Identifying galaxies within 1 MPc

In a robot story of Asimov, the first Jump was done over a distance of
0,3 MPc. The ship had distance indicator showing distance from Earth
between 0 to 1 MPc.

Assume that you know you have Jumped to somewhere less than 1 MPc
distant, but do NOT have any indicator how far or in which direction.

How do you find your way within the local group of galaxies?

There are 2 giant spirals - Milky Way and Andromeda.

If you are somewhere within the disc of Andromeda, which clues would
you have that you are not in a distant part of the disc of our own
Milky Way?

A difference between Milky Way and Andromeda is said to be fewer young
stars in Andromeda - ratio of young stars to old is about 1/5 to 1/10
that of Milky Way.

Now looking around deep sky...

Milky Way should be a naked eye object in Andromeda, but even dimmer
than Andromeda from Milky Way (which is not bright to begin with).

Andromeda is at Milky Way galactic latitude of about 20. Andromeda is
itself oblique to line of sight... what is the angle between Andromeda
disc and direction to Milky Way?

Then there are the satellites. Where are the satellites of Andromeda
looking from Andromeda, and how bright are they?

And where is Triangulum? It is closer to Andromeda than Milky Way (how
close?) - where is it looking from Andromeda, and how bright?

"c" == chornedsnorkack writes:

c In a robot story of Asimov, the first Jump was done over a distance
c of 0,3 MPc. The ship had distance indicator showing distance from
c Earth between 0 to 1 MPc.

c Assume that you know you have Jumped to somewhere less than 1 MPc
c distant, but do NOT have any indicator how far or in which
c direction.

c How do you find your way within the local group of galaxies?

Ill-posed question. What do I have with me? 10-m optical telescope
and 100-m radio telescope? How quickly do I want to locate my position?


Quickest thing to do might be to locate the M87, 3C 84 (the center
galaxy in the Perseus cluster), and a couple of other nearby
distinctive sources to use as navigational aids.


c There are 2 giant spirals - Milky Way and Andromeda.

c If you are somewhere within the disc of Andromeda, which clues
c would you have that you are not in a distant part of the disc of
c our own Milky Way?

c A difference between Milky Way and Andromeda is said to be fewer
c young stars in Andromeda - ratio of young stars to old is about 1/5
c to 1/10 that of Milky Way.

Yes, but finding stellar ages is a laborious process. You'd have to
obtain the spectra of numerous stars, then compare them with
isochrones (tracks of constant age) within an HR diagram. Unless one
had an array of telescopes, fast computers, and an army of technicians
at one's disposal, this could take a while.


c Now looking around deep sky...

c Milky Way should be a naked eye object in Andromeda, but even
c dimmer than Andromeda from Milky Way (which is not bright to begin
c with).

Easily viewed with even a modest visible light telescope though.

c Andromeda is at Milky Way galactic latitude of about 20. Andromeda
c is itself oblique to line of sight... what is the angle between
c Andromeda disc and direction to Milky Way?

It's thought to be about 75 degrees.

c Then there are the satellites. Where are the satellites of
c Andromeda looking from Andromeda, and how bright are they?

c And where is Triangulum? It is closer to Andromeda than Milky Way
c (how close?) - where is it looking from Andromeda, and how bright?

Have you seen URL: http://www.seds.org/messier/more/local.html and
looked at the links off it?

An alternate manner of position finding might be to start with a table
of positions of major galaxies in the Local Group (Andromeda, Milky
Way, Triangulum, probably M32 and the Large Magellanic Cloud). After
the Jump, do a quick survey of the sky to see if one can locate these
galaxies, then see if one can correlate a possible position within the
Local Group with the observed distribution of galaxies.

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Joseph Lazio kirjutas:
"c" == chornedsnorkack writes:

c In a robot story of Asimov, the first Jump was done over a distance
c of 0,3 MPc. The ship had distance indicator showing distance from
c Earth between 0 to 1 MPc.

c Assume that you know you have Jumped to somewhere less than 1 MPc
c distant, but do NOT have any indicator how far or in which
c direction.

c How do you find your way within the local group of galaxies?

Ill-posed question. What do I have with me? 10-m optical telescope
and 100-m radio telescope? How quickly do I want to locate my position?


Quickest thing to do might be to locate the M87, 3C 84 (the center
galaxy in the Perseus cluster), and a couple of other nearby
distinctive sources to use as navigational aids.


c There are 2 giant spirals - Milky Way and Andromeda.

c If you are somewhere within the disc of Andromeda, which clues
c would you have that you are not in a distant part of the disc of
c our own Milky Way?

c A difference between Milky Way and Andromeda is said to be fewer
c young stars in Andromeda - ratio of young stars to old is about 1/5
c to 1/10 that of Milky Way.

Yes, but finding stellar ages is a laborious process. You'd have to
obtain the spectra of numerous stars,

Not necessarily. In young spiral galaxies, most light comes from young
heavy stars on main sequence, which are blue and white. Young light
stars also exist, but the young heavy stars are much brighter and
outshine them. Whereas in Milky Way globular clusters and in elliptical
galaxies, young massive stars do not exist, so those bodies consist of
individually dim old main sequence light stars, which are yellow or
red, and a few brighter old red giants which also are red.

Elliptical galaxies are said to be visibly yellow in contrast to
bluish-white spirals. Is Andromeda also perceptibly yellower than Milky
Way?

then compare them with
isochrones (tracks of constant age) within an HR diagram. Unless one
had an array of telescopes, fast computers, and an army of technicians
at one's disposal, this could take a while.


c Now looking around deep sky...

c Milky Way should be a naked eye object in Andromeda, but even
c dimmer than Andromeda from Milky Way (which is not bright to begin
c with).

Easily viewed with even a modest visible light telescope though.

c Andromeda is at Milky Way galactic latitude of about 20. Andromeda
c is itself oblique to line of sight... what is the angle between
c Andromeda disc and direction to Milky Way?

It's thought to be about 75 degrees.

c Then there are the satellites. Where are the satellites of
c Andromeda looking from Andromeda, and how bright are they?

c And where is Triangulum? It is closer to Andromeda than Milky Way
c (how close?) - where is it looking from Andromeda, and how bright?

Have you seen URL: http://www.seds.org/messier/more/local.html and
looked at the links off it?

An alternate manner of position finding might be to start with a table
of positions of major galaxies in the Local Group (Andromeda, Milky
Way, Triangulum, probably M32 and the Large Magellanic Cloud). After
the Jump, do a quick survey of the sky to see if one can locate these
galaxies, then see if one can correlate a possible position within the
Local Group with the observed distribution of galaxies.

Or rather - survey the sky to locate what nebulae exist in the sky,
then see if any of them can be identified as known major galaxies.

Not necessarily. In young spiral galaxies, most light comes from young
heavy stars on main sequence, which are blue and white. Young light
stars also exist, but the young heavy stars are much brighter and
outshine them. Whereas in Milky Way globular clusters and in elliptical
galaxies, young massive stars do not exist, so those bodies consist of
individually dim old main sequence light stars, which are yellow or
red, and a few brighter old red giants which also are red.

Elliptical galaxies are said to be visibly yellow in contrast to
bluish-white spirals. Is Andromeda also perceptibly yellower than Milky
Way?

The principle is valid, yes, but I don't think M31 is significantly
different in color from the Milky Way. I suspect you could measure a
difference, but as I said in my earlier post, there doesn't seem to be
much point, since you can just look for M31's two prominent satellite
galaxies around the other galaxy. If they're there, you're in the Milky
Way; if they're not, you're in M31.

Incidentally, I have an essay on finding your way through the Milky Way
at

http://astro.isi.edu/games/dimension.html

which may be of some relevance.

--
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The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
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Brian Tung kirjutas:
Not necessarily. In young spiral galaxies, most light comes from young
heavy stars on main sequence, which are blue and white. Young light
stars also exist, but the young heavy stars are much brighter and
outshine them. Whereas in Milky Way globular clusters and in elliptical
galaxies, young massive stars do not exist, so those bodies consist of
individually dim old main sequence light stars, which are yellow or
red, and a few brighter old red giants which also are red.

Elliptical galaxies are said to be visibly yellow in contrast to
bluish-white spirals. Is Andromeda also perceptibly yellower than Milky
Way?

The principle is valid, yes, but I don't think M31 is significantly
different in color from the Milky Way. I suspect you could measure a
difference, but as I said in my earlier post, there doesn't seem to be
much point, since you can just look for M31's two prominent satellite
galaxies around the other galaxy.

If you find and manage to identify "the other galaxy".

The two brightest galaxies seen from Milky Way are the Magellanic
Clouds. The Large Magellanic Cloud is by far brighter than Andromeda,
and the Small Magellanic Cloud is also significantly brighter.

Large Magellanic Cloud is also intrinsically the brightest galaxy in
Local Group after the three of Andromeda, Milky Way and Triangulum. It
is intrinsically significantly brighter than either M32 or M110. So,
looking from Andromeda, the Large Magellanic Cloud should be far more
prominent than M32 or M110 looking from Milky Way.

If they're there, you're in the Milky
Way; if they're not, you're in M31.

Incidentally, I have an essay on finding your way through the Milky Way
at

http://astro.isi.edu/games/dimension.html

which may be of some relevance.

And several points where I would do things differently.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

If you find and manage to identify "the other galaxy".

If you're in either M31 or the Milky Way, an amateur astronomer would
find it trivial to locate the other galaxy, and identify it on the basis
of the whether M32 and M110 are present around the other galaxy.

The two brightest galaxies seen from Milky Way are the Magellanic
Clouds. The Large Magellanic Cloud is by far brighter than Andromeda,
and the Small Magellanic Cloud is also significantly brighter.

Large Magellanic Cloud is also intrinsically the brightest galaxy in
Local Group after the three of Andromeda, Milky Way and Triangulum. It
is intrinsically significantly brighter than either M32 or M110. So,
looking from Andromeda, the Large Magellanic Cloud should be far more
prominent than M32 or M110 looking from Milky Way.

All true, but irrelevant. Take a look at photos of M31. Its satellite
galaxies don't look anything like the Magellanic Clouds. The fact that
they're dimmer than the Magellanic Clouds doesn't make it any harder to
identify them. Any amateur astronomer would have little difficulty
making the distinction.

Incidentally, I have an essay on finding your way through the Milky Way
at

http://astro.isi.edu/games/dimension.html

which may be of some relevance.

And several points where I would do things differently.

I never said that it was the only way. I also never said it was the
quickest way. If you don't like it, you are welcome to write your own
essay.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

On 14 Dec 2006 09:29:56 -0800, wrote:

The two brightest galaxies seen from Milky Way are the Magellanic
Clouds. The Large Magellanic Cloud is by far brighter than Andromeda,
and the Small Magellanic Cloud is also significantly brighter.

I suspect there are places in our galaxy where one or both of these
might be very hard to see.

On 14 Dec 2006 09:29:56 -0800, wrote:


Brian Tung kirjutas:
Not necessarily. In young spiral galaxies, most light comes from young
heavy stars on main sequence, which are blue and white. Young light
stars also exist, but the young heavy stars are much brighter and
outshine them. Whereas in Milky Way globular clusters and in elliptical
galaxies, young massive stars do not exist, so those bodies consist of
individually dim old main sequence light stars, which are yellow or
red, and a few brighter old red giants which also are red.

Elliptical galaxies are said to be visibly yellow in contrast to
bluish-white spirals. Is Andromeda also perceptibly yellower than Milky
Way?

The principle is valid, yes, but I don't think M31 is significantly
different in color from the Milky Way. I suspect you could measure a
difference, but as I said in my earlier post, there doesn't seem to be
much point, since you can just look for M31's two prominent satellite
galaxies around the other galaxy.

If you find and manage to identify "the other galaxy".

The two brightest galaxies seen from Milky Way are the Magellanic
Clouds. The Large Magellanic Cloud is by far brighter than Andromeda,
and the Small Magellanic Cloud is also significantly brighter.

Large Magellanic Cloud is also intrinsically the brightest galaxy in
Local Group after the three of Andromeda, Milky Way and Triangulum. It
is intrinsically significantly brighter than either M32 or M110. So,
looking from Andromeda, the Large Magellanic Cloud should be far more
prominent than M32 or M110 looking from Milky Way.


Yes, but prominently irregular. Nobody who has any business attempting
to navigate an intergalatic spacecraft, is going to be confused by the
difference between a spiral, elliptical, and irregular galaxies. The
Milky Way, Andromeda, and Triangulum, are spirals. Andromeda's two
prominent satellites, are dwarf ellipticals. The Milky way's prominent
satellites, are dwarf irregulars. Triangulum, doesn't have prominent
satellites.

If you're in the local group, if you've got a good pair of binoculars,
and if your line of sight isn't blocked, that should be enough to point
the way home.


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*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
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Brian Tung wrote:
Not necessarily. In young spiral galaxies, most light comes from young
heavy stars on main sequence, which are blue and white. Young light
stars also exist, but the young heavy stars are much brighter and
outshine them. Whereas in Milky Way globular clusters and in elliptical
galaxies, young massive stars do not exist, so those bodies consist of
individually dim old main sequence light stars, which are yellow or
red, and a few brighter old red giants which also are red.

Elliptical galaxies are said to be visibly yellow in contrast to
bluish-white spirals. Is Andromeda also perceptibly yellower than Milky
Way?

The principle is valid, yes, but I don't think M31 is significantly
different in color from the Milky Way. I suspect you could measure a
difference, but as I said in my earlier post, there doesn't seem to be
much point, since you can just look for M31's two prominent satellite
galaxies around the other galaxy. If they're there, you're in the Milky
Way; if they're not, you're in M31.

Incidentally, I have an essay on finding your way through the Milky Way
at

http://astro.isi.edu/games/dimension.html

which may be of some relevance.


Another reference which may help in orientation is *The Astronomical
Companion* by Guy Ottewell. It's a little out of date but the
progressive zoom-out charts are very informative--intra galactic or
inter.
http://www.universalworkshop.com
or maybe Amazon or your local library, perhaps.

Brian Tung wrote:
Incidentally, I have an essay on finding your way through the Milky Way
at

http://astro.isi.edu/games/dimension.html

which may be of some relevance.

Very interesting. It seems to be relevant to the current sci-fi series
Battlestar Galactica who are trying to find the Earth too, from some
other point in the galaxy.

Yousuf Khan