passes the meridian it is 00:00
Sidereal Time. When Orion is at its maximum altitude, it is between
5h and 6h Sidereal Time; when the Big Dipper can be seen close to the
zenith it is about 12h Sidereal Time; and when Sagittarius, with all
its glories close to the center of our Galaxy, reaches maximum
altitude it is around 18h Sidereal Time. The Sidereal Time at a
particular place and location is the same as the local Mean Solar
Time, plus 12 hours, plus the Right Ascension of the Mean Sun (which
is the same as the Mean Longitude of the true sun). It can be
computed from this formula:

LST(hours) = 6.6974 + 2400.051336 * T + 24 * FRAC(JD+0.5) + long/15

whe

LST = Local Sidereal Time in hours
JD = the Julian Day Number for the moment, including fractions of a day
Note that a new Julian Day starts at Greenwich Noon
T = ( JD - 2451545.0 ) / 36525.0
long = your local longitude: east positive, west negative
FRAC = a function discarding the integral part and returning only the
fractional part of a real number.

STANDARD TIME ZONES: Some 100+ years ago the railway made fast
transportation possible for the first time. Quite soon it became
awkward for the travellers to continually have to adjust their clocks
when travelling between different places, and the railway companies
had the problem to select which city's time to use for their own
schedules. An interim solution was to use a specific "railway time,"
but soon standard time zones were created. At first the time to be
used within a country was the local time of the capital of the
country. A few very large countries employed several time zones. It
took a few decades to arrive at a worldwide agreement here, and in
particular there was a "battle" between England and France whether the
world's prime meridian was to be the meridian of the Greenwich or the
Paris observatory. England won this battle, and "Greenwich Mean Time"
(GMT) was universally agreed upon as the world's standard time zones.
Almost all other parts of the world were assigned time zones, which
usually differ from GMT by an integral number of hours. Some
countries (e.g., India) use differences that are not an integral
number of hours.

GMT (Greenwich Mean Time): This term is a historic term which is in a
strict sense obsolete, though often used (although not in astronomy,
e.g., BBC still uses this abbreviation for patriotic reasons ;-) as a
synonym for UTC. In 1972, an international atomic time scale has been
introduced and since then, the time on the zero meridian, which goes
through the old observatory in Greenwich, London, UK, has been called
Universal Time (UT). Prior to 1925, it was reckoned for astronomical
purposes from Greenwich mean noon (12h UT). Sometimes GMT is referred
to as Z ("Zulu"). (This arises from the military custom of writing
times as hours and minutes run together and suffixed with a single
letter designating the time zone: 2100Z = 21:00 UTC. The word "zulu"
is the phonetic word associated with the letter "z.")

UT (Universal time): Defined by the Earth's rotation and determined by
astronomical observations. This time scale is slightly irregular.
There are several different definitions of UT, but the difference
between them is always less than about 0.03 s. Usually one means UT2
when saying UT. UT2 is UT corrected for pole wandering and seasonal
variations in the Earth's rotational speed.

If you are interested in time more precisely than 1 s, then you'll
have to differentiate between the following versions of Universal
Time:

UT0 is the precise solar local time on the zero meridian. It is today
measured by radio telescopes which observe quasars.

UT1 is UT0 corrected by a periodic effect known as Chandler wobble or
"polar wandering", i.e., small changes in the longitude/latitude
of all places on the Earth due to the fact that the geographical
poles of the Earth "wander" in semi-regular patterns: the poles
follow (very approximately) small circles, about 10--20 meters in
diameter, with a period of approximately 400--500 days. The
changes in the longitude/latitude of all places of Earth due to
this amounts to fractions of an arc second
(1 arc second = 1/3600 degree).

UT2 is an even better corrected version of UT0 which accounts for
seasonal variations in the Earth's rotation rate and is sometimes
used in astronomy.

UTC is a time defined not by the movement of the earth, but by a
large collection of atomic clocks located all over the world, the
atomic time scale TAI. When UTC and UT1 are about to drift apart
more than 0.9 s, a leap second will be inserted (or deleted, but
this never has happened) into UTC to correct this. When necessary,
leap seconds are inserted as the 61th second of the last UTC
minute of June or December. During a leap second, a UTC clock
(e.g., a radio clock such as MSF, HBG, or DCF77) shows:

1995-12-31 23:59:59
1995-12-31 23:59:60
1996-01-01 00:00:00

Today, practically all national civil times are defined relative
to UTC and differ from UTC by an integral number of hours
(sometimes also half- or quarter-hours). UTC is defined in ITU-R
Recommendation TF.460-4 and was introduced in 1972.

If you are interested in UTC more precisely than a microsecond,
then you also have to consider the following differences:

The abbreviation UTC can be followed by an abbreviation of the
organization who publishes this time reference signal.
For example, UTC(USNO) is the US reference time published by the
US Naval Observatory, UTC(PTB) is the official German reference
time signal published (via a 77.5 kHz long-wave broadcast) by the
Physikalisch Technische Bundesanstalt in Braunschweig and
UTC(BIPM) is the most official time published by the Bureau
International des Poids et Mesures in Paris, however UTC(BIPM) is
only a filtered paper clock published each year that is used by
the other time maintainers to resynchronize their clocks against
each other. All these UTC versions do not differ by more than a
few nanoseconds.

The acronym UTC stands for Coordinated Universal Time. In 1970
when this system was being developed by the International
Telecommunication Union, it felt it was best to designate a single
abbreviation for use in all languages in order to minimize
confusion. Unanimous agreement could not be achieved on using
either the English word order, CUT, or the French word order, TUC,
so a compromise using neither, UTC, was adopted.

DUT1 is the difference between UTC and UT1 as published by the US
Naval Observatory rounded to 0.1 s each week. This results in the
UT1 which is used e.g., for space navigation.

ET (Ephemeris Time): Somewhere around 1930--1940, astronomers noticed
that errors in celestial positions of planets could be explained by
assuming that they were due to slow variations on the Earth's
rotation. Starting in 1960, the time scale Ephemeris Time (ET) was
introduced for astronomical purposes. ET closely matches UT in the
19th century, but in the 20th century ET and UT have been diverging
more and more. Currently ET is running almost precisely one minute
ahead of UT. In 1984, ET was replaced by Dynamical Time and TT. For
most purposes, ET up to 1983-12-31 and TDT from 1984-01-01 can be
regarded as a continuous time-scale.

TT and Dynamical Time: Introduced in 1984 as a replacement for ET, it
defines a uniform astronomical time scale more accurately, taking
relativistic effects into account. There are two kinds of Dynamical
Time: TDT (Terrestrial Dynamical Time), which is a time scale tied to the
Earth, and TDB (Barycentric Dynamical Time), used as a time reference
for the barycenter of the solar system. The difference between TDT and
TDB is always smaller than a few milliseconds. When the difference
TDT-TDB is not important, TDT is referred to as TT. For most purposes,
TDT can be considered equal to TAI + 32.184 seconds.

TAI (Temps Atomique International = International Atomic Time):
Defined by the same worldwide network of atomic clocks that defines
UTC. In contrast to UTC, TAI has no leap seconds. TAI and UTC were
identical in the late 1950s. The difference between TAI and UTC is
always an integral number of seconds. TAI is the most uniform time
scale we currently have available.


RELATION BETWEEN THE TIME SCALES
--------------------------------

TDT = TAI+32.184s == UT-UTC = TAI-UTC - (TDT-UT) + 32.184s

Starting at TAI-UTC ET/TDT-UT UT-UTC

1972-01-01 +10.00 +42.23 -0.05
1972-07-01 +11.00 +42.80 +0.38
1973-01-01 +12.00 +43.37 +0.81
1973-07-01 -"- +43.93 +0.25
1974-01-01 +13.00 +44.49 +0.69
1974-07-01 -"- +44.99 +0.19
1975-01-01 +14.00 +45.48 +0.70
1975-07-01 -"- +45.97 +0.21
1976-01-01 +15.00 +46.46 +0.72
1976-07-01 -"- +46.99 +0.19
1977-01-01 +16.00 +47.52 +0.66
1977-07-01 -"- +48.03 +0.15
1978-01-01 +17.00 +48.53 +0.65
1978-07-01 -"- +49.06 +0.12
1979-01-01 +18.00 +49.59 +0.59
1979-07-01 -"- +50.07 +0.11
1980-01-01 +19.00 +50.54 +0.64
1980-07-01 -"- +50.96 +0.22
1981-01-01 -"- +51.38 -0.20
1981-07-01 +20.00 +51.78 +0.40
1982-01-01 -"- +52.17 +0.01
1982-07-01 +21.00 +52.57 +0.61
1983-01-01 -"- +52.96 +0.22
1983-07-01 +22.00 +53.38 +0.80
1984-01-01 -"- +53.79 +0.39
1984-07-01 -"- +54.07 +0.11
1985-01-01 -"- +54.34 -0.16
1985-07-01 +23.00 +54.61 +0.57
1986-01-01 -"- +54.87 +0.31
1986-07-01 -"- +55.10 +0.08
1987-01-01 -"- +55.32 -0.14
1987-07-01 -"- +55.57 -0.39
1988-01-01 +24.00 +55.82 +0.36
1988-07-01 -"- +56.06 +0.12
1989-01-01 -"- +56.30 -0.12
1989-07-01 -"- +56.58 -0.40
1990-01-01 +25.00 +56.86 +0.32
1990-07-01 -"- +57.22 -0.04
1991-01-01 +26.00 +57.57 +0.61
1991-07-01 -"- +57.94 +0.24
1992-01-01 -"- +58.31 -0.13
1992-07-01 +27.00 +58.72 +0.46
1993-01-01 -"- +59.12 +0.06
1993-07-01 +28.00 +59.5 +0.7
1994-01-01 -"- +59.9 +0.3
1994-07-01 +29.00 +60.3 +0.9
1995-01-01 -"- +60.7 +0.5
1995-07-01 -"- +61.1 +0.1
1996-01-01 +30.00 +61.63 +0.55
1996-07-01 -"- +62.0 +0.2
1997-01-01 -"- +62.4 -0.2
1997-07-01 +31.00 +62.8 +0.4
1998-01-01 -"- +63.3 -0.1
1998-07-01 -"- +63.7 -0.5
1999-01-01 +32.00 +64.1 +0.1

Additional information about the world time standard UTC (e.g., when
will the next leap second be inserted in time) is available from the
US Naval Observatory and the International Earth Rotation Service
(IERS):

URL:http://tycho.usno.navy.mil/time.html
URL:http://tycho.usno.navy.mil/gps_datafiles.html
URL:http://maia.usno.navy.mil/
URL:ftp://maia.usno.navy.mil/ser7/tai-utc.dat
URL:ftp://tycho.usno.navy.mil/pub/series/ser14.txt
URL:ftp://maia.usno.navy.mil/ser7/deltat.preds

URL:ftp://mesiom.obspm.fr/iers/.
URL:ftp://hpiers.obspm.fr/iers/bul/bulc/BULLETINC.GUIDE

Also URL:http://www.eecis.udel.edu/~ntp/ is a good start if you want
to learn more about time standards.

------------------------------

Subject: C.03 How do I compute astronomical phenomena for my location?
Author: Paul Schlyter

COMPUTING AZIMUTH AND ELEVATION
-------------------------------

To compute the azimuth and elevation of an object, you first must
compute the Local Sidereal Time of the place and time in question.
First convert your local time to UT (Universal Time), with the date
adjusted if needed. Now suppose that the time is Y,M,D,UT where Y,M,D
is the calendar Year, Month (1--12) and Date (1--31), and UT is the
Universal Time in hours+fractions. Also suppose your position is
lat,long, where lat is counted as + if north and - if south, and long
is counted as + if east and - if west. Now, first compute a "day
number", d:

7*(Y + INT((M+9)/12))
d = 367*Y - INT(---------------------) + INT(275*M/9) + D - 730530 + UT/24
4

where INT is a function that discards the fractional part and returns the
integer part of a function. d is zero at 2000 Jan 0.0

Now compute the Local Sidereal Time, LST:

LST = 98.9818 + 0.985647352 * d + UT*15 + long

(east long. positive). Note that LST is here expressed in degrees,
where 15 degrees corresponds to one hour. Since LST really is an angle,
it's convenient to use one unit---degrees---throughout.

Now, suppose your object resides at a known RA (Right Ascension) and
Dec (Declination). Convert both RA and Dec to degrees + decimals,
remembering that 1 hour of RA corresponds to 15 degrees of RA.

Next, compute the Hour Angle:

HA = LST - RA

Now you can compute the Altitude, h, and the Azimuth, az:

sin(h) = sin(lat) * sin(Dec) + cos(lat) * cos(Dec) * cos(HA)

sin(HA)
tan(az) = --------------------------------------------
cos(HA) * sin(lat) - tan(Dec) * cos(Lat)

Here az is 0 deg in the south, 90 deg in the west etc. If you prefer
0 deg in the north and 90 deg in the east, add 180 degrees to az.


A NOTE ON TRIGONOMETRIC FUNCTIONS ON YOUR COMPUTER
--------------------------------------------------

If you have an atan2() function (or equivalent) available on your
computer, compute the numerator and denominator separately and feed
them both to your atan2() function, instead of dividing and feeding
them to your atan() function---then you'll get the correct quadrant
immediately. In the "C" language you would thus write:

az = atan2( sin(HA), cos(HA)*sin(lat)-tan(Dec)*cos(Lat) );

instead of:

az = atan( sin(HA) / (cos(HA)*sin(lat)-tan(Dec)*cos(Lat)) );

On a scientific calculator, there is often a "rectangular to polar"
coordinate conversion function that does the same thing.

Users of Pascal and other programming languages that lack an atan2()
function are strongly encouraged to write such a function of their
own. In Pascal it would be (pi is assumed to have been assigned an
appropriate value---one way is to compute: pi := 4.0*arctan(1) ):

function atan2( y : real, x : real ) real;
(* Compute arctan(y/x), selecting the correct quadrant *)
begin
if x 0
atan2 := arctan(y/x)
else if x 0
atan2 := arctan(y/x) + pi
(* Below x is zero *)
else if y 0
atan2 := pi/2
else if y 0
atan2 := -pi/2
/* Below both x and y are zero *)
else
atan2 := 0.0 (* atan2( 0.0, 0.0 ) is really an error though.. *)
end

Another trick I also use is to add a set of trig functions that work
in degrees instead of radians to my function library---that will make
life a lot easier when you're working in degrees as the basic unit. I
name them sind, cosd, atan2d, etc. If you don't do that, you'll have
to convert between degrees and radians when calling the standard trig
functions.

COMPUTING RISE AND SET TIMES
----------------------------

To compute when an object rises or sets, you must compute when it
passes the meridian and the HA of rise/set. Then the rise time is
the meridian time minus HA for rise/set, and the set time is the
meridian time plus the HA for rise/set.

To find the meridian time, compute the Local Sidereal Time at 0h local
time (or 0h UT if you prefer to work in UT) as outlined above---name
that quantity LST0. The Meridian Time, MT, will now be:

MT = RA - LST0

where "RA" is the object's Right Ascension (in degrees!). If negative,
add 360 deg to MT. If the object is the Sun, leave the time as it is,
but if it's stellar, multiply MT by 365.2422/366.2422, to convert from
sidereal to solar time. Now, compute HA for rise/set, name that
quantity HA0:

sin(h0) - sin(lat) * sin(Dec)
cos(HA0) = ---------------------------------
cos(lat) * cos(Dec)

where h0 is the altitude selected to represent rise/set. For a purely
mathematical horizon, set h0 = 0 and simplify to:

cos(HA0) = - tan(lat) * tan(Dec)

If you want to account for refraction on the atmosphere, set h0 = -35/60
degrees (-35 arc minutes), and if you want to compute the rise/set times
for the Sun's upper limb, set h0 = -50/60 (-50 arc minutes).

When HA0 has been computed, leave it as it is for the Sun but multiply
by 365.2422/366.2422 for stellar objects, to convert from sidereal to
solar time. Finally compute:

Rise time = MT - HA0
Set time = MT + HA0

convert the times from degrees to hours by dividing by 15.

If you'd like to check that your calculations are accurate or just
need a quick result, check the USNO's Sun or Moon Rise/Set Table,
URL:http://aa.usno.navy.mil/AA/data/docs/RS_OneYear.html.

COMPUTING THE SUN'S POSITION
----------------------------

To be able to compute the Sun's rise/set times, you need to be able to
compute the Sun's position at any time. First compute the "day
number" d as outlined above, for the desired moment. Next compute:

oblecl = 23.4393 - 3.563E-7 * d

w = 282.9404 + 4.70935E-5 * d
M = 356.0470 + 0.9856002585 * d
e = 0.016709 - 1.151E-9 * d

This is the obliquity of the ecliptic, plus some of the elements of
the Sun's apparent orbit (i.e., really the Earth's orbit): w =
argument of perihelion, M = mean anomaly, e = eccentricity.
Semi-major axis is here assumed to be exactly 1.0 (while not strictly
true, this is still an accurate approximation). Next compute E, the
eccentric anomaly:

E = M + e*(180/pi) * sin(M) * ( 1.0 + e*cos(M) )

where E and M are in degrees. This is it---no further iterations are
needed because we know e has a sufficiently small value. Next compute
the true anomaly, v, and the distance, r:

r * cos(v) = A = cos(E) - e
r * sin(v) = B = sqrt(1 - e*e) * sin(E)

and

r = sqrt( A*A + B*B )
v = atan2( B, A )

The Sun's true longitude, slon, can now be computed:

slon = v + w

Since the Sun is always at the ecliptic (or at least very very close to
it), we can use simplified formulae to convert slon (the Sun's ecliptic
longitude) to sRA and sDec (the Sun's RA and Dec):

sin(slon) * cos(oblecl)
tan(sRA) = -------------------------
cos(slon)

sin(sDec) = sin(oblecl) * sin(slon)

As was the case when computing az, the Azimuth, if possible use an
atan2() function to compute sRA.

REFERENCES
----------

"Practical Astronomy with your Calculator", Peter Duffet-Smith, 3rd
edition. Cambridge University Press 1988. ISBN 0-521-35699-7.

A good introduction to basic concepts plus many useful algorithms.
The third edition is much better than the two previous editions. This
book is also preferable to Duffet-Smith's "Practical Astronomy with
your Computer", which has degenerated into being filled with Basic
program listings.

"Astronomical Formulae for Calculators", Jean Meeus, 4th ed,
Willmann-Bell 1988, ISBN 0-943396-22-0

"Astronomical Algorithms", Jean Meeus, 1st ed, Willmann-Bell 1991,
ISBN 0-943396-35-2

Two standard references for many kinds of astronomical computations.
Meeus' is an undisputed authority here---many other authors quote his
books. "Astronomical Algorithms" is the more accurate and more modern
of the two, and one can also buy a floppy disk containing software
implementations (in Basic or C) to that book.

------------------------------

Subject: C.04 What's a Julian date? modified Julian date?
Author: Edward Wright ,
William Hamblen

It's the number of days since noon GMT 4713 BC January 1. What's so
special about this date?

Joseph Justus Scaliger (1540--1609) was a noted Italian-French
philologist and historian who was interested in chronology and
reconciling the dates in historical documents. Before the western
civil calendar was adopted by most countries, each little city or
principality reckoned dates in its own fashion, using descriptions
like "the 5th year of the Great Poo-bah Magnaminus." Scaliger wanted
to make sense out of these disparate references so he invented his own
era and reckoned dates by counting days. He started with 4713 BC
January 1 because that was when solar cycle of 28 years (when the days
of the week and the days of the month in the Julian calendar coincide
again), the Metonic cycle of 19 years (because 19 solar years are
roughly equal to 235 lunar months) and the Roman indiction of 15 years
(decreed by the Emperor Constantine) all coincide. There was no
recorded history as old as 4713 BC known in Scaliger's day, so it had
the advantage of avoiding negative dates. Joseph Justus's father was
Julius Caesar Scaliger, which might be why he called it the Julian
Cycle. Astronomers adopted the Julian cycle to avoid having to
remember "30 days hath September ...."

For reference, Julian day 2450000 began at noon on 1995 October 9.
Because Julian dates are so large, astronomers often make use of a
"modified Julian date"; MJD = JD - 2400000.5. (Though, sometimes
they're sloppy and subtract 2400000 instead.)

------------------------------

Subject: C.05 Was 2000 a leap year?
Author: Steve Willner

Yes.

Oh, you wanted to know more?

The reason for leap days is that the year---the time it takes the
Earth to go round the Sun---is not an integral multiple of the
day---the time it takes the Earth to rotate once on its axis. In this
case, the year of interest is the "tropical year," which controls the
seasons. The tropical year is defined as the interval from one spring
equinox to the next: very close to 365.2422 days.

The Julian calendar, instituted by the Roman Emperor Julius Caesar
(who else? , has a 365-day ordinary year with a 366-day leap year
every fourth year. This gives a mean year length of 365.25 years, not
a very large error. However, the error builds up, and by the
sixteenth century, reform was considered desirable. A new calendar
was established in most Roman Catholic countries in 1582 under the
authority of Pope Gregory XIII; in that year, the date October 4 was
followed by October 15---a correction of 10 days. Most non-Catholic
countries adopted this "Gregorian" calendar somewhat later (Great
Britain and the American colonies in 1752), and by then the difference
between Julian and Gregorian dates was even greater than 10 days.
(Russia didn't adopt the Gregorian calendar until after the "October
Revolution"---which took place in November under the new calendar!)
Many of the calendar changeovers elicited strong emotional reactions
from the populations involved; people objected to "losing ten (or
more) days of our lives."

The rule for leap years under the Gregorian calendar is that all years
divisible by four are leap years EXCEPT century years NOT divisible by
400. Thus 1700, 1800, and 1900 were not leap years, while 2000 will be
one. This rule gives 97 leap years in 400 years or a mean year length
of exactly 365.2425 days.

The error in the Gregorian calendar will build up to a full day in
roughly 3000 years, by which time another reform will be necessary.
Various schemes have been proposed, some taking account of the changing
lengths of the day and/or the tropical year, but none has been
internationally recognized. Leaving a reform to our descendants seems
reasonable, since there is no obvious need to make a correction now.

------------------------------

Subject: C.06 When will the new millennium start?
Author: Steve Willner ,
Paul Schlyter

There is a difference of opinion. Steve Willner writes:

Big "end of millennium" parties were held on 1999-12-31. The
psychological significance of changing the first digit in the year
must not be discounted. (Preceeding these parties were the big
headaches that occurred as everybody rushed to ensure---appropriately
enough---that the date code in everybody's computer did not break on
the next day.) However, the third millennium A.D. in fact begins on
2001-01-01; there was no year zero, and thus an interval of 2000 years
from the arbitrary beginning of "A.D." dates will not have elapsed
until then.

More details may be found in an article by Ruth Freitag in the 1995
March newsletter of the American Astronomical Society. I am seeking
permission to include the article in the FAQ.

A view to the contrary is expressed by Paul Schlyter :

On 2000 January 1 of course! Some people argue that it should be 2001
January 1 just because Roman Numerals lacks a symbol for zero, but I
find that irrelevant, because:

1. Our year count wasn't introduced until A.D. 525---thus the people
who lived at A.D. 1 were completely unaware that we label that
year "A.D. 1."

2. No real known event occurred at either 1 B.C. or A.D. 1---Jesus
was born some 6--7 years earlier. Thus the new millennium
should _really_ have been celebrated already, at least of we
want to celebrate 2000 years since the event that supposedly
started our way of counting years....

(Yes, the Julian calendar _was_ around at 1 B.C. and 1 A.D., but at that
time the years was counted since the "foundation of Rome.")

Interested readers may also want to check the Web sites of The Royal
Observatory Greenwich URL:http://www.rog.nmm.ac.uk/ and the US Naval
Observatory URL:http://www.usno.navy.mil/.

------------------------------

Subject: C.07 Easter:

------------------------------

Subject: C.07.1 When is Easter?
Author: Jim Van Nuland ,
John Harper

The "popular" rule (for Roman Catholics and most Protestant
denominations) is that Easter is on the first Sunday after the first
full moon after the March equinox.

The actual rule is similar, except that the astronomical equinox is
not used; the date is fixed at March 21. And the astronomical full
moon is not used; an "ecclesiastical" new moon is determined by
adopted tables based on the Metonic cycle, and "full" is taken as the
14th day of that lunation. There are auxiliary rules that make March
22 the earliest possible date for Easter and April 25 the latest. The
intent of these rules is that the date will be incontrovertibly fixed
and determinable indefinitely in advance. In addition it is
independent of longitude or time zones.

The popular rule works surprisingly well. When the two rules give
different dates, that occurs in only part of the world because two dates
separated by the international date line are simultaneously in progress.

The Eastern Churches (most Orthodox and some others, e.g., Uniate
Churches in Palestine) use the same system, but based on the old
(Julian) calendar. In that calendar, Easter Day is also between March
22 and April 25, but in the western (Gregorian) calendar those days
are at present April 3 and May 8. Whenever the Gregorian calendar
skips a leap year, those dates advance one day.

Some Eastern Churches find both movable feasts like Easter and fixed
ones like Christmas with the Julian calendar; some use the Julian for
movable and the Gregorian for fixed feasts; and the Finnish Orthodox
use the Gregorian for all purposes.

To explain the Eastern system one must begin with the Jews in
Alexandria at the time of the Christian Council of Nicaea in 325, who
appear to have been celebrating Passover on the first "full moon"
after March 21, as specified by the 19-year Metonic cycle and the
Julian calendar (with its leap year every 4 years, end of century or
not). The Bishop of Alexandria was made responsible for the Christian
calendar; he specified that Easter be the Sunday after that Passover.
Eastern Christians still say that Easter must follow Passover, but
that Passover is the one that is meant, not the Passover defined by
the present Jewish calendar.

Subsequently the Jews reformed their calendar (in 358 or in the early
6th century according to different sources; possibly at different
times in different places), in order to improve the fit between
astronomy and their arithmetic, but the Christians did not follow
suit. In 1996, for example, Passover was on April 4 but the Orthodox
Easter was on Sunday April 14, not April 7 (which as it happens was
the Western Easter.)

The Eastern Easter is 0, 1, 4, or 5 weeks after the Western
Easter. The Western Easter can precede the (modern) Jewish Passover,
as in 1967, 1970, 1978, 1986, 1989 and 1997, and can even coincide
with it, as in 1981.

Much of this information was taken from the Explanatory Supplement to
the Astronomical Ephemeris, page 420, 1974 reprint of the 1961
edition. There is more in the Explanatory Supplement, specifically a
series of tables that can be used to determine the Easter date for
both the Julian (Eastern and pre-1582 Western) and Gregorian
calendars. However, the Explanatory Supplement is misleading on the
subject of the Eastern Easters, though its tables are correct.

Jean Meeus has published a program to compute Easter in "Astronomical
Algorithms," also see below. Simon Kershaw has written one in C,
available at URL:http://www.ely.anglican.org/cgi-bin/easter.

The most easily available published source for what the Jews
and Christians were doing in ancient Alexandria appears to be Otto
Neugebauer's "Ethiopic Easter Computus" in his _Astronomy and History
Selected Essays_, Springer, New York, 1983, pp. 523--538.

John Harper acknowledges the help of Archimandrite Kyril Jenner, Simon
Kershaw, and Dr. Brian Stewart concerning Eastern Easters.

------------------------------

Subject: C.07.2 Can I calculate the date of Easter?
Author: Bill Jefferys

John Horton Conway (the Princeton mathematician who is responsible for
"the Game of Life") wrote a book with Guy and Berlekamp, _Winning
Ways_, that describes in Volume 2 a number of useful calendrical
rules, including How to Calculate the Day of the Week, Given The Date,
and Easter. Here's a brief precis of how to calculate Easter:

G(the Golden Number) = Year_{mod 19} + 1 (never forget to add the 1!)

C(the Century term) = +3 for all Julian years (i.e., if using the
Julian Calendar)

-4 for 15xx, 16xx }
-5 for 17xx, 18xx } Gregorian
-6 for 19xx, 20xx, 21xx }

The general formula for C in a Gregorian year Hxx is

C = -H + [H/4] + [8*(H+11)/25] (brackets [] mean integer part)

1) The Paschal Full Moon is given by the formula

(Apr 19 = Mar 50) - (11*G+C)_{mod 30}

Except when the formula gives Apr 19 you should take Apr 18, and when it
gives Apr 18 and G=12 you should take Apr 17. Easter is then the
following Sunday, since Easter always falls on the next Sunday that is
_strictly later_ than the Paschal Full Moon.

Example: 1945 = 7 mod 19, so G = 8 and we find for the Paschal Full Moon

Mar 50 - (88-6)_{mod 30} = Mar 50 - 22 = Mar 28.

This happens to be a Wednesday (by Horton's "Doomsday" rule for Day of
the Week, see below). Therefore, Easter 1945 took place on Sunday,
April 1.

Conway's "Doomsday" method for finding the day of the week, given the
date, is needed for his Easter method.

To every year there is a distinguished day of the week, which Conway
calls the "Doomsday", D. In any year, if March 0 (the last day of
February) falls on a particular DOW, then the following dates also
fall on the same DOW: 4/4, 6/6, 8/8, 10/10, 12/12. Also 5/9, 9/5,
7/11, 11/7 (for which he has devised the mnemonic "I went to my
nine-to-five job at the Seven-Eleven. Note to non-US readers:
"Seven-Eleven" is the name of a ubiquitous chain of convenience
stores.) In non-leap years, Jan 3 and Feb 0 (Jan 31) also fall on
that DOW; in leap years, Jan 4 and Feb 1. Conway calls this DOW the
"doomsday" for that year.

For example, in 1995 Doomsday is Tuesday. Columbus Day (10/12) is two
days after 10/10, a Tuesday, so 10/12 is a Thursday.

All that remains is a rule for calculating the Doomsday for any year.
In any century, this is done by taking the last two digits of the
year, call them xx, dividing by 12 to get a quotient Q and remainder
R. Divide R by 4 to get a second quotient Q2. Then this century,
the Doomsday for that year is given by Wednesday + Q + R + Q2. In
1995, for example, we have 95/12 = 7 with remainder 11; 11/4 gives
quotient 2; Wednesday + 7 + 11 + 2 = Tuesday (cf. above).

In other years on the Gregorian calendar, one uses instead of
Wednesday, the century day as follows: 16xx and 20xx: Tuesday; 17xx
and 21xx: Sunday; 18xx and 22xx: Friday; 15xx, 19xx and 23xx:
Wednesday. The cycle repeats over a 4 century period.

If you need the DOW on the Julian calendar, the rules are the same
except that the century rule is different: for a date in the year ccxx,
use -cc for the century day of week, where Sunday = 0. For example,
October 4, 1582 (the last day of the Julian calendar in countries that
followed Pope Gregory's institution of the Gregorian calendar) took
place as follows:

82/12 = 6 remainder 10; 10/4 gives remainder 2; 6+10+2-15= 3,
which is Wednesday. 10/10 was Wednesday, 10/3 was Wednesday, so
10/4/1582 (Julian) was a Thursday.

The following day was October 15, 1582 (Gregorian). Again we
can check: 6+10+2+Wed = Sunday. 10/10 was a Sunday (Gregorian)
so 10/15/1582 (Gregorian) was a Friday.

The nice thing about these algorithms is that they can easily be done in
one's head with a little practice (OK, mod 19 for the Golden Number is a
bit hairy for me, but I can still do it!). The DOW calculation is very
useful if you are caught without a calendar, and it makes a good party
trick.

Additional information is available at
URL:http://quasar.as.utexas.edu/BillInfo/doomsday.html and
URL:http://quasar.as.utexas.edu/BillInfo/ReligiousCalendars.html.

------------------------------

Subject: C.08 What is a "blue moon?"
Author: Steve Willner ,
Jay Respler

Colloquially the term "blue moon" is used to mean "a very long time."
In fact, there have been at least seven different uses of the term
"blue moon" in the past several hundred years.

The alt.usage.english FAQ discusses these different meanings of the
term "blue moon." The two definitions most relevant to astronomy are
the following:

1. Under certain conditions of atmospheric haze, the moon may actually
look blue. A notable example occurred after the explosion of the
volcano Krakatoa. The appropriate conditions are extremely rare.

2. The second full moon in a calendar month. Since the synodic month
is 29.53 days, this kind of blue moon occurs roughly once out of 60
30-day months and once out of 21 31-day months or about once in 2.5
years on average. It can occur in January and the following March if
there is no full moon at all in February. There are some indications
that some calendars used to put the first moon in the month in red,
the second in blue, hence the origin of the term.

Philip Hiscock, writing in the 1999 March issue of Sky & Telescope,
expands upon the history of this definition. This definition of "blue
moon" is of fairly recent vintage and came into widespread use in the
late 1980s as a result of the board game Trivial Pursuit. He was able
to trace its origin to an (incorrect) entry in the 1937 edition of the
_Maine Farmer's Almanac_.

The alt.usage.english FAQ is available from
URL:
ftp://rtfm.mit.edu/
pub/usenet-by-group/alt.usage.english/alt.usage.english_FAQ
or
URL:
http://www.cis.ohio-state.edu/
hypertext/faq/usenet/alt-usage-english-faq/faq.html.

------------------------------

Subject: C.09 What is the Green Flash (or Green Ray)?
Author: Steve Willner ,
Geoffrey A. Landis

When the sun sets, sometimes the last bit of light from the disk itself
is an emerald green. The same is true of the first bit of light from
the rising sun. This phenomenon is known as the "green flash" or "green
ray." It is not an optical illusion.

The green flash is common and will be visible any time the sun is
rises or sets on a *clear*, *unobstructed*, and *low* horizon. From
our observatory at Mt. Hopkins, I (SW) see the sunset green flash
probably 90% of the evenings that have no visible clouds on the
western horizon. It typically lasts one or two seconds (by estimate,
not stopwatch) but on rare occasions much longer (5 seconds??). I've
seen the dawn green flash only once, but a) I'm seldom outside
looking, b) the topography is much less favorable, and c) it takes
luck to be looking in exactly the right place. If you'd like to see
the green flash, the higher you can go, the better (see below).

The explanation for the green flash involves refraction, scattering,
and absorption. First, the most important of these processes,
refraction: light is bent in the atmosphere with the net effect that
the visible image of the sun at the horizon appears roughly a solar
diameter *above* the geometric position of the sun. This refraction
is mildly wavelength dependent with blue light being refracted the
most. Thus if refraction were the only effect, the red image of the
sun would be lowest in the sky, followed by yellow, green, and blue
highest. If I've understood the refraction table properly, the
difference between red and blue (at the horizon) is about 1/40 of a
solar diameter.

Now scattering: the blue light is Rayleigh scattered away (not Compton
or Thomson scattering).

Now absorption: air has a very weak absorption band in the yellow.
When the sun is overhead, this absorption hardly matters, but near the
horizon, the light travels through something like 38 "air masses," so
even a weak absorption becomes significant.

The explanation for the green flash is thus, 1) refraction separates
the solar images by color; 2) at just the right instant, the red image
has set, 3) the yellow image is absorbed; and 4) the blue image is
scattered away. We are left with the upper limb of the green image.

Because the green flash is primarily a refraction effect, it lasts
longer and is easier to see from a mountain top than from sea level.
The amount of refraction is proportional to the path length through
the atmosphere times the density gradient (in a linear approximation
for the atmosphere's index of refraction). This product will scale
like 1+(h/a)^(0.5), where h is your height and a the scale height of
the atmosphere. The density scale height averaged over the bottom
10 km of the atmosphere is about 9.2 km, so for a 2 km mountain the
increase in refraction is about a factor 1.5; a 3 km mountain gives
1.6 and a 4.2 km mountain (e.g., Mauna Kea) gives 1.7.

More details can be found in _The Green Flash and Other Low Sun
Phenomena_, by D. J. K. O'Connell and the classic _Light and Color in
the Open Air_. A refraction table appears in _Astrophysical
Quantities_, by C. W. Allen. There's also an on-line resource at
URL:http://mintaka.sdsu.edu/GF.

------------------------------

Subject: C.10 Why isn't the earliest Sunrise (and latest Sunset) on the
longest day of the year?
Author: Steve Willner

This phenomenon is called the "equation of time." This is just a
fancy name for the fact that the Sun's speed along the Earth's equator
is not constant. In other words, if you were to measure the Sun's
position at exactly noon every day, you would see not only the
familiar north-south change that goes with the seasons but also an
east-west change in the Sun's position. A graphical representation of
both positional changes is the analemma, that funny figure 8 that most
globes stick in the middle of the Pacific ocean.

The short explanation of the equation of time is that it has two
causes. The slightly larger effect comes from the obliquity of the
ecliptic---the Earth's equator is tilted with respect to the orbital
plane. Constant speed along the ecliptic---which is how the "mean
sun" moves---translates to varying speed in right ascension (along the
equator). This gives the overall figure 8 shape of the analemma.
Almost as large is the fact that the Earth's orbit is not circular,
and the Sun's angular speed along the ecliptic is therefore not
constant. This gives the inequality between the two lobes of the
figure 8.

Some additional discussion, with illustrations, is provided by Nick
Strobel at URL:http://www.astronomynotes.com/nakedeye/s9.htm, though
you may want to start with the section on time at
URL:http://www.astronomynotes.com/nakedeye/s7.htm. Mattthias
Reinsch provides an analytic expression for determining the number of
days between the winter solstice and the day of the latest sunrise for
Northern Hemisphere observers,
URL:http://arXiv.org/abs/astro-ph/?0201074.

The Earth's analemma will change with time as the Earth's orbital
parameters change. This is described by Bernard Oliver (1972 July,
_Sky and Telescope_, pp. 20--22)

An article by David Harvey (1982 March, _Sky and Telescope_,
pp. 237--239) shows the analemmas of all nine planets. A simulation
of the Martian analemma is at
URL:http://apod.gsfc.nasa.gov/apod/ap030626.html, and illustrations
of other planetary analemmas is at URL:http://www.analemma.com/.

------------------------------

Subject: C.11 How do I calculate the phase of the moon?
Author: Bill Jefferys

John Horton Conway (the Princeton mathematician who is responsible for
"the Game of Life") wrote a book with Guy and Berlekamp, _Winning
Ways_, that describes in Volume 2 a number of useful calendrical
rules. One of these is an easy "in your head" algorithm for
calculating the phase of the Moon, good to a day or better depending
on whether you use his refinements or not.

In the 20th century, calculate the remainder upon dividing the
last two digits of the year by 19; if greater than 9, subtract
19 from this to get a number between -9 and 9.

Multiply the result by 11 and reduce modulo 30 to obtain a
number between -29 and +29.

Add the day of the month and the number of the month (except
for Jan and Feb use 3 and 4 for the month number instead of
1 and 2).

Subtract 4.

Reduce modulo 30 to get a number between 0 and 29. This is
the age of the Moon.

Example: What was the phase of the Moon on D-Day (June 6,
1944)?

Answer: 44/19=2 remainder 6.

6*11=66, reduce modulo 30 to get 6.

Add 6+6 to this and subtract 4: 6+6+6-4=14; the Moon was (nearly)
full. I understand that the planners of D-day did care about the phase
of the Moon, either because of illumination or because of tides. I
think that Don Olsen recently discussed this in _Sky and Telescope_
(within the past several years).

In the 21st century use -8.3 days instead of -4 for the last number.

Conway also gives refinements for the leap year cycle and also
for the slight variations in the lengths of months; what I have
given should be good to +/- a day or so.

------------------------------

Subject: C.12 What is the time delivered by a GPS receiver?
Author: Markus Kuhn

Navstar GPS (global positioning system) is a satellite based
navigation system operated by the US Air Force. The signals broadcast
by GPS satellites, contain all information required by a GPS receiver
in order to determine both UTC and TIA highly accurately. Commercial
GPS receivers can provide a time reference that is closer than 340 ns
to UTC(USNO) in 90% of all measurements, classified military versions
are even better.

------------------------------

Subject: C.13 Why are there two tides a day and not just one?
Author: Joseph Lazio ,
Paul Zander

An easy way to think of the Moon's effect on the Earth is the
following. The Moon exerts a gravitational force on the Earth. The
strength of the gravitational force decreases with increasing
distance. So, because the surface of the ocean is closer to the Moon
than the sea floor, the surface water is attracted more strongly to
the Moon. That's the tide that occurs (nearly) under the Moon.

What's happening on the other side of the Earth? On the other side of
the Earth from the Moon, the sea floor is being pulled more strongly
toward the Moon than the surface water. In essence, the surface water
is being left behind. Voila, another bulge in the surface water and
another tide.

In principle, there should be two tides of equal height in a day. In
practice, many parts of the earth do not experience two tides of equal
height in a day.

First, because the Moon's orbit is at an angle to the Earth's equator,
one tidal bulge may be in the northern hemisphere, while the other is
in the southern hemisphere.

Except around Antarctica, the shape of the Earth's continents prevent
the tidal bulges from simply following the moon. Each ocean basin has
its own individual pattern for the tidal flow. In the South Atlantic
Ocean, the tides travel from south to north, taking about 12 hours to
go from the tip of Africa to the equator.

In the North Atlantic, the tides travel in a counter-clockwise
direction going around once in about 12 hours. The effect is similar
to water sloshing around in a bowl. Because the two tides are roughly
equal, they are called semidaily or semidiurnal.

In some parts of the Gulf of Mexico, there is only one high tide and
one low tide a day. These are called daily or diurnal tides. In much
of the Pacific Ocean, there are two high tides and two low tides each
day, but they are of unequal height. These are called mixed tides.

The traditional way to predict tides has been to collect data for
several years to have enough combinations of positions of the moon and
sun to allow accurate extrapolation. More recently, computer models
have been made taking into account detailed shapes of the ocean
bottoms and coastlines.

Even the best predictions can have difficulties. The extremely heavy
snow fall during the winter of 1994--95 in California and the
associated run-off as it melted were not part of the model for San
Francisco Bay. Sail boat races scheduled to take advantage of tidal
currents coming into the Golden Gate found the current was still going
out!

Ref: Oceanography, A View of the Earth, M. Grant Gross, Prentice Hall,
Englewood Cliffs, New Jersey, 1972.

For even more details, see
URL:ftp://d11t.geo.tudelft.nl/pub/ejo/tides and
URL:http://www.co-ops.nos.noaa.gov/restles1.html.

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2005 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults. Any
express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
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Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/sci.astro.html and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: D.00 Astrophysics

[Dates in brackets are last edit.]

D.01 Do neutrinos have rest mass? What if they do? [2002-05-04]
D.02 Have physical constants changed with time? [1997-02-04]
D.03 What is gravity? [1998-11-04]
D.04 Does gravity travel at the speed of light? [1998-05-06]
D.05 What are gravitational waves? [1997-06-10]
D.06 Can gravitational waves be detected? [2000-08-31]
D.07 Do gravitational waves travel at the speed of
light? [1996-07-03]
D.08 Why can't light escape from a black hole? [1995-10-05]
D.09 How can gravity escape from a black hole? [1996-01-24]
D.10 What are tachyons? Are they real? [1995-10-02]
D.11 What are magnetic monopoles? Are they real? [1996-07-03]
D.12 What is the temperature in space? [1998-04-14]
D.13 Saturn's rings, proto-planetary disks, accretion disks---Why
are disks so common? [1999-07-18]

[Interesting note: The Astrophysical Journal was founded in 1895 by
George Hale and James Keeler. Professor Edward Wright points out that
these men would not have understood most of these questions---let
alone have known any of the answers.]


------------------------------

Subject: D.01 Do neutrinos have rest mass? What if they do?
Author: Joseph Lazio

First, it is worth remembering what a neutrino is. During early
studies of radioactivity it was discovered that a neutron could decay.
The decay products appeared to be just a proton and electron.
However, if these are the only decay products, an ugly problem rears
its head. If one considers a neutron at rest, it has a certain amount
of energy. (Its mass is equivalent to a rest energy because of E =
mc^2.) If one then sums the energies of the decay products---the
masses of the electron and proton and their kinetic energy---it never
equals that of the rest energy of a neutron. Thus, one has two
choices, either energy is not conserved or there is a third decay
product.

Wolfgang Pauli was uncomfortable with abandoning the principle of
energy conservation so he proposed, in 1930, that there was a third
particle (which Enrico Fermi called the "little neutral one" or
neutrino) produced in the decay of a neutron. It has to be neutral,
i.e., carry no charge or have charge 0, because a neutron is neutral
whereas an electron has charge -1 and a proton has a charge +1. In
1956 Pauli and Fermi were vindicated when a neutrino was detected
directly by Reines & Cowan. (For his experimental work, Reines
received the 1995 Nobel Prize in Physics.)

The long gap between the Pauli's proposal and the neutrino's discovery
is due to the way that a neutrino interacts. Unlike the electron and
protron that can interact via the electromagnetic force, the neutrino
interacts only via the weak force. (The electron can also interact
via the weak force.) As its name suggests, weak force interactions
are weak. A neutrino can pass through our planet without a problem.
Indeed, as you read this, billions of neutrinos are passing through
your body. As one might imagine, building an experimental appartus to
detect neutrinos is challenging.

Since 1956, additional kinds of neutrinos have been discovered. The
electron has more massive counterparts, the muon and tau lepton. Each
of these has an associated neutrino. Thus there is an electron
neutrino, mu neutrino, and tau neutrino. (In addition, each has an
anti-particle as well, so there is an electron anti-neutrino, mu
anti-neutrino, and tau anti-neutrino. Furthermore, it was realized
that in order to get the equations to balance, the decay of a neutron
actually produces an electron, a protron, and electron anti-neutrino.)
Early work assumed that the neutrino had no mass and experiments
revealed quickly that, if the electron neutrino and anti-neutrino have
any mass, it must be quite small.

In the 1960s Raymond Davis, Jr., realized that the Sun should be a
copious source of neutrinos, *if* it shines by nuclear fusion.
Various fusion reactions that are thought to be important in producing
energy in the core of the Sun produce neutrinos as a by-product. In a
now-famous experiment at the Homestake Mine, he set out to detect some
of these solar neutrinos. John Bahcall has collaborated with Davis to
write a history of this experiment at
URL:http://www.sns.ias.edu/~jnb/. Although quite difficult, in a
few years, it became evident that there was a discrepancy. The number
of neutrinos detected at Homestake was far lower than what models of
the Sun predicted. Moreover, as new experiments came online in the
late 1980s and early 1990s, the problem became even more severe. Not
only was the number of neutrinos lower than expected, their energies
were not what was predicted.

There are three ways to resolve this problem. (1) Our models of the
Sun are wrong. In particular, if the temperature of the Sun's core is
just slightly lower than predicted that reduces the fusion reaction
rates and therefore the number of neutrinos that should be detected at
the Earth. (2) Our understanding of neutrinos is incomplete and,
namely, the neutrino has mass. (3) Both.

Astronomers were uncomfortable with explanation (1). The fusion
reaction rate in the Sun's core is *quite* sensitive to its
temperature. Adopting explanation (1) seemed to require some
elaborate "fine-tuning" of the model. (Observations of the Sun in the
1990s have supported this initial reluctance of astronomers. Using
helioseismology, URL:http://antwrp.gsfc.nasa.gov/apod/ap990615.html,
astronomers have a second way of probing beneath the Sun's surface, and
it does appear that the temperature of the Sun's core is just about
what our best models predict.)

In contrast explanation (2) seemed reasonable. After all, just
detecting neutrinos was challenging. The possibility that they might
have mass was not unreasonable. In the 1970s Vera Rubin and her
collaborators were also demonstrating that spiral galaxies appeared to
have a lot of unseen matter in them. If neutrinos has mass, one might
be able to solve two problems at once, both matching the solar
neutrino observations and accounting for some of the "missing matter"
or dark matter.

Explanation (2) is the following. Suppose the neutrino has mass.
Then the neutrinos we observe, the electron neutrino, mu neutrino, and
tau neutrino, might not be the "true" neutrinos. The true neutrinos,
call them nu1, nu2, and nu3, would combine in various ways to produce
the observed neutrinos. Moreover, various properties of quantum
mechanics would allow the observed neutrinos to "oscillate" between
the various flavors. Thus, an electron neutrino could be produced in
the core of the Sun but oscillate to become a mu neutrino by the time
it reached the Earth. Because the early experiments detected only
electron neutrinos, if the electron neutrinos were changing to a
different kind of neutrino, the apparent discrepancy would be
resolved. This explanation is known as the MSW effect after
the three physicists Mikheyev, Smirnov, and Wolfenstein who proposed
it first.

The second explanation now appears correct. Various terrestrial
experiments, such as the Sudbury Neutrino Observatory (SNO), the
Super-Kamiokande Observatory, the Liquid Scintillator Neutrino
Detector (LSND) experiment, and Main Injector Neutrino Oscillation
Search (MINOS), appear to be detecting neutrino oscillations directly.

The mass required to explain neutrino oscillations is quite small.
The mass is sufficiently small that all of the neutrinos in the
Universe are unlikely to make a substantial contribution to the
density of the Universe. However, it does appear to be sufficient to
resolve the solar neutrino problem.

Additional information on neutrinos is at
URL:http://wwwlapp.in2p3.fr/neutrinos/aneut.html.

------------------------------

Subject: D.02 Have physical constants changed with time?
Author: Steve Carlip

The fundamental laws of physics, as we presently understand them, depend
on about 25 parameters, such as Planck's constant h, the gravitational
constant G, and the mass and charge of the electron. It is natural to
ask whether these parameters are really constants, or whether they vary
in space or time.

Interest in this question was spurred by Dirac's large number
hypothesis. The "large number" in question is the ratio of the
electric and the gravitational force between two electrons, which is
about 10^40; there is no obvious explanation of why such a huge number
should appear in physics. Dirac pointed out that this number is
nearly the same as the age of the Universe in atomic units, and
suggested in 1937 that this coincidence could be understood if
fundamental constants---in particular, G---varied as the Universe
aged. The ratio of electromagnetic and gravitational interactions
would then be large simply because the Universe is old. Such a
variation lies outside ordinary general relativity, but can be
incorporated by a fairly simple modification of the theory. Other
models, including the Brans-Dicke theory of gravity and some versions
of superstring theory, also predict physical "constants" that vary.

Over the past few decades, there have been extensive searches for
evidence of variation of fundamental "constants." Among the methods
used have been astrophysical observations of the spectra of distant
stars, searches for variations of planetary radii and moments of
inertia, investigations of orbital evolution, searches for anomalous
luminosities of faint stars, studies of abundance ratios of radioactive
nuclides, and (for current variations) direct laboratory measurements.

One powerful approach has been to study the "Oklo Phenomenon," a uranium
deposit in Gabon that became a natural nuclear reactor about 1.8 billion
years ago; the isotopic composition of fission products has permitted a
detailed investigation of possible changes in nuclear interactions.
Another has been to examine ratios of spectral lines of distant quasars
coming from different types of atomic transitions (resonant, fine
structure, and hyperfine). The resulting frequencies have different
dependences on the electron charge and mass, the speed of light, and
Planck's constant, and can be used to compare these parameters to their
present values on Earth. Solar eclipses provide another sensitive test
of variations of the gravitational constant. If G had varied, the
eclipse track would have been different from the one we calculate today,
so the mere fact that a total eclipse occurred at a particular location
provides a powerful constraint, even if the date is poorly known.

So far, these investigations have found no evidence of variation of
fundamental "constants." The current observational limits for most
constants are on the order of one part in 10^10 to one part in 10^11 per
year. So to the best of our current ability to observe, the
fundamental constants really are constant.

References:

For a good short introduction to the large number hypothesis and the
constancy of G, see:

C.M. Will, _Was Einstein Right?_ (Basic Books, 1986)

For more technical analyses of a variety of measurements, see:

L. L. Cowie & A. Songaila, Astrophysical Journal (1995) v. 453,
p. 596 also available online at
URL:
http://adsabs.harvard.edu/cgi-bin/nph-article_query?1995ApJ...453..596C

P. Sisterna & H. Vucetich, Physical Review D41 (1990) 1034 and
Physical Review D44 (1991) 3096

E.R. Cohen, in _Gravitational Measurements, Fundamental Metrology and
Constants_, V. De Sabbata & V.N. Melnikov, editors (Kluwer
Academic Publishers, 1988)

"The Constants of Physics," Philosophical Transactions of the Royal
Society of London A310 (1983) 209--363

------------------------------

Subject: D.03 What is gravity?
Author: Steve Carlip

Hundreds of years of observation have established the existence of a
universal attraction between physical objects. In 1687, Isaac Newton
quantified this phenomenon in his law of gravity, which states that
every object in the Universe attracts every other object, with a force
between any two bodies that is proportional to the product of their
masses and inversely proportional to the square of the distance between
them. If M and m are the two masses, r is the distance, and G is the
gravitational constant, we can write:
F = GMm/r^2 .
The gravitational constant G can be measured in the laboratory and has a
value of approximately 6.67x10^{-11} m^3/kg sec^2. Newton's law of
gravity was one of the first great "unifications" of physics, explaining
both the force we experience on Earth (the fall of the proverbial apple)
and the force that causes the planets to orbit the Sun with a single,
simple rule.

Gravity is actually an extremely weak force. The electrical repulsion
between two electrons, for example, is some 10^40 times stronger than
their gravitational attraction. Nevertheless, gravity is the dominant
force on the large scales of interest in astronomy. There are two
reasons for this. First, gravity is a "long range" force---the strong
nuclear interactions, for instance, fall off with distance much faster
than gravity's inverse square law. Second, gravity is additive.
Planets and stars are very nearly electrically neutral, so the forces
exerted by positive and negative charges tend to cancel out. As far as
we know, however, there is no such thing as negative mass, and no such
cancellation of gravitational attraction. (Gravity may sometimes feel
strong, but remember that you have the entire 6x10^24 kg of the Earth
pulling on you.)

For most purposes, Newton's law of gravity is extremely accurate.
Newtonian theory has important limits, though, both observational (small
anomalies in Mercury's orbit, for example) and theoretical
(incompatibility with the special theory of relativity). These limits
led Einstein to propose a revised theory of gravity, the general theory
of relativity ("GR" for short), which states (roughly) that gravity is a
consequence of the curvature of spacetime.

Einstein's starting point was the principle of equivalence, the
observation that any two objects in the same gravitational field that
start with the same initial velocities will follow exactly the same
path, regardless of their mass and internal composition. This means
that a theory of gravity is really a theory of paths (strictly
speaking, paths in spacetime), which picks out a "preferred" path
between any two points in space and time. Such a description sounds
vaguely like geometry, and Einstein proposed that it *was*
geometry---that a body acting under the influence of gravity moves in
the "straightest possible line" in a curved spacetime.

As an analogy, imagine two ships starting at different points on the
equator and sailing due north. Although the ships do not steer
towards each other, they will find themselves drawn together, as if a
mysterious force were pulling them towards each other, until they
eventually meet at the North Pole. We know why, of course---the
"straightest possible lines" on the curved surface of the Earth are
great circles, which converge. According to general relativity,
objects in gravitational fields similarly move in the "straightest
possible lines" (technically, "geodesics") in a curved spacetime,
whose curvature is in turn determined by the presence of mass or
energy. In John Wheeler's words, "Spacetime tells matter how to move;
matter tells spacetime how to curve."

Despite their very different conceptual starting points, Newtonian
gravity and general relativity give nearly identical predictions. In
the few cases that they differ measurably, observations support GR. The
three "classical tests" of GR are anomalies in the orbits of the inner
planets (particularly Mercury), bending of light rays in the Sun's
gravitational field, and the gravitational red shift of spectral lines.
In the past few years, more tests have been added, including the
gravitational time delay of radar and the observed motion of binary
pulsar systems. Further tests planned for the future include the
construction of gravitational wave observatories (see D.05) and the
planned launch of Gravity Probe B, a satellite that will use sensitive
gyroscopes to search for "frame dragging," a relativistic effect in
which the Earth "drags" the surrounding space along with it as it
rotates.

References:

For introductions to general relativity, try:
K.S. Thorne, _Black Holes and Time Warps_ (W.W. Norton, 1994)
R.M. Wald, _Space, Time, and Gravity_ (Univ. of Chicago Press, 1977)
J.A. Wheeler, _A Journey into Gravity and Spacetime_ (Scientific
American Library, 1990)

For experimental evidence, see:
C.M. Will, _Was Einstein Right?_ (Basic Books, 1986)
or, for a more technical source,
C.M. Will, _Theory and Experiment in Gravitational Physics, revised
edition (Cambridge Univ. Press, 1993)

You can find out about Gravity Probe B at
URL:http://einstein.stanford.edu/ and
URL:http://www.nap.edu/readingroom/books/gpb/.

------------------------------

Subject: D.04 Does gravity travel at the speed of light?
Author: Steve Carlip ,
Matthew P Wiener
Geoffrey A Landis

To begin with, the speed of gravity has not been measured directly in
the laboratory---the gravitational interaction is too weak, and such
an experiment is beyond present technological capabilities. The
"speed of gravity" must therefore be deduced from astronomical
observations, and the answer depends on what model of gravity one uses
to describe those observations.

In the simple Newtonian model, gravity propagates instantaneously: the
force exerted by a massive object points directly toward that object's
present position. For example, even though the Sun is 500 light
seconds from the Earth, Newtonian gravity describes a force on Earth
directed towards the Sun's position "now," not its position 500
seconds ago. Putting a "light travel delay" (technically called
"retardation") into Newtonian gravity would make orbits unstable,
leading to predictions that clearly contradict Solar System
observations.

In general relativity, on the other hand, gravity propagates at the
speed of light; that is, the motion of a massive object creates a
distortion in the curvature of spacetime that moves outward at light
speed. This might seem to contradict the Solar System observations
described above, but remember that general relativity is conceptually
very different from Newtonian gravity, so a direct comparison is not
so simple. Strictly speaking, gravity is not a "force" in general
relativity, and a description in terms of speed and direction can be
tricky. For weak fields, though, one can describe the theory in a
sort of Newtonian language. In that case, one finds that the "force"
in GR is not quite central---it does not point directly towards the
source of the gravitational field---and that it depends on velocity as
well as position. The net result is that the effect of propagation
delay is almost exactly cancelled, and general relativity very nearly
reproduces the Newtonian result.

This cancellation may seem less strange if one notes that a similar
effect occurs in electromagnetism. If a charged particle is moving at
a constant velocity, it exerts a force that points toward its present
position, not its retarded position, even though electromagnetic
interactions certainly move at the speed of light. Here, as in
general relativity, subtleties in the nature of the interaction
"conspire" to disguise the effect of propagation delay. It should be
emphasized that in both electromagnetism and general relativity, this
effect is not put in _ad hoc_ but comes out of the equations. Also,
the cancellation is nearly exact only for *constant* velocities. If a
charged particle or a gravitating mass suddenly accelerates, the
*change* in the electric or gravitational field propagates outward at
the speed of light.

Since this point can be confusing, it's worth exploring a little
further, in a slightly more technical manner. Consider two
bodies---call them A and B---held in orbit by either electrical or
gravitational attraction. As long as the force on A points directly
towards B and vice versa, a stable orbit is possible. If the force on
A points instead towards the retarded (propagation-time-delayed)
position of B, on the other hand, the effect is to add a new component
of force in the direction of A's motion, causing instability of the
orbit. This instability, in turn, leads to a change in the mechanical
angular momentum of the A-B system. But *total* angular momentum is
conserved, so this change can only occur if some of the angular
momentum of the A-B system is carried away by electromagnetic or
gravitational radiation.

Now, in electrodynamics, a charge moving at a constant velocity does
not radiate. (Technically, the lowest order radiation is dipole
radiation, which depends on the acceleration.) So to the extent that
that A's motion can be approximated as motion at a constant velocity,
A cannot lose angular momentum. For the theory to be consistent,
there *must* therefore be compensating terms that partially cancel the
instability of the orbit caused by retardation. This is exactly what
happens; a calculation shows that the force on A points not towards
B's retarded position, but towards B's "linearly extrapolated"
retarded position. Similarly, in general relativity, a mass moving at
a constant acceleration does not radiate (the lowest order radiation
is quadrupole), so for consistency, an even more complete cancellation
of the effect of retardation must occur. This is exactly what one
finds when one solves the equations of motion in general relativity.

While current observations do not yet provide a direct
model-independent measurement of the speed of gravity, a test within
the framework of general relativity can be made by observing the
binary pulsar PSR 1913+16. The orbit of this binary system is
gradually decaying, and this behavior is attributed to the loss of
energy due to escaping gravitational radiation. But in any field
theory, radiation is intimately related to the finite velocity of
field propagation, and the orbital changes due to gravitational
radiation can equivalently be viewed as damping caused by the finite
propagation speed. (In the discussion above, this damping represents
a failure of the "retardation" and "non-central, velocity-dependent"
effects to completely cancel.)

The rate of this damping can be computed, and one finds that it
depends sensitively on the speed of gravity. The fact that
gravitational damping is measured at all is a strong indication that
the propagation speed of gravity is not infinite. If the
calculational framework of general relativity is accepted, the damping
can be used to calculate the speed, and the actual measurement
confirms that the speed of gravity is equal to the speed of light to
within 1%. (Measurements of at least one other binary pulsar system,
PSR B1534+12, confirm this result, although so far with less
precision.)

Are there future prospects for a direct measurement of the speed of
gravity? One possibility would involve detection of gravitational
waves from a supernova. The detection of gravitational radiation in
the same time frame as a neutrino burst, followed by a later visual
identification of a supernova, would be considered strong experimental
evidence for the speed of gravity being equal to the speed of light.
However, unless a very nearby supernova occurs soon, it will be some
time before gravitational wave detectors are expected to be sensitive
enough to perform such a test.

References:

There seems to be no nontechnical reference on this subject. For a
technical reference, see

T. Damour, in _Three Hundred Years of Gravitation_, S.W. Hawking and
W. Israel, editors (Cambridge Univ. Press, 1987)

For a good reference to the electromagnetic case, see

R.P. Feynman, R.B. Leighton, and M. Sands, _The Feynman Lectures on
Physics_, chapter II-21 (Addison-Wesley, 1989)

------------------------------

Subject: D.05 What are gravitational waves?
Author: Bradford Holden

General Relativity has a set of equations that give results for how a
lump of mass-energy changes the space-time around it. (See D.03.) One
of the solutions to these equations is the infamous black hole, another
solution is the results used in modern cosmology, and the third common
solution is one that leads to gravitational waves.

Over a hundred years ago Maxwell realized that a solution to the
equations governing electricity and magnetism would create waves.
These waves move at the same speed that light does, and, hence, he
realized that light is an electro-magnetic wave. In general,
electromagnetic waves are created whenever a charge is accelerated,
that is, whenever its velocity changes.

Gravitational waves are analogous. However, instead of being
disturbances in electric and magnetic fields, they are disturbances in
spacetime. As such, they affect things like the distance between two
points or the amount of time perceived to pass by an observer.
Moreover, since there is no "negative mass," and momentum is
conserved, any acceleration of mass is balanced by an equal and
opposite change of momentum of some other mass. This implies that the
lowest order gravitational wave is quadrupole, and gravitational waves
are produced when an acceleration changes.

Because gravitational waves are waves, they should exhibit many other
properties of waves. For example, gravitational waves can, in
principle, be scattered or exhibit a redshift. (But see the next
question on the difficulty of testing this prediction.)

[Note, *gravitational* waves...gravity waves are something else
entirely (they occur in a medium when gravity is the restoring force)
and are commonly seen in the atmosphere and oceans.]

------------------------------

Subject: D.06 Can gravitational waves be detected?
Author: Bradford Holden ,
Steve Willner

The effects of gravitational waves are ridiculously weak, and direct
evidence for their existence has (probably) not been found with the
detectors built to date. However, no known type of source would emit
gravitational waves strong enough for detection, so no one is worried.

In the 60's and early 70's, Joe Weber at the University of Maryland
attempted to detect gravitational waves using large aluminum bars,
which would vibrate if a gravitational wave came by. Because local
causes also created vibrations, the technique was to look for
coincidences between two or more detectors some distance apart. Weber
claimed to see more coincidences than expected statistically and even
to see a correlation with sidereal time. Unfortunately, other groups
have used far more sensitive detectors operating on the same
principles and found nothing.

Two new experiments, far more sensitive than those using metal bars, are
being built now. These are LIGO in the US and Virgo in Italy. They
will work by detecting displacements between two elements separated by
several kilometers.

An indirect measurement of gravitational waves has been made, however.
Gravitational waves are formed when a mass undergoes change of
acceleration. They are stronger if the mass is dense and the
acceleration changes rapidly. One place where this might happen would
be two pulsars circling each other. A couple of systems like this
exist, and one has been studied actively over the past 20 years or so.
Pulsars make good clocks so you can time the orbital period of the
pulsars quite easily. As the pulsars circle, they emit gravitational
waves, and these waves remove energy (and angular momentum) from the
system. The energy released has to come from somewhere, and that
somewhere is the orbital energy of the pulsars themselves. This leads
to the pulsars becoming closer and closer over time. A formula was
worked out for this effect, and the observed pulsars match it amazingly
well. So well, in fact, that if you plot the data on top of the
prediction, there is no apparent deviation. (It's actually rather
disgusting, none of my results ever come out that well.) Anyway, Joe
Taylor of Princeton and a student of his, Russell Hulse, shared the
Nobel Prize in Physics for, in part, this work.

Useful references are given in section D.03.

V. M. Kaspi discusses pulsar timing in 1995 April Sky & Telescope, p. 18.

The conference proceedings volume _General Relativity and Gravitation
1989_, eds. Ashby, Bartlett, & Wyss, (Cambridge U. Press 1990) contains
a summary of the aluminum bar results.

_General Relativity and Gravitation 1992_, eds. Gleiser, Kozameh, &
Moreschi (IOP Publishing 1993) contains an article by Joe Taylor
summarizing the pulsar results.

An example of recent pulsar research is the article by Kaspi, Taylor,
and Ryba, 1994 ApJ 428, 713, who give instructions for obtaining their
archival timing data via Internet.

Some references to Weber's work a
1969 Phys. Rev. Lett. 22, 1320.
1970 Phys. Rev. Lett. 24, 276.
1971 Nuovo Cimento 4B, 199.

Information on gravitational wave detection experiments can be found
on the Web for LIGO URL:http://www.ligo.caltech.edu/, VIRGO
URL:http://www.virgo.infn.it/, GEO 600
URL:http://www.geo600.uni-hannover.de/, and TAMA
URL:http://tamago.mtk.nao.ac.jp/.

------------------------------

Subject: D.07 Do gravitational waves travel at the speed of light?

See sci.physics FAQ part 2,
URL:ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/answers/physics-faq,
(for North American sites)
URL:http://math.ucr.edu/home/baez/physics/faq.html,
URL:http://www.public.iastate.edu/~physics/sci.physics/faq/faq.html,
URL:http://www-hpcc.astro.washington.edu/mirrors/physicsfaq/faq.html,
(European sites)
URL:http://www.desy.de/user/projects/Physics/faq.html, and
(Australia)
URL:http://www.phys.unsw.edu.au/physoc/physics_faq/faq.html.

Short answer: yes in GR, not necessarily in other theories of gravity;
experimental limits require speed very close to c.

------------------------------

Subject: D.08 Why can't light escape from a black hole?
Author: William H. Mook, Jr.

P.S. Laplace wrote in 1798:
"A luminous star, of the same density of Earth, and whose diameter
should be two hundred and fifty times larger than that of the Sun
would not in consequence of its attraction, allow any of its rays
to arrive at us; it is therefore possible that the largest luminous
bodies in the universe may, through this cause, be invisible."

_Gravitation_ by Misner, Thorne & Wheeler presents a dialog explaining
why black holes deserve their name. (It is on pp 872--875 in the 1978
paperback edition, ISBN 0-7167-0344-0.)

As explained in D.03, light rays follow geodesics in spacetime. To
describe things fully you need Eddington-Finkelstein coordinates. In
these coordinates it's pretty easy to see there is a 'surface of last
influence'. In fact, page 873 of MTW has a pretty good graphic showing
just that. The surface of last influence is the 'birthpoint' of the
black hole. It's also clear that in the normal sense of things, 'up'
doesn't exist on the surface of a black hole. As a matter of fact,
black holes don't really have solid surfaces as you might be thinking.

Black holes have horizons, but that's a region in space, not a solid
surface. If you draw various world lines of observers travelling in and
around black holes you will see that the light cones of observers who
don't cross the event horizon have some segment of those cones above the
horizon. Those observers who do cross the event horizon of a black hole
are constrained to fall toward the center eventually. There simply are
not any geodesics that cross the horizon in the outward direction.

At the center there is a region of infinite density and zero volume
where everything ends up. This is a problem in the classical
understanding of black holes.

Recent attempts to understand black holes on a quantum level have
indicated that they radiate thermally (they have a finite temperature,
though one incredibly low if the black hole is of reasonable size) that
is proportional to the gradient of the gravity field. This is due to
the capture of virtual particles decaying from the vacuum at the
horizon. These are created in pairs and one of them is caught in the
black hole and the other is radiated externally. This has been
interpreted by Hawking as a tunneling effect and as a form of Unruh
radiation. This may give some clever and knowledgeable researcher
enough information to figure out what's happening at the center someday.

Another way to think about things is to consider basic geometry. The
surface area of a ball is related to its diameter by pi. A = pi*d^2.
But any gravitating body distorts space so that a light beam travelling
through the center of the body measures a diameter slightly larger than
that indicated by the surface from which it is measured. In the case of
a black hole the diameter measured in this way is infinite, while the
surface area is finite.

------------------------------

Subject: D.09 How can gravity escape from a black hole?
Author: Matthew P Wiener ,
Steve Carlip

In a classical point of view, this question is based on an incorrect
picture of gravity. Gravity is just the manifestation of spacetime
curvature, and a black hole is just a certain very steep puckering
that captures anything that comes too closely. Ripples in the
curvature travel along in small undulatory packs (radiation---see
D.05), but these are an optional addition to the gravitation that is
already around. In particular, black holes don't need to radiate to
have the fields that they do. Once formed, they and their gravity
just are.

In a quantum point of view, though, it's a good question. We don't
yet have a good quantum theory of gravity, and it's risky to predict
what such a theory will look like. But we do have a good theory of
quantum electrodynamics, so let's ask the same question for a charged
black hole: how can a such an object attract or repel other charged
objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if
quantum gravity ends up looking like quantum electrodynamics) are
mediated by the exchange of *virtual* particles. This allows a
standard loophole: virtual particles can pretty much "do" whatever they
like, including travelling faster than light, so long as they disappear
before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must
exceed the speed of light in order to escape, and thus are trapped.
The horizon is meaningless to a virtual particle with enough speed.
In particular, a charged black hole is a source of virtual photons
that can then do their usual virtual business with the rest of the
universe. Once again, we don't know for sure that quantum gravity
will have a description in terms of gravitons, but if it does, the
same loophole will apply---gravitational attraction will be mediated
by virtual gravitons, which are free to ignore a black hole event
horizon.

See R Feynman QED (Princeton, ???) for the best nontechnical account
of how virtual photon exchange manifests itself as long range
electrical forces.

------------------------------

Subject: D.10 What are tachyons? Are they real?
Author: William H. Mook, Jr.

See also the sci.physics FAQ part 4:
ftp://rtfm.mit.edu/pub/usenet-by-hierarchy/sci/physics/
sci.physics_Frequently_Asked_Questions_(4_4)]

Tachyons are theoretical particles that always travel faster than
light. Tachy meaning "swift."

There is a formula that relates mass to speed in the special theory
of relativity:

m = m0 / SQR(1 - v^2/c^2)

where m = energy divided by c^2 (sometimes called "relativistic mass")
m0 = rest mass
v = velocity of mass relative to you
c = velocity of light (constant in all frames of reference)

So, as you see an object moving faster and faster, its mass
increases. A simple experiment with electrons in a vacuum tube can
convince you that mass increases in this way. So you get something
like:

v/c m/m0

0.0 1.000
.2 1.021
.4 1.091
.6 1.250
.8 1.667
.9 2.294
.95 3.203
.99 7.089
.995 10.013
.999 22.366
1.000 infinity

This led Einstein and others to conclude that it was impossible for
any material object to travel at or beyond the speed of light.
Because as you increase speed mass increases. With increased mass,
there's a requirement for increased energy to accelerate the mass.
In the end, an infinite amount of energy is needed to move any object
*at* the speed of light. Nothing would move you faster than the
speed of light, according to this type of analysis.

But, some researchers noted that light has no trouble moving at the
speed of light. Furthermore, objects with mass have no trouble
converting to light. Light has no trouble converting to objects with
mass. So, you have tardyons and photons. Tardy meaning slow. These
classes of objects can easily be converted into one another.

Now, in terms of the equation given above, if you start out with
*any* mass you are constrained to moving less than the speed of
light. If you start out with zero mass, you stay at zero mass. This
describes the situation with respect to photons. You have zero over
zero, and end up with zero....

But, what if you started out faster than the speed of light? Then
the equation above would give you an imaginary mass, since v^2 / c^2
would be greater than 1 and that would be subtracted from 1 to
produce a negative number. Then you'd take the square root of the
negative number and end up with an imaginary number. So, normal
matter moving faster than the speed of light ends up with imaginary
mass, whatever that may be.

Imaginary mass travelling faster than the speed of light would show
up as regular mass to an observer at rest.

v/c m/m0 (m/m0)*i

infinity 0+0.000i 0.000
1,000 0-0.001i 0.001
100 0-0.010i 0.010
10 0-0.101i 0.101
8 0-0.126i 0.126
6 0-0.169i 0.169
4 0-0.258i 0.258
2 0-0.577i 0.577
1.5 0-1.118i 1.118
1.1 0-2.182i 2.182
1.05 0-3.123i 3.123
1.01 0-7.053i 7.053
1.000 0-inf*i infinity

So, if there was such a thing as imaginary mass, it would look like
normal mass but it would always travel *faster* than c, the speed of
light. When it lost energy it would move faster. When it gained
energy it would move slower. So, in addition to tardyons and
photons, there might exist tachyons.

Description Tardyon Photon Tachyon

Gain energy faster c slower
Lose energy slower c faster
Zero energy rest c infinity
Infinite energy c c c

Now, do tachyons exist?

If tachyons exist they can easily be detected by the presence of
Cerenkov radiation in a vacuum. Cerenkov radiation is radiation
emitted when a charged particle travels through a medium at a speed
greater than the velocity of light in the medium. This occurs when
the refractive index of the medium is high.

Cerenkov radiation is like the bow wave of a boat, or the shock wave
of a supersonic airplane. Photons bunch up in front of the tachyon
and they're radiated away at an angle determined by the speed of the
tachyon.

Cerenkov detectors are useful in atomic physics for determining the
speed of particles moving through a medium. Light slows as it passes
through a medium. That's what's responsible for optical effects.
There's nothing mysterious about Cerenkov radiation in a medium. So,
folks know how to make an operate Cerenkov detectors because they're
a useful speedometer when you're working with subatomic particles

Now, there have been a few studies looking for Cerenkov radiation in
a vacuum. This would indicated the reality of tachyons. Cerenkov
radiation has never been detected in vacuum. So, most people believe
that tachyons don't exist.

------------------------------

Subject: D.11 What are magnetic monopoles? Are they real?

Short answer is that magnetic monopoles are the magnetic equivalent of
point electric charges. Like the electron and positron (which can be
considered to carry one unit of electric charge, negative and
positive, respectively), one could imagine that there might be
magnetic particles which have only a north or south magnetic pole.

See J. D. Jackson, _Classical Electrodynamics_, for an extensive
discussion.

------------------------------

Subject: D.12 What is the temperature in space?
Author: Steve Willner

Empty space itself cannot have a temperature, unless you mean some
abstruse question about quantum vacuums.

However, if you put a physical object into space, it will reach a
temperature that depends on how efficiently it absorbs and emits
radiation and on what heating sources are nearby. For example, an
object that both absorbs and emits perfectly, put at the Earth's
distance from the Sun, will reach a temperature of about 280 K or 7 C.
If it is shielded from the Sun but exposed to interplanetary and
interstellar radiation, it reaches about 5 K. If it were far from all
stars and galaxies, it would come into equilibrium with the microwave
background at about 2.7 K.

Spacecraft (and spacewalking astronauts) often run a bit hotter than
280 K because they generate internal energy. Arranging for them to
run at the desired temperature is an important aspect of design.

Some people also consider the "temperature" of high energy particles
like the solar wind or cosmic rays or the outer parts of the Earth's
atmosphere. These particles are not in thermal equilibrium, so it's
not correct to speak of a single temperature for them, but their
energies correspond to temperatures of thousands of kelvins or higher.
Generally speaking, these particles are too tenuous to affect the
temperature of macroscopic objects. There simply aren't enough
particles around to transfer much energy. (It's the same on the
ground. There are cosmic rays going through your body all the time,
but there aren't enough to keep you warm if the air is cold. The air
at the Earth's surface is dense enough to transfer plenty of heat to
or from your body.)

------------------------------

Subject: D.13 Saturn's rings, proto-planetary disks, accretion
disks---Why are disks so common?
Author: Michael Richmond ,
Peter R. Newman

Disks are common in astronomical objects: The rings around the giant
planets, most notably Saturn; the disks surrounding young stars; and
the disks thought to surround neutron stars and black holes. Why are
they so common? First a simple explanation, then a more detailed one.

Consider a lot of little rocks orbiting around a central point, with
orbits tilted with respect to each other. If two rocks collide, their
vertical motions will tend to cancel out (one was moving downwards,
one upwards when they hit), but, since they were both orbiting around
the central point in roughly the same direction, they typically are
moving in the same direction "horizontally" when they collide.

Over a long enough period of time, there will be so many collisions
between rocks that rocks will lose their "vertical" motions---the
average vertical motion will approach zero. But the "horizontal"
motion around the central point, i.e., a disk, will remain.

A more detailed explanation starts with the following scenario:
Consider a "gas" of rubber balls (molecules) organized into a huge
cylindrical shape rotating about the axis of the cylinder. Make some
astrophysically-reasonable assumptions:

- The laws of conservation of angular momentum and conservation of
linear momentum hold (this is basic, well-tested Newtonian mechanics).

- The cylinder is held together by gravity, so the gas doesn't just
dissipate into empty space.

- The main motion of each ball is in rotation about the cylinder's
axis, but each ball has some random motion too, so the balls all run
into each other occasionally. The sum of the angular momentum of the
whole system is thus not zero, but the sum of the linear momentum is
zero (relative to the centre of mass of the entire cylinder).

- The balls are not perfectly bouncy, so that collisions between balls
results in some of the energy of collision going to heating each ball.

Now, consider the motion of the balls in two directions: perpendicular
to the cylinder axis, and parallel to the axis.

First, perpendicular to the axis: conservation of the non-zero angular
momentum will tend to keep the diameter of the cylinder stay
relatively constant. When the balls bounce off each other, some are
thrown towards the axis and some away. In a more realistic model,
some balls are, indeed, ejected from the system entirely, and others
(to conserve angular momentum) will fall into the center (i.e., the
central object).

Parallel to the axis, however, the net linear momentum is zero, and
this, too, is conserved. Balls falling from the top and bottom (due
to the gravity of all the other balls) will again hit each other and
get heated. They don't bounce back as far as they fall, so the length
of the axis is continuously (if slowly) shortened.

Continue with both sets of changes for long enough, and the cylinder
collapses to a disk (i.e., a cylinder with small height). A similar
explanation works for a rotating gas organized into any initial shape
such as a sphere. The subsequent evolution of the initial disk starts
to get complicated in the astrophysical setting, because of things
like magnetic fields, stellar wind, and so on.

So, in short, what makes the disk is the rotation. If an initial
spherical cloud were not rotating, it would simple collapse as a
sphere and no disk would form.

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2000 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
  #6  
Old February 2nd 06, 02:36 AM posted to sci.astro,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)


Last-modified: $Date: 2003/01/27 00:00:01 $
Version: $Revision: 3.14 $
URL: http://sciastro.astronomy.net/
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part5

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/sci.astro.html and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: E.00 Sun, Moon, and Planets

[Dates in brackets are last edit.]

E.01 How did the solar system form? [2000-07-15]
E.02 Has anyone attempted to discern details of the star that went
supernova and formed our local group of stars? [2002-05-04]
E.03 What is the "Solar Neutrino Problem"? [1997-07-01]
E.04 Could the Sun be part of a binary (multiple) star system?
[1995-08-27]
E.05 When will the Sun die? How? [1995-08-23]
E.06 What happens to the planets when the Sun dies? [2000-03-17]
E.07 Could the Sun explode? [1995-07-07]
E.08 How are solar system objects and features named? [1995-11-29]
E.09 Where can I find pictures and planetary data? (ref)
E.10 Could Jupiter become a star? [1995-07-07]
E.11 Is Pluto a planet? Is Ceres? Is Titan? [1995-08-18]
E.12 Additional planets:
12.1 What about a planet (Planet X) outside Pluto's orbit?
[2000-05-21]
12.2 What about a planet inside Mercury's orbit? [1996-11-20]
E.13 Won't there be catastrophes when the planets align in the
year 2000? [2000-07-15]
E.14 Earth-Moon system:
14.1 Why doesn't the Moon rotate? [1997-10-01]
14.2 Why does the Moon always show the same face to the
Earth? [1997-10-01]
14.3 Is the Moon moving away from the Earth? (and why is Phobos
moving closer to Mars?) [1997-06-04]
14.4 What was the origin of the Moon? [1998-11-04]
E.15 What's the difference between a solar and lunar eclipse?
Where can I find more information about eclipses?
[2001-01-17]
E.16 What's the Oort Cloud and Kuiper Belt? [1998-02-28]
E.17 Asteroid Impacts
17.1 What would be the effects of an asteroid impact on the
Earth? [1998-04-14]
17.2 What can we do about avoiding impacts? [2000-01-26]
17.3 I heard that an asteroid was going to hit the Earth?!
[2000-01-26]
E.18 What's the difference between meteoroids, meteors, and
meteorites? [1998-04-14]
E.19 How do we know that meteorites are from the Mars? (or the
Moon?) [2002-05-04]

------------------------------

Subject: E.01 How did the solar system form?
Author: Joseph Lazio

Any theory of the formation of the solar system must explain at least
the following two observations: First, the planets, with the exception
of Pluto, orbit in almost the same plane (the "ecliptic"). Second,
the inner four planets are small and rocky, while the outer four
planets are large and gaseous. One theory that does a reasonably good
job of explaining these observations is the disk model.

The Sun is thought to have formed by the collapse of a large
interstellar gas cloud. The original cloud was probably thousands of
times larger than the present solar system. Initially the cloud had a
very slow rotation rate (it's essentially impossible for one of these
clouds to have a rotation rate of exactly zero). As it collapsed, it
began rotating faster (much like a skater will spin faster if she
pulls her arms to her sides---this principle is known as the
"conservation of angular momentum"). The collapse process is not 100%
efficient, though, so some of the material did not fall into the
proto-Sun. This rotating gas that was left behind settled into a
disk.

In addition to gas, interstellar clouds can also contain dust.
Therefore, the rotating disk consisted of dust grains and gas. In the
process of settling into a disk---and even after the disk had
formed---the dust grains began to collide and stick together.
Initially quite small, this process of colliding dust grains sticking
together (known as "accretion") began to build up larger dust grains.
The accretion process continued with large dust grains accreting to
form small pebbles, small pebbles accreting to form large pebbles,
pebbles forming rocks, rocks forming boulders, etc. Initially this
process is quite random: Two dust grains collide only if their paths
happen to cross. However, as particles became larger, they exert a
larger gravitational force and attract smaller particles to them.
Hence, once started, the accretion process can actually speed up.

The collapse process itself can generate considerable heat.
Furthermore, as the Sun's mass grew, it eventually reached the point
at which fusion reactions in its core could be sustained. The result
was that there was a heat source in the middle of the disk: the inner
parts of the disk were warmer than the outer parts.

In the inner part of the disk, only those materials which can remain
solid at high temperatures could form the planets. That is, the dust
grains were composed of materials such as silicon, iron, nickel, and
the like; as these materials accrete they form rocks. Farther from
the early Sun, where the disk was cooler, there were not only dust
grains but also snowflakes---primarily ice flakes of water, methane,
and ammonia. In the outer parts of the disk, not only could dust
grains accrete to form rocks, but these snowflakes could accrete to
form snowballs.

Water, methane, and ammonia are relatively abundant substances,
particularly compared to substances formed from silicon, iron, etc.
In the inner part of the solar system, where only rocks could remain
solid, we therefore expect small planets, whereas in the outer solar
system, where both rocks and ices could remain solid, we therefore
expect large planets. (Not only did the gaseous planets form from
more abundant substances, they also had more raw material from which
to form. Just compare the size of Earth's orbit to that of Jupiter's
orbit.)

The formation of the giant planets, particularly Jupiter and Saturn,
deserves an additional comment. It is currently thought that they
formed from a run-away accretion process. They started accreting
slowly and probably initially were quite rocky. However, once their
mass reached about 10--15 times that of Earth, their gravitational
force was so strong that they could attract not only other rocks and
snowballs around them, but also some of the gas in the disk that had
not frozen into an ice. As they attracted more material, their
gravitational force increased, thereby attracting even more material
and increasing their gravitational force even more. The result was
run-away accretion and large planets.

One of the problems with this scenario for the formation of Jupiter,
though, is that it seems to take longer than the disk may have
existed. The conventional scenario predicts that Jupiter might have
taken several million years to form. Alan Boss (2000, Astrophysical
Journal, vol. 536, p. L101) has suggested that the conventional model
for the formation of Jupiter is wrong. His work indicates that a
giant planet might also form from small, unstable clumps in the disk.
Rather than being "bottom-up," like the conventional model, his
"top-down" idea is that an entire region of the disk might become
unstable and collapse quite quickly, perhaps in only a few hundred
years.

One of the results of finding planets around other stars is the
realization that this model does not require the planets to always
have been in the same orbits as they have today. Interactions between
the planets, particularly the giant planets, and the disk of material
could have resulted *migration*. The giant planets may moved inward
or outward from their current locations during their formation. If
planets can migrate during or shortly after their formation, it makes
it easier to explain the presence of Uranus and Neptune. A
straightforward application of the above model encounters a slightly
embarrassing problem: The time to form Uranus and Neptune is longer
than the age of the solar system. If, however, these planets formed
at a closer distance, then migrated outward, it may be easier to
understand why Uranus and Neptune are at their current distances from
the Sun. (See Science magazine, vol. 286, 1999 December 10 for more
details.)

------------------------------

Subject: E.02 Has anyone attempted to discern details of the star that went
supernova and formed our local group of stars?
Author: Joseph Lazio

There's one reason, and possibly two, why this cannot be done.

First, our local group of stars is not the group of stars near the Sun
when it formed. All stars have some small random motion, in addition
to their general revolution about the center of the Milky Way Galaxy.
This random motion is typically 10 km/s. Moreover, in the solar
neighborhood, stars tend to have roughly the same velocity (~ 200
km/s), but stars slightly closer to the Galactic center have a smaller
orbit than stars slightly farther away from the Galactic center. The
combination of these factors means that, over the roughly 20 Galactic
orbits that the Sun has completed since it first began fusing hydrogen
in some molecular cloud, its sister stars have dispersed all over the
Galaxy. They are all probably at roughly the same distance from the
Galactic center as the Sun, but some might be on the other side of the
Galaxy by now.

Second, when referring to a supernova and the formation of the Sun,
most people have in mind the hypothesis that the solar system's
formation began as the result of a supernova shock wave impinging on a
molecular cloud. This hypothesis was proposed to account for the
presence of very short-lived isotopes in meteorites. For instance,
the decay products of Aluminum-26 have been found in meteorites. The
half-life of Al-26 is less than 1 million years. Thus, the hypothesis
asserts that, in order for any substantial amount of Al-26 to have
been incorporated into solar system meteorites, there must have been a
supernova (within which Al-26 can be made) quite close to the nascent
solar system.

This hypothesis is being challenged. Recent Chandra X-ray Observatory
observations have shown that young stars may be much more energetic
than the Sun is currently,
URL:http://chandra.harvard.edu/press/01_releases/press_090601solar.html.
If so, then it is possible that some of the X-ray flares produced by
the young Sun might have been enough to explain some or all of the
unusual isotopes found in meteorites. Thus, no supernova might be
required to explain the presence of the solar system.


------------------------------

Subject: E.03 What is the "Solar Neutrino Problem?"
Author: Bruce Scott TOK ,
Joseph Lazio

A middle-aged main-sequence star like the Sun is in a slowly-evolving
equilibrium, in which pressure exerted by the hot gas balances the
self-gravity of the gas mass. Slow evolution results from the star
radiating energy away in the form of light, fusion reactions occurring
in the core heating the gas and replacing the energy lost by
radiation, and slow structural adjustment to compensate the changes in
entropy and composition.

We cannot directly observe the center, because the mean-free path of a
photon against absorption or scattering is very short, so short that
the radiation-diffusion time scale is of order 10 million years. In
other words, the energy produced in the Sun's center and carried by
photons takes about 10 million years to make its way to the Sun's
surface. But the main proton-proton reaction (PP1) in the Sun
involves emission of a neutrino:

PP1: p + p -- D + positron + neutrino(0.26 MeV),

which is directly observable since the cross-section for interaction
with ordinary matter is so small (0.26 MeV is the average energy
carried away by the neutrino). Essentially all the neutrinos escape
the Sun. Of course, this property also makes it difficult to detect
the neutrinos. The first experiments by Davis and collaborators,
involving large tanks of chloride fluid placed underground, could only
detect higher-energy neutrinos from small side-chains in the solar
fusion:

PP2: Be(7) + electron -- Li(7) + neutrino(0.80 MeV),
PP3: B(8) -- Be(8) + positron + neutrino(7.2 MeV).

Recently, however, the GALLEX experiment, using a gallium-solution
detector system, has observed the PP1 neutrinos to provide the first
unambiguous confirmation of proton-proton fusion in the Sun.

There are some discrepancies, however.

1. The first, and most well-known, "solar neutrino problem" is that
every experiment has measured a shortfall of neutrinos. About one- to
two-thirds of the neutrinos expected are observed, depending on
experimental error. In the case of GALLEX, the data read 80 units
where 120 are expected, and the discrepancy is about two standard
deviations.

2. The second solar neutrino problem arises when one compares the
number of neutrinos detected at various detectors. The Kamiokande
experiment detects neutrinos by their interaction with water while the
experiment by Davis uses chlorine. One can use the Kamiokande
experiment to predict how many neutrinos can be detected in Davis'
experiment. The observed number is only 80% that of the predicted
number.

3. The third problem arises when one compares how many neutrinos are
expected from the various processes shown above. The observed number
of neutrinos in the gallium experiments can be compared with the
number expected from the PP1 process and from the PP3 process, after
accounting for the fact that the gallium experiments only see a
fraction of the PP3 process neutrinos. The observed number agrees
with the expected number. But that means that the PP2 process cannot
contribute any neutrinos.

To explain these various shortfall, one of two things must be the
case: (1) the temperature in the Sun's core is slightly less than we
think it is, or (2) something happens to the neutrinos during their
flight over the 150-million-km journey to Earth. A third possibility
is that the Sun undergoes relaxation oscillations in central
temperature on a time scale shorter than 10 Myr, but since no one has
a credible mechanism this alternative is not seriously entertained.

(1) The fusion reaction rate is a very strong function of the
temperature, because particles much faster than the thermal average
account for most of it. Reducing the temperature of the standard solar
model by 6 per cent would entirely explain GALLEX; indeed, Bahcall has
ublished an article arguing that there may be no solar
neutrino problem at all. However, the community of solar
seismologists, who observe small oscillations in spectral line
strengths due to pressure waves traversing through the Sun, argue that
such a change is not permitted by their results.

(2) A mechanism (called MSW, after its authors) has been proposed, by
which the neutrinos self-interact to periodically change flavor
between electron, muon, and tau neutrino types. Here, we would only
expect to observe a fraction of the total, since only electron
neutrinos are detected in the experiments. (The fraction is not
exactly 1/3 due to the details of the theory.) Efforts continue to
verify this theory in the laboratory. The MSW phenomenon, also called
"neutrino oscillation", requires that the three neutrinos have finite
and differing mass, which is also still unverified.

To use explanation (1) with the Sun in thermal equilibrium generally
requires stretching several independent observations to the limits of
their errors, and in particular the earlier chloride results must be
explained away as unreliable (there was significant scatter in the
earliest ones, casting doubt in some minds on the reliability of the
others). Further data over longer times will yield better statistics
so that we will better know to what extent there is a
problem. Explanation (2) depends of course on a proposal whose
veracity has not been determined. Until the MSW phenomenon is observed
or ruled out in the laboratory, the matter will remain open.

In summary, fusion reactions in the Sun can only be observed through
their neutrino emission. Fewer neutrinos are observed than expected,
by two standard deviations in the best result to date. This can be
explained either by a slightly cooler center than expected or by a
particle-physics mechanism by which neutrinos oscillate between
flavors. The problem is not as severe as the earliest experiments
indicated, and further data with better statistics are needed to
settle the matter.

References:

[0] The main-sequence Sun: D. D. Clayton, Principles of Stellar Evolution
and Nucleosynthesis, McGraw-Hill, 1968. Still the best text.
[0] Solar neutrino reviews: J. N. Bahcall and M. Pinsonneault, Reviews of
Modern Physics, vol 64, p 885, 1992; S. Turck-Chieze and I. Lopes,
Astrophysical Journal, vol 408, p 347, 1993. See also J. N. Bahcall,
Neutrino Astrophysics (Cambridge, 1989); J. N. Bahcall, "Solar
Neutrinos: Where We Are, Where We Are Going," 1996, Astrophysical
Journal, vol. 467, p. 475.
[1] Experiments by R. Davis et al: See October 1990 Physics Today, p 17.
[2] The GALLEX team: two articles in Physics Letters B, vol 285, p 376
and p 390. See August 1992 Physics Today, p 17. Note that 80 "units"
correspond to the production of 9 atoms of Ge(71) in a solution
containing 12 tons Ga(71), after three weeks of run time!
[3] Bahcall arguing for new physics: J. N. Bahcall and H. A. Bethe,
Physical Review D, vol 47, p 1298, 1993; against new physics: J. N.
Bahcall et al, "Has a Standard Model Solution to the Solar Neutrino
Problem Been Found?", preprint IASSNS-94/13 received at the National
Radio Astronomy Observatory, 1994.
[4] The MSW mechanism, after Mikheyev, Smirnov, and Wolfenstein: See the
second GALLEX paper.
[5] Solar seismology and standard solar models: J. Christensen-Dalsgaard
and W. Dappen, Astronomy and Astrophysics Reviews, vol 4, p 267, 1992;
K. G. Librecht and M. F. Woodard, Science, vol 253, p 152, 1992. See
also the second GALLEX paper.

------------------------------

Subject: E.04 Could the Sun be part of a binary (multiple) star system?
Author: Bill Owen ,
Steve Willner

Very unlikely. In the 1980's there was proposed a small companion, nicknamed
Nemesis, in a 26-million-year highly eccentric orbit, to explain apparent
periodicities in the fossil extinction record. However, these periodicities
have turned out to be more imagined than real, so the driver for the existence
of Nemesis is gone.

Furthermore, such an object would be relatively close by, bright enough in the
infrared to have been detected easily by IRAS, and its high proper motion
should have been detected by astrometrists long ago.

One very slim possibility is that a very faint companion now located
near the aphelion of an eccentric orbit is not ruled out. Such an
object would be hard to detect because its proper motion would be
small. It's not clear, however, that an orbit consistent with the
lack of detection would be stable for the Sun's lifetime.

So the chances are that there exist no stellar companions to our Sun.

------------------------------

Subject: E.05 When will the Sun die? How?
Author: Erik Max Francis

The Sun is a yellow, G2 V main sequence dwarf. Yellow dwarfs live
about 10 billion years (from zero-age main sequence to white dwarf
formation), and our Sun is already about 5 billion years old.

Main sequence stars (like our Sun) are those that fuse hydrogen into
helium, though the exact reactions vary depending on the mass of the
star. The main sequence phase is by far the most stable and
long-lived portion of a star's lifetime; the remainder of a star's
evolution is almost an afterthought, even though the results of that
evolution are what are most visible in the night sky. As the Sun
ages, it will increase steadily in luminosity. In approximately 5
billion years, when the hydrogen in the Sun's core is mostly
exhausted, the core will collapse---and, consequently, its temperature
will rise---until the Sun begins fusion helium into carbon. Because
the helium fuel source will release more energy than hydrogen, the
Sun's outer layers will swell, as well as leaking away some of its
outer atmosphere to space. When the conversion to the new fuel source
is complete, the Sun will be slightly decreased in mass, as well as
extending out to the current orbit of Earth or Mars (both of which
will then be somewhat further out due to the Sun's slightly decreased
mass). Since the Sun's fuel source will not have increased in
proportion to its size, the blackbody power law indicates that the
surface of the Sun will be cooler than it is now, and will become a
cool, deep red. The Sun will have become a red giant.

A few tens or hundreds of millions of years after the Sun enters its
red giant phase (or "helium main sequence"; the traditional main
sequence is occasionally referred to as the hydrogen main sequence to
contrast the other main sequences that a massive star enters), the Sun
will begin to exhaust its fuel supply of helium. As before, when the
Sun left the (hydrogen) main sequence, the core will contract, which
will correspondingly lead to an increase in temperature in the core.

For very massive stars, this second core collapse would lead to a
carbon main sequence, where carbon would fuse into even heavier
elements, such as oxygen and nitrogen. However, the Sun is not
massive enough to support the fusion of carbon; instead of finding
newer fuel sources, the Sun's core will collapse until degenerate
electrons---electrons which are in such a compressed state that their
freedom of movement is quantum mechanically restricted---smashed
together in the incredible pressures of the gravitational collapse,
will halt the core's collapse. Due to the energy radiated away during
the process process of the formation of this electron-degenerate core,
the atmosphere of the Sun will be blown away into space, forming what
astronomers call a planetary nebula (named such because it resembles a
planetary disk in the telescope, not because it necessarily has
anything to do with planets). The resulting dense, degenerate core is
called a white dwarf, with a mass of something like the Sun compressed
into a volume about that of the Earth's.

White dwarfs are initially extremely hot. But since the white dwarf
is supported by degenerate electrons, and has no nuclear fuel to speak
of to create more heat, they have no alternative but to cool. Once
the white dwarf has cooled sufficiently---a process which will take
many billions of years---it is called an exhausted white dwarf, or a
black dwarf.

------------------------------

Subject: E.06 What happens to the planets when the Sun dies?
Author: Joseph Lazio

A couple of possibilities exist. Prior to forming a planetary nebula,
a low-mass star (i.e., one with a mass similar to that of the Sun)
forms a red giant. Planets close to the star are engulfed in the
expanding star, spiral inside it, and are destroyed. In our own solar
system, Mercury and Venus are doomed.

As the star expands to form a red giant, it also starts losing mass.
All stars lose mass. For instance, the Sun is losing mass. However,
at the rate at which the Sun is currently losing mass, it would take
over 1 trillion years (i.e., 100 times longer than the age of the
Universe) for the Sun to disappear.

When a star enters the red giant phase, the rate at which it loses
mass can accelerate. The mass of a star determines how far a planet
orbits from it. Thus, as the Sun loses mass, the orbits of the other
planets will expand. The orbit of Mars will almost certainly expand
faster than the Sun does, thus Mars will probably not suffer the same
fate as Mercury and Venus. It is currently an open question as to
whether the Earth will survive or be engulfed.

The orbits of planets farther out (Jupiter, Saturn, Uranus, Neptune,
and Pluto) will also expand. However, they will not expand by much
(less than double in size), so they will remain in orbit about the Sun
forever, even after it has collapsed to form a white dwarf.

(Any planets around a high-mass star would be less lucky. A high-mass
star loses a large fraction of its mass quickly in a massive explosion
known as a supernova. So much mass is lost that the planets are no
longer bound to the star, and they go flying off into space.)

As for the material in the planetary nebula, it will have little
impact on the planets themselves. The outer layers of a red giant are
extremely tenuous; by terrestrial standards they are a fairly decent
vacuum!

------------------------------

Subject: E.07 Could the Sun explode?
Author: Erik Max Francis

The short answer is no; the detailed answer depends entirely on what is
meant by "explode." The Sun doesn't have anything like enough mass to
form a Type 2 supernova (whose progenitors are supergiants), which
require more than about 8 solar masses; thus the Sun will not become a
supernova on its own.

"Novae" arise from an accumulation of gases on a collapsed object,
such as a white dwarf or a neutron star. The gas comes from a nearby
companion (usually a distended giant). Although nova explosions are
large by human standards, they are not nearly powerful enough to
destroy the star involved; indeed, most novae are thought to explode
repeatedly on time scales of years to millenia. Since the Sun is not
a collapsed object, nor does it have a companion---let alone a
collapsed one---the Sun cannot go (or even be involved in) a nova.

Under conditions not well understood, the accumulation of gases on a
collapsed object may produce a Type 1 supernova instead of an ordinary
nova. This is similar in principle to a nova explosion but much larger;
the star involved is thought to be completely destroyed. The Sun will
not be involved in this type of explosion for the same reasons it will
not become a nova.

When the Sun evolves from a red giant to a white dwarf, it will shed its
atmosphere and form a planetary nebula; but this emission could not
really be considered an explosion.

------------------------------

Subject: E.08 How are solar system objects and features named?
Author: Bill Owen ,
Gareth Williams

Comets are named for their discoverers, up to three names per comet.

Minor planets are named by the Small Bodies Names Committee of the
International Astronomical Union Commission 20. Discoverers of minor
planets may propose names to the SBNC and minor planets have been
named to honor all sorts of famous (and some not so famous) people and
animals in all walks of life.

Planetary satellites are named by the Working Group for Planetary
System Nomenclature of the IAU, in consultation with the SBNC (mainly
to avoid conflicts of names), and they *usually* defer to the
discoverer's wishes. Names of satellites are usually taken from Greek
mythology or classical literature.

Features on Solar System bodies are named by the same commission, generally
following a specific theme for each body. For instance, most features on Venus
are named in honor of famous women, and volcanos on Io are named for gods and
goddesses of fire.

For additional discussion, see
URL:http://seds.lpl.arizona.edu/billa/tnp/names.html.

The IAU Planetary System Nomenclature Working Group's Web site,
URL:http://wwwflag.wr.usgs.gov/nomen/nomen.html, has an extensive
discussion, as well as lists of names.

------------------------------

Subject: E.09 Where can I find pictures and planetary data?

See Part 1 of this FAQ, and
URL:http://seds.lpl.arizona.edu/billa/tnp/,
URL:ftp://phobos.sscl.uwo.ca/pub/Space,
URL:http://bang.lanl.gov/solarsys/,
URL:http://www-pdsimage.wr.usgs.gov/PDS/public/mapmaker/mapmkr.htm,
and URL:http://wwwflag.wr.usgs.gov/USGSFlag/Space/.

------------------------------

Subject: E.10 Could Jupiter become a star?
Author: Erik Max Francis

A star is usually defined as a body whose core is hot enough and under
enough pressure to fuse light elements into heavier ones with a
significant release of energy. The most basic (and easiest, in terms of
the temperatures and pressures required) type of fusion involve the
fusion of four hydrogen nuclei into one helium-4 nucleus, with a
corresponding release of energy (in the form of high-frequency photons).
This reaction powers the most stable and long-lived class of stars, the
main sequence stars (like our Sun and nearly all of the stars in the
Sun's immediate vicinity).

Below certain threshold temperatures and pressures, the fusion reaction
is not self-sustaining and no longer provides a sufficient release of
energy to call said object a star. Theoretical calculations indicate
(and direct observations corroborate) that this limit lies somewhere
around 0.08 solar masses; a near-star below this limit is called a brown
dwarf.

By contrast, Jupiter, the largest planet in our solar system, is only
0.001 masses solar. This makes the smallest possible stars roughly 80
times more massive than Jupiter; that is, Jupiter would need something
like 80 times more mass to become even one of the smallest and feeblest
red dwarfs. Since there is nothing approaching 79 Jupiter masses of
hydrogen floating around anywhere in the solar system where it could be
added to Jupiter, there is no feasible way that Jupiter could become a
star.

------------------------------

Subject: E.11 Is Pluto a planet? Is Ceres? Is Titan?
Author: Andy Rivkin

While on the face of it, this seems a reasonably easy question with a
simple answer, like the "When does the 21st Century begin?" question
there is no hard and fast rule, no committee of astronomers who decide
these things. The best rule of thumb is that if people think
something's a planet, it is. Common criteria include orbiting the Sun
rather than another body (although sticklers find this troublesome)
and being "large". Some have suggested using "world" as a neutral
term for an interesting solar system body. The word "planet"
originally meant "wanderer", so using a strict definition, everything
in the solar system is a planet!

When Pluto was discovered in 1930, there was no question as to whether
it was a planet. The predictions made at the time imagined it to be
at least the size of the Earth. As better data became available with
the discovery of Pluto's moon Charon allowing the determination of a
mass for Pluto, and with Pluto and Charon eclipsing each other in the
late 1980's--early 1990's, it was found that Pluto is much smaller
than the Earth, with a diameter of roughly 2300 km (or about 1400
mi.). In the last several years, a number of small bodies at about
the same distance from the Sun as Pluto have been discovered,
prompting some to call Pluto the "King of the Kuiper Belt" (the Kuiper
Belt is a postulated population of comets beyond Neptune's orbit) and
rally for its demotion from bona-fide planet to overgrown comet.

Is Pluto a planet? It depends on what one thinks is necessary to
bestow planetary status. Pluto has an atmosphere and a satellite. Of
course, Titan has a much larger atmosphere, and the tiny asteroid Ida
has a satellite. Most astronomers would probably consider stripping
Pluto of its status akin to stripping [the U.S. states of] Connecticut
or Vermont of statehood because Texas and Alaska later joined.

Is Ceres a planet? Like Pluto, when it was first discovered there was
no doubt that it was. Within a few years, however, Pallas, Vesta and
Juno were discovered. While Ceres is the largest asteroid, the
second, third and fourth largest asteroids are roughly half its size,
compared to Pluto, which is about ten times larger than the Kuiper
Belt objects found so far. Ceres is also not thought to have
undergone large-scale geological processes such as vulcanism, although
Vesta has. The consensus is probably that neither Ceres nor any other
asteroid is a "planet", though they are interesting bodies in their
own right.

Is Titan a planet? In the 1940's a methane atmosphere was discovered
around Titan, making it the only satellite with a substantial
atmosphere. This atmosphere has long prevented observations of the
surface, frustrating the attempts of Voyager 1 and 2 and leading
theorists to suggest a Titan-wide global ocean of carbon compounds.
Recent observations have been able to penetrate to the surface of
Titan, showing tantalizing glimpses of what may be continents on the
surface. The atmosphere combined with Titan's large size have led
some to consider Titan a "planet", but what about Ganymede, which is
larger, or Mercury which is smaller and has no atmosphere? Again, the
general consensus is that satellites are not planets.

------------------------------

Subject: E.12 Additional planets:

In addition to the questions answered here, addition info is at
URL:http://seds.lpl.arizona.edu/billa/tnp/hypo.html

------------------------------

Subject: E.12.1 What about a planet (Planet X) outside Pluto's orbit?
Author: Ron Baalke ,
contributions by Bill Owen ,
edited by Steve Willner

Pluto was discovered from discrepancies in the orbits of Uranus and
Neptune. The search was for a large body to explain the
discrepancies, but Pluto was discovered instead (by accident, if you
will, though Clyde Tombaugh's search was systematic and thorough).
Pluto's mass is too small to cause the apparent discrepancies, so the
obvious hypothesis was that there is another planet waiting to be
discovered.

The orbit discrepancies go away when you use the extremely accurate
measurements of the masses of Uranus and Neptune made by Voyager 2
when it flew by those planets in 1986 and 1989. Uranus is now known
to be 0.15% less massive and Neptune 0.51% less massive, than was
previously believed.

[N.B. These numbers come from comparing the post-Voyager masses to those in
the 1976 IAU standard.]

When the new values for these masses is factored into the equations,
the outer planets are shown to be moving as expected, going all the
way back to the early 1800's.

The positional measurements do not bode too well for the existence of
Planet X. They do not entirely rule out the existence of a Planet X,
but they do indicate that it will not be a large body.

Reference:
Standish, E. M., Jr. 1993, "Planet X: No Dynamical Evidence in the
Optical Observations," Astronomical Journal, vol. 105, p. 2000--2006

------------------------------

Subject: E.12.2 What about a planet inside Mercury's orbit?
Author: Paul Schlyter

The French mathematician Urbain Le Verrier, co-predictor with
J.C. Adams of the position of Neptune before it was seen, in an 1860
lecture announced that the problem of observed deviations of the
motion of Mercury could be solved by assuming a planet or a second
asteroid belt inside Mercury's orbit. The only ways to observe this
planet (or asteroids) was if/when it transited the Sun or during total
solar eclipses.

In 1859, Le Verrier had received a letter from the amateur astronomer
Lescarbault, who reported having seen a round black spot on the Sun on
1859 March 26, looking like a planet transiting the Sun. From
Lescarbault's observations, Le Verrier estimated a mean distance from
the Sun of 0.1427 AU (period of 19.3 days). The diameter was
considerably smaller than Mercury's and its mass was estimated at 1/17
of Mercury. This was too small to account for the deviations of
Mercury's orbit, but perhaps this was the largest member of an
asteroid belt? Additional support for such objects was provided by
Prof. Wolf and others at the Zurich sunspot data center, who
identified a total of two dozen spots on the Sun which fit the pattern
of two intra-Mercurial orbits, one with a period of 26 days and the
other of 38 days.

Le Verrier fell in love with the planet and named it Vulcan. In 1860
Le Verrier mobilized all French and some other astronomers to find
Vulcan during a total solar eclipses---nobody did. Wolf's suspicious
"spots" revived Le Verrier's interest, and just before Le Verrier's
death in 1877, there were more "detections." On 1875 April 4, a
German astronomer, H. Weber, saw a round spot on the Sun. Le Verrier's
orbit indicated a possible transit on April 3 that year, and Wolf
noticed that his 38-day orbit also could have performed a transit at
about that time. That "round dot" was also photographed at Greenwich
and Madrid.

There was one more flurry of "detections" after the total solar
eclipse at 1878 July 29: Small illuminated disks which could only be
small planets inside Mercury's orbit. J.C. Watson (professor of
astronomy at the Univ. of Michigan) believed he'd found *two*
intra-Mercurial planets! Lewis Swift (co-discoverer of Comet
Swift-Tuttle, which returned 1992) also saw "Vulcan"---but at a
different position than either of Watson's two "intra-Mercurials." In
addition, neither Watson's nor Swift's Vulcans could be reconciled
with Le Verrier's or Lescarbault's Vulcan.

After this, nobody ever saw Vulcan again, in spite of several searches
at different total solar eclipses. In 1916, Albert Einstein published
his General Theory of Relativity, which explained the deviations in
the motions of Mercury without invoking an additional planet. In 1929
Erwin Freundlich photographed the total solar eclipse in Sumatra. A
comparison with plates taken six months later showed no unknown object
brighter than 9th magnitude near the Sun.

What did these people really see? Lescarbault had no reason to tell a
fairy tale, and even Le Verrier believed him. It is possible that
Lescarbault happened to see a small asteroid passing just inside
Earth's orbit. Such asteroids were unknown at that time. Swift and
Watson could, during the hurry to obtain observations during totality,
have misidentified some stars, believing they had seen Vulcan.

"Vulcan" was briefly revived around 1970-1971, when a few researchers
thought they had detected several faint objects close to the Sun
during a total solar eclipse. These objects might have been faint
comets, and comets have been observed to collide with the Sun.

------------------------------

Subject: E.13 Won't there be catastrophes when the planets align in
the year 2000?
Author: Laz Marhenke ,
Chris Marriott

Obviously there were no catastrophes in May (05-05-2000), nor were
there any in the year 1982.

For starters, the planets only "align" in a very rough fashion. They
don't orbit the Sun in the same plane, so it's impossible to get very
many of the planets in a straight line. Nevertheless, any time they
all get within about 90 degrees of each other, someone will claim
they're "aligned." The last time this happened was 1982 when dire
predictions were heard about how the "Jupiter effect" would lead to
world-wide disaster.

Second, even if they *were* all aligned, the effect on the Earth would
be miniscule. It's true that the other planets' gravity does affect
the orbit of the Earth, but the effect is small, and lining up all the
planets doesn't even come close to making it big enough for anyone to
notice. The effect on the Earth is dominated by Jupiter and Venus
anyway (Jupiter because it's massive, Venus because it's occasionally
very close to us). All the other planets put together only affect us
about 10% as much as those two, so the fact that they're all in the
same general direction as Jupiter and Venus doesn't make much
difference.

Third, even if all the planets could produce a strong gravitational
effect on the Earth (which they can't, unless they find a way to
increase their mass by a factor of 10--100), it wouldn't result in the
"crust spinning over the magma" or some other dire effect, since their
gravity would be pulling on every part of the Earth (almost) equally.

The "(almost)" is because the other planets do exert tidal forces on
the Earth, which means they pull on different parts of the Earth very
slightly differently. However, tidal forces decrease *rapidly* with
distance (as the third power), so these forces are very small: The
tidal force from Venus at its closest approach to Earth is only
1/17,000th as large as the Moon's, and we seem to survive the Moon's
tides well enough twice a day. If the Moon raises tides of 1 meter
(three feet) where you live, Venus at its closest will raise tides of
1/20th of a millimeter, or about the thickness of a hair. The other
planets have even smaller tidal effects on the Earth than Venus does.

Finally, it's worth remembering that the Earth is about 4.5 billion
years old. Whilst these "alignments" may be rare in terms of a human
lifetime (occurring once every few decades), they've occurred numerous
times during the time that life has existed on this planet, and many,
many times in the comparatively brief time that humans have been
around. Brian Monson found ten such "alignments" between AD 1000 and
AD 2000, URL:http://drumright.ossm.edu/astronomy/conjunctions.html.
Thus, over the history of this planet there have been about 45 million
such "alignments." The fact that we're still here to talk about it is
proof enough that nothing *too* terrible happens!

------------------------------

Subject: E.14 Earth-Moon system

Related questions include
B.11 Why does the Moon look so big when it's near the horizon?
B.12 Is it O.K. to look at the Sun or solar eclipses using exposed
film? CDs?
C.07 Easter
C.08 What is a "blue moon?"
C.11 How do I calculate the phase of the moon? and
C.13 Why are there two tides a day and not just one?

------------------------------

Subject: E.14.1 Why doesn't the Moon rotate?
Author: Laz Marhenke

In fact the Moon *does* rotate: It rotates exactly once for every
orbit it makes about the Earth. The fact that the Moon is rotating
may seem counterintuitive: If it's always facing towards us, how can
it be rotating at all? To see how this works, put two coins on a
table, a large one to represent the Earth, and a small one to
represent the Moon. Choose a particular place on the edge of the
"Moon" as a reference point. Now, move the Moon around the Earth in a
circle, but be careful to always keep the spot you picked pointed at
the Earth (this is analogous to the Moon always keeping the same face
pointed at the Earth). You should notice that as you do this, you
have to slowly rotate the Moon as it circles the Earth. By the time
the Moon coin goes once around the Earth coin, you should have had to
rotate the Moon exactly once.

This exact equality between the Moon's rotation period and orbital
period is sometimes seen as a fantastic coincidence, but, in fact,
there is a physical process which slowly changes the rotation period
until it matches the orbital period. See the next entry.

------------------------------

Subject: E.14.2 Why does the Moon always show the same face to the
Earth?
Author: Laz Marhenke

When it first formed, the Moon probably did not always show the same
face to the Earth. However, the Earth's gravity distorts the Moon,
producing tides in it just as the Moon produces tides in the Earth.
As the Moon rotated, the slight elongation of its tidal bulge was
dragged a bit in the direction of its rotation, providing the Earth
with a "handle" to slow down the Moon's rotation. More specifically,
the tidal bulge near the Earth is attracted to the Earth more strongly
than the bulge away from the Earth. Unless the bulge points toward
the Earth, a torque is produced on the Moon.

If we imagine looking down on the Earth-Moon system from the north
pole, here's what we'd see with the Moon rotating at the same rate as
it goes around the Earth:

Earth Moon
__
/ \ ____ ^
| | / \ |
\__/ \____/ Orbiting
this way
Tidal bulge *greatly*
exaggerated.


What if the Moon were rotating faster? Then the picture would look like:

Earth Moon
__
/ \ ___ ^
| | / ) |
\__/ (___/ Orbiting
this way
Rotating
counterclockwise;
Tidal bulge *greatly*
exaggerated.

If it isn't clear why the tidal bulge should move the way the picture
shows, think about it this way: Take the Moon in the top picture, with
its tidal bulges lined up with the Earth. Now, grab it and rotate it
counterclockwise 90 degrees. Its tidal bulge is now lined up the
"wrong" way. The Moon will eventually return to a shape with tidal
bulges lined up with the Earth, but it won't happen instantly; it will
take some time. If, instead of rotating the Moon 90 degrees, you did
something less drastic, like rotating it one degree, the tidal bulge
would still be slightly misaligned, and it would still take some time
to return to its proper place. If the Moon is rotating faster than
once per orbit, it's like a constant series of such little
adjustments. The tidal bulge is perpetually trying to regain its
correct position, but the Moon keeps rotating and pushing it a bit out
of the way.

Returning to the second picture above, the Earth's gravitational
forces on the Moon look like this:
___
F1 -----/ )
F2 -------(___/

F2 is larger than F1, because that part of the Moon (the "bottom" half
in the drawing, or the half that's "rearward" in the orbit) is a bit
closer to the Earth. As a result, the two forces together tend to
twist the Moon clockwise, slowing its spin. Over time, the result is
that the Moon ends up with one face always facing, or "locked," to the
Earth. If you drew this picture for the first case, (where the Moon
rotates at the same rate that it orbits, and the tidal bulges are in
line with the Earth), the forces would be acting along the same line,
and wouldn't produce any twist.

Another way to explain this is to say that the Moon's energy of
rotation is dissipated by internal friction as the Moon spins and its
tidal bulge doesn't, but I think the detailed force analysis above
makes things a little clearer.

This same effect occurs elsewhere in the solar system as well. The
vast majority of satellites whose rotation rates have been measured
are tidally locked (the jargon for having the same rotation and
orbital periods). The few exceptions are satellites whose orbits are
very distant from their primaries, so that the tidal forces on them
are very small. (There could be, in principle, other exceptions among
some of the close-in satellites whose rotation rates haven't been
measured, but this is unlikely as tidal forces grow stronger the
closer to the planet the satellite is.)

Pluto's satellite Charon is so massive (compared to Pluto) that it has
locked Pluto, as well as Pluto locking Charon. This will happen to
the Earth eventually too, assuming we survive the late stages of the
Sun's evolution intact. :')

------------------------------

Subject: E.14.3 Is the Moon moving away from the Earth? (and why is
Phobos moving closer to Mars?)
Author: Richard A. Schumacher ,
Michael Dworetsky ,
Joseph Lazio

Yes, at a rate of about 3--4 cm/yr.

The tidal bulges on the Earth (largely in the oceans), raised by the
Moon, are rotated forward (ahead of) the Earth-Moon line by the
Earth's rotation since it is faster than the Moon's orbital motion.

Using a similar picture as from the previous question, we'd see
(looking down from the north pole):

Earth Moon
____
/ ) ___ ^
/ / / \ |
(____/ \___/ Moon's orbit &
Earth's rotation
(Ocean) Tidal bulge this way
*greatly* exaggerated.

The gravity from these leading and trailing bulges impels the Moon
mostly forward along the direction of its motion in orbit (the Moon's
orbit is not exactly in the plane of the Earth's equator). This force
transfers momentum from the rotating Earth to the revolving Moon,
simultaneously dragging the Earth and accelerating the Moon.

In addition to causing the Moon to recede from the Earth, this process
also causes the Earth's rotation to slow and days to become longer (at
a rate of about 0.002 seconds every century). Eventually the result
will be that the Earth will show only one face to the Moon (much like
the Moon only shows one face to the Earth). A lower limit to how long
it will take for the Earth and Moon to become tidally locked is 50
billion years, at which point the month and the Earth's "day" will
both be approximately 50 (of our current) days long. However, this
estimate is based on the assumption that liquid water seas would be
present on Earth's surface to provide the tidal interactions
necessary. But as the Sun evolves, the seas will evaporate and tidal
interactions will be much slower (solid planet distortions only). The
oceans will evaporate about 1--2 billion years from now, so the actual
time will probably be much longer.

Considerably more detail on the evolution of the Earth-Moon system can
be found in an article by J. Burns in the book _Planetary Satellites_
(ed. J. Burns [Tucson: University of Arizona]) and in Sir Harold
Jeffries' _The Earth_, 3rd ed (Cambridge Univ Press, 1952).

It is also interesting to consider what would happen if a satellite
orbits its planet *faster* than the planet rotates. This is not the
case for the Earth and Moon, but it is true for Mars and Phobos. In
this case, Phobos also raises (crustal) tides on Mars. But now,
Phobos is in front of the tidal bulge, so the gravitational action of
the tidal bulge slows Phobos and Phobos moves *inward*. Thus, at some
point in the future, Phobos will hit Mars. The most recent estimate
is that the impact will occur in 40 million years, by A. T. Sinclair
(1989, Astronomy & Astrophysics, vol. 220, p. 321).

------------------------------

Subject: E.14.4 What was the origin of the Moon?
Author: George Cummings
Joseph Lazio ,

The Moon presents a curious problem. Of the terrestrial planets
(Mercury, Venus, Earth, and Mars) only Earth and Mars have satellites.
Mars' satellites are much smaller than the Moon, both in absolute size
and in comparison to their primary. (The Moon is 3476 km in diameter
while Phobos is 23 km in diameter; the Moon's diameter is 27% that of
the Earth while Phobos' diameter is 0.34% that of Mars.)

Furthermore, the Moon's chemical composition is peculiar. In many
respects it is quite similar to the Earth's, except that the Moon
seems to have less iron (and similar elements like nickel) and
considerably less water (it's quite dry!).

Until recently there were three competing theories to explain the
Moon's origin. (1) The Moon formed elsewhere in the solar system and
was captured eventually by the Earth. (2) The Moon and Earth formed
together at the same time in essentially the same place. (3) The
early Earth was spinning so fast that a portion of it broke off and
formed the Moon (possibly leaving the Pacific Ocean basin as a
result). All theories had their difficulties, though.

If the Moon formed elsewhere in the solar system (like between the
orbits of Venus and Earth or between the orbits of Earth and Mars),
how did it get disturbed into the orbit that took it near the Earth?
Furthermore, it is actually quite difficult for an object that is not
initially orbiting the Earth to begin doing so. The incoming object
must lose energy. In the case of Mars, its small satellites could
have gotten close enough to skim the upper part of its atmosphere,
which would cause them to lose energy from air resistance. Because
the Moon is so big, it probably would have hit the Earth rather than
passing just close enough to lose just enough energy to be captured
into orbit.

If the Earth and Moon formed simultaneously at nearly the same
location in the solar system, then the differing chemical compositions
of the two are quite difficult to understand. Why are they similar
yet so different?

Finally, there isn't much evidence to suggest that the early Earth was
spinning anywhere near fast enough for it to break apart.

With the realization in the 1980s that impacts (of comets, asteroids,
etc.) have played a major role in the history of the solar system, a
new theory emerged:

The Moon was formed when a Mars-sized object collided with the Earth
when the Earth was very young, about 4.5 billion years ago. Much of
the Earth's crust and mantle, along with most of the colliding object,
disintegrated and was blown into orbit thousands of kilometers high.
About half of this debris fell back to Earth. The rest coalesced into
the Moon. (Loose material in orbit can coalesce if it is outside the
"Roche limit," otherwise it will be pulled apart by tidal forces. The
Roche limit for the Earth is approximately 3 Earth radii. The
material outside this limit formed the Moon, the material inside the
limit fell back to Earth.) Since the time of its original formation,
the Moon has slowly moved farther from the Earth to its present
position.

This theory does a good job of explaining why only the Earth has a
large moon and why the Moon's chemical composition is similar yet
different. Impacts are random events, and there almost certainly were
not a lot of large objects left in the solar system as the planets
were nearly the end of their formation. The Earth just happened to be
the planet struck by this large, rogue planetoid. If we could start
over the formation of the solar system, it might be Venus or Mars that
would end up with a large moon. The chemical composition of the Earth
and Moon are clearly predicted to be similar in this model, since a
portion of the Earth went into forming the Moon and a portion of the
impactor remained in the Earth. The Moon would be deficient in iron
and similar metals if the impact occurred after those elements had
largely sunk to the center of the Earth (i.e., after the Earth
differentiated). The Moon should also be quite dry because the
material from which the Moon formed was heated to a high temperature
in the impact, thereby evaporating all of the water.

Computer models of this event indicate that the Moon coalesced in only
about a year. Also interesting is that a large percentage of
simulations result in the formation of two moons. Some of the more
recent simulations suggest that the colliding object might have had to
have been much larger, about three times the size of Mars.

More information on this theory of Moon formation can be found at
URL:http://www.earthsky.com/specials/moonformation.html.

------------------------------

Subject: E.15 What's the difference between a solar and lunar
eclipse? Where can I find more information about eclipses?
Author: Joseph Lazio

A solar eclipse occurs when the Moon passes between the Earth and Sun
and the Moon's shadow crosses the Earth, viz. (not to scale!)

Sun Moon Earth

Solar eclipses can be total, partial, or annular. A total eclipse is
when the Moon obscures the Sun entirely. A partial eclipse is when
the Moon only covers a portion of the Sun. Because the Moon's orbit
about the Earth is not perfectly circular, sometimes it is slightly
farther away from the Earth. If a solar eclipse occurs when the Moon
is at the far point in its orbit, the Moon will not cover the Sun
entirely. A thin ring, or annulus, of sunlight will be visible around
the Moon. This kind of eclipse is called an annular eclipse.

**Solar eclipses can be damaging to one's eyesight, unless proper
precautions are taken!** See FAQ Question B.11 and the Eclipse Home
Page, URL:http://sunearth.gsfc.nasa.gov/eclipse/.

A lunar eclipse occurs when the Earth passes between the Moon and Sun,
viz. (again, not to scale)

Sun Earth Moon

Lunar eclipses are either total or partial, depending upon whether the
Moon moves completely into the Earth's shadow or not. Lunar eclipses
are always safe to view.

Eclipses do not happen once a month because the Earth's orbit about
the Sun and the Moon's orbit about the Earth are not in the same
plane. The above "pictures" are if one is looking "down" on the Earth
from the North Pole (or "up" on the South Pole). If we look at the
system from the side (looking at the Earth's equator), the typical
situation is

Sun Earth
Moon

(with the angle shown exaggerated greatly, the actual angle is about 5
degrees). Only when the three bodies are in the same plane can an
eclipse occur. The total number of eclipses, both lunar and solar,
never exceeds seven in a year. Because the Moon is so much smaller
than the Earth, and casts a smaller shadow, solar eclipses are more
infrequent than lunar eclipses; in a year, between 2 to 4 lunar
eclipses will occur and at least 2 solar eclipses will occur. *Total*
solar eclipses happen only every 1.5 years or so.

For additional information see the Eclipse Home Page,
URL:http://sunearth.gsfc.nasa.gov/eclipse/.

------------------------------

Subject: E.16 What's the Oort Cloud and Kuiper Belt?
Author: Joseph Lazio

Comets have highly elliptical orbits. When at perihelion or closest
approach to the Sun, they are typically about the same distance from
the Sun as the Earth is. When at aphelion or farthest distance from
the Sun, they can be well outside the orbit of Pluto. If a comet is
observed for a sufficient period of time, its motion on the sky allows
us to estimate when it is at perihelion and how far away aphelion is
(more precisely, we can estimate the major axis of its orbit).

In 1950 Jan Oort was analyzing the comets whose orbits had been
determined. He discovered that many comets had their aphelia at
roughly the same distance from the Sun, about 50,000 AU. (For
reference, the Earth is at a distance of 1 AU from the Sun, Neptune is
at a distance of 40 AU, and the nearest star is at a distance of
270,000 AU.) So Oort proposed that the Sun was surrounded by a vast
swarm of comets, stretching nearly 1/5 of the distance to the nearest
star.

At these large distances from the Sun, these comets are only loosely
gravitationally bound to the Sun. A slight gravitational nudge, from
a star passing within a couple of light years or so perhaps, is enough
to change their orbits dramatically. The gravitational tug can result
in a comet either (1) becoming gravitationally unbound from the Sun
and drifting into interstellar space never to return or (2) falling
into the inner solar system. This is the currently accepted
explanation for the origin of so-called "long-period" comets. These
comets orbit the Sun at great distances, until a slight gravitational
nudge changes their orbit and causes them to fall into the inner solar
system, where we see them. Because their aphelia remain at large
distances, it can take hundreds, thousands, or maybe even 1 million
years before they return to the inner solar system. Comet Hale-Bopp
is an example of such a comet.

Theorizing that comets originate from the Oort cloud doesn't explain
the properties of all comets, however. "Short-period" comets, those
with periods less than 200 years, have orbits in or near the
ecliptic---the plane in which the Earth and other planet orbit.
Long-period comets appear to come from all over the sky. Short-period
comets can be explained if there is a disk of material, probably left
over from the formation of the solar system, extending from the orbit
of Neptune out to 50 AU or more. Collisions between objects in such a
disk and gravitational tugs from the gas giants in our solar system
would be enough to cause some of the objects to fall into the inner
solar system occasionally where we would see them. Comet Halley is
probably an example of such a comet.

Direct detection of Kuiper Belt objects occurred in the early 1990s
with the detection of 1992/QB1, see
URL:http://www.ifa.hawaii.edu/faculty/jewitt/qb1.html. Additional
indirect evidence for a disk of material around the Sun comes from
images of nearby stars which have disks around them. These disks
around other stars are several times larger than the Kuiper Belt has
thus far been observed to extend, but they might be qualitatively
similar to the Kuiper Belt. See
URL:http://galileo.ifa.hawaii.edu/users/jewitt/Origins-bpic.html.

Interestingly, current theories for the origin of the Oort Cloud and
Kuiper Belt indicate that the Kuiper Belt probably formed first. The
Kuiper Belt is the detritus from the formation of the solar system.
Objects from it that make it into the inner solar system can interact
gravitationally with the giant planets, particularly Jupiter. Some
objects would have had their orbits changed so that they impacted with
one of the planets (like Comet Shoemaker-Levy 9 did in 1994); some
objects would be ejected from the solar system entirely; and some
objects would be kicked into very large orbits and into the Oort
cloud.

------------------------------

Subject: E.17 Asteroid Impacts

Much of the material in this section is drawn from the SpaceGuard
Survey report, URL:http://ccf.arc.nasa.gov/sst/spaceguard.html.

A crucial point about asteroid impacts is that they are random. Below
are various estimates of the frequency with which the Earth is struck
by objects of various sizes. These estimates are, roughly speaking,
averages over the Earth's history. For instance, the average time
between the impact of a 100 m diameter object is roughly 100--200 yr.
The actual time between the impacts of such objects could be shorter
than 10 yr or longer than 1000 yr.

For more information about Near-Earth Objects, those asteroids (or
minor planets) that have orbits similar to Earth's, see the following.
A list of "Potentially Hazardous Asteroids" (PHAs) is at
URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html. These have
a projected closest distance to Earth of less than 0.05 AU (7.5
million km, about 1000 Earth radii). A list of closest approaches to
the Earth by PHAs between 1999 and 2099 is available at
URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html. A list
of moderately close (to within 0.2 AU) approaches to the Earth by
asteroids and comets between 1999 and 2032 is available at
URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html. It is worth
emphasizing that, at the moment, *none* of the known objects presents
a serious risk of collision.

------------------------------

Subject: E.17.1 What would be the effects of an asteroid impact on
the Earth?
Author: Joseph Lazio

The Earth is constantly pelted by bits of cosmic debris. Most of this
simply burns up in the atmosphere (as one can attest by simply
watching meteors on a dark night). However, if an object is big
enough it can survive passage through the atmosphere. The damage done
by a meteorite (an object that strikes the Earth) depends upon its
initial size.

10--100 m: Objects in this size range can produce devastation similar
to that of an atomic blast (leading to them occasionally being called
"city-busters"). Effects include severe damage to or collapse of
standing buildings and the ignition of flammable materials leading to
widespread fires. The radius over which such effects occur would vary
depending upon the size and composition of the object, but could
easily exceed 10 km. The Tunguska event, in Siberia, of 1908 is
thought to have been from an object about 60 m in size; it led to
trees being flattened out to 20 km and trees 40 km away being damaged.

At the small end of this size range, objects about 10 m strike the
Earth about once a decade. Fortunately, only the densest objects,
those containing iron, survive to the surface; most of the objects of
this size explode sufficiently high in the atmosphere that there are
no effects (other than maybe a loud noise) on the ground. At the
larger end of this size range, it is estimated that the Earth is
struck several times a millennium or about 1 impact every 100--200 yr.

100 m--1 km: Objects in this size range are likely to cause severe
damage over a regional area, possibly as large as a continent (hence
the name "continent-busters"). If they strike land, they will almost
certainly produce a crater, while an ocean impact will generate large
tidal waves. A 150 m object might produce a crater 3 km in diameter,
an ejecta blanket 10 km in diameter, and a zone of destruction
extending much farther out. For a 1 km impactor the zone of
destruction might reasonably extend to cover countries. The death
toll could be in the tens to hundreds of millions. A 1 km impactor
could begin to have minor global consequences, including global
cooling caused by vast amounts of dust in the atmosphere.

Estimates from the geologic record suggest that craters are formed on
the Earth roughly once every 5000 yr.

1--10 km: Objects in this size range are likely to cause severe global
effects ("species-busters"). An impact 65 million years ago by an
object of 5--10 km in diameter is thought to have been partially or
fully responsible for the extinction of half the living species of
animals and plants at the time, including the dinosaurs. The crater
alone from such an impact will be 10--15 times larger than the object
itself. World-wide crop failures from dust injected into the
atmosphere could imperil civilization, and the largest-sized objects
could make the human species extinct.

The frequency with which the Earth is struck by such objects has to be
estimated from the geological and paleontological record. At the low
end of this size range, estimates are that such impacts occur roughly
every 300 000 yr; at the upper end of the size range, impacts occur
about every 10 million years.

------------------------------

Subject: E.17.2 What can we do about avoiding impacts?
Author: Joseph Lazio

A number of papers on the risks, potential damages from impacts, and
ways to mitigate the danger is at
URL:http://www.llnl.gov/planetary/.

Our ability to prevent impacts depends upon several things, the size
of the object, its orbit, and the amount of time until impact.
Generally speaking, the more time the better. It is perhaps
counter-intuitive, but we could mount the best defense against objects
in orbits similar to that of Earth. Such an object would pass close
to Earth several times, giving us many chances to discover it,
calculate an extremely accurate orbit, and launch one or more missions
to it. We might have decades or even centuries to plan. Conversely,
a comet on an impact course might be discovered only a month or so
away from impact, giving us little or no time to act.

The optimum approach to avoiding an impact is to discover an object
well before impact and gently nudge it. If discovered long enough
before impact, only small nudges are sufficient to change the object's
orbit so that it will no longer strike Earth. There are a number of
strategies to nudge an asteroid including landing a rocket engine on
the asteroid or vaporizing a small portion of it with a laser or
stand-off nuclear blast or reflected, concentrated sunlight.

Popular depictions of laser beams or nuclear weapons being used to
blast asteroids into pieces are usually unrealistic; moreover, if
actually used, such "solutions" would probably make the situation
worse. First, it is unlikely that the firepower exists to blow apart,
say, a 5 km asteroid. Second, even if we could blow apart an
asteroid, most of the pieces would stay on essentially the same orbit,
i.e., on target to hit the Earth. A rain of 1000 100-m--sized objects
could still cause considerable damage.

------------------------------

Subject: E.17.3 I heard that an asteroid was going to hit the Earth?!
Author: Louis Strous

These such questions typically occur after a news report of a future
close encounter between the Earth and an asteroid. To date, all such
reports have resulted from (1) Astronomers did not yet know well
enough the orbit of a newly-discovered asteroid to say with any
certainty that it would not hit the Earth; (2) Reporters not checking
their stories or misunderstanding what they were told; or (3) both.

Objects that can potentially come close to the Earth are referred to
as Near-Earth Objects (NEOs). The International Astronomical Union
maintains lists of such objects. About 100 asteroids are classified as
"Potentially Hazardous Asteroids" (PHAs), at
URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html; they all
have a projected closest distance to Earth of less than 0.05 AU (7.5
million km). A list of closest approaches to the Earth by PHAs
between 1999 and 2099 is available at
URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html. A list
of moderately close (to within 0.2 AU) approaches to the Earth by
asteroids and comets between 1999 and 2032 is available at
URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html. At the
moment, NONE of these encounters is thought to pose a serious risk.

The "potential hazard" of PHAs lies in their orbits and the
perturbations on those orbits from the planets and the Moon currently
not being known with sufficient accuracy to completely exclude the
possibility of a collision, but, generally, labeling these asteroids
as PHAs is erring on the side of extreme caution. It is not worth
losing any sleep over them.

------------------------------

Subject: E.18 What's the difference between meteoroids, meteors, and
meteorites?

Briefly, a meteoroid is piece of cosmic debris in the solar system.
It becomes a meteor when it enters Earth's atmosphere and begins to
glow brightly. It becomes a meteorite if it survives and hits the
ground.

Three FAQs on different aspects of meteors and meteorites are
maintained by the American Meteor Society at
URL:http://www.serve.com/meteors/.

------------------------------

Subject: E.19 How do we know that meteorites are from the Mars? (or
the Moon?)

[This question comes up most frequently with reference to ALH 84001,
the Martian meteorite that has been suggested as carrying evidence of
past Martian life.]

Most meteorites are thought to originate from collisions between
asteroids in the asteroid belt. However, a small number have
characteristics suggestive of a Martian or lunar origin. Why do we
think this?

The short explanation is that we can compare the composition of a
meteorite to what various space probes and missions have told us about
the composition of Mars (or the Moon). Moreover, in the case of a
candidate Martian meteorite, it may have small pockets of gas trapped
within it, which can be compared to the Viking measurements of the
Martian atmosphere. Finally, it is possible to simulate launching a
small piece of rock from Mars or the Moon (say, from an asteroid
impact) and determine its path through space. Because of
gravitational perturbations from other planets (notably Jupiter and
the Earth), such a small rock could find its way to Earth, on fairly
short time scales even (a few million years or so).

For more details, see "On the Question of the Mars Meteorite,"
URL:http://cass.jsc.nasa.gov/pub/lpi/meteorites/mars_meteorite.html
and Michael Richmond's archive of postings by James Head (from the
Lunar and Planetary Institute) on this topic,
URL:http://a188-l009.rit.edu/richmond/answers/martian.html.

Finally, the meteorite Northwest Africa #11 (NWA011) has a composition
similar to that of many Martian and lunar meteorites, but some
important differences as well (notably in the amount of oxygen). This
has led some to speculate that NWA011 might be from Mercury(!).

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2000 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
  #7  
Old February 2nd 06, 02:37 AM posted to sci.astro,sci.astro.seti,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro] ET Life (Astronomy Frequently Asked Questions) (6/9)


Last-modified: $Date: 2003/04/27 01:49:47 $
Version: $Revision: 4.3 $
URL: http://sciastro.astronomy.net/
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part6

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/ and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: F.00 Extraterrestrial Life

[Dates in brackets are last edit.]

F.01 What is life? [1997-09-03]
F.02 Life in the Solar System
02.1 Is there life on Mars? [1996-09-03]
02.2 Is there life in Jupiter (or Saturn)? [1996-09-03]
02.3 Is there life on Jupiter's moon Europa? [1996-09-03]
02.4 Is there life on Saturn's moon Titan? [1997-08-05]
F.03 What is the Drake equation? [1995-10-04]
F.04 What is the Fermi paradox? [1995-12-28]
F.05 Could we detect extraterrestrial life? [1999-09-15]
F.06 How far away could we detect radio transmissions?
[2000-07-19]
F.07 What's a Dyson sphere? [1997-06-04]
F.08 What is happening with SETI now? [2998-01-31]
F.09 Why search for extraterrestrial intelligence using radio?
Why not fill in the blank method? [2000-01-01]
F.10 Why do we assume that other beings must be based on carbon?
Why couldn't organisms be based on other substances?
[2001-03-20]
F.11 Could life occur on an interstellar planet? [2003-04-27]

See also the entry in Section G of the FAQ on the detection of
extrasolar planets.

------------------------------

Subject: F.01 What is life?
Author: T. Joseph W. Lazio

This material is extracted from the review article by Chyba &
MaDonald (1995, Annual Review of Earth and Planetary Science).

How might we tell if a future mission to another body in the solar
system had discovered life? How do we separate living from
non-living? A simple set of criteria for doing so might be,
Something that is alive must (1) acquire nutrients from its
environment, (2) respond to stimuli in its environment, and
(3) reproduce. Unfortunately, with this definition we would conclude
that mules are not alive while fire is. Other attempts to define
life---based on genetic, chemical, or thermodynamic criteria---suffer
from similar failings.

A working definition used by many attempting to understand the origin
of life on the Earth is something like, "Life is a self-sustained
chemical system capable of undergoing Darwinian evolution." (Note
that this definition, *chemical* systems, would exclude computer life
or A-life, but other definitions exist which would not.) Again this
definition is not without its difficulties. The emphasis on evolving
systems implicitly assumes a collection of entities; Victor
Frankenstein's creation would not have been classified as alive.
Further, how long must one wait before concluding that a system was
not evolving? A recent definition that focusses on individual
entities is that a living organism must be (1) self-bounded, (2)
self-generating, and (3) self-perpetuating.

Perhaps it is not possible to provide necessary and sufficient
criteria to distinguish "alive" from "not alive." Indeed, if life can
arise from natural physical and chemical processes, there may be a
continuous spectrum of "aliveness," with some entities clearly
"alive"---humans, trees, dogs---some entities clearly "not
alive"---rocks, pop bottles---and some entities somewhere in
between---viruses.

Operationally, at our current stage of exploration of the solar
system, all of the above definitions are probably too detailed. On
Earth, we have entities we clearly identify as "alive." Liquid water
appears to be a requirement for these living things. Hence, the focus
in solar system studies of life has been to target those bodies where
liquid water either is possibly now or may have once been present.

------------------------------

Subject: F.02 Life in the Solar System

Within the past 100--150 years, the conventional wisdom regarding life
in the solar system (beside the Earth) has been on a roller coaster
ride. Life on other planets used to be considered likely.
Suggestions for sending messages to other planets included cutting
down huge tracts in the Siberian forests or filling and setting afire
trenches of kerosene in the Sahara. Lowell believed that he could see
evidence for a civilization on Mars.

During the Space Age the planets were explored with robotic craft.
The images and other measurements sent back by these craft convinced
most scientists that only the Earth harbored life.

With even more recent findings, the possibility of life that life
exists or existed elsewhere in the solar system is now being
re-examined.

------------------------------

Subject: F.02.1 Is there life on Mars?
Author: Steve Willner

The Viking landers found conditions on the surface of Mars unlikely to
support life as we know it. The mass spectrometer found too little
carbon, which is the basis for organic molecules. The chemistry is
apparently highly oxidizing as well. Some optimists have nevertheless
argued that there still might be life on Mars, either below the
surface or in surface regions not sampled by the landers, but most
scientists consider life on Mars quite unlikely. Evidence of surface
water suggests, however, that Mars had a wetter and possibly warmer
climate in the past, and life might have existed then. If so, there
might still be remnants (either living or fossil) today, but close
examination will be necessary to find out.

More recently, McKay et al. have invoked biological activity to
explain a number of features detected in a meteorite from Mars. See
URL:http://www.fas.org/mars/ for additional information.

------------------------------

Subject: F.02.2 Is there life in Jupiter (or Saturn)?

Jupiter (and Saturn) has no solid surface, like the Earth. Rather the
density and temperature increase with depth. The lack of solid
surface need not be a deterrent to life, though, as many aquatic
animals (e.g., fish, jellyfish) never touch a solid surface.

There has been speculation that massive gas-bag organisms could exist
in Jupiter's atmosphere. These organisms might be something like
jellyfish, floating upon the atmospheric currents and eating either
each other or the organic materials formed in Jupiter's atmosphere.

------------------------------

Subject: F.02.3 Is there life on Jupiter's moon, Europa?

This article is adapted from NASA Press Releases.

In the late 1970's, NASA Voyager spacecraft imaged Europa. Its
surface was marked by complicated linear features, appearing like
cracks or huge fractures in the surface. No large craters (more than
five kilometers in diameter) were easily identifiable. One
explanation for this appearance is that the surface is a thin ice
crust overlying water or softer ice and that the linear features are
fractures in that crust. Galileo images have reinforced the idea that
Europa's surface is an ice-crust, showing places on Europa that
resemble ice floes in Earth's polar regions, along with suggestions of
geyser-like eruptions.

Europa's appearance could result from the stresses of the contorting
tidal effects of Jupiter's strong gravity (possibly combined with some
internal heat from decay of radioactive elements). If the warmth
generated by tidal heating is (or has been) enough to liquefy some
portion of Europa, then the moon may have environmental niches warm
and wet enough to host life. These niches might be similar to those
found near ocean-floor vents on the Earth.

------------------------------

Subject: F.02.4 Is there life on Saturn's moon Titan?
Author: T. Joseph W. Lazio

This material is extracted from the review article by Chyba &
McDonald (1995, Annual Review of Earth and Planetary Science).

Titan's atmosphere is a rich mix of nitrogen and methane, from which
organic molecules (i.e., those containing carbon, not necessarily
molecules in living organisms) can be formed. Indeed, there has been
speculation that Titan's atmosphere resembles that of Earth some 4
billion years ago. Complex organic chemistry can result from the
ultraviolet light from the Sun or from charged particle impacts on the
upper atmosphere. Unfortunately, Titan's great distance from the Sun
means that the surface temperature is so low that liquid water is
probably not present globally. Since we believe that liquid water is
probably necessary for the emergence of life, Titan is unlikely to
harbor any life. The impact of comets or asteroids on Titan may,
however, warm the surface enough that any water ice could melt. Such
"impact pools" could persist for as long as 1 thousand years,
potentially allowing life-like chemical reactions to occur.

------------------------------

Subject: F.03 What is the Drake equation?
Author: John Pike , Bill Arnett ,
Steve Willner

There are various forms of it, but basically it is a means of doing
boundary calculations for the prevalence of intelligent life in the
universe. It might take the form of saying that if there a

X stars in the Galaxy, of which
Y % have planets, of which
Z % can support life, on which
A % intelligent life has arisen, with
B representing the average duration of civilizations

then you fool around with the numbers to figure out how close on average
the nearest civilization is. There are various mathematical expressions
for this formula (see below), and there are variations on how many terms
the equations include.

The problem, of course, is that some of the variables are easy to pick
(e.g., stars in the Galaxy), some are under study (e.g., how many
stars have terrestrial-like planets), and others are just flat-out
wild guesses (e.g., duration of civilization, where we are currently
running an experiment to test this here on Terra of Sol).


One useful form says the number of detectable civilizations is:
N = R * fp * ne * fl * fi * fc * L
where
R = "the average rate of star formation in the region in question",
fp = "the fraction of stars that form planets"
ne = "the average number of planets hospitable to life per star"
fl = "the fraction of those planets where life actually emerges"
fi = "the fraction of life-bearing planets where life evolves into
intelligent beings"
fc = "the fraction of planets with intelligent creatures capable
of interstellar communication"
L = "the length of time that such a civilization remains
detectable".

(If you want some definition of civilization other than detectability,
just change your definition of fc and L accordingly.)

Can we provide reasonable estimates for any of the above numbers? The
"social/biological" quantities are at best speculative and aren't
appropriate for this newsgroup anyway. (For arguments that they are
quite small, see biologist Ernst Mayr's article in _Bioastronomy
News_, Quarter 1995, URL:http://planetary.org/tps/mayr.html.) Even
the "astronomical" numbers, though determinable in principle, have
considerable uncertainty. Nevertheless, I will attempt to provide
reasonable estimates. I'll take the "region in question" to be the
Milky Way Galaxy and consider only cases "similar to" our solar
system.

For R, I'm going to use only stars with luminosities between half and
double that of the Sun. Dimmer stars have a very small zone where
Earth-like temperatures will be found, and more luminous stars have
relatively short lifetimes. Near the Sun, there are about 4.5E-3 such
stars in a cubic parsec. I'm only going to consider stars in the
Galactic disk, which I take to have a scale height of 660 pc and scale
length of between 5 and 8 kpc. (Stars outside the disk either have
lower metallicity than the Sun or live in a very different environment
and may have formed in a different way.) The Sun is about 8 kpc from
the Galactic center, and thus in a region of lower than maximum star
density. Putting everything together, there ought to be around 1.4E9
stars in the class defined. This represents about 1% of the total mass
of the Galaxy. The age of the Sun is about 4.5E9 years, so the average
rate of formation R is about 0.3 "solar like stars" per year.

Planets are more problematic, since extrasolar planets cannot generally
be detected, but it is thought that their formation is a natural and
indeed inevitable part of star formation. For stars like the Sun, in
fact, there is either observational evidence or clear theoretical
justification for every stage of the planet formation process as it is
currently understood. We might therefore be tempted to take fp=1 (for
stars in the luminosity range defined), but we have to consider binary
stars. A second star may disrupt planetary orbits or may somehow
prevent planets forming in the first place. Because about 2/3 of the
relevant stars are in binary systems, I'm going to take fp=1/3.

Now we are pretty much out of the range of observation and into
speculation. It seems reasonable to take ne=1 or even 1.5 on the basis
of the Solar system (Earth and Mars), but a pessimist could surely take
a smaller number. You can insert your own values for the probabilities,
but if we arbitrarily set all of them equal to one
N = 0.1 L
seems consistent with all known data.

A more detailed discussion of interpretation of the Drake equation and
the factors in it can be found in Issue 5 of SETIQuest.

------------------------------

Subject: F.04 What is the Fermi paradox?
Author: John Pike ,
Steve Willner

One of the problems that the Drake Equation produces is that if you take
reasonable (some would say optimistic) numbers for everything up to the
average duration of technological civilizations, then you are left with
three possibilities:

1. If such civilizations last a long time, "They" should be _here_
(leading either the the Flying Saucer hypothesis---they are here and
we are seeing them, or the Zoo Hypothesis---they are here and are
hiding in obedience to the Prime Directive, which they observe with
far greater fiqdelity than Captain Kirk could ever muster). -or-

2. If such civilizations last a long time, and "They" are not "here"
then it becomes necessary to explain why each and every technological
civilization has consistently chosen not to build starships. The
first civilization to build starships would spread across the entire
Galaxy on a timescale that is short relative to the age of the Galaxy.
Perhaps they lose interest in space flight and building starships
because they are spending all their time surfing the net. (Think about
it---the whole point of space flight is the proposition that there are
privileged spatial locations, and the whole point of the net is that
physical location is more or less irrelevant.) -or-

3. Such civilizations do not last a long time, and blow themselves up
or otherwise fall apart pretty quickly (... film at 11).

Thus the Drake Equation produces what is called the Fermi Paradox
(i.e., "Where are They?"), in that the implications of #3 and #2 are
not terribly encouraging to some folks, but the two flavors of #1 are
kinda hard to come to grips with.


An alternate version of 2 is that interstellar travel is far more
difficult than we think it is. Right now, it doesn't seem much beyond
the boundaries of current technology to launch "generation ships," which
amount to an O'Neill colony plus propulsion and power systems. An
alternative is robot probes with artificial intelligence; these don't
seem so difficult either. The Milky Way galaxy is well under 10^5 light
years in diameter and over 10^9 years old, so even travel beginning
fairly recently in Galactic history and proceeding well under the speed
of light ought to have filled the Galaxy by now. (Travel very near the
speed of light still seems very hard, but such high speed isn't
necessary to fill the Galaxy with life.) The paradox, then, is that we
don't observe evidence of anybody besides us.

------------------------------

Subject: F.05 Could we detect extraterrestrial life?
Author: Steve Willner

Yes, although present observations can do so only under optimistic
assumptions. Radio and optical searches currently underway are aimed
at detecting "beacons" built by putative advanced civilizations and
intended to attract attention. More sensitive searches (e.g., Project
Cyclops) that might detect normal activities of an advanced
civilization (similar for example to our military radars or TV
stations) have been proposed but so far not funded. No funding of
these is likely until the search for beacons is far closer to being
complete. Why get involved with the difficult until you are done with
the easy?

Ordinary astronomical observations are most unlikely to detect life.
The kinds of life we speculate about would be near stars, and the
light from the star would conceal most signs of life unless a special
effort is made to look for them.

Within the solar system, the Viking landers found conditions on the
surface of Mars unlikely to support life as we know it. The mass
spectrometer found too little carbon, which is the basis for organic
molecules. The chemistry is apparently highly oxidizing as well.
Some optimists have nevertheless argued that there still might be
life on Mars, either below the surface or in surface regions not
sampled by the landers, but most scientists consider life on Mars
quite unlikely. Evidence of surface water suggests, however, that
Mars had a wetter and possibly warmer climate in the past, and life
might have existed then. If so, there might still be remnants
(either living or fossil) today, but close examination will be
necessary to find out.

Other sites that conceivably could have life include the atmosphere
of Jupiter (and perhaps Saturn) and the presumed liquid water under
the surface ice of Jupiter's satellite Europa. Organisms living in
either place would have to be very different from anything we know on
Earth, and it's hard to know how one would even start to look for
them.

Concepts for specialized space missions that could detect Earth-like
planets and return spectral information on their atmospheres have been
suggested, and either NASA or ESA may launch such a mission some time
in the next two decades (see
URL:http://techinfo.jpl.nasa.gov/www/ExNPS/HomePage.html and
URL:http://ast.star.rl.ac.uk/darwin/). The evidence for life would
be detection of ozone (implying oxygen) in the planet's atmosphere.
While this would be strong evidence for life---oxygen in Earth's
atmosphere is thought to have come from life---it would not be
ironclad proof, as there may be some way an oxygen atmosphere could
form without life.

For more information, see references at the end of F.06. Also, check
out the SETI Institute Web site at URL:http://www.seti-inst.edu.

------------------------------

Subject: F.06 How far away could we detect radio transmissions?
Author: Al Aburto ,
David Woolley

Representative results are presented in Tables 1 and 2. The short
answer is
(1) Detection of broadband signals from Earth such as AM radio, FM
radio, and television picture and sound would be extremely
difficult even at a fraction of a light-year distant from the
Sun. For example, a TV picture having 5 MHz of bandwidth and 5
MWatts of power could not be detected beyond the solar system
even with a radio telescope with 100 times the sensitivity of the
305 meter diameter Arecibo telescope.

(2) Detection of narrowband signals is more resonable out to
thousands of light-years distance from the Sun depending on the
transmitter's transmitting power and the receiving antenna size.

(3) Instruments such as the Arecibo radio telescope could detect
narrowband signals originating thousands of light-years from the
Sun.

(4) A well-designed 12 ft diameter amateur radio telescope could
detect narrowband signals from 1 to 100 light-years distance
assuming the transmitting power of the transmitter is in the
terawatt range.

What follows is a basic example for the estimation of radio and
microwave detection ranges of interest to SETI. Minimum signal
processing is assumed. For example an FFT can be used in the
narrowband case and a bandpass filter in the broadband case (with
center frequency at the right place of course). In addition it is
assumed that the bandwidth of the receiver (Br) is constrained such
that it is greater than or equal to the bandwidth of the transmitted
signal (Bt) (that is, Br = Bt).

Assume a power Pt (watts) in bandwidth Bt (Hz) radiated isotropically.
At a distance of R (meters), this power will be uniformly distributed
(reduced) over a sphere of area: 4 * pi * R^2. The amount of this
power received by an antenna of effective area Aer with bandwidth Br
(Hz), where Br = Bt, is therefo

Pr = Aer * (Pt / (4 * pi * R^2))

If the transmitting antenna is directive (that is, most of the
available power is concentrated into a narrow beam) with power gain Gt
in the desired direction then:

Pr = Aer * ((Pt * Gt) / (4 * pi * R^2))

The antenna gain G (Gt for transmitting antenna) is given by the
following expression. (The receiving antenna has a similar expression
for its gain, but the receiving antenna's gain is not used explicitly
in the range equation. Only the effective area, Aer, intercepting the
radiated energy at range R is required.)

Gt = Aet * (4 * pi / (w^2)), where

Aet = effective area of the transmitting antenna (m^2), and
w = wavelength (m) the antenna is tuned to.
f = c / w, where f is the frequency and c is the speed of light.
c = 2.99792458E+08 (m/sec)
pi = 3.141592654...

For an antenna (either transmiting or receiving) with circular apertures:

Ae = eta * pi * d^2 / 4

etar = efficiency of the antenna,
d = diameter (m) of the antenna.

The Nyquist noise, Pn, is given by:

Pn = k * Tsys * Br, where

k = Boltzmann's constant = 1.38054E-23 (joule/kelvin)
Tsys = is the system temperature (kelvins), and
Br = the receiver bandwidth (hertz).

The signal-to-noise ratio, snr, is given by:

snr = Pr / Pn.

If we average the output for a time t, in order to reduce the variance
of the noise, then one can improve the snr by a factor of
sqrt(Br * t). Thus:

snr = Pr * sqrt(Br * t) / Pn.

The factor Br*t is called the "time bandwidth product," of the receive
processing in this case, which we'll designate as:

twp = Br * t.

We'll designate the integration or averaging gain as:

twc = sqrt(twp).

Integration of the data (which means: twp = Br * t 1, or
t (1 / Br) ) makes sense for unmodulated "CW" signals that are
relatively stable over time in a relatively stationary (steady) noise
field. On the other hand, integration of the data does not make
sense for time-varying signals since this would distroy the
information content of the signal. Thus for a modulated signal
twp = Br * t = 1 is appropriate.

In any case the snr can be rewritten as:

snr = (Pt * Gt) * Aer * twc / (4 * pi * R^2 * Br * k * Tsys)

Pt * Gt is called the Effective Isotropic Radiated Power (EIRP) in
the transmitted signal of bandwidth Bt. So:

EIRP = Pt * Gt, and

snr = EIRP * Aer * twc / (4 * pi * R^2 * Br * k * Tsys)

This is a basic equation that one can use to estimate SETI detection
ranges.

################################################## #####################
# If Rl is the number of meters in a light year (9.46E+15 [m/LY]), #
# then the detection range in light years is given by #
# #
# R = sqrt[ EIRP * Aer * twc / (4 * pi * snr * Br * k * Tsys) ] / Rl #
# #
# If we wanted the range in Astronomical Units then replace Rl #
# with Ra = 1.496E+11 (m/AU). #
################################################## #####################

Note that for maximum detection range (R) one would want the transmit
power (EIRP), the area of the receive antenna (Aer), and the time
bandwidth product (twp) to be as big as possible. In addition one
would want the snr, the receiver bandwidth (Br), and thus transmit
signal bandwidth (Bt), and the receive system temperature (Tsys) to be
as small as possible.

(There is a minor technical complication here. Interstellar space
contains a plasma. Its effects on a propagating radio wave including
broadening the bandwidth of the signal. This effect was first
calculated by Drake & Helou and later by Cordes & Lazio. The
magnitude of the effect is direction, distance, and frequency
dependent, but for most lines of sight through the Milky Way a typical
value might be 0.1 Hz at a frequency of 1000 MHz. Thus, bandwidths
much below this value are unnecessary because there will be few, if
any, signals with narrower bandwidths.)

Now we are in a position to carry out some simple estimates of
detection range. These are shown in Table 1 for a variety of radio
transmitters. We'll assume the receiver is similar to Arecibo, with
diameter dr = 305 m and an efficiency of 50% (etar = 0.5). We'll
assume snr = 25 is required for detection (The META project used a snr
of 27--33 and SETI@home uses 22; more refined signal processing might
yield increased detection ranges by a factor of 2 over those shown in
the Table 1.) We'll also assume that twp = Br * Tr = 1. An
"educated" guess for some of the parameter values, Tsys in particular,
was taken as indicated by the question marks in the table. As a
reference note that Jupiter is 5.2 AU from the Sun and Pluto 39.4 AU,
while the nearest star to the Sun is 4.3 LY away. Also any signal
attenuation due to the Earth's atmosphere and ionosphere have been
ignored; AM radio, for example, from Earth, is trapped within the
ionosphere.

The receive antenna area, Aer, is

Aer = etar * pi * dr^2 / 4 = 36.5E3 m^2.

(Scientific notation is being used here; 1E1 = 10, 1E2 = 100, 1E3 =
1000, so 36.5E3 is 36.5 times 1000.) Hence the detection range (light
years) becomes

R = 3.07E-04 * sqrt[ EIRP / (Br * Tsys) ].

Table 1 Detection ranges of various EM emissions from Earth and the
Pioneer spacecraft assuming a 305 meter diameter circular
aperture receive antenna, similar to the Arecibo radio
telescope. Assuming snr = 25, twp = Br * Tr = 1, etar =
0.5, and dr = 305 meters.
-------------+--------------+-----------+--------+--------+-----------+
Source | Frequency | Bandwidth | Tsys | EIRP | Detection |
| Range | (Br) |(Kelvin)| | Range (R) |
-------------+--------------+-----------+--------+--------+-----------+
AM Radio | 530-1605 kHz | 10 kHz | 68E6 | 100 KW | 0.007 AU |
-------------+--------------+-----------+--------+--------+-----------+
FM Radio | 88-108 MHz | 150 kHz | 430 | 5 MW | 5.4 AU |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV | 470-806 MHz | 6 MHz | 50 ? | 5 MW | 2.5 AU |
Picture | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV | 470-806 MHz | 0.1 Hz | 50 ? | 5 MW | 0.3 LY |
Carrier | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
WSR-88D | 2.8 GHz | 0.63 MHz | 40 | 32 GW | 0.01 LY |
Weather Radar| | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 22 TW | 720 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 1 TW | 150 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 1 GW | 5 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Pioneer 10 | 2.295 GHz | 1.0 Hz | 40 | 1.6 kW | 120 AU |
Carrier | | | | | |
-------------+--------------+-----------+--------+--------+-----------+

It should be apparent then from these results that the detection of AM
radio, FM radio, or TV pictures much beyond the orbit of Pluto will be
extremely difficult even for an Arecibo-like 305 meter diameter radio
telescope! Even a 3000 meter diameter radio telescope could not
detect the "I Love Lucy" TV show (re-runs) at a distance of 0.01
Light-Years!

It is only the narrowband high intensity emissions from Earth
(narrowband radar generally) that will be detectable at significant
ranges (greater than 1 LY). Perhaps they'll show up very much like
the narrowband, short duration, and non-repeating, signals observed by
our SETI telescopes. Perhaps we should document all these
"non-repeating" detections very carefully to see if any long term
spatial detection patterns show up.

Another question to consider is what an Amateur SETI radio telescope
might achieve in terms of detection ranges using narrowband FFT
processing. Detection ranges (LY) are given in Table 2 assuming a 12
ft (3.7 m) dish antenna operating at 1.42 GHz, for various FFT
binwidths (Br), Tsys, snr, time bandwidth products (twp = Br*t), and
EIRP values. It appears from the table that effective amateur SETI
explorations can be conducted out beyond approximately 30 light years
provided the processing bandwidth is near the minimum (approximately
0.1 Hz), the system temperature is minimal (20 to 50 Degrees Kelvin),
and the EIRP of the source (transmitter) is greater than approximately
25 terawatts.


Table 2 Detection ranges (LY) for a 12 foot diameter amateur
radio telescope SETI system, operating at 1.420 GHz.
+-------------------------------+
| EIRP |
+-------+--------+------+-------+
| 100TW | 25TW | 1TW | 100GW |
-------+-------+----------+------+-------+--------+------+-------+
Br | Br*t | Tsys | snr | Detection Range |
(Hz) | | (kelvin) | | (LY) |
-------+-------+----------+------+-------+--------+------+-------+
0.1 | 2 | 50 | 25 | 28 | 17 | 3.4 | 1.1 |
-------+-------+----------+------+-------+--------+------+-------+
0.1 | 1 | 50 | 25 | 20 | 12 | 2.4 | 0.76 |
-------+-------+----------+------+-------+--------+------+-------+
0.5 | 2 | 50 | 25 | 12.7 | 6.4 | 1.3 | 0.4 |
-------+-------+----------+------+-------+--------+------+-------+
0.5 | 1 | 50 | 25 | 9 | 4.5 | 0.9 | 0.3 |
-------+-------+----------+------+-------+--------+------+-------+
0.1 | 20 | 50 | 25 | 90 | 54 | 11 | 3.4 |
-------+-------+----------+------+-------+--------+------+-------+
1.0 | 200 | 50 | 25 | 90 | 54 | 11 | 3.4 |
-------+-------+----------+------+-------+--------+------+-------+


REFERENCES:
Radio Astronomy, John D. Kraus, 2nd edition, Cygnus-Quasar
Books, 1986, P.O. Box 85, Powell, Ohio, 43065.

Radio Astronomy, J. L. Steinberg, J. Lequeux, McGraw-Hill
Electronic Science Series, McGraw-Hill Book Company, Inc,
1963.

Project Cyclops, ISBN 0-9650707-0-0, Reprinted 1996, by the
SETI League and SETI Institute.

Extraterrestrial Civilizations, Problems of Interstellar
Communication, S. A. Kaplan, editor, 1971, NASA TT F-631
(TT 70-50081), page 88.


------------------------------

Subject: F.07 What's a Dyson spheres?
Author: Anders Sandberg

Freeman Dyson noted that one of the limiting resources for
civilizations is the amount of energy they can harness. He proposed
that an advanced civilization could harness a substantial fraction of
its sun's energy by enclosing the star in a shell which would capture
most of the radiation emitted by the star. That energy could then be
used to do work.

As originally proposed a Dyson sphere consisted of many solar
collectors in independent orbits. Many science fiction writers have
modified the idea to make a Dyson sphere one complete shell. In
addition to capturing all of the available energy from the star, such
a shell would have a huge surface area for living space. While
Dyson's original proposal of a number of solar collectors is stable,
this later idea of a complete shell is not stable. Without some
stablizing mechanism, even small forces, e.g., a meteor hit, would
cause the shell to drift and eventually hit the star. Also, the
stresses on a complete shell Dyson sphere are huge and no known
material has enough strength to be used in the construction of such a
shell.

There have been searches for Dyson spheres. Such searches typically
take place in the infrared. Because the shell is trapping energy from
the star, it will begin to heat up. At some point it will radiate as
much energy as it receives from the star. For a Dyson sphere with a
radius about the radius of Earth's orbit, most of the radiation
emitted by the shell should be in the infrared. Thus far, no search
has been successful.

Considerably more discussion of Dyson spheres is in the Dyson sphere
FAQ, URL:http://www.student.nada.kth.se/~nv91-asa/dysonFAQ.html.

------------------------------

Subject: F.08 What is happening with SETI now?
Author: Larry Klaes

Some of the following material is from SETIQuest Magazine, copyright
Helmers Publishing, and used by permission.

Project BETA (Billion-channel ExtraTerrestrial Assay) is a radio
search begun 1995 October 30. It is sponsored by the Planetary
Society and is an upgraded version of Project META (Million...).
(Actually META I; see below for META II.) META I/BETA's observatory
is the 26-meter radio antenna at Harvard, Massachusetts. Their Web
site is URL:http://planetary.org/BETA/.

META II uses a 30-meter antenna at the Argentine Institute for Radio
Astronomy, near Buenos Aires, Argentina, and provides coverage of the
southern sky. URL:http://seti.planetary.org/META2/

META I/II monitored 8.4 million channels at once with a spectral
resolution of 0.05 Hz, an instantaneous bandwidth of 0.4 MHz, a total
frequency coverage of 1.2 MHz, a maximum sensitivity of 7x10^-24 W m^-2,
and a combined sky coverage of 93 percent. After five years of
observations from the northern hemisphere and observing 6x10^13
different signals, META I found 34 candidates, or "alerts".
Unfortunately, the data are insufficient to determine their real origin.
Interestingly, the observed signals seem to cluster near the galactic
plane, where the major density of Milky Way stars dwell. META II, after
three years of observations and surveying the southern hemisphere sky
almost three times, found nineteen signals with similar characteristics
to the META I results. META II has also observed eighty nearby, main
sequence stars (less than fifty light years from the Sun) that have the
same physical characteristics as Earth's star. These observations were
performed using the tracking mode for periods of one hour each at two
different epochs.

On 1992 October 12, NASA began its first SETI program called
HRMS---High-Resolution Microwave Survey. Unfortunately for all,
Congress decided the project was spending way too much money---even
though it received less funds per year than your average big league
sports star or film actor---and cut all money to NASA for SETI work.
This act saved our national deficit by all of 0.0002 percent.

Fortunately, NASA SETI was saved as a private venture called Project
Phoenix and run by The SETI Institute. It operates between 1.0 and
3.2 GHz with 1 Hz resolution and 2.8E7 channels at a time. Rather
than trying to scan the entire sky, this survey focusses on
approximately 1000 nearby stars. They began observations in 1995
February using the Parkes 64 m radio telescope in New South Wales,
Australia, and have since moved to the 42 m radio telescope in Green
Bank, West Virginia. After completing about 1/3 of their targets,
they had found no evidence of ET transmissions. More details are in
SETIQuest issue 3 and at the Project Phoenix home page
URL:http://www.seti-inst.edu/phoenix/Welcome.html. The Web site has
lots of general information about SETI as well as details of the
survey.

Since 1973, Ohio State University had conducted a radio search with a
telescope consisting of a fixed parabolic reflector and a tiltable
flat reflector, each about 110 m wide and 30 m high. Information is
available at URL:http://everest.eng.ohio-state.edu/~klein/ro/ or a
longer version in SETIQuest issue 3. The "wow!" signal, detected in
1977, had the appearance of an extraterrestrial signal but was seen
only briefly and never repeated. However, the Ohio State University
administration decided to let the landlord who owns the property on
which Big Ear resides tear down the radio telescopes and put up condos
and a golf course instead. OSU SETI is considering its next step,
Project Argus, at an undetermined location.

The UC Berkeley SETI Program, SERENDIP (Search for Extraterrestrial
Radio Emissions from Nearby Developed Intelligent Populations) is an
ongoing scientific research effort aimed at detecting radio signals
from extraterrestrial civilizations. The project is the world's only
"piggyback" SETI system, operating alongside simultaneously conducted
conventional radio astronomy observations. SERENDIP is currently
piggybacking on the 300 m dish at Arecibo Observatory in Puerto Rico,
the largest radio telescope in the world. Information at
URL:http://albert.ssl.berkeley.edu/serendip/, from which this
paragraph was extracted. SERENDIP operates at 430 MHz; more
information is given in SETIQuest issue 3.

Project BAMBI is an amateur SETI effort operating at a radio frequency
of 4 GHz. See SETIQuest issue 5 and
URL:http://wbs.net/sara/bambi.htm for status reports.

The Columbus Optical SETI Observatory uses visible light instead of
radio waves. The COSETI Observatory is a prototype observatory
located in Bexley, Ohio, USA. Telescope aperture size is 30 cm. More
information in SETIQuest issue 4 and at URL:http://www.coseti.org/.
Much of the work on "Optical SETI" comes from Dr. Stuart A. Kingsley
, who also maintains BBS on
Optical SETI.

The Planetary Society maintains a list of online SETI-related material
at URL:http://seti.planetary.org/.

And of course SETIQuest magazine, Larry Klaes, Editor. For
subscription or other information, contact Helmers Publishing, 174
Concord Street, Peterborough, NH 03458-0874. Phone (603) 924-9631,
FAX (603) 924-7408, Internet: or see
URL:http://www.setiquest.com/.


Other references:

Frank Drake, Dava Sobel, Is Anyone Out The The Scientific
Search For Extraterrestrial Intelligence, 1992, Delacorte
Press, ISBN 0-385-30532-X.

Frank White, The SETI Factor, 1990, Walker Publishing Company,
Inc., ISBN 0-8027-1105-7.

Donald Goldsmith and Tobias Owen, The Search For Life in the
Universe, Second Edition, 1992, Addison-Wesley Publishing
Company, Inc., ISBN 0-201-56949-3.

Walter Sullivan, We Are Not Alone: The Continuing Search for
Extraterrestrial Intelligence, 1993, Dutton, ISBN
0-525-93674-2.

G. Seth Shostak, Editor, Progress In The Search For
Extraterrestrial Life, 1993 Bioastronomy Symposium, Santa
Cruz, California, 16--20 August 1993. Published in 1995 by The
Astronomical Society of the Pacific (ASP). ISBN 0-937707-93-7.

The journals Icarus, URL:http://astrosun.tn.cornell.edu/Icarus/, and
Astronomy & Geophysics often feature papers concerning SETI.

------------------------------

Subject: F.09 Why search for extraterrestrial intelligence using
radio? Why not fill in the blank method?
Author: Joseph Lazio

There are two possibilities for sending information to other
technological civilizations over interstellar distances: send matter
or send radiation. The focus in SETI has been on detecting
electromagnetic radiation, particularly radio, because compared to all
other known possibilities, it is cheap, easy to produce, and can
travel across the Milky Way Galaxy.

Compared to radiation, most matter has a distinct disadvantage: it is
slow. Radiation can travel at the speed of light whereas (most)
matter is constrained to travel slower. Distances between stars are
so large, it makes no sense to use a slow mode of communication when a
faster one is available. The speed at which spacecraft travel is the
primary justification why there is little effort spent within the SETI
community searching for interstellar spacecraft (that and the fact
that there is no evidence that there are any such interstellar
spacecraft from other civilizations in our vicinity). A secondary
justification is that spacecraft are relatively expensive. The launch
of a single Earth-orbiting spacecraft can cost US $100 million. It
is difficult to imagine building and launching a fleet of interstellar
spacecraft for US $500 million, yet this is the estimated cost of a
next-generation radio telescope capable of detecting TV signals over
interstellar distances. It is possible that future technology will
make spacecraft cheaper. It is difficult to imagine a technology that
would make spacecraft cheaper without also lowering the cost of a new
telescope.

Although chunks of matter, i.e., spacecraft, seem a rather inefficient
way to communicate across interstellar space, what about a beam of
matter. Most often suggested in this context is a beam of neutrinos.
Neutrinos are nearly massless so they travel at almost the speed of
light. They also interact only weakly with matter, so a beam of
neutrinos could cross the Milky Way Galaxy without any significant
absorption by interstellar gas and dust clouds. This advantage is
also a disadvantage: The weakness of their interaction makes it
difficult to detect a beam of neutrinos, far more difficult than
detecting a beam of electromagnetic radiation.

(A beam of electrons or protons could be accelerated to nearly the
speed of light and would be far easier to detect. However, electrons
and protons are charged particles. When travelling through
interstellar space, the direction of their travel is influenced by the
magnetic field of the Milky Way Galaxy. The Milky Way's magnetic
field has "small-scale" irregularities in it that would divert and
scatter such a beam. The result is that one could not "aim" such a
beam in any particular direction [except possibly to the very closest
stars] because its actual path would be influenced by the [unknown]
direction[s] of the magnetic field it would encounter.)

The known forms of radiation are electromagnetic and gravitational.
Electromagnetic radiation results from the acceleration of charged
particles and is used commonly: Radio and TV broadcasts are radio
radiation, microwave ovens produce microwave radiation, X-ray machines
produce X-ray radiation, overhead lights produce visible radiation,
etc. Gravitational radiation results from the acceleration of massive
objects. Gravitational radiation has never been detected directly,
and its indirect detection resulted in the 1993 Nobel Prize. Gravity is
a much weaker force than electromagnetism. Thus, detectable amounts
of gravitational radiation result only from events like the explosion
of a massive star or the gravitational interaction between two closely
orbiting neutron stars or black holes. Again, it is possible that a
future technology might result in gravitational radiation becoming
easier to detect. It is still difficult to imagine that it would not
also result in electromagnetic radiation.

Of the various forms of electromagnetic radiation---radio, microwave,
infrared, visible, ultraviolet, X-ray, and gamma-ray---only radio and
gamma-ray can cross the Milky Way Galaxy. The other forms suffer
varying amounts of absorption by interstellar dust and gas clouds
(though they could still be used to communicate over shorter
distances). Gamma rays are extremely energetic and are produced by
events like the explosion of nuclear bombs. Radio radiation is far
less energetic. Thus, to send the same amount of information requires
far less energy (i.e., it's cheaper) to send it via radio than gamma
ray.

The above are merely plausibility arguments to suggest why radio is
likely to be a preferred method of communication among technological
civilizations. Of course, they may reason that they are only
interested in communicating with other civilizations technologically
advanced enough to transmit and detect neutrino beams or gravitational
radiation (or maybe even some undiscovered method). If so, the
existing radio SETI programs are doomed to failure. Nonetheless, it
does seem sensible to search first using the most simple technology.

------------------------------

Subject: F.10 Why do we assume that other beings must be based on
carbon? Why couldn't organisms be based on other substances?
Author: Joseph Lazio

[A portion of this entry is based on a lecture by Alain Leger (IAS) at
the SPIE Astronomical Telescopes and Instrumentation 2000 Conference.]

As far as SETI, the search for extraterrestrial intelligence, is
concerned, we do not assume that other being must be based on carbon.
In fact, SETI is a bit of a misnomer. We are searching for
extraterrestrial *technological* intelligences, technological
intelligences capable of broadcasting their existence over
interstellar distances. Whether the technological civilizations is
based on carbon or some other substance is largely irrelevant. (Of
course, one might worry that intelligences based on some substance
other than carbon might have such different perspectives on the
Universe that, even if they broadcast electromagnetic radiation, they
would do so in a fashion that we would never consider.)

However, when one moves to finding life on other bodies in the solar
system or traces of life on extrasolar planets, there is a definite
carbon chauvinism in our thinking. The most commonly mentioned
alternate to carbon (C) is silicon (Si). It has similar chemical
properties as C, lying just below C in the periodic table of the
elements.

Carbon chauvinism has arisen because C is able to form quite
complicated molecules, in part because its atomic structure is such
that C can bond with up to four other elements. Not only can it bond
with up to four other elements, but C can form multiple bonds with
other elements, particularly itself. (Atoms bond by sharing
electrons, when two atoms share more than one electron they have a
multiple bond. For instance, water is formed by an oxygen atom
sharing the two electrons from two hydrogen atoms. In contrast, there
are many C compounds in which a single C atom shares multiple
electrons with other atom.)

A clear indication of the versatility of C is found in interstellar
chemistry. Interstellar chemistry typically occurs on the surface of
microscopic dust grains contained with large clouds of gas between the
stars. The physical conditions are much different than anything on
the surface of a habitable planet. Nonetheless, of the molecules
identified in interstellar space as of 1998, 84 are based on C and 8
are based on Si. Moreover of the eight Si-based compounds, 4 also
include C.

Thus, while there is definitely a C bias in our thinking, there is at
least some evidence from Nature supporting this bias.

------------------------------

Subject: F.11 Could life occur on an interstellar planet?
Author: Joseph Lazio

This question has taken on increased importance with the discovery of
giant planets close to their primary stars. It is thought that these
giant planets did not form this close to their host stars but
migrated. (See the FAQ entry on the formation of the solar system.)
In general, the possibility of migration has alerted (or re-awakened)
astronomers to the possibility that a planetary system can change over
time. If a giant planet migrates inward from the position at which it
formed, it can scatter terrestrial planets. These terrestrial planets
might plunge into the host star or be kicked into interstellar space.
(Another possibility, though probably even less likely, is for a
passing star to disrupt a planetary system.)

What would happen if the Earth were kicked into interstellar space?
Life on the surface would certainly be doomed as it gets its energy to
survive from the Sun. In fairly short order, the oceans would freeze
over. However, the Earth is still generating heat by radioactive
decay in its interior. Some of this heat leaks out through
hydrothermal vents on the floors of the oceans. Thus, the lower
levels of the oceans would remain liquid, and the hydrothermal vents
would remain active. Organisms that depend only on the hydrothermal
vents could survive probably quite happily for several billion years
after the Earth was ejected from the solar system. (Indeed, since the
oceans will probably boil away in the next few billion years as the
Sun's luminosity increases, these organisms might prefer the Earth to
be ejected into interstellar space!)

For additional reading see "The Frozen Earth" by Adams & Laughlin,
URL:
http://adsabs.harvard.edu/cgi-bin/np...AS...194.1511A
and Stevenson, "Life-sustaining planets in interstellar space?",

Nature, v. 400, 1 Jul 1999, p. 32.


------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2003 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
  #8  
Old February 2nd 06, 02:37 AM posted to sci.astro,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro] Stars (Astronomy Frequently Asked Questions) (7/9)


Last-modified: $Date: 2003/10/18 00:00:02 $
Version: $Revision: 4.5 $
URL: http://sciastro.astronomy.net/
Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part7

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/ and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: G.00 Stars

[Dates in brackets are last edit.]

G.01 What are all those different kinds of stars?
01.1 General overview and main sequence stars [1996-01-02]
01.2 White dwarfs [2003-04-27]
01.3 Neutron stars [2003-04-27]
01.4 Black holes [2003-04-27]
G.02 Are there any green stars? [1995-12-28]
G.03 What are the biggest and smallest stars? [1998-06-03]
G.04 What fraction of stars are in multiple systems? [1995-06-27]
G.05 Where can I get stellar data (especially distances)?
[2003-05-08]
G.06 Which nearby stars might become supernovae? [1995-12-29]
G.07 What will happen on Earth if a nearby star
explodes? [2000-02-20]
G.08 How are stars named? Can I name/buy one? [1995-12-28]
G.09 Do other stars have planets?
G.10 What happens to the planets when a planetary nebula is
formed? Do they get flung out of the solar system?
[2002-05-04]
G.11 How far away is the farthest star? [1999-01-01]
G.12 Do star maps (or galaxy maps) correct for the motions of the
stars? [2003-10-18]

For an overall sense of scale when talking about stars, see the Atlas
of the Universe, URL:http://anzwers.org/free/universe/.

------------------------------

Subject: G.01.1 What are all those different kinds of stars?
General overview and main sequence stars
Author: Steve Willner ,
Ken Croswell

There are lots of different ways to classify stars. The most important
single property of a star is its mass, but alas, stellar masses for most
stars are very hard to measure directly. Instead stars are classified
by things that are easier to measure, even though they are less
fundamental.

There are three separate classification criteria commonly used: surface
temperature, surface gravity, and heavy element abundance. The familiar
"spectral sequence" OBAFGKM is a _temperature_ sequence from the hottest
to the coolest stars. Strictly speaking, the letters describe the
appearance of a star's spectrum, but because most stars are made out of
the same stuff, temperature has the biggest effect on the spectrum. O
stars are hotter than 30000 K and show ionized helium in their spectra.
M stars are cooler than 4000 K and show molecular bands of TiO. Others
are in between.

The ordinary spectral classes are divided into subclasses denoted by
numbers; thus G5 is a medium temperature star a little cooler than G2.
The Sun is generally considered a G2 star. Not all the subclasses are
used, or at least generally accepted; G3 and G4 are absent, for example.

For historical reasons, hotter stars are said to have "earlier"
spectral types, and cool stars to have "later" spectral types. An
"early A" star might mean somewhere between A0 and A3, while "late A"
might denote roughly A5--A8. Or "early type stars" might mean
everything from O through A or F. There's nothing terribly wrong with
this bit of jargon, but it can be confusing if you haven't seen it
before.

There are several spectral types that don't fit the scheme above. One
reason is abnormal composition. For example, some stars are cool enough
for molecules to form in their atmospheres. The most stable molecule at
high temperatures is carbon monoxide. In most stars, oxygen is more
abundant than carbon, and if the star is cool enough to form molecules,
virtually all the carbon combines with oxygen. Leftover oxygen can form
molecules like titanium oxide and vanadium oxide (neither of which is
particularly abundant but both of which have prominent spectral bands at
visible wavelengths), but no carbon-containing molecules other than CO
can form. (This is only approximately true. Weak CN lines can often be
seen, for example, and all kinds of stuff will show up if you look hard
enough. This article just gives a summary of the big picture.) In a
minority of stars, however, the situation is reversed, and there is no
(or rather very little) oxygen to form molecules other than CO. These
stars show lines of CH, CC, and CN, and they are called (not
surprisingly!) "carbon stars." They are nowadays given spectral
classifications of C(x,y) where x is a temperature index and y is
related to heavy element abundance and surface gravity. These stars
were formerly given "R" and "N" spectral types, and you occasionally
still see those used. Roughly speaking, R stars have temperatures in
the same range as K stars and N stars in the same range as M, though the
correspondence is far from exact.

Another interesting group is the S stars. In these, the atmospheric
carbon and oxygen abundances are nearly equal, and neither C nor O (or
at least not much of either) is available to form other molecules.
These stars show zirconium oxide and unusual metal lines such as barium.

There are other stars with unusual abundances: CH, CN, SC, and probably
more. They are rare. There are also stars that are peculiar in one way
or another and have spectral types followed by "p." The "Ap" stars are
one popular class. And finally, some stars have extended atmospheres
and show emission lines instead of the normal absorption lines. These
get an "e" or "f."

The second major classification is by surface gravity, which is
proportional to the stellar mass divided by radius squared. This is
useful because spectra can measure the gas pressure in the part of the
atmosphere where the spectral lines are formed; this pressure depends
closely on surface gravity. But because surface gravity is related to
stellar radius, it is also related to the stellar luminosity. Every
unit of stellar surface area emits an amount of radiation that mostly
depends on the temperature, and for a given temperature the total
luminosity thus depends on surface area which is proportional to radius
squared hence inversely proportional to surface gravity. The upshot of
all this is that we have "dwarf" stars of relatively high surface
gravity, small radius, and low luminosity, and "giant" stars of low
surface gravity, large radius, and high luminosity _and their spectra
look different_. In fact, many "luminosity classes" are identified in
spectra. For normal stars, these are designated by Roman numerals and
lower case letters following the spectral class in the order: Ia+, Ia,
Iab, Ib, II, III, IV, V. Class I stars are also called "supergiants,"
class II "bright giants," class III "giants," class IV "subgiants," and
class V either "dwarfs" or more commonly "main sequence stars." By the
way, not all luminosity classes exist for every spectral type.

The importance of all this is that the luminosity classes are closely
related to the evolution of the stars. Stars spend most of their
lives burning hydrogen in their cores. For stars in this evolutionary
stage, the surface temperature and radius, hence spectral type and
luminosity class, are determined by stellar mass. If we draw a
diagram of temperature or spectral type on one axis and luminosity
class on the other and plot each star as a point in the correct
position, we find nearly all stars fall very close to a single line;
this line is called the "main sequence." (This kind of diagram is
called a "Hertzsprung-Russell" or "H-R" diagram after two astronomers
who were among the first to use it.) Stars at the low mass end of the
main sequence are very cool (spectral type M) and are called "red
dwarfs." This term is not very precise and may include K-type stars
as well.

As stars age, they expand and cool off; stars in this stage of evolution
account for the brighter luminosity classes mentioned above. If they
happen to be cool, they are called "red giants" or perhaps "red
supergiants." One interesting special case is for the hottest stars,
spectral classes O and early B. Normally main sequence stars are hotter
if they have more mass, but not once they reach such high temperatures.
Instead more massive stars have larger radii but about the same surface
temperature, so an O I star is likely more massive but no more evolved
than an O V star. These stars are called "blue giants" or "blue
supergiants."

After stars finally burn out their nuclear fuel, any of several thing
can happen, depending mainly on their initial mass and perhaps on
whether they had a nearby companion. Some stars explode and are
entirely destroyed, but most leave remnants: white dwarfs, neutron
stars, or black holes.

White dwarfs have high density because they are supported by "electron
degeneracy pressure." This is a kind of pressure that arises from the
Fermi exclusion principle in nuclear physics. A white dwarf has roughly
the radius of the Earth but a mass close to that of the Sun. No white
dwarf can have a mass greater than the "Chandrasekhar limit," about 1.4
solar masses. White dwarfs are given spectral type designations DA, DB,
and DC according to the spectral lines seen. These lines represent the
composition of just a thin layer on the star's surface, so the spectral
classifications aren't terribly fundamental.

White dwarfs radiate solely by virtue of their stored heat. As they
radiate, they cool off, eventually turning into "black dwarfs." Because
their radii are so small, though, white dwarfs take billions of years to
cool. There may be few or no black dwarfs in our galaxy simply there
has not been time for many white dwarfs to cool off. Of course it's not
obvious how one would detect black dwarfs if they exist.

Neutron stars are even more compact; the mass of the Sun in a radius of
order only 10 km. These stars are supported by "neutron degeneracy
pressure," in which Fermi exclusion acts on neutrons. Neutron stars
have a maximum mass of around 2 solar masses, although the exact
theoretical value depends on properties of the neutron that are not
known terribly accurately. Because the radius is so small, these stars
don't emit significant visible light from their surfaces. They may emit
radio energy as pulsars.

Some properties of black holes are discussed elsewhere in the FAQ.

All types of "compact remnants," white dwarfs, neutron stars, and black
holes, may emit energy from an accretion disk around them if a nearby
companion is transferring mass to the compact remnant. The emission
often comes out at X-ray and ultraviolet wavelengths.

The third classification is by composition and specifically by "heavy
element abundance." In astronomy, "heavy elements" or "metals" refers
to all elements heavier than helium. Since heavy elements are created
in stars, stars formed later in the life of the galaxy have more heavy
elements than found in older stars.

The term "subdwarf" or occasionally "luminosity class VI" refers to
stars of low metallicity. Because they have so few metals, they look a
little hotter than they "ought" to be for their masses or equivalently
have lower luminosity than main sequence stars of the same color.
Physically, these stars are burning hydrogen in their cores and are
similar to main sequence stars except for the lower metallicities.
Since all these stars are old, they are of low luminosity. Their higher
luminosity counterparts no doubt existed but have long since evolved
away, most of them presumably into some form of compact remnant.


The following material is adapted from Ken Croswell's book The Alchemy
of the Heavens (Doubleday/Anchor, 1995) and is reprinted here with
permission of the author.

The terms "Population I" and "Population II" originated with Baade,
who in 1943 divided stars into these two broad groups. Today, we
know the Galaxy is considerably more complicated, and we recognize
four different stellar populations. To make a long story short, the
modern populations a

THIN DISK metal-rich, various ages
THICK DISK old and somewhat metal-poor
STELLAR HALO old and very metal-poor; home of the subdwarfs
BULGE old and metal-rich

To make a long story longer: as astronomers presently understand the
Milky Way, every star falls into one of these four different stellar
populations. The brightest is the thin-disk population, to which the
Sun and 96 percent of its neighbors belong. Sirius, Vega, Rigel,
Betelgeuse, and Alpha Centauri are all members. Stars in the thin
disk come in a wide variety of ages, from newborn objects to stars
that are 10 billion years old. As its name implies, the thin-disk
population clings to the Galactic plane, with a typical member lying
within a thousand light-years of it. Kinematically, the stars revolve
around the Galaxy fast, having fairly circular orbits and small U, V,
W velocities. (These are the intrinsic space velocities with respect
to the average of nearby stars. Zero in all components means rotating
around the center of the Galaxy at something like 220 km/s but no
other motion.) Thin-disk stars are also metal-rich, like the Sun.

The second stellar population in the Galaxy is called the thick disk.
It accounts for about 4 percent of all stars near the Sun. Arcturus is
a likely member. The thick disk is old and forms a more distended
system around the Galactic plane, with a typical star lying several
thousand light-years above or below it. The stars have more elliptical
orbits, higher U, V, W velocities, and metallicities around 25 percent
of the Sun's.

The third stellar population is known as the halo. Halo stars are old
and rare, accounting for only 0.1 to 0.2 percent of the stars near the
Sun. Kapteyn's Star is the closest halo star to Earth. These stars
make up a somewhat spherical system, so most members of the halo lie far
above or far below the Galactic plane. Kinematically, halo stars as a
group show little if any net rotation around the Galaxy, and a typical
member therefore has a very negative V velocity. (This is a reflection
of the Sun's motion around the Galactic center in the +V direction.)
The halo stars often have extremely elliptical orbits; some of them may
lie 100,000 light-years from the Galactic center at apogalacticon but
venture within a few thousand at perigalacticon. Metallicities are even
lower than in the thick disk, usually between 1 and 10 percent of the
Sun's. Subdwarfs are members of this population.

The fourth and final stellar population is the bulge, which lies at the
center of the Galaxy. Other galaxies have bulges too; some can be seen
in edge-on spiral galaxies as the bump that extends above and below the
galaxy's plane at the center. The Galactic bulge is old and metal-rich.
Most of its stars lie within a few thousand light-years of the Galactic
center, so few if any exist near the Sun. Consequently, the bulge is
the least explored stellar population in the Milky Way.

References:

Ken Croswell, _The Alchemy of the Heavens_ (Doubleday/Anchor, 1995)
(See http://www.ccnet.com/~galaxy)

James B. Kaler, _Stars and their Spectra: an Introduction to the
Spectral Sequence (Cambridge U. Press, 1989)

Most any introductory astronomy book.

------------------------------

Subject: G.01.2 What are all those different kinds of stars?
White Dwarfs How are white dwarfs classified? What
do the spectral types DA, DC, etc. mean?
Author: Mike Dworetsky

The MK classification system for the vast majority of stars works
remarkably well for one simple reason: most stars in the Galactic disk
have surface chemical compositions that are broadly similar to each
other and the Sun's composition. They are 71 percent hydrogen, 27
percent helium, and 2 percent "metals" (Li--U). Thus, the differences
in spectral line strengths that give rise to the familiar OBAFGKM
sequence are due to their vast range in surface temperature. The MK
system can also classify by absolute stellar brightness: the more
subtle differences in the strengths of certain lines at various
classes, caused by the different surface gravities of main sequence
and supergiant stars, for example, are spoken of as luminosity
criteria, because they depend on the size of the star (big stars
radiate much more energy than small stars, but their atmospheres are
much less dense).

The name "white dwarf" for these stars comes from the observed colors
of the first examples discovered. They caught the attention of
astronomers because they had large masses comparable to the Sun but
were hot and very faint, hence extremely small and dense. We now know
that there are a few "white dwarfs" that are actually cool enough to
look red.

The first spectroscopic investigators of white dwarfs tried to fit
them into a descriptive system parallel to the MK classes, using the
letter D plus a suffix OBAFGK or M, with the letter C added for the
cases when the spectra showed no lines (continuous spectra). The
types were sometimes supplemented by cryptic abbreviations like "wk"
for weak; "s" for sharp-lined, and so on.

When the spectra of white dwarfs were investigated in more detail, it
proved impossible to categorize them neatly for one increasingly
apparent reason: the surface compositions of white dwarfs varied
enormously from star to star. Astronomers needed a new scheme to
reflect this. In the revised classification scheme, white dwarf
designations still start with the letter D to indicate dwarf or
"degenerate" stellar structure. A second letter indicates the main
spectral features visible: C for a continuous spectrum with no lines,
A for Balmer lines of hydrogen with nothing else, B for He I (neutral
helium) lines, O for He II with or without He I or H, Z for metal
lines (often, strong Ca II lines are seen), and Q for atomic or
molecular lines of carbon (C is used for continuous spectra; K for
Karbon could be confused with the K stars; so try to think of
Qarbon!).

These basic types can sometimes mix; DAQ stars are known, for example.

A further suffix can be added: P for magnetic stars with polarized
light, H for magnetic stars that do not have polarized light, and V
for variable. (There is a class of short-period pulsating white
dwarfs, called ZZ Ceti stars.) There may be emission lines (E). And if
an unusual star still defies classification, it goes into type X.

Finally, a number is appended that classifies the star according to
its effective temperature based on formulae which use the observed
colors: the number is 50400/T rounded to the nearest 0.5, i.e., the
value of 50400/temperature, rounded. If white dwarfs with T much
higher than 50,000 K are ever found, they could have the number 0 or
0.5 appended. The coolest designation is open-ended; there is a star
classified as DC13, for example, which is actually rather red, not
white.

Thus a hot white dwarf with neutral helium lines might be described as
DB2.5; a cooler white dwarf with hydrogen lines, a magnetic field,
polarized light, and a trace of carbon might be DAQP6.

This system can provide good summary descriptions of the vast majority
of white dwarf stars. However, it is a definite move away from the
original concept of spectral classification, because it requires
photometry and polarimetry as well as visual inspection of a spectrum,
in order to make an assignment. But most leading experts on the
subject have agreed it was necessary to move in this direction.

Some references:
Sion, E.M., et al. 1983. Astrophys. J., 269, 253--257
Greenstein, J. 1986. Astrophys. J., 304, 334--355
Wesemael, F. et al. 1993. Publ. Astr. Soc. Pacif., 105, 761--778

(Electronic versions of journal articles can be found on the WWW in
postscript and pdf formats via the Astronomical Data Center and its
mirrors in Europe, South America and Asia. Start from
http://adswww.harvard.edu/ and locate the best mirror for your location.)

------------------------------

Subject: G.01.3 What are all those different kinds of stars?
Neutron Stars
Author: Joseph Lazio

Neutron stars are the remnants of massive stars. Sufficiently massive
stars form iron in their cores during the process of nuclear fusion.
Iron proves problematic for the star, though, as iron is among the
most tightly bound nuclei. Nuclear fusion involving iron actually
requires energy to occur, as opposed to nuclear fusion involving
lighter nuclei in which the fusion produces energy. At some point so
much iron accumulates in the core of the star that its nuclear
reactions do not produce enough heat (i.e., pressure) to
counter-balance the force of gravity due to the star's mass. The star
implodes in a supernova, blowing off much of its outer layers and
leaving an NS as a remnant. A star has to be (roughly) at least 8
times as massive as the Sun and not more than 25--50 times as massive
as the Sun to form an NS. (The upper limit is quite uncertain.)

(There has been a second mechanism postulated as a way to form neutron
stars. There is an upper limit to the mass of a white dwarf, 1.4
times the mass of the Sun, called the Chandrasekhar limit after
Subrahmanyan Chandrasekhar who first described it. Above this mass
the force of gravity overwhelms the internal pressure provided by the
electrons in the WD. If one had a WD that was quite close to the
Chandrasekhar limit and a small amount of mass was added to it, it
might collapse to form an NS. This process is called
"accretion-induced collapse." It is not clear if this mechanism
actually occurs, however.)

NSs can be divided into three broad classes, rotation-powered pulsars,
accretion-powered pulsars, and magnetars.

Rotation-powered pulsars are the kind of pulsars most commonly
described and were the first kind of NSs observed. These NSs have
powerful magnetic fields and rotate. If the axes of the star's
rotation and magnetic field are not aligned, this rotating magnetic
field produces an electric field; in the case of NSs, the electric
fields are strong enough to rip particles from the crust of the NS and
accelerate them. The accelerated particles radiate. The magnetic
field collimates the accelerated particles, so the radiation from the
NS is emitted in two narrow beams. If one of the beams sweeps across
the Earth, we observe a pulsating source---a pulsar. Most of the
known rotation-powered pulsars are observed in the radio (though the
radio emission itself is a usually just a tiny fraction of the
rotation energy of the NS).

Rotation-powered pulsars are often further sub-divided into
strong-field and recycled pulsars. Strong-field pulsars have magnetic
fields of about 10^8 Tesla and observed pulse periods about 1 second.
As the pulsars lose energy, their rates of spin slow down. At some
point, the rotating magnetic field is no longer produces electric
fields strong enough to power the pulsar mechanism, and the pulsar
"shuts off." However, if the NS is a member of a binary system, its
companion star, during the course of its own evolution, increase in
size and start spilling matter onto the NS. As the matter spills onto
the NS, if it hits the NS in the same direction that the NS is
rotating, it can increase the rate at which the NS is spinning or
"spin-up" the NS. If this spin-up process goes on for a long enough
period of time, the NS may "turn on" as a pulsar again. The process
of matter spilling onto the pulsar tends to suppress the magnetic
field, though. With a weaker magnetic field, the spun-up pulsar
doesn't spin down as fast as before. So, these recycled pulsars are
distinguished by having very slow spin-down rates. As it turns out,
they also tend to have very short pulse periods, typically less than
0.1 seconds, with the shortest being 0.00156 seconds.

Accretion-powered pulsars are NSs onto which matter is spilling. The
gravity well around an NS is so deep, it is actually fairly difficult
for matter to fall onto the NS. Only matter that starts at rest with
respect to the NS can fall directly onto its surface. If the matter
has any velocity relative to the NS, as it falls toward the NS, it
will begin to orbit the NS. (This is the same principle that causes a
skater to spin faster as she pulls in her arms.) If a lot of matter
is falling toward the NS, a disk is formed around the NS. Due to
"frictional" forces within the disk, matter slowly works its way
closer to the NS until finally falling a short distance onto its
surface. The process of the matter falling onto the NS' surface is
known as accretion, so the disk is called an accretion disk. The
gravitational potential of a NS is so deep that a lot of energy can be
released as the matter forms an accretion disk and spills onto the NS'
surface. Consequently, accretion-powered NSs are typically seen as
X-ray sources.

Magnetars are a recently recognized class of NSs. It is thought that
rotation-powered pulsars only work if the magnetic field is not too
strong. If the magnetic field is too strong, it can effectively shut
down the process by which the particles are produced. The critical
field seems to be about 10^10 Tesla. Only a few examples of magnetars
are known. These generally appear as fairly constant X-ray sources,
though magnetars have also been suggested to be responsible for
sources known as soft-gamma ray repeaters.

------------------------------

Subject: G.01.4 What are all those different kinds of stars?
Black Holes
Author: Joseph Lazio

A black hole is any object for which its entire mass M is contained
within a radius
2GM
R = ---
c^2
where G is the universal gravitation constant (G = 6.67 x 10^-11
m^3/kg/s^2) and c is the speed of light. An object this compact will
have an escape velocity larger than light so nothing can escape from
it. (For an object with the mass of the Sun, this radius is 3 km.)

BHs can be divided into (at least) three classes: primordial,
stellar-mass, and supermassive. Primordial BHs, if they exist, were
formed during the initial instants of the Big Bang. The initial
Universe was not perfectly smooth, there were slight fluctuations in
its density. Some of these density fluctuations could have satisfied
the above criterion. In that case, BHs would have formed. These
primordial BHs could have a range of masses, anywhere from milligrams
to 10^17 times the mass of the Sun. Currently, however, there is
little evidence to suggest that any primordial BHs did form. (In
fact, the available evidence suggests that no primordial BHs formed.)

Stellar-mass BHs are those with masses of roughly 10 times the mass of
the Sun. These are formed from processes involving one or a few
stars. For instance, a star more massive than 50 solar masses will
also start to form a iron core. Unlike a less massive star that forms
an NS during the supernova, though, the iron core becomes so massive
that it collapses to form a BH. Another possibility for the formation
of a stellar-mass BH is the collision of two stars, such as might
happen in the center of dense globular cluster of stars or two
orbiting NSs. A Stellar-mass BH is identified typically when it is
orbited by a lower mass star. Some of the material from the companion
star may be stripped away from it and fall into the BH, producing
copious amounts of radio and X-ray emission in the process.

Supermassive BHs are those with masses exceeding roughly 1 million
times that of the Sun. These are found at the center of galaxies. It
is not clear how these form, but it probably involves the accumulation
of many smaller mass BHs, NSs, and perhaps interstellar gas during the
formation of galaxies. Recent work shows a correlation between the
mass of the central parts of galaxies and the mass of the central BH.
This has led to some speculation at to whether the central BHs form
first and "seed" the formation of galaxies or if there is a symbotic
process in which the central BH and the galaxy are created
simultaneously.

There have also been suggestions of "intermediate mass" BHs. These
would be objects whose mass is roughly 100--1000 times that of the
Sun. The suggestions that such intermediate mass BHs might exist
arise from X-ray observations of other galaxies showing strong X-ray
sources not associated with the centers of the galaxies. Certain
assumptions must be used in relating the X-ray brightness of the
objects to their mass, though, so whether such intermediate mass BHs
actually exist is still somewhat controversial.

------------------------------

Subject: G.02 Are there any green stars?
Author: Paul Schlyter ,
Steve Willner

The color vision of our eyes is a pretty complicated matter. The
colors we perceive depend not only of the wavelength mix the eye
receives at a perticular spot, but also on a number of other factors.
For instance the brightness of the light received, the brightness and
wavelength mix received simultaneously in other parts of the field
of view (sometimes visible as "contrast effects"), and also the
brightness/wavelength mix that the eye previously received (sometimes
visible as afterimages).

One isolated star, viewed by an eye not subjected to other strong
lights just before, and with very little other light sources in the
field of view, will virtually never look green. But put the same
star (which we can assume to appear white when viewed in isolation)
close to another, reddish, star, and that same star may immediately
look greenish, due to contrast effects (the eye tries to make the
"average" color of the two stars appear white).

Also, stars generally have very weak colors. The only exception is
perhaps those cool "carbon" stars with a very low temperature---they
often look quite red, but still not as red as a stoplight. Very hot
stars have a faint bluish tinge, but it's always faint---"blue" stars
never get as intense in their colors as the reddest stars. Once the
temperature of a star exceeds about 20,000 K, its temperature doesn't
really matter to the perceived color (assuming blackbody
radiation)---the star will appear to have the same blue-white color no
matter whether the temperature is 20,000, 100,000 or a million degrees K.

Old novae in the "nebular" phase often look green. This is because
they are surrounded by a shell of gas that emits spectral lines of
doubly ionized oxygen (among other things). Although these object
certainly look like green stars in a telescope---the gas shell cannot
usually be resolved---the color isn't coming from a stellar
photosphere.

------------------------------

Subject: G.03 What are the biggest and smallest stars?
Author: Ken Croswell,
John E. Gizis

[Table reflects most recent distances from Hipparcos.]
The most luminous star within 10 light-years is Sirius.
The most luminous star within 20 light-years is Sirius.
The most luminous star within 30 light-years is Vega.
The most luminous star within 40 light-years is Arcturus.
The most luminous star within 50 light-years is Arcturus.
The most luminous star within 60 light-years is Arcturus.
The most luminous star within 70 light-years is Aldebaran.
The most luminous star within 80 light-years is still Aldebaran.
The most luminous star within 100 light-years is still...Aldebaran.
The most luminous star within 1000 light-years is Rigel.
(Honorable mentions: Canopus, Hadar, gamma Velae, Antares, and
Betelgeuse.)
The most luminous star within 2000 light-years is Rigel.
The most luminous star in the whole Galaxy is *drum roll, please*
.... Cygnus OB2 number 12, with an absolute magnitude around -10.
(also known as VI Cygni No 12).

A table listing the nearest stars (within 12 light years) may be found
at http://www.ccnet.com/~galaxy/tab181.html. The faintest star
within that distance is Giclas 51-15 with absolute visual magnitude
16.99 and spectral type M6.5.

Wielen et al. published the following as the local luminosity function
(total number of stars within 20 parsecs = 65 lightyears). At the faint
end (abs. magnitude 12) this table is bit out of date and the numbers
are probably too high. Everything from abs. magnitude 9 to 18 is
considered an M dwarf (shows TiO and other molecules) or a white dwarf.

abs. mag Number
-1 1
0 4
1 14
2 24
3 43
4 78
5 108 Sun is here!
6 121
7 102
8 132
9 159
10 245
11 341
12 512
13 597
14 427
15 427
16 299
17 299
18 16

------------------------------

Subject: G.04 What fraction of stars are in multiple systems?
Author: John E. Gizis

According to the work of A. Duquennoy and M. Mayor, 57% of systems
have two or more stars. They were working with a sample of F and G
stars, i.e., stars like the Sun. It appears that for the coolest,
low-luminosity stars (the M-dwarfs) there are fewer binaries. Fischer
and Marcy found that only 42% of M-dwarfs are binaries. Neill Reid
and I have used HST images to find that for the coolest stars in the
Hyades cluster (absolute magnitude 12, or mass 0.3 solar masses)
only 30% are binaries.

[There's also the tongue-in-cheek answer that three out of every two
stars is in a binary. TJWL]

References:
Gizis, J. & Reid, I. Neill 1995, "Low-Mass Binaries in the Hyades,"
Astronomical Journal, v. 110, p. 1248

------------------------------

Subject: G.05 Where can I get stellar data (especially distances)?
Author: Steve Willner ,
John Ladasky Jr.

Two key sites for stellar data are the Astronomical Data Center,
URL:http://adc.gsfc.nasa.gov/adc.html, and the CDS Service for
Astronomical Catalogues,
URL:http://cdsweb.u-strasbg.fr/cats/Cats.htx, both of which maintain
large inventories of astronomical catalogs, including star catalogs.
Another important site is SIMBAD,
URL:http://simbad.u-strasbg.fr/sim-fid.pl, as one can use it to find
alternate names for a star. (For instance, what is another name for
the variable star V* V645 Cen?)

Distances in astronomy are always problematic, and it is important to
keep in mind that all astronomical data have uncertainties. It is
vital to understand what the uncertainties are. Moreover, if one is
interested in constructing 3-D star maps, one should recognize that
astronomical data are not stored in XYZ coordinates. Science-fiction
writers and people who want to make 3-D maps of local space like them,
but astronomers don't use them. Astronomers need polar coordinates
(right ascension and declination) centered on Earth, so that they know
where to point their telescopes.

Three useful sites for distance data are

* One large (3803 stars) compilation of nearby stars is the
"Preliminary Version of the Third Catalogue of Nearby Stars," which
aims to catalog all known stars within 25 pc (~ 75 light years) of
the Sun. The "ReadMe" file for the catalog is at
URL:ftp://adc.gsfc.nasa.gov/pub/adc/archives/catalogs/5/5070A/ReadMe.

* The Internet Stellar Database
URL:http://www.stellar-database.com/ attempts to synthesize
information about the nearest stars from various catalogs.

* Recent research on refining astronomical data for the nearby stars
can be found at the Research Consortium on Nearby Stars (RECONS),
URL:http://tarkus.pha.jhu.edu/%7Ethenry/RECONS.html.

(Note that these sites tend to focus on *nearby* stars---that's a
result of the difficulty of obtaining accurate distances for distant
stars.)

If an object is close enough to Earth to have a significant parallax
(an apparent yearly wobble in the sky that results from the change in
observing position of the Earth), then its distance can be determined
by triangulation. With two angles and a distance, you can compute
Cartesian coordinates if you want them. If you'd like to use the
astronomical data, say, to calculate distances between stars, a useful
reference is URL:http://www.projectrho.com/starmap.html. (Note that
many astronomical catalogs do not include parallax measurements.)

The best parallax data collected thus far comes from the European
astrometry satellite, Hipparcos,
URL:http://astro.estec.esa.nl/Hipparcos/, and it represents a
gigantic improvement both in systematic accuracy and in precision over
previous catalogs, but it is limited to fairly bright stars (magnitude
limit around 11).

Both the CDS and the Hipparcos Web site offer online tools for
searching the Hipparcos catalog as well as the full catalog itself.
Two important aspects of the Hipparcos catalog are how distances are
described and the names given to stars. First, distances are
described by the parallax in milliarcseconds. The distance d in
parsecs is given by d = 1000/p for a parallax p in milliarcseconds.
To obtain a distance in light years, multiply by 3.26. Thus, a star
with a parallax of 100 milliarcseconds is at a distance of 10 pc (~ 30
light years).

Second, all of the Hipparcos catalog "names" will be unfamiliar to
you, as they are just numbers. One can use SIMBAD to convert from
Hipparcos catalog names to more familiar names.

------------------------------

Subject: G.06 Which nearby stars might become supernovae?
Author: Steve Willner

Obvious candidates are alpha Orionis (Betelgeuse, M1-2 Ia-Iab), alpha
Scorpii (Antares, M1.5 Iab-Ib), and alpha Herculis (Rasalgethi, M5
Ib-II). Spectral types come from the Bright Star Catalog. Although
trigonometric parallaxes are listed in the catalog, they will not be
very accurate for stars this far away. I derive photometric distances
of around 400 light years for the first two and 600 light years for
alpha Her. (Anybody have better sources, or do we have to wait for
Hipparcos?) Anybody want to suggest more?

------------------------------

Subject: G.07 What will happen on Earth if a nearby star explodes?

A nice article by Michael Richmond may be found at
URL:http://a188-L009.rit.edu/richmond/answers/snrisks.txt. His
conclusion is:

"I suspect that a type II explosion must be within a few parsecs of
the Earth, certainly less than 10 pc, to pose a danger to life on
Earth. I suspect that a type Ia explosion, due to the larger amount
of high-energy radiation, could be several times farther away. My
guess is that the X-ray and gamma-ray radiation are the most important
at large distances."

------------------------------

Subject: G.08 How are stars named? Can I name/buy one?
Author: Kevin D. Conod

Official names for celestial objects are assigned by the International
Astronomical Union. Procedures vary depending on the type of object.
Often there is a system for assigning temporary designations as soon as
possible after an object is discovered and later on a permanent name.
See E.05 of this FAQ.

Some commercial companies purport to allow you to name a star.
Typically they send you a nice certificate and a piece of a star atlas
showing "your" star. The following statement on star naming was
approved by the IPS Council June 30, 1988.

The International Planetarium Society's Guidelines on Star Naming

SELLING STAR NAMES

The star names recognized and used by scientists are those that have
been published by astronomers at credible scientific institutions. The
International Astronomical Union, the worldwide federation of
astronomical societies, accepts and uses _only_ those names. Such names
are never sold.

Private groups in business to make money may claim to "name a star for
you or a loved one, providing the perfect gift for many occasions." One
organization offers to register that name in a Geneva, Switzerland,
vault and to place that name in their beautiful copyrighted catalog.
However official-sounding this procedure may seem, the name and the
catalog are not recognized or used by any scientific institution.
Further, the official-looking star charts that commonly accompany a
"purchased star name" are the Becvar charts excerpted from the _Atlas
Coeli 1950.0_. [Other star atlases such as _Atlas Borealis_ may be used
instead.] While these are legitimate charts, published by Sky
Publishing Corporation, they have been modified by the private "star
name" business unofficially. Unfortunately, there are instances of news
media describing the purchase of a star name, apparently not realizing
that they are promoting a money-making business only and not science.
Advertisements and media promotion both seem to increase during holiday
periods.

Planetariums and museums occasionally "sell" stars as a way to raise
funds for their non-profit institutions. Normally these institutions
are extremely careful to explain that they are not officially naming
stars and that the "naming" done for a donation is for amusement only.

OFFICIAL STAR-NAMING PROCEDURES

Bright stars from first to third magnitude have proper names that have
been in use for hundreds of years. Most of these names are Arabic.
Examples are Betelgeuse, the bright orange star in the constellation
Orion, and Dubhe, the second-magnitude star at the edge of the Big
Dipper's cup (Ursa Major). A few proper star names are not Arabic. One
is Polaris, the second-magnitude star at the end of the handle of the
Little Dipper (Ursa Minor). Polaris also carries the popular name, the
North Star.

A second system for naming bright stars was introduced in 1603 by
J. Bayer of Bavaria. In his constellation atlas, Bayer assigned
successive letters of the Greek alphabet to the brighter stars of each
constellation. Each Bayer designation is the Greek letter with the
genitive form of the constellation name. Thus Polaris is Alpha Ursae
Minoris. Occasionally Bayer switched brightness order for serial order
in assigning Greek letters. An example of this is Dubhe as Alpha Ursae
Majoris, with each star along the Big Dipper from the cup to handle
having the next Greek letter.

Faint stars are designated in different ways in catalogs prepared and
used by astronomers. One is the _Bonner Durchmusterung_, compiled at
Bonn Observatory starting in 1837. A third of a million stars to a
faintness of ninth magnitude are listed by "BD numbers." The
_Smithsonian Astrophysical Observatory (SAO) Catalog_, _The Yale Star
Catalog_, and _The Henry Draper Catalog_ published by Harvard College
Observatory all are widely used by astronomers. The Supernova of 1987
(Supernova 1987A), one of the major astronomical events of this century,
was identified with the star named SK -69 202 in the very specialized
catalog, the _Deep Objective Prism Survey of the Large Magellanic
Cloud_, published by the Warner and Swasey Observatory.

These procedures and catalogs accepted by the International Astronomical
Union are the only means by which stars receive long-lasting names. Be
aware that no one can buy immortality for anyone in the form of a star
name.

------------------------------

Subject: Do other stars have planets?
Author: needed

Yes!

This is an active area of research, and since 1992 astronomers have
found planets around two pulsars (PSR 1257+12 and 0329+54) and about a
half-dozen main-sequence stars.

See
URL:http://cannon.sfsu.edu/~gmarcy/planetsearch/planetsearch.html,
URL:http://www.obspm.fr/planets,
URL:http://techinfo.jpl.nasa.gov/WWW/ExNPS/HomePage.html, and
URL:http://ast.star.rl.ac.uk/darwin/ for more information.

------------------------------

Subject: G.10 What happens to the planets when a planetary nebula is
formed? Do they get flung out of the solar system?
Author: Joseph Lazio

A couple of possibilities exist. Prior to forming a planetary nebula,
a low-mass star (i.e., one with a mass similar to that of the Sun)
forms a red giant. Planets close to the star are engulfed in the
expanding star, spiral inside it, and are destroyed. In our own solar
system, Mercury and Venus are doomed.

As the star expands to form a red giant, it also starts losing mass.
All stars lose mass. For instance, the Sun is losing mass. However,
at the rate at which the Sun is currently losing mass, it would take
over 1 trillion years (i.e., 100 times longer than the age of the
Universe) for the Sun to disappear. When a star enters the red giant
phase, the rate at which it loses mass can accelerate. The mass of a
star determines how far a planet orbits from it. Thus, as the Sun
loses mass, the orbits of the other planets will expand. The orbit of
Mars will almost certainly expand faster than the Sun does, thus Mars
will probably not suffer the same fate as Mercury and Venus. It is
currently an open question as to whether the Earth will survive or be
engulfed.

The orbits of planets farther out (Jupiter, Saturn, Uranus, Neptune,
and Pluto) will also expand. However, they will not expand by much
(less than double in size), so they will remain in orbit about the Sun
forever, even after it has collapsed to form a white dwarf.

(Any planets around a high-mass star would be less lucky. A high-mass
star loses a large fraction of its mass quickly in a massive explosion
known as a supernova. So much mass is lost that the planets are no
longer bound to the star, and they go flying off into space.)

As for the material in the planetary nebula, it will have little
impact on the planets themselves. The outer layers of a red giant are
extremely tenuous; by terrestrial standards they are a fairly decent
vacuum!

------------------------------

Subject: G.11 How far away is the farthest star?
Author: Joseph Lazio

This question can have a few answers.

1. The Milky Way galaxy is about 120,000 light years in diameter.
We're about 25,000 light years from the center. Thus, the most
distant stars that are still in Milky Way galaxy are about 95,000
light years away, on the opposite side of the center from us. Because
of absorption by interstellar gas and dust, though, we cannot see any
of these stars.

2. The most distant object known has a redshift of just over 5. That
means that the light from this object started its journey toward us
when the Universe was only 30% of its current age. The exact age of
the Universe is not known, but is probably roughly 12 billion years.
Thus, the light from this object left it when the Universe was a few
billion years old. Its distance is roughly 25 billion light years.

3. Existing observations suggest that the Universe may be infinite
in spatial extent. If so, then the farthest star would actually
be infinitely far away!

------------------------------

Subject: G.12 Do star maps (or galaxy maps) correct for the motions of the
stars?
Author: Joseph Lazio

In general, no.

The reason is that stellar distances are so large. Over human time
spans, the typical velocity of a star is so low that its distance does
not change appreciably.

Let's consider a star with a velocity of 10 km/s, typical of most
stars. In 1000 yrs, this star moves about 300 billion kilometers (or
3E11 km). Suppose the star is 100 light years (about 1E15 km or 1
quadrillion kilometers) distant. Thus, in 1000 yrs, the star moves
about 0.03% of its distance from the Sun. This is such a small
change, it's not worth worrying about it.

The situation is even more extreme in the case of galaxies. Typical
galaxy velocities might be hundreds to thousands of kilometers per
second. However, their distances are measured in the millions to
billions of light years.

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2003 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
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Archive-name: astronomy/faq/part8

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/ and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)


Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: H.00 Galaxies, Clusters, and Quasars (QSOs)

[Dates in brackets are last edit.]

H.01 How many stars, galaxies, clusters, QSO's etc. in the
Universe? [1997-08-06]
H.02 Is there dark matter in galaxies? [1997-12-02]
H.03 What is the Hubble constant? What is the best value? [1995-07-19]
H.04 How are galaxy distances measured? [1995-06-29]
H.05 When people speak of galaxies X billion light years, does
this mean they are that far away now or were that far away
when the light left them? [1997-08-06]
H.06 What are QSO's ("quasars")? [1995-06-29]
H.07 Are the QSO's really at their redshift distances? [2003-02-18]
H.08 What about apparent faster-than-light motions? [1995-06-29]
H.09 What's the Local Group? [1999-05-19]

For an overall sense of scale when talking about galaxies, see the
Atlas of the Universe, URL:http://anzwers.org/free/universe/.

------------------------------

Subject: H.01 How many stars, galaxies, clusters, QSO's etc. in the Universe?

The various parts of this question will be considered separately.
Also, rather consider how many stars there are in the Universe, we'll
consider how many stars there are in the Milky Way. The number of
stars in the Universe can be estimated by multiplying the number of
stars in the Milky Way by the number of galaxies in the Universe.

------------------------------

Subject H.01.1 How many stars are there in the Milky Way?
Author: William Keel

My standard answer in introductory astronomy classes is "about as many
as the number of hamburgers sold by McDonald's." Being more precise
requires an extrapolation, because we can't see all the individual
stars in the Milky Way for two reasons---distance and dust absorption.

Both factors make stars appear dimmer. Observations at visible
wavelengths are limited to a region of (more or less) 5000 light-years
radius about the Sun, with a few windows in the intervening dust
giving us glimpses of more distant areas (especially near the Galactic
center). Our map of the Galaxy gets correspondingly more sketchy with
distance. Guided somewhat by observations of other spiral galaxies, we
think that the overall run of star density with radius is fairly well
known. Getting a total stellar head count is more of a problem,
because the stars that we can see to the greatest distances are also
the rarest. Measurements of the relative numbers of stars with
different absolute brightness (known in the trade as the luminosity
function) shows that, for example, for every Sun-like star there are
about 200 faint red M dwarfs. These are so faint that the closest,
Proxima Centauri, despite being closer to the Sun than any other
(known) star, takes very large binoculars or a telescope to find. So,
to get the total stellar population in the Milky Way, we must take the
number of luminous stars that we can see at large distances and assume
that we know how many fainter stars go along with them. Recent numbers
give about 400,000,000,000 (400 billion) stars, but a 50% error either
way is quite plausible. Much of the interest in "brown dwarfs" stems
from a similar issue---a huge number of brown dwarfs would not change
how bright the Galaxy appears (at visible wavelengths), but would
change its total mass quite substantially. Oddly enough, within a
particular region, we probably know the total mass and luminosity
rather more accurately than we do just how many stars are producing
that light (since the most common stars are by far the dimmest).

------------------------------

Subject: H.01.2 How many galaxies in the Universe?
Author: William Keel

A widely-distributed press release about the Hubble Deep Field
observations, URL:http://oposite.stsci.edu/pubinfo/PR/96/01.html,
reported the discovery of a vast number of new galaxies. The
existence of many galaxies too faint to be hitherto detected was no
surprise, and calculations of the number of galaxies in the observable
Universe and searches for how they change with cosmic time must always
allow for the ones we can't detect, through some combination of
intrinsic faintness and great distance. What was of great interest in
the Hubble Deep field (and similar) data was just how any faint
galaxies were detected and what their colors and forms are. Depending
on just what level of statistical error can be tolerated, catalogs of
galaxies in the Hubble Deep Field list about 3000. This field covers
an area of sky of only about 0.04 degrees on a side, meaning that we
would need 27,000,000 such patches to cover the whole sky. Ignoring
such factors as absorption by dust in our own Galaxy, which make it
harder to see outside in some directions, the Hubble telescope is
capable of detecting about 80 billion galaxies (although not all of
these within the foreseeable future!). In fact, there must be many
more than this, even within the observable Universe, since the most
common kind of galaxy in our own neighborhood is the faint dwarfs
which are difficult enough to see nearby, much less at large
cosmological distances. For example, in our own local group, there are
3 or 4 giant galaxies which would be detectable at a billion
light-years or more (Andromeda, the Milky Way, the Pinwheel in
Triangulum, and maybe the Large Magellanic Cloud). However, there are
at least another 20 faint members, which would be difficult to find at
100 million light-years, much less the billions of light years to
which the brightest galaxies can be seen.

------------------------------

Subject: H.01.3 How many globular clusters in the Milky Way?
Author: William Keel

We are on firmer ground with this one, since globular clusters are
fairly large and luminous. The only places where our census in the
Milky Way is incomplete are regions close to the galactic disk and
behind large amounts of absorbing dust, and for the fainter clusters
that are farthest from the Milky Way just now. The electronic version
of the 1981 Catalogue of Star Clusters and Associations. II. Globular
Clusters by J. Ruprecht, B. Balazs, and R.E. White lists 137 globular
clusters in and around the Milky Way. More recent discoveries have
added a handful, especially in the heavily reddened regions in the
inner Galaxy. As a rough estimate accounting for the regions that
cannot yet be searched adequately, our galaxy should have perhaps 200
total globulars, compared with the approximately 250 actually found
for the larger and brighter Andromeda galaxy.

------------------------------

Subject: H.01.4 How many open clusters?
Author: William Keel

Here we must extrapolate again, since open clusters can be difficult
to find against rich star fields in the plane of the Milky Way, and
since richer clusters may be identified farther away than poor
ones. The electronic version of the catalogue of open cluster data
compiled by Gosta Lynga, Lund Observatory, Box 43, S-221 00 Lund,
Sweden, 1987 version, lists 1111 identified open clusters in our
galaxy. There are certainly at least ten times this number, since we
have trouble seeing even rich open clusters more than about 7000
light-years away in most directions through the obscuring dust in the
plane of our Galaxy. This effect is especially acute since young star
clusters are strongly concentrated to this plane (no coincidence since
the gas from which new clusters are formed is associated with dust).

------------------------------

Subject: H.02 Is there dark matter in the Universe?
Author: Will Sutherland ,
William Keel

Dark matter is matter that is detected by its gravitational effect on
other matter rather than because of its electromagnetic radiation
(i.e., light). This might be because of one of two reasons: 1. The
matter may emit light, but the light is so faint that we cannot detect
it; an example of this kind of matter is interstellar planets. 2.
The matter might not interact with light at all; an example of this
kind of matter is neutrinos.

The first astronomical instances of "dark matter" were probably the
white dwarf Sirius B and the planet Neptune. The existence of both
objects was inferred by their gravitational effects on a nearby object
(Sirius A and the planet Uranus, respectively) before they were seen
directly.

------------------------------

Subject: H.02.1 Evidence for dark matter

There are many independent lines of evidence that most of the matter
in the universe is dark. Essentially, many of these measurements rely
on "weighing" an object such as a galaxy or a cluster of galaxies by
observing the motions of objects within it, and calculating how much
gravity is required to prevent it flying apart.

(1) Rotation patterns in spiral galaxies.
(2) Velocities of galaxies in clusters.
(3) Gravitational lensing.
(4) Hot gas in galaxies and clusters.
(5) Large-scale motions.

(1) Rotation patterns in spiral galaxies. The disks of spirals are
full of stars and gas in nearly circular coplanar orbits, making them
wonderful tracers for the gravitational field in which they move. In
centrally-concentrated masses, such as within the solar system (where
most of the mass is concentrated in the Sun), the
velocity-vs.-distance relation approaches Kepler's 3rd Law, velocity^2
= constant * central mass / distance. Once we sample outside the
central concentration of stars, using observations of the 21cm line
emitted by neutral hydrogen clouds, spiral galaxies violate this
velocity-distance relation quite flagrantly; velocity=constant is a
good approximation (hence the moniker "flat rotation curves"). A
sample picture and rotation curve is at
URL:http://crux.astr.ua.edu/gifimages/ngc5746.html. To get this
pattern, one needs a mass distribution that goes as density
proportional to 1/radius^2, much fluffier than the observable stars
and gas in the galaxy, and in an amount that may be 10 or more times
the total mass we can account for with stars, dead stellar remnants,
gas, and dust. There were hints of this issue for a while, but it was
a series of observations by Vera Rubin and collaborators in the
mid-1970's that really rubbed our noses in it.

(2) Velocities of galaxies in clusters. Galaxies in clusters have
random orbits. By measuring the dispersion for, e.g., 100 galaxies in
the cluster, one finds typical dispersions of 1000 km/s. The clusters
must be held together by gravity, otherwise the galaxies would escape
in less than 1 billion years; cluster masses are required to be at
least 10 times what the galaxies' stars can account for. This problem
was first demonstrated in 1938 by Fritz Zwicky who studied the
galaxy-rich Coma cluster. Zwicky was very bright, very arrogant, and
highly insulting to anyone he felt was beneath him, so this took a
long while to sink in. Today we know that virtually all clusters of
galaxies show the same thing.

(3) Gravitational lensing. General relativity shows that we can treat
gravity (more precisely than in Newtonian dynamics) by considering it
as a matter-induced warping of otherwise flat spacetime. One of the
consequences of this is that, viewed from a distance, a large enough
mass will bend the paths of light rays. Thus, background objects seen
past a large mass (galaxy or cluster of galaxies) are either multiply
imaged or distorted into "arcs" and "arclets." Some beautiful
examples can be seen at
URL:http://www.stsci.edu/pubinfo/PR/96/10/A.html,
URL:http://www.stsci.edu/pubinfo/PR/95/14.html, and
URL:http://www.stsci.edu/pubinfo/PR/95/43.html. When we know the
distances of foreground and background objects, the mass inside the
lensing region can be derived (and for some of these multi-lens
clusters, its radial distribution). Same old story - we need a lot
more mass in invisible than visible form.

(4) Hot gas in galaxies and clusters. A real shocker once X-ray
astronomy became technologically possible was the finding that
clusters of galaxies are intense X-ray sources. The X-rays don't come
from the galaxies themselves, but from hot, rarefied gas at typically
10,000,000 K between the galaxies. To hold this stuff together
against its own thermal motions requires - you guessed it, huge
amounts of unseen material.

It is worth noting that these last three methods all give about the
same estimate for the amount of dark matter in clusters
of galaxies.

(5) Less direct evidence also exists: On larger scales, there is
evidence for large-scale "bulk motions" of galaxies towards
superclusters of galaxies, e.g., the Great Attractor. There is also
the question of reconciling the very small (1 part in 100,000)
observed fluctuations in the cosmic microwave background with the
"lumpy" galaxy distribution seen at the present day; dark matter helps
nicely to match these two facts because the density fluctuations grow
more rapidly with time in a higher-density Universe. Finally, the
theory of inflation (which is an "optional extra" to the standard big
bang model) usually predicts that the universe should have exactly the
critical density, which could require as much as 95% of the mass in
the Universe to be dark.

It is worth mentioning the possibility of non-standard gravity
theories, which attempt to explain the above list of observations
without dark matter. It turns out that modifying the inverse-square
law of gravity does not work well, essentially because the dark matter
problem extends over so many different lengthscales. Modifying the F =
ma law has been tried, e.g., by Milgrom, but relativistic versions of
this theory have not been found, and most cosmologists are reluctant
to abandon Einstein's GR which is elegant and well tested (at least on
solar system scales).

------------------------------

Subject: H.02.2 How much dark matter is there?

A convenient way of quoting mass estimates is via Omega, the ratio of
the density contributed by some objects to the "critical density" = 3
H^2 / 8 pi G, where H is the Hubble constant and G is the universal
constant of gravitation. The critical density is the amount of matter
that would be just sufficient to stop the expansion of the Universe
and is 10^{-29} g/cm^3. (Of course, portions of the Universe have a
higher density than this, e.g., you, but this is an average density.)
The visible stars in galaxies contribute about 1 percent of critical
density, i.e., Omega_stars ~ 0.01; dark halos around galaxies
contribute Omega_halos ~ 0.05; mass estimates from clusters tend to
give Omega_clus ~ 0.2 (assuming the ratio of dark matter to stars is
the same in clusters as everywhere else); and theoretical
considerations (i.e., inflation) favor Omega_total = 1. The gap
between 0.05 and 0.2 can be explained if galaxy halos extend further
out than we can measure the rotation curves, but if Omega_total = 1 we
may require extra dark matter in intergalactic space.

It's also interesting to consider the dark matter density "locally."
Within a few hundred parsecs of the Sun, this is about 0.01 Solar
masses per cubic parsec, or about 0.3 proton masses per cm^3; that's
only about 1/10 of the density of visible matter (mostly stars);
though it's much larger than critical density because we live in a
galaxy. However, because the stars are in a thin disk while the dark
matter is more spherical, if you take an 8 kpc radius sphere centred
on the Galaxy and passing through the Sun, roughly half the mass in
this sphere is dark matter If you consider a larger sphere, e.g., out
to the Large Magellanic Cloud at 50 kpc radius, over 80% of the mass
in it is dark matter. This estimate was first made by Jan Oort, and
the estimate of the *total* mass density near the Sun is today termed
the Oort limit in his honor.

------------------------------

Subject: H.02.3 What is the dark matter?

Since it's detected in a negative sense---not visible in gamma rays,
X-rays, ultraviolet, visible light, infrared, millimeter, or radio
regimes, and it doesn't block light either---it's a theoretical happy
hunting ground. First, let's list some things that can't make the
dark matter. Most forms of gas are excluded, because atomic hydrogen
would be seen in 21cm radiation, and hot gas would be seen in X-rays
and/or distort the spectrum of the CMB. Cold molecular gas is a
possibility, but it would tend to collapse into visible stars.
"Snowballs" made of solid hydrogen would evaporate due to the CMB, and
larger snowballs would leave too many craters on the Moon or be seen
as high-speed comets. "Rocks" are unlikely because there haven't been
enough stars to make the heavy elements. Faint red stars are excluded
because they're not seen in deep images e.g., the Hubble Deep Field.

This leaves two main classes of dark-matter candidate: large objects
called MACHOs and subatomic particles, some of which are called WIMPs.

MACHOs stands for Massive Compact Halo Objects; examples are
"interstellar Jupiters" or "brown dwarfs," which are lumps of mostly
hydrogen less than 0.08 Solar masses; objects this small don't get hot
enough to fuse hydrogen into helium, and so would be extremely faint
and hard to find. Other varieties of MACHOs are dead stars, such as
old white dwarfs or neutron stars, and black holes.

The second class is some form of sub-atomic particle; if so, there'd
be millions of these passing through us every second, but they'd
hardly ever interact with normal matter, hence the term "weakly
interacting massive particles" or WIMPs. Many varieties of these have
been suggested; the only one of these that certainly exists is the
neutrino, but neutrinos may not have any mass. The number of
neutrinos made in the Big Bang is similar to the number of CMB photons
(few hundred per cm^3), so if they have a small mass (around 30 eV = 6
x 10^-5 electron masses) they could contribute most of the dark
matter. However, computer models indicate that galaxies form much too
late in a neutrino-dominated universe. Another possibility is the
"axion" which is a hypothetical particle invented to solve a strange
"coincidence" in particle physics (called the strong CP problem).

The most popular WIMP at the moment is the "neutralino" or "lightest
supersymmetric particle"; supersymmetry is a popular way to unify the
strong and electroweak forces (also known as a Grand Unified Theory),
which has some (tentative) experimental support. Supersymmetry
predicts an unobserved new particle or "superpartner" for every known
particle; the lightest of these should be stable, and lots of them
would be left over from the Big Bang. These probably weigh about
30-500 proton masses.

An important piece of evidence here is "primordial nucleosynthesis,"
which explains the abundances of He-4, Deuterium, He-3 and Li-7
produced a few minutes after the Big Bang; in order to obtain the
observed abundances of these elements, the density of baryons (i.e.,
"ordinary" matter) must be Omega_baryon ~ 0.02--0.1. Since Omega_stars
~ 0.01, there are probably some dark baryons, but if Omega_total = 1
(as inflation predicts) most of the dark matter is probably WIMPs.

------------------------------

Subject: H.02.4 Searches for Dark Matter

There are many searches now underway for the dark matter.

For MACHOs, the most promising method is "gravitational microlensing,"
where we wait for a MACHO to pass between us and a distant star, and
the gravity of the MACHO bends the starlight into two images. These
images are too close together to resolve, but add up to more light, so
the star appears to brighten and then fade back to normal as the MACHO
passes by. The shape is quite distinctive, and the brightening
happens only once so does not look like a variable star. The
probability of such a close-enough approach is very low, so millions
of stars must be monitored to have a chance of finding these
events. The Large Magellanic Cloud is the most popular target. A
number of groups---MACHO, EROS, OGLE, among others---have been doing
this for several years, and have found a number of good candidate
microlensing events. At the moment, it is too early to say that
MACHOs have definitely been discovered, but it looks as though the
"brown dwarf" objects are just about excluded, while perhaps as much
as 50% of the dark matter could be in larger objects roughly 0.5 solar
masses, e.g., white dwarfs.

There is an axion search recently started at Lawrence Livermore Labs,
which uses a huge superconducting magnet to convert axions (if they
exist) into microwave photons. For the big bang neutrinos, there is
currently no hope of detecting them because they have far less energy
than the well-known solar neutrinos (see FAQ entry E.01). However, if
a neutrino mass could be measured by lab experiments, we could
calculate their contribution to the dark matter.

For the supersymmetric particles, there are broadly three ways at
detecting them: i) Direct detection by watching a crystal down a deep
mine, and waiting for a WIMP to bounce off a nucleus in it with
observable results such as scintillation or heating of the crystal.
Very roughly 1 WIMP per day should hit each kg of detector, but the
tricky part is discriminating these from natural radioactivity. The
WIMPS should have a preferred direction (due to the orbit of the Sun
around the galaxy), but we'll have to wait for next-generation
experiments to measure this. ii) Indirect detection, whereby WIMPs
get captured in the Sun, and then a WIMP + anti-WIMP annihilate into
super-high energy (GeV) neutrinos which could be detected in huge
volume detectors, e.g., Antarctic ice or ocean water. iii) Create
WIMPs directly at next-generation accelerators like LHC, measure their
properties and then calculate how many should have been produced in
the Big Bang.

With all these searches, there is a good chance that in the next 10
years or so we may find out what constitutes dark matter.

Further reading:

Astronomy magazine, Oct. 1996 issue contains many dark matter articles.

The Center for Particle Astrophysics home page at
URL:http://physics7.berkeley.edu/ has several links including the
Question of Dark Matter page.

The MACHO home page at URL:http://wwwmacho.mcmaster.ca/ has info on
the MACHO project and links to many other dark matter searches.

For cosmology background, see Ned Wright's Cosmology Tutorial at
URL:http://www.astro.ucla.edu/~wright/cosmoall.htm.

A more technical conference summary is at
URL:http://xxx.lanl.gov/abs/astro-ph/9610003.

Krauss, L., _The Fifth Essence_, Basic Books, NY 1989.

Silk, J., _The Big Bang_, Freeman, San Francisco, 1988.

Peebles, P.J.E., _Principles of Physical Cosmology_, Princeton, 1992
(advanced)

------------------------------

Subject: H.03 What is the Hubble constant? What is the best value?
Author: Steve Willner ,
Joseph Lazio

By 1925, V. M. Slipher had compiled radial velocities for 41 galaxies.
He noticed that their velocities were quite a bit larger than typical
for objects within our Galaxy and that most of the velocities
indicated recession rather than approach. In 1929, Edwin Hubble (and
others) recognized the simple relationship that recession velocity is
on average proportional to the galaxy's distance. (His distance
measure was the apparent magnitude of the brightest individually
recognizable stars.) This proportionality is now called "Hubble's
Law," and the constant of proportionality is known as the "Hubble
constant," H (often written "Ho," i.e., H subscript zero).

The Hubble constant also has the property of being related to the age
of the Universe, which undoubtedly explains some of the interest in
its value. It is a constant of proportionality between a speed
(measured in km/s) and a distance (measured in Mpc), so its units are
(km/s)/Mpc. Since kilometers and megaparsecs are both units of
distance, with the correct factor, we can convert megaparsecs to
kilometers, and we're left with a number whose units are (km/s)/km.
If we take 1/H, we see that it has units of seconds, that is 1/H is a
time. We might consider 1/H to be the time it takes for a galaxy
moving at a certain velocity (in km/s) to have moved a certain
distance (in Mpc). If the galaxies have always been moving exactly as
they now are, 1/H seconds ago all of them were on top of us!

Of course the proportionality isn't exact for individual galaxies. Part
of the problem is uncertainties in measuring the distances of galaxies,
and part is that galaxies don't move entirely in conformity with the
"Hubble Flow" but have finite "peculiar velocities" of their own. These
are presumably due to gravitational interactions with other, nearby
galaxies. Some nearby galaxies indeed have blue shifts; M 31 (the
Andromeda galaxy) is a familiar example.

In order to measure the Hubble constant, all one needs a distance and a
redshift to a galaxy that is distant enough that its peculiar velocity
does not matter. Measuring redshifts for galaxies is easy, but
measuring distances is hard. (See the next question.) The Hubble
constant is therefore not easy to measure, and it is not surprising that
there is controversy about its value. In fact, there are generally two
schools of thought: one group likes a Hubble constant around 55
(km/s)/Mpc, and another prefers values around 90 (km/s)/Mpc.

When converted to an age of the Universe, H = 55 (km/s)/Mpc corresponds
to an age of about 19 billion years and H = 90 (km/s)/Mpc is an age of
11 billion years (again if the velocities are constant).

A measure of how difficult it is to determine the Hubble constant
accurately can be seen by examining the different values reported. A
search by Tim Thompson for the period
1992--1994 found 39 reported values for H in the range
40--90 (km/s)/Mpc.

The linear relation between distance and recession velocity breaks down
for redshifts around 1 and larger (velocities around 2E5 km/s). The
true relation depends on the curvature of space, which is a whole other
topic in itself (and has no clear answer). The sense, though, is that
infinite redshift, corresponding to a recession velocity equal to the
speed of light, occurs at a finite distance. This distance is the
"radius of the observable Universe." Nothing more distant than this can
be observed, even in principle.

------------------------------

Subject: H.04 How are galaxy distances measured?
Author: Martin Hardcastle

Galaxy distances must be measured by a complicated series of inferences
known as the distance ladder. We can measure the distances to the
nearest stars by parallax, that is by the apparent motion of the star in
the sky as a result of the Earth's motion round the Sun. This technique
is limited by the angular resolution that can be obtained. The
satellite Hipparcos will provide the best measurements, giving the
parallax for around 100,000 stars. At present parallax can be used
accurately to determine the distances of stars within a few tens of
parsecs from the Sun. [ 1 parsec = 3.26 lt yrs.]

Statistical methods applied to clusters of stars can be used to extend
the technique further, as can `dynamical parallax' in which the
distances of binary stars can be estimated from their orbital
parameters and luminosities. In this way, or by other methods, the
distance to the nearest `open clusters' of stars can be estimated;
these can be used to determine a main sequence (unevolved
Hertzsprung-Russell diagram) which can be fitted to other more distant
open clusters, taking the distance ladder out to around 7 kpc.
Distances to `globular clusters', which are much more compact clusters
of older stars, can also have their distances determined in this way
if account is taken of their different chemical composition; fitting
to the H-R diagram of these associations can allow distance estimates
out to 100 kpc. All of these techniques can be checked against one
another and their consistency verified.

The importance of this determination of distance within our own galaxy
is that it allows us to calibrate the distance indicators that are used
to estimate distances outside it. The most commonly used primary
distance indicators are two types of periodic variable stars (Cepheids
and RR Lyrae stars) and two types of exploding stars (novae and
supernovae). Cepheids show a correlation between their period of
variability and their mean luminosity (the colour of the star also plays
a part) so that if the period and magnitude are known the distance can
in principle be calculated. Cepheids can be observed with ground-based
telescopes out to about 5 Mpc and with the Hubble space telescope to at
least 15 Mpc. RR Lyrae stars are variables with a well-determined
magnitude; they are too faint to be useful at large distances, but they
allow an independent measurement of the distance to galaxies within 100
kpc, such as the Magellanic Clouds, for comparison with Cepheids. Novae
show a relationship between luminosity at maximum light and rate of
magnitude decline, though not a very tight one; however, they are
brighter than Cepheids, so this method may allow distance estimates for
more distant objects. Finally, supernovae allow distance determination
on large scales (since they are so bright), but the method requires some
input from theory on how they should behave as they expand. The
advantage of using supernovae is that the derived distances are
independent of calibration from galactic measurements; the disadvantage
is that the dependence of the supernova's behaviour on the type of star
that formed it is not completely understood.

The best primary distance indicators (generally Cepheids) can be used
to calibrate mainly empirical secondary distance indicators; these
include the properties of H II regions, planetary nebulae, and
globular clusters in external galaxies and the Tully-Fisher relation
between the width of the 21-cm line of neutral hydrogen and the
absolute magnitude of a spiral galaxy. These can all be used in
conjunction with type Ia supernovae to push the distance ladder out to
the nearest large cluster of galaxies (Virgo, at around 15--20 Mpc)
and beyond (the next major goal is the Coma cluster at around 5 times
farther away). Other empirical estimators such as a galaxy
size-luminosity relation or a constant luminosity for brightest
cluster galaxies are of uncertain value.

The goal in all of this is to get out beyond the motions of our local
group of galaxies and determine distances for much more distant
objects which can reasonably be assumed to be moving along with the
expansion of the universe in the Big Bang cosmology. Since we know
their velocities from their redshifts, this would allow us to
determine Hubble's constant, currently the `holy grail' of
observational cosmology; if this were known we would know the
distances to _all_ distant galaxies directly from their recession
velocity. Sadly different methods of this determination, using
different steps along the distance ladder, give different results;
this leads to a commonly adopted range for H of between 50 and 100
km/s/Mpc, with rival camps supporting different values. There are a
number of ongoing attempts to reduce the complexity of the distance
ladder and thus the uncertainty in H. One has been the recent (and
continuing) use of the Hubble Space Telescope to measure Cepheid
variables directly in the Virgo cluster, thereby eliminating several
steps; this leads to a high (80--100) value of H, although with large
uncertainty (which should hopefully be reduced as more results
arrive). Other groups are working on eliminating the distance ladder,
with its large uncertainty and empirical assumptions, altogether, and
determining the distances to distant galaxies or clusters directly,
for example using the Sunyaev-Zeldovich effect together with X-ray
data on distant clusters or using the time delays in gravitational
lenses. The early results tend to support lower values of H, around
50.

------------------------------

Subject: H.05 When people speak of galaxies X billion light years
away, does this mean they are that far away now or were that
far away when the light left them?
Author: William Keel

Distance is indeed a slippery thing in an expanding universe such as ours.
There are at least three kinds of distances:

* angular-diameter distance---the one you need to make the usual
relation
sine(angular size) = linear size/distance
work;

* luminosity distance---makes the typical relationship
observed flux = luminosity / 4 pi (distance**2)
work; and

* proper distance---the piece-by-piece distance the light actually
travelled.

Of the three, the proper distance is perhaps the most sensible of the
three. In this case, distance doesn't mean either when the light was
emitted or received, but how far the light travelled. Since the
Universe expands, we have been moving away from the emitting object so
the light is catching up to us (at a rate set by the rate of expansion
and our separation from the quasar or whatever at some fiducial
time). You can of course turn this distance into an extrapolated
distance (where the quasar or it descendant object is "today") but
that gets very slippery. Both special and general relativity must be
taken into account, so simultaneity, i.e., "today," has only a limited
meaning. Nearby galaxies are pretty much where we see them; for
example, the light from the Andromeda galaxy M31 has been travelling
only about 0.01% of the usually estimated age of the Universe, so its
distance from us would have changed by about that fraction, if nothing
but the Hubble expansion affected its measured distance (which is not
the case, because gravitational interactions between the Andromeda
galaxy and our Galaxy affect the relative velocity of the two
galaxies).

To muddy the waters further, observers usually express distances (or
times) not in light-years (or years) but by the observable quantity
the redshift. The redshift is, by definition, the amount by which
light from an object has been shifted divided by the emitted or
laboratory wavelength of the light and is usually denoted by z. For
an object with a redshift z, one can show that (1+z) is the ratio of
the scale size of distances in the Universe between now and the epoch
when the light was given off. Turning this into an absolute distance
(i.e., some number of light-years) requires us to plug in a rate for
the expansion (the Hubble constant) and its change with time (the
deceleration parameter), neither of which is as precisely known as we
might like.

As a result ages and distances are usually quoted in fairly round
numbers. If the expansion rate has remained constant (the unrealistic
case of an empty Universe), the age of the Universe is the reciprocal
of the Hubble constant. This is from 10--20 billion (US, 10^9) years
for the plausible range of Hubble constants. If we account for the
matter in the Universe, the Universe's age drops to 7--15 billion
years. A quick estimate of the look-back time (i.e., how long the
light from an object has been travelling to us) for something at
redshift z is
t = (z/[1+z])*1/H0
for Hubble constant H0. For example, the author has published a paper
discussing a cluster of galaxies at z=2.4. For the press release we
quoted a distance of 2.4/3.4 x 15 billion light-years (rounded to 11
since that 15 is fuzzy).

------------------------------

Subject: H.06 What are QSO's ("quasars")?
Author: Martin Hardcastle

"Quasi-stellar objects" (or QSO's) are defined observationally as
objects that appear star-like on photographic plates but have high
redshifts (and thus appear extragalactic; see above). The luminosity
(if we accept that the redshift correctly indicates the distance) of a
QSO is much larger than that of a normal galaxy, and many QSO's vary on
time scales as short as days, suggesting that they may be no more than a
few light days in size. QSO spectra typically contain strong emission
lines, both broad and narrow, so that the redshift can be very well
determined. In a few cases, a nebulosity reminiscent of stars in a
normal galaxy has been detected around a QSO. Quasars (a shortened
version of "quasi-stellar radio source") were originally discovered as
the optical counterparts to radio sources, but the vast majority of
QSO's now known are radio-quiet. Some authors reserve the term "quasar"
for the radio-loud class and use the term "QSO" generically; others
(especially in the popular literature) use "quasar" generically.

In the standard model, QSO's are assumed to lie at the centre of
galaxies, and to form the most extreme example of the class of active
galactic nuclei (AGN); these are compact regions in the centre of
galaxies which emit substantially more radiation in most parts of the
spectrum than would be expected from starlight. From the energy
output in QSO's, together with some guess at their lifetime (about
10^8 years) the mass of the central engine can be estimated as of
order 10^7 solar masses or more (this is consistent with estimates of
the masses of other, related types of AGN). A compact, massive object
of this kind is most likely (on our current understanding of physics)
to be a black hole, and most astronomers would accept this as the
standard assumption. The luminosity ultimately derives from matter
falling into the black hole and gravitational potential energy being
converted to other forms, but the details are unexplained and very
much an active research topic.

------------------------------

Subject: H.07 Are the QSO's really at their redshift distances?
Author: Martin Hardcastle

It's often suggested that QSOs are not at the distances that would be
inferred from their redshifts and from Hubble's law; this would avoid
the enormous powers and necessity for general-relativistic physics in
the standard model. Many arguments of this type are flawed by a lack
of consideration of the other types of galaxies and active galactic
nuclei (AGN): unless it's believed that _no_ galaxy is at its redshift
distance, i.e., that the whole concept of redshift is wrong, then we
know that there are objects very similar to QSOs which _are_ at their
redshift distances. (Cosmological theories that overthrow the whole
idea of redshift and the big bang are beyond the scope of this
discussion, although several have been proposed based on the apparent
spatial association of objects with very different redshifts.)

Another argument favoring QSOs being at their redshift distance comes
from gravitational lensing. Gravitational lenses occur when two
objects are nearly aligned, and the mass of the foreground object
lenses (magnifies and/or distorts) the background object. In every
gravitational lens for which redshifts are known, the galaxy (or
galaxies) acting as the lens has a lower redshift than the galaxy
being lensed.

A recent analysis of data available from the 2-degree field (2dF
survey) also showed no evidence for a connection between galaxies and
QSOs. This analysis is particularly significant because the people
who carried out the analysis spoke to proponents on both sides of the
argument *before* conducting their analysis (Hawkins, Maddox, &
Merrifield 2002, Mon. Not. R. Astron. Soc., vol. 336, p. L13).

More generally, though, like many arguments in science, this one also
has an element of aesthetics. The proponents of the standard model
argue that the physics we know (general relativity, special
relativity, electromagnetism) is sufficient to explain QSOs, and that,
by Occam's razor, no model introducing new physics is necessary. Its
opponents argue either that there are features of QSOs which cannot be
explained by the standard model or that the predictions of the
standard model (and, in particular, its reliance on supermassive black
holes) are so absurd as clearly to require some new physics. A good
deal of bad science has been put forward (on both sides) on sci.astro.
Readers should be aware that the scientific community isn't as
insanely conservative as some posters would have them believe, and
that a number of other possibilities for QSO physics were considered
and rejected when they were first discovered. For example, the
frequent suggestion that the redshifts of QSOs are gravitational does
not work in any simple model. Species having different ionization
potentials ought to exist at different distances from the central
source and thus should have different redshifts, but in fact emission
lines from all species are observed to have the same redshift.

For examples of claims of galaxy-QSO associations, see papers by
Stockton, either of the Burbidges, or Arp. For additional, technical
discussions of why these conclusions are not valid, see papers by
Newman & Terzian; Newman, Terzian, & Haynes; and Hawkins, Maddox, &
Merrifield (2002).

------------------------------

Subject: H.08 What about apparent faster-than-light motions?
Author: Martin Hardcastle

The apparently faster-than-light motions observed in the jets of some
radio-loud quasars have misled a number of people into believing that
the speed of light is not really a limit on velocity and that
astrophysics has provided a disproof of the theory of relativity. In
fact, these motions can be easily understood without any new physics;
you just need trigonometry and the idea of the constancy of the speed of
light.

Consider the situation shown in the diagram below. A blob B of
radio-emitting plasma starts at O and moves with velocity v at some
angle a to our line of sight. At a time t, B has moved across the sky
a distance vt sin a. The light from when it was at O has travelled a
distance ct towards us (c is the speed of light). But the light from
its position at time t only has to travel an additional distance
(ct - vt cos a) to reach us. Thus we measure the time between the two
events as (distance / speed of light) = t(1 - (v/c) cos a). If we
derive an apparent velocity by dividing the (measurable) transverse
motion of the source by the measured time difference, we get

vt sin a v sin a
v(apparent) = ------------------ = ---------------
t(1 - (v/c) cos a) 1 - (v/c) cos a


^ O ^
| |\ |
| | \ |
| | \ vt cos a
| | a \ |
ct | \ |
| | \ |
| | B v
| | ^
| | ct - vt cos a
v | v



\_____I_____/
(Earth, radio telescope)

This apparent velocity can clearly be greater than c if a is small and
v is close to c. There are other independent reasons for believing
that the jets in radio-loud quasars have velocities close to c and are
aligned close to the line of sight, so that this explanation is a
plausible one.

------------------------------

Subject: H.09 What's the Local Group?
Author: Hartmut Frommert ,
Christine Kronberg

This is "our" group of galaxies. It was first recognized by Hubble,
in the time of the first distance determinations and redshift
measurements.

The Local Group contains the Andromeda Galaxy (M31) and its satellites
M32 and M110, as well as the Triangulum galaxy (M33). Other members
(over 30 in all) include our Milky Way Galaxy, the Large and the Small
Magellanic Cloud (LMC and SMC), which have been known before the
invention of the telescope (as was the Andromeda Galaxy), as well as
several smaller galaxies which were discovered more recently. These
galaxies are spread in a volume of nearly 10 million light years
diameter, centered somewhere between the Milky Way and M31.
Membership is not certain for all these galaxies, and there are
possible other candidate members.

Of the Local Group member galaxies, the Milky Way and M31 are by for
the most massive, and therefore dominant members. Each of these two
giant spirals has accumulated a system of satellite galaxies, where

* the system of the Milky Way contains many (nearby) dwarf galaxies,
spread all over the sky, namely Sag DEG, LMC, SMC, and the dwarf
galaxies in Ursa Minor, Draco, Carina, Sextans (dwarf), Sculptor,
Fornax, Leo I and Leo II; and

* the system of the Andromeda galaxy is seen from outside, and thus
grouped around its main galaxy M31 in Andromeda, containing bright
nearby M32 and M110 as well as fainter and more far-out NGC 147 and
185, the very faint systems And I, And II, And III, and, possibly, And
IV.

The third-largest galaxy, the Triangulum spiral M33, may or may not be
an outlying gravitationally bound companion of M31, but has itself
probably the dwarf LGS 3 as a satellite.

The other members cannot be assigned to one of the main subgroups, and
float quite alone in the gravitational field of the giant group
members. The substructures of the group are probably not
stable. Observations and calculations suggest that the group is highly
dynamic and has changed significantly in the past: The galaxies around
the large elliptical Maffei 1 have probably been once part of our
galaxy group.

As this shows, the Local Group is not isolated, but in gravitational
interaction, and member exchange, with the nearest surrounding groups,
notably:

* the Maffei 1 group, which besides the giant elliptical galaxy Maffei
1 also contains smaller Maffei 2, and is associated with nearby IC
342. This group is highly obscured by dark dust near the Milky Way's
equatorial plane.

* the Sculptor Group or South Polar Group (with members situated
around the South Galactic pole), dominated by NGC 253;

* the M81 group; and

* the M83 group.

In the future, interaction between the member galaxies and with the
cosmic neighborhood will continue to change the Local Group. Some
astronomers speculate that the two large spirals, our Milky Way and
the Andromeda Galaxy, may perhaps collide and merge in some distant
future, to form a giant elliptical. In addition, there is evidence
that our nearest big cluster of galaxies, the Virgo Cluster, will
probably stop our cosmological recession away from it, accelerate the
Local Group toward itself so that it will finally fall and merge into
this huge cluster of galaxies.

A table of the currently known Local Group member galaxies is at
URL:http://www.seds.org/messier/more/local.html. A (somewhat
technical) review of the Local Group is at
URL:http://arXiv.org/abs/astro-ph/?0001040.

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2003 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
  #10  
Old February 2nd 06, 02:37 AM posted to sci.astro,sci.answers,news.answers
external usenet poster
 
Posts: n/a
Default [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)


Posting-frequency: semi-monthly (Wednesday)
Archive-name: astronomy/faq/part9
Last-modified: $Date: 2000/08/03 00:23:14 $
Version: $Revision: 4.0 $
URL: http://sciastro.astronomy.net/

------------------------------

Subject: Introduction

sci.astro is a newsgroup devoted to the discussion of the science of
astronomy. As such its content ranges from the Earth to the farthest
reaches of the Universe.

However, certain questions tend to appear fairly regularly. This
document attempts to summarize answers to these questions.

This document is posted on the first and third Wednesdays of each
month to the newsgroup sci.astro. It is available via anonymous ftp
from URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/astronomy/faq/,
and it is on the World Wide Web at
URL:http://sciastro.astronomy.net/ and
URL:http://www.faqs.org/faqs/astronomy/faq/. A partial list of
worldwide mirrors (both ftp and Web) is maintained at
URL:http://sciastro.astronomy.net/mirrors.html. (As a general note,
many other FAQs are also available from
URL:ftp://rtfm.mit.edu/pub/usenet/news.answers/.)

Questions/comments/flames should be directed to the FAQ maintainer,
Joseph Lazio ).

------------------------------

Subject: Table of Contents

[All entries last edited on 1998-02-28, unless otherwise noted.]

I.01 What do we know about the properties of the Universe?
I.02 Why do astronomers favor the Big Bang model of the Universe?
I.03 Where is the center of the Universe?
I.04 What do people mean by an "open," "flat," or "closed" Universe?
I.05 If the Universe is expanding, what about me? or the Earth? or
the Solar System?
I.06 What is inflation?
I.07 How can the Big Bang (or inflation) be right? Doesn't it
violate the idea that nothing can move faster than light?
(Also, can objects expand away from us faster than the speed
of light?)
I.08 If the Universe is only 10 billion years old, how can we see
objects that are now 30 billion light years away? Why isn't
the most distant object we can see only 5 billion light years
away?
I.09 How can the oldest stars in the Universe be older than the
Universe?
I.10 What is the Universe expanding into?
I.11 Are galaxies really moving away from us or is space-time just
expanding?
I.12 How can the Universe be infinite if it was all concentrated into
a point at the Big Bang?
I.13 Why haven't the cosmic microwave background photons outrun the
galaxies in the Big Bang?
I.14 Can the cosmic microwave background be redshifted starlight?
I.15 Why is the sky dark at night? (Olbers' paradox) [2001-10-02]
I.16 What about objects with discordant redshifts?
I.17 Since energy is conserved, where does the energy of redshifted
photons go? [1998-12-03]
I.18 There are different ways to measure distances in cosmology?
[1999-07-06]

This section of the FAQ is largely extracted from Ned Wright's
Cosmology Tutorial,
URL:http://www.astro.ucla.edu/%7Ewright/cosmolog.htm, and was
written jointly by Ned Wright and Joseph Lazio, unless otherwise noted.

------------------------------

Subject: I.01. What do we know about the properties of the Universe?

There are three key facts we know about the properties of the
Universe: galaxies recede, there's a faint microwave glow coming from
all directions in the sky, and the Universe is mostly hydrogen and
helium.

In 1929 Edwin Hubble published a claim that the radial velocities of
galaxies are proportional to their distance. His claim was based on
the measurement of the galaxies' redshifts and estimates of their
distances. The redshift is a measure of how much the wavelength of a
spectral line has been shifted from the value measured in
laboratories; if assumed to occur because of the Doppler effect, the
redshift of a galaxy is then a measure of its radial velocity. His
estimates of the galaxies' distances was based on the brightness of a
particular kind of star (a pulsating star known as a Cepheid).

The constant of proportionality in Hubble's relationship (v = H * d,
where v is a velocity and d is a distance) is known as Hubble's
parameter or Hubble's constant. Hubble's initial estimate was that
the Hubble parameter is 464 km/s/Mpc (in other words, a galaxy 1 Mpc =
3 million light years away would have a velocity of 464 km/s). We
know now that Hubble didn't realize that there are two kinds of
Cepheid stars. Various estimates of the Hubble parameter today are
between 50--100 km/s/Mpc.

Hubble also measured the number of galaxies in different directions
and at different brightness in the sky. He found approximately the
same number of faint galaxies in all directions (though there is a
large excess of bright galaxies in the northern sky). When a
distribution is the same in all directions, it is isotropic. When
Hubble looked for galaxies four times fainter than a particular
brightness, he found approximately 8 times more galaxies than he found
that were brighter than this cutoff. A brightness 4 times smaller
implies a doubled distance. In turn, doubling the distance means one
is looking into a volume that is 8 times larger. This result
indicates that the Universe is close to homogeneous or it has a
uniform density on large scales. (Of course, the Universe is not
really homogeneous and isotropic, because it contains dense regions
like the Earth. However, if you take a large enough box, you will
find about the same number of galaxies in it, no matter where you
place the box. So, it's a reasonable approximation to take the
Universe to be homogeneous and isotropic.) Surveys of very large
regions confirm this tendency toward homogeneity and isotropy on the
scales larger than about 300 million light years.

The case for an isotropic and homogeneous Universe became much
stronger after Penzias & Wilson announced the discovery of the Cosmic
Microwave Background in 1965. They observed an excess brightness at a
wavelength of 7.5 cm, equivalent to the radiation from a blackbody
with a temperature of 3.7+/-1 degrees Kelvin. (The Kelvin temperature
scale has degrees of the same size as the Celsius scale, but it is
referenced at absolute zero, so the freezing point of water is 273.15
K.) A blackbody radiator is an object that absorbs any radiation that
hits it and has a constant temperature. Since then, many astronomers
have measured the intensity of the CMB at different wavelengths.
Currently the best information on the spectrum of the CMB comes from
the FIRAS instrument on the COBE satellite. The COBE data are
consistent with the radiation from a blackbody with T = 2.728 K. (In
effect, we're sitting in an oven with a temperature of 2.728 K.) The
temperature of the CMB is almost the same all over the sky. Over the
distance from which the CMB travels to us, the Universe must be
exceedingly close to homogeneous and isotropic. These observations
have been combined into the so-called Cosmological Principle: The
Universe is *homogeneous* and *isotropic*.

If the Universe is expanding---as the recession of galaxies
suggests---and it is at some temperature today, then in the past
galaxies would have been closer together and the Universe would have
been hotter. If one continues to extrapolate backward in time, one
reaches a time when the temperature would be about that of a star's
interior (millions of degrees; galaxies at this time would have been
so close that they would not retain their form as we see them today).
If the temperature was about that of a star's interior, then fusion
should have been occurring.

The majority of the Universe is hydrogen and helium. Using the
known rate of expansion of the Universe, one can figure out how long
fusion would have been occurred. From that one predicts that,
starting with pure hydrogen, about 25% of it would have been fused to
form deuterium (heavy hydrogen), helium (both helium-4 and helium-3),
and lithium; the bulk of the fusion products would helium-4.
Observations of very old stars and very distant gas show that the
abundance of hydrogen and helium is about 75% to 25%.

------------------------------

Subject: I.02. Why do astronomers favor the Big Bang model of the
Universe?

The fundamental properties of the Universe, summarized above, one can
develop a simple model for the evolution of the Universe. This model
is called the Big Bang.

The essential description of the Big Bang model is that it predicts
the Universe was hotter and denser in the past. For most of the 20th
century, astronomers argued about the best description of the
Universe. Was the BB right? or was another model better? Today, most
astronomers think that the BB is essentially correct, the Universe was
hotter and denser in the past. Why?

When Einstein was working on his theory of gravity, around 1915, he
was horrified to discover that it predicted the Universe should either
be expanding or collapsing. The prevailing scientific view at the
time was that the Universe was static, it always had been and always
would be. He ended up modifying his theory, introducing a long-range
force that cancelled gravity so that his theory would describe a
static Universe.

When Hubble announced that galaxies were receding from us, astronomers
realized quickly that this was consistent with the notion that the
Universe is expanding. If you could imagine "running the clock
backwards" and looking into the past, you would see galaxies getting
closer together. In effect, the Universe would be getting denser.

If the Universe was denser in the past, then it was also hotter. At
some point in the past, the conditions in the Universe would have
resembled the interior of a star. If so, we should expect that
nuclear fusion would occur. Detailed predictions of how much nuclear
fusion would have occurred in the early Universe were first undertaken
by George Gamow and his collaborators. Since then, the calculations
have been refined, but the essential result is still the same. After
nuclear fusion stopped, about 1000 seconds into the Universe's
history, there should be about one Helium-4 atom for every 10 Hydrogen
atoms, one Deuterium atom (heavy hydrogen) for every 10,000 H atoms,
one Helium-3 atom for every 50,000 H atoms, and one Lithium-7 atom for
every 10 billion H atoms. These predicted abundances are in very good
agreement with the observed abundances.

As the Universe expanded and cooled, the radiation in it should have
also lost energy. In 1965 Arlo Penzias and Robert Wilson were annoyed
to discover that no matter what direction they pointed a telescope,
they kept picking up faint glow. Some physicists at Princeton
recognized that this faint glow was exactly what was expected from a
cooling Universe. Since then, the COBE satellite has measured the
temperature of this radiation to be 2.728 +/- 0.002 K.

It is the combination of these excellent agreements between prediction
and observation that lead most astronomers to conclude that the Big
Bang is a good model for describing the Universe.

------------------------------

Subject: I.03. Where is the center of the Universe?

Often when people are told that galaxies are receding from us, they
assume that means we are at the center of the Universe. However,
remember that the Universe is homogeneous and isotropic. No matter
where one is, it looks the same in all directions. Thus, all galaxies
see all other galaxies receding from them. Hubble's relationship is
compatible with a Copernican view of the Universe: Our position is not
a special one.

So where is the center? *There isn't one*. Although apparently
nonsensical, consider the same question about the *surface* of a
sphere (note the *surface*). Where's the center of a sphere's
surface? Of course, there isn't one. One cannot point to any point
on a sphere's surface and say that, here is the center. Similarly,
because the Universe is homogeneous and isotropic, all we can say is
that, in the past, galaxies were closer together. We cannot say that
galaxies started expanding from any particular point.

------------------------------

Subject: I.04. What do people mean by an "open," "flat," or "closed"
Universe?

These different descriptions concern the future of the Universe,
particularly whether it will continue to expand forever. The future
of the Universe hinges upon its density---the denser the Universe is,
the more powerful gravity is. If the Universe is sufficiently dense,
at some point in the (distant) future, the Universe will cease to
expand and begin to contract. This is termed a "closed" Universe. In
this case the Universe is also finite in size, though unbounded. (Its
geometry is, in fact, similar to the *surface* of a sphere. One can
walk an infinite distance on a sphere's surface, yet the surface of a
sphere clearly has a finite area.)

If the Universe is not sufficiently dense, then the expansion will
continue forever. This is termed an "open" Universe. One often hears
that such a Universe is also infinite in spatial extent. This is
possibly true; recent research suggests that it may be possible for
the Universe to have a finite volume, yet expand forever.

One can also imagine a Universe in which gravity and the expansion are
exactly equal. The Universe stops expanding only after an infinite
amount of time. This Universe is also (possibly) infinite in spatial
extent and is termed a "flat" Universe, because the sum of the
interior angles of a triangle sum to 180 degrees---just like in the
plane or "flat" geometry one learns in (US) high school. For an open
Universe, the geometry is negatively curved so that the sum of the
interior angles of a triangle is less than 180 degrees; in a closed
Universe, the geometry is positively curved and the sum of the
interior angles of a triangle is more than 180 degrees.

The critical density that separates an open Universe from a closed
Universe is 1.0E-29 g/cm^3. (This is an average density; there are
clearly places in the Universe more dense than this, e.g., you, the
reader with a density of about 1 g/cm^3, but this density is to be
interpreted as the density if all matter were spread uniformly
throughout the Universe.) Current observational data are able to
account for about 10--30% of this value, suggesting that the Universe
is open. However, motivated by inflationary theory, many theorists
predict that the actual density in the Universe is essentially equal
to the critical density and that observers have not yet found all of
the matter in the Universe.

------------------------------

Subject: I.05. If the Universe is expanding, what about me? or the
Earth? or the Solar System?

You, the reader, are not expanding, even though the Universe in which
you live is. There are two ways to understand this.

The simple way to understand the reason you're not expanding is that
you are held together by electromagnetic forces. These
electromagnetic forces are strong enough to overpower the expansion of
the Universe. So you do not expand. Similarly, the Earth is held
together by a combination of electromagnetic and gravitational forces,
which again are strong enough to overpower the Universe's expansion.
On even larger scales---those of the Solar System, the Milky Way, even
the Local Supercluster of galaxies (also known as the Virgo
Supercluster)---gravity alone is still strong enough hold these
objects together and prevent the expansion. Only on the very largest
scales does gravity become weak enough that the expansion can win
(though, if there's enough gravity in the Universe, the expansion will
eventually be halted).

A second way to understand this is to appreciate the assumption of
homogeneity. A key assumption of the Big Bang is that the Universe is
homogeneous or relatively uniform. Only on large enough scales will
the Universe be sufficiently uniform that the expansion occurs. You,
the reader, are clearly not uniform---inside your body the density is
about that of water, outside is air. Similarly, the Earth and its
surroundings are not of uniform density, nor for the Solar System or
the Milky Way.

This latter way of looking at the expansion of the Universe is similar
to common assumptions in modelling air or water (or other fluids). In
order to describe air flowing over an airplane wing or water flowing
through a pipe, it is generally not necessary to consider air or water
to consist of molecules. Of course, on very small scales, this
assumption breaks down, and one must consider air or water to consist
of molecules. In a similar manner, galaxies are often described as
the "atoms" of the Universe---on small scales, they are important, but
to describe the Universe as a whole, it is not necessary to consider
it as being composed of galaxies.

Also note that the definitions of length and time are not changing in
the standard model. The second is still 9192631770 cycles of a Cesium
atomic clock and the meter is still the distance light travels in
9192631770/299792458 cycles of a Cesium atomic clock.

------------------------------

Subject: I.06. What is inflation?

The "inflationary scenario," developed by Starobinsky and by Guth,
offers a solution to two apparent problems with the Big Bang. These
problems are known as the flatness-oldness problem and the horizon
problem.

The flatness problem has to do with the fact that density of the
Universe appears to be roughly 10% of the critical density (see
previous question). This seems rather fortuitous; why is it so close
to the critical density? We can imagine that the density might be
0.0000001% of the critical value or 100000000% of it. Why is it so
close to 100%?

The horizon problem relates to the smoothness of the CMB. The CMB is
exceedingly smooth (if one corrects for the effects caused by the
Earth and Sun's motions). Two points separated by more than 1 degree
or so have the same temperature to within 0.001%. However, two points
this far apart today would not have been in causal contact at very
early times in the Universe. In other words, the distance separating
them was greater than the distance light could travel in the age of
the Universe. There was no way for two such widely separated points
to communicate and equalize their temperatures.

The inflationary scenario proposes that during a brief period early in
the history of the Universe, the scale size of the Universe expanded
rapidly. The scale factor of the Universe would have grown
exponentially, a(t) = exp(H(t-t0)), where H is the Hubble parameter,
t0 is the time at the start of inflation, and t is the time at the end
of inflation. If the inflationary epoch lasts long enough, the
exponential function gets very large. This makes a(t) very large, and
thus makes the radius of curvature of the Universe very large.

Inflation, thus, solves the flatness problem rather neatly. Our
horizon would be only a very small portion of the whole Universe.
Just like a football field on the Earth's surface can appear flat,
even though the Earth itself is certainly curved, the portion of the
Universe we can see might appear flat, even though the Universe as a
whole would not be.

Inflation also proposes a solution for the horizon problem. If the
rapid expansion occurs for a long enough period of time, two points in
the Universe that were initially quite close together could wind up
very far apart. Thus, one small region that was at a uniform
temperature could have expanded to become the visible Universe we see
today, with its nearly constant temperature CMB.

The onset of inflation might have been caused by a "phase change." A
common example of a phase change (that also produces a large increase
in volume) is the change from liquid water to steam. If one was to
take a heat-resistant, extremely flexible balloon filled with water
and boil the water, the balloon would expand tremendously as the water
changed to steam. In a similar fashion, astronomers and physicists
have proposed various ways in which the cooling of the Universe could
have led to a sudden, rapid expansion.

It is worth noting that the inflationary scenario is not the same as
the Big Bang. The Big Bang predicts that the Universe was hotter and
denser in the past; inflation predicts that as a result of the physics
in the expanding Universe, it suddenly underwent a rapid expansion.
Thus, inflation assumes that the Big Bang theory is correct, but the
Big Bang theory does not require inflation.

------------------------------

Subject: I.07. How can the Big Bang (or inflation) be right? Doesn't
it violate the idea that nothing can move faster than light?
(Also, can objects expand away from us faster than the speed
of light?)

In the Big Bang model the *distance* between galaxies increases, but
the galaxies don't move. Since nothing's moving, there is no
violation of the restriction that nothing can move faster than light.
Hence, it is quite possible that the distance between two objects is
so great that the distance between them expands faster than the speed
of light.

What does it mean for the distance between galaxies to increase
without them moving? Consider two galaxies in a one-dimensional Big
Bang model:
*-|-|-|-*
0 1 2 3 4

There are four distance units between the two galaxies. Over time the
distance between the two galaxies increases:

* - | - | - | - *
0 1 2 3 4

However, they remain in the same position, namely one galaxy remains
at "0" and the other remains at "4." They haven't moved.

(Astronomers typically divide the distance between two galaxies into
two parts, D = a(t)*R. The function a(t) describes how the size of
the Universe increases, while the distance R is independent of any
changes in the size of the Universe. The coordinates based on R are
called "co-moving coordinates.")

------------------------------

Subject: I.08. If the Universe is only 10 billion years old, how can
we see objects that are now 30 billion light years away? Why
isn't the most distant object we can see only 5 billion light
years away?

When talking about the distance of a moving object, we mean the
spatial separation NOW, with the positions of both objects specified
at the current time. In an expanding Universe this distance NOW is
larger than the speed of light times the light travel time due to the
increase of separations between objects as the Universe expands. This
is not due to any change in the units of space and time, but just
caused by things being farther apart now than they used to be.

What is the distance NOW to the most distant thing we can see? Let's
take the age of the Universe to be 10 billion years. In that time
light travels 10 billion light years, and some people stop here. But
the distance has grown since the light traveled. Half way along the
light's journey was 5 billion years ago. For the critical density
case (i.e., flat Universe), the scale factor for the Universe is
proportional to the 2/3 power of the time since the Big Bang, so the
Universe has grown by a factor of 22/3 = 1.59 since the midpoint of
the light's trip. But the size of the Universe changes continuously,
so we should divide the light's trip into short intervals. First take
two intervals: 5 billion years at an average time 7.5 billion years
after the Big Bang, which gives 5 billion light years that have grown
by a factor of 1/(0.75)2/3 = 1.21, plus another 5 billion light years
at an average time 2.5 billion years after the Big Bang, which has
grown by a factor of 42/3 = 2.52. Thus with 1 interval we get 1.59*10
= 15.9 billion light years, while with two intervals we get
5*(1.21+2.52) = 18.7 billion light years. With 8192 intervals we get
29.3 billion light years. In the limit of very many time intervals we
get 30 billion light years.

If the Universe does not have the critical density then the distance
is different, and for the low densities that are more likely the
distance NOW to the most distant object we can see is bigger than 3
times the speed of light times the age of the Universe.

------------------------------

Subject: I.09. How can the oldest stars in the Universe be older than
the Universe?

Obviously, the Universe has to be older than the oldest stars in
it. So this question basically asks, which estimate is wrong:

* The age of the Universe?
* The age of the oldest stars? or
* Both?

The age of the Universe is determined from its expansion rate: the
Hubble constant, which is the ratio of the radial velocity of a
distant galaxy to its distance. The radial velocity is easy to
measure, but the distances are not. Thus there is currently a 15%
uncertainty in the Hubble constant.

Determining the age of the oldest stars requires a knowledge of their
luminosity, which depends on their distance. This leads to a 25%
uncertainty in the ages of the oldest stars due to the difficulty in
determining distances.

Thus the discrepancy between the age of the oldest things in the
Universe and the age inferred from the expansion rate is within the
current margin of error.

------------------------------

Subject: I.10. What is the Universe expanding into?

This question is based on the ever popular misconception that the
Universe is some curved object embedded in a higher dimensional space,
and that the Universe is expanding into this space. This
misconception is probably fostered by the balloon analogy that shows a
2-D spherical model of the Universe expanding in a 3-D space.

While it is possible to think of the Universe this way, it is not
necessary, and---more importantly---there is nothing whatsoever that
we have measured or can measure that will show us anything about the
larger space. Everything that we measure is within the Universe, and
we see no edge or boundary or center of expansion. Thus the Universe
is not expanding into anything that we can see, and this is not a
profitable thing to think about. Just as Dali's Crucifixion is just a
2-D picture of a 3-D object that represents the surface of a 4-D cube,
remember that the balloon analogy is just a 2-D picture of a 3-D
situation that is supposed to help you think about a curved 3-D space,
but it does not mean that there is really a 4-D space that the
Universe is expanding into.

------------------------------

Subject: I.11. Are galaxies really moving away from us or is
space-time just expanding?

This depends on how you measure things, or your choice of coordinates.
In one view, the spatial positions of galaxies are changing, and this
causes the redshift. In another view, the galaxies are at fixed
coordinates, but the distance between fixed points increases with
time, and this causes the redshift. General relativity explains how
to transform from one view to the other, and the observable effects
like the redshift are the same in both views.

------------------------------

Subject: I.12. How can the Universe be infinite if it was all
concentrated into a point at the Big Bang?

Only the *observable* Universe was concentrated into a point at the
time of the Big Bang, not the entire Universe. The distinction
between the whole Universe and the part of it that we can see is
important.

We can see out into the Universe roughly a distance c*t, where c is
the speed of light and t is the age of the Universe. Clearly, as t
becomes smaller and smaller (going backward in time toward the Big
Bang), the distance to which we can see becomes smaller and smaller.
This places no constraint on the size of the entire Universe, though.

------------------------------

Subject: I.13. Why haven't the CMB photons outrun the galaxies in
the Big Bang?

Once again, this question assumes that the Big Bang was an explosion
from a central point. The Big Bang was not an explosion from a single
point, with a center and an edge. The Big Bang occurred everywhere.
Hence, no matter in what direction we look, we will eventually see to
the point where the CMB photons were being formed. (The CMB photons
didn't actually form in the Big Bang, they formed later when the
Universe had cooled enough for atoms to form.)

------------------------------

Subject: I.14. Can the CMB be redshifted starlight?

No! The CMB radiation is such a perfect fit to a blackbody that it
cannot be made by stars. There are two reasons for this.

First, stars themselves are at best only pretty good blackbodies, and
the usual absorption lines and band edges make them pretty bad
blackbodies. In order for a star to radiate at all, the outer layers
of the star must have a temperature gradient, with the outermost
layers of the star being the coolest and the temperature increasing
with depth inside the star. Because of this temperature gradient, the
light we see is a mixture of radiation from the hotter lower levels
(blue) and the cooler outer levels (red). When blackbodies with these
temperatures are mixed, the result is close to, but not exactly equal
to a blackbody. The absorption lines in a star's spectrum further
distort its spectrum from a blackbody.

One might imagine that by having stars visible from different
redshifts that the absorption lines could become smoothed out.
However, these stars will be, in general, different temperature
blackbodies, and we've already seen from above that it is the mixing
of different apparent temperatures that causes the deviation from a
blackbody. Hence more mixing will make things worse.

How does the Big Bang produce a nearly perfect blackbody CMB? In the
Big Bang model there are no temperature gradients because the Universe
is homogeneous. While the temperature varies with time, this
variation is exactly canceled by the redshift. The apparent
temperature of radiation from redshift z is given by T(z)/(1+z), which
is equal to the CMB temperature T(CMB) for all redshifts that
contribute to the CMB.

------------------------------

Subject: I.15. Why is the sky dark at night? (Olbers' paradox)

If the Universe were infinitely old, infinite in extent, and filled
with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun. This is known as Olbers' Paradox after Heinrich
Wilhelm Olbers (1757--1840) who wrote about it in 1823--1826 (though
it had been discussed earlier). A common suggestion for resolving the
paradox is to consider interstellar dust, which blocks light by
absorping it. However, absorption by interstellar dust does not
circumvent this paradox, as dust reradiates whatever radiation it
absorbs within a few minutes, which is much less than the age of the
Universe.

The resolution of Olbers' paradox comes by recognizing that the
Universe is not infinitely old and it is expanding. The latter effect
reduces the accumulated energy radiated by distant stars. Either one
of these effects acting alone would solve Olbers' Paradox, but they
both act at once.

------------------------------

Subject: I.16. What about objects with discordant redshifts?

A common objection to the Big Bang model is that redshifts do not
measure distance. The logic is that if redshifts do not measure
distance, then maybe the Hubble relation between velocity and distance
is all wrong. If it is wrong, then one of the three pillars of
observational evidence for the Big Bang model collapses.

One way to show that redshifts do not measure distance is to find two
(or more) objects that are close together on the sky, but with vastly
different redshifts. One immediately obvious problem with this
approach is that in a large Universe, it is inevitable that some very
distant objects will just happen to lie behind some closer objects.

A way around this problem is to look for "connections"---for instance,
a bridge of gas---between two objects with different redshifts.
Another approach is to look for a statistical "connection"---if high
redshift objects tend to cluster about low redshift objects that might
suggest a connection. Various astronomers have claimed to find one or
the other kind of connection. However, their statistical analyses
have been shown to be flawed, or the nature of the apparent "bridge"
or "connection" has been widely disputed.

At this time, there's no unambiguous illustration of a "connection" of
any kind between objects of much different redshifts.

------------------------------

Subject: I.17 Since energy is conserved, where does the energy of
redshifted photons go?
Author: Peter Newman

The energy of a photon is given by E = hc/lambda, where h is Planck's
constant, c is the speed of light, and lambda is its wavelength. The
cosmological redshift indicates that the wavelength of a photon
increases as it travels over cosmological distances in the Universe.
Thus, its energy decreases.

One of the basic conservation laws is that energy is conserved. The
decrease in the energy of redshifted photons seems to violate that
law. However, this argument is flawed. Specifically, there is a flaw
in assuming Newtonian conservation laws in general relativistic
situations. To quote Peebles (_Principles of Physical Cosmology_,
1995, p. 139):

Where does the lost energy go? ... The resolution of this
apparent paradox is that while energy conservation is a good
local concept ... and can be defined more generally in the
special case of an isolated system in asymptotically flat space,
there is not a general global energy conservation law in general
relativity theory.

In other words, on small scales, say the size of a cluster of
galaxies, the notion of energy conservation is a good one. However,
on the size scales of the Universe, one can no longer define a
quantity E_total, much less a quantity that is conserved.

------------------------------

Subject: There are different ways to measure distances in cosmology?
Author: Joseph Lazio

Yes!

There are at least three ways one can measure the distance to objects:

* parallax;
* angular size; or
* brightness.

The parallaxes of cosmologically-distant objects are so small that
they will remain impossible to measure in the foreseeable future (with
the possible exception of some gravitationally-lensed quasars).

Suppose there exists an object (or even better a class of objects)
whose intrinsic length is known. That is, the object can be treated
as a ruler because its length known to be exactly L (e.g., 1 m, 100
light years, 10 kiloparsecs, etc.). When we look at it, it has an
*angular diameter* of H. Using basic geometry, we can then derive its
distance to be
L
D_L = ---
H

Suppose there exists an object (or even better a class of objects)
whose intrinsic brightness is known. That is, the object can be
treated as a lightbulb because the amount of energy it is radiating is
known to be F (e.g., 100 Watts, 1 solar luminosity, etc.). When we
look at it, we measure an *apparent* flux of f. The distance to the
object is then
F
D_F =sqrt( ------ )
4*pi*f

In general, D_L *is not equal to* D_F!

For more details, see "Distance Measures in Cosmology" by David Hogg,
URL:http://xxx.lanl.gov/abs/astro-ph/9905116, and references within.
Plots showing how to convert redshifts to various distance measures
are included in this paper, and the author will provide C code to do
the conversion as well. Even more details are provided in "A General
and Practical Method for Calculating Cosmological Distances" by Kayser
et al., URL:http://xxx.lanl.gov/abs/astro-ph/9603028 or URL:
http://multivac.jb.man.ac.uk:8000/helbig/Research/Publications/info/angsiz.html.
Fortran code for calculating these distances is provided by the second
set of authors.

------------------------------

Subject: Copyright

This document, as a collection, is Copyright 1995--2000 by T. Joseph
W. Lazio ). The individual articles are copyright
by the individual authors listed. All rights are reserved.
Permission to use, copy and distribute this unmodified document by any
means and for any purpose EXCEPT PROFIT PURPOSES is hereby granted,
provided that both the above Copyright notice and this permission
notice appear in all copies of the FAQ itself. Reproducing this FAQ
by any means, included, but not limited to, printing, copying existing
prints, publishing by electronic or other means, implies full
agreement to the above non-profit-use clause, unless upon prior
written permission of the authors.

This FAQ is provided by the authors "as is," with all its faults.
Any express or implied warranties, including, but not limited to, any
implied warranties of merchantability, accuracy, or fitness for any
particular purpose, are disclaimed. If you use the information in
this document, in any way, you do so at your own risk.
 




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