|
|
|||||||
|
|
Thread Tools | Display Modes |
|
#1
February 1st 06, 06:28 AM
posted to sci.astro,sci.astro.seti,sci.answers,news.answers
|
|||
|
|||
[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. 
|
|
#3
February 2nd 06, 02:35 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[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 dustresponsible 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
February 2nd 06, 02:36 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[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 yearevery 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.
|
|
#5
February 2nd 06, 02:36 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[sci.astro] Astrophysics (Astronomy Frequently Asked Questions) (4/9)
Last-modified: $Date: 2002/05/04 00:00:01 $ Version: $Revision: 4.2 $ URL: http://sciastro.astronomy.net/ Posting-frequency: semi-monthly (Wednesday) Archive-name: astronomy/faq/part4 ------------------------------ 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
February 2nd 06, 02:36 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[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
February 2nd 06, 02:37 AM
posted to sci.astro,sci.astro.seti,sci.answers,news.answers
|
|||
|
|||
|
[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
February 2nd 06, 02:37 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[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.
|
|
#9
February 2nd 06, 02:37 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9)
Last-modified: $Date: 2003/04/27 00:12:18 $ Version: $Revision: 4.3 $ URL: http://sciastro.astronomy.net/ Posting-frequency: semi-monthly (Wednesday) 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
February 2nd 06, 02:37 AM
posted to sci.astro,sci.answers,news.answers
|
|||
|
|||
|
[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.
|
| Thread Tools | |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Thread Starter | Forum | Replies | Last Post |
| question about how many people read sci groups | Kurt stocklmeir | Astronomy Misc | 3 | April 15th 04 05:50 PM |
| Read Shawn's Postings from Trollsareus | AcuraEL2001 | Amateur Astronomy | 4 | February 19th 04 02:22 AM |
| Something to read | Aladar | Astronomy Misc | 2 | December 7th 03 02:52 PM |
Linear Mode
