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.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

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.

You don't even really need to know that. Just look around the other
spiral for M32 and M110; if you don't see them, you must be in M31, the
Andromeda Galaxy.

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?

Also in the neighborhood of 20 degrees, since it generally appears about
three times as long as it is wide.

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

The Triangulum Galaxy (assuming you mean M33), is about 3 million
light-years from us, and M31 is about 2.5 million light-years from us.
Both distances have significant error bars on them, but assuming they
are essentially correct, their angular separation of 15 degrees yields a
distance between them of about 750,000 light-years (230,000 parsecs).
From M31, it would be about 2.5 magnitudes brighter than it is here
(where it has magnitude 5.7), so it would be magnitude 3.2 or so.

However, it would have that relatively bright magnitude mostly by virtue
of its covering a large amount of sky; magnitudes of extended sources
like nearby galaxies often convey a misleading impression of how easy
they are to see because they're computed on the basis of all the light
coming from the object. M33 can be devilishly difficult to see even
from suburban skies because its magnitude 5.7 light is spread out over a
comparatively large area. It can be entirely invisible, even through a
good-sized telescope, even when magnitude 5.7 *stars* are easily seen.
Its brightness per unit area--its so-called "surface brightness"--is a
somewhat better measure of how easily it would be seen, and that does
not go up as you get closer to the object.

As a result: M33 would be easily seen from space (in M31) and from a
dark observing site; however, it would be difficult to make out from a
hypothetical planet in M31 experiencing substantial light pollution.

--
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

In message . com,
writes
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?

I hear that Andromeda has significantly more globular clusters than the
Milky Way. It's satellite galaxies are different - the most prominent
satellites of the Milky Way are the Magellanic Clouds, which are
irregular galaxies, whilst, IIRC, Andromeda has a couple of dwarf
ellipticals (M32, M110).

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?


--
Stewart Robert Hinsley

wrote:

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?
Two questions immediately popped into my mind:
A: how does one know one is in the same time period as one left?
and
B: assuming - which I cannot prove - that a FTL hyperspace jump leaves one
in the same time period, what differences in stellar and galactic motion,
etc, would result?

Eg, I take a jump into the Large Magellanic Cloud, same time period. The
last time I saw the Large Magellanic Cloud it was as it was 160 thousand
years ago, and it's moved somewhat since then.

I assume one would use the most powerful radio, ultraviolet and X-Ray
emitters to triangulate oneself - since I doubt that quasars are going to
look any different from the Andromeda galaxy than they do from the Milky
Way - but the angles will be different.

Then you can think of the pulsars. But they are more problematical because
landing up in a galaxy 2.5 million light years away, you are not going to
be able to see some of the more prominent Milky Way pulsars, because of
their relative youth.

Ditto for the satellite galaxies. You're going to need damn good
spectrographic records of them, because I can't see any way of recognizing
them otherwise.

Just my 0.02c - heavily inflated of course!

Wesley Parish

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.

--
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

Brian Tung kirjutas:
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.

You don't even really need to know that. Just look around the other
spiral for M32 and M110; if you don't see them, you must be in M31, the
Andromeda Galaxy.


Where IS M32?

It is between Milky Way and Andromeda. Does anyone have a clue of how
far M32 is from Andromeda?

Looking from Andromeda, Milky Way should be behind and seen through
M32... what does it look like?

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?

Also in the neighborhood of 20 degrees, since it generally appears about
three times as long as it is wide.

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

The Triangulum Galaxy (assuming you mean M33), is about 3 million
light-years from us, and M31 is about 2.5 million light-years from us.
Both distances have significant error bars on them,

Exactly. The distance from Andromeda to M33 includes the difference
between two similar values that are both rather imprecise.

but assuming they
are essentially correct, their angular separation of 15 degrees yields a
distance between them of about 750,000 light-years (230,000 parsecs).
From M31, it would be about 2.5 magnitudes brighter than it is here
(where it has magnitude 5.7), so it would be magnitude 3.2 or so.

However, it would have that relatively bright magnitude mostly by virtue
of its covering a large amount of sky; magnitudes of extended sources
like nearby galaxies often convey a misleading impression of how easy
they are to see because they're computed on the basis of all the light
coming from the object. M33 can be devilishly difficult to see even
from suburban skies because its magnitude 5.7 light is spread out over a
comparatively large area. It can be entirely invisible, even through a
good-sized telescope, even when magnitude 5.7 *stars* are easily seen.
Its brightness per unit area--its so-called "surface brightness"--is a
somewhat better measure of how easily it would be seen, and that does
not go up as you get closer to the object.

Does anyone know where the plane of M33 is? Milky Way is far from that
plane, but how far?

Are the Milky Way and Andromeda visible exclusively because we happen
to be inside the plane of one and near the plane of the other? And
would they vanish from sight if viewed from higher galactic latitudes?

As a result: M33 would be easily seen from space (in M31) and from a
dark observing site; however, it would be difficult to make out from a
hypothetical planet in M31 experiencing substantial light pollution.

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

Where IS M32?

It is between Milky Way and Andromeda. Does anyone have a clue of how
far M32 is from Andromeda?

It's unclear that it lies between us and M31. We don't know its
distance precisely enough. It's possible that kinematics would tell us
where it is in relation to M31, but I'm not aware of any reliable
results.

Looking from Andromeda, Milky Way should be behind and seen through
M32... what does it look like?

No, the Milky Way would not be seen through M32, unless you are in the
correct outskirts of M31--the part of M31 that is behind M32 as seen
from our vantage point. From all other vantage points in M31 (most of
them, in other words), the Milky Way might seem close to M32 in the sky,
but it would not appear actually behind it.

Does anyone know where the plane of M33 is? Milky Way is far from that
plane, but how far?

M33 is about twice as long as it is wide, so a guess would be that the
Milky Way is 30 degrees off M33's galactic plane.

Are the Milky Way and Andromeda visible exclusively because we happen
to be inside the plane of one and near the plane of the other? And
would they vanish from sight if viewed from higher galactic latitudes?

No, if anything, they would appear easier to see. Google "zone of
avoidance" and galaxy.

--
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