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External
Since: Sep 08, 2005
Posts: 24
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(Msg. 1) Posted: Wed Dec 13, 2006 9:08 am
Post subject: Identifying galaxies within 1 MPc
Archived from groups: rec>arts>sf>science, others (more info?)
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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? |
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External
Since: Dec 13, 2003
Posts: 5
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(Msg. 2) Posted: Wed Dec 13, 2006 6:51 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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In message <1166029695.490201.85990 DeleteThis @t46g2000cwa.googlegroups.com>,
chornedsnorkack DeleteThis @hushmail.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 |
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External
Since: Sep 08, 2005
Posts: 24
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(Msg. 3) Posted: Thu Dec 14, 2006 9:05 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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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.
> >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Dec 27, 2005
Posts: 40
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(Msg. 4) Posted: Thu Dec 14, 2006 12:09 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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On 14 Dec 2006 09:29:56 -0800, chornedsnorkack RemoveThis @hushmail.com 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. >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Mar 23, 2004
Posts: 24
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(Msg. 5) Posted: Fri Dec 15, 2006 2:00 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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chornedsnorkack.RemoveThis@hushmail.com 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 >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Sep 08, 2005
Posts: 24
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(Msg. 6) Posted: Fri Dec 15, 2006 9:02 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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Brian Tung kirjutas:
> > 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 has been said that M32 should be nearer because its spectra does not
show absorption in Andromeda. But this does not tell us how far from
Andromeda.
> 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.
>
Do we know the direction? In the other words, do we have a clue of
which edge is the near edge? Same question about Andromeda.
> > 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.
>
Ah yes, it is additional complucation.
For example, M33 is about twice as long as it is wide - and pretty low
surface brightness even though total brightness is 5,7.
How would M33 look from a vantage point at exact same distance, but
from its axis?
It would be exactly as long as seen from Earth. It would be as wide as
it is long, so twice wider than when seen from Earth. Would it have the
same total brightness spread over twice the area, so as to be much
dimmer? Or would it show a lot of light blocked from us, so as to be
even brighter than seen from Earth? >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Apr 06, 2005
Posts: 16
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(Msg. 7) Posted: Fri Dec 15, 2006 6:53 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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On 14 Dec 2006 09:29:56 -0800, chornedsnorkack RemoveThis @hushmail.com 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.
--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
*schillin@spock.usc.edu * for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition * >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Sep 09, 2005
Posts: 3
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(Msg. 8) Posted: Fri Dec 15, 2006 7:10 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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chornedsnorkack.RemoveThis@hushmail.com wrote:
> Do we know the direction? In the other words, do we have a clue of
> which edge is the near edge? Same question about Andromeda.
For M31 you can ascertain the answer immediately by looking at a
photograph. For M33 it's not obvious; I don't know offhand how you'd go
about finding out.
--
"Always look on the bright side of life."
To reply by email, replace no.spam with my last name. >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Sep 08, 2005
Posts: 24
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(Msg. 9) Posted: Sat Dec 16, 2006 3:09 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
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Brian Tung kirjutas:
> > > 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.
> >
> > Do we know the direction? In the other words, do we have a clue of
> > which edge is the near edge? Same question about Andromeda.
>
> I don't happen to know it. I think it is known, though.
Yes, fortunately it is obvious for Andromeda, at least from our
viewpoint.
>
> > How would M33 look from a vantage point at exact same distance, but
> > from its axis?
> >
> > It would be exactly as long as seen from Earth. It would be as wide as
> > it is long, so twice wider than when seen from Earth. Would it have the
> > same total brightness spread over twice the area, so as to be much
> > dimmer? Or would it show a lot of light blocked from us, so as to be
> > even brighter than seen from Earth?
>
> There's *some* additional extinction due to gas in M33's arms, but not
> much. It would have essentially the same surface brightness, but over
> twice the apparent area, so our eyes would receive twice the light from
> M33 that they do now.
Wait, how?
If there is no or little extinction due to gas, how would the light
shining to our eyes depend on where we see from?
Looking at M33 along axis, it ought to have the same total brightness,
over twice the apparent area, and therefore half the surface
brightness.
Or what?
> That would make it marginally easier to see,
> though it would still be easily lost from light-polluted locales.
>
> -- >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Jul 08, 2003
Posts: 76
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(Msg. 10) Posted: Sun Dec 17, 2006 12:39 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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In alt.books.isaac-asimov Howard Brazee <howard.DeleteThis@brazee.net> wrote:
: On 14 Dec 2006 09:29:56 -0800, chornedsnorkack.DeleteThis@hushmail.com 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.
I don't know about you, but I am sitting in one of them as I write this.
-----
Richard Schultz schultr.DeleteThis@mail.biu.ac.il
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Opinions expressed are mine alone, and not those of Bar-Ilan University
-----
"Logic is a wreath of pretty flowers which smell bad." >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Jan 02, 2007
Posts: 4
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(Msg. 11) Posted: Tue Jan 02, 2007 11:03 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
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Joseph Lazio wrote:
> YK> Very interesting. It seems to be relevant to the current sci-fi
> YK> series Battlestar Galactica who are trying to find the Earth too,
> YK> from some other point in the galaxy.
>
> I don't think so. You may recall at the end of the pilot episode an
> exchange between Adama and the President. It's just after the funeral
> service, when Adama steps forward and announces that he knows the
> location of Earth and that that is their destination. Later, when the
> two of them are alone, the President expresses surprise that he knows
> the location, and he all but admits that he doesn't really know it.
That was the case at the end of the pilot episode, yes. But we're now
into season 3, and clues have been coming fast and furious about how to
really find Earth. Adama's little white lie turned out to be the truth.
A lot of the clues are referring to celestial landmarks, like pulsars
and stuff, the clues being encoded into the vagaries of religious texts.
Yousuf Khan >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Jan 02, 2007
Posts: 4
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(Msg. 12) Posted: Tue Jan 02, 2007 7:23 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
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Brian Tung wrote:
> chornedsnack wrote:
>>> There are indeed galaxies that one can use as signposts on either side
>>> of the galactic plane. I chose to fix on M31 to keep the essay of a
>>> manageable length. One could use M83 on the other side of the galactic
>>> plane.
>> It is eighth magnitude!
>
> And therefore trivial to see with a small telescope. You really ought
> to try observing a little bit. Your comments indicate that you haven't
> done that, and any navigator worth his or her salt would be intimately
> familiar with the night sky--both with the unaided eye and with a
> telescope.
On a slightly different note, what is the feasibility of using the
pulsar locator system that's been used on the Pioneer and Voyager space
probes? That's the map that's been etched on the plaques of those
probes, listing 14 pulsars seen from here identifying them by their
rotation periods.
My guess is that system wouldn't be useful beyond a few hundred light
years of the Solar system, as pulsars are highly directional and there's
no guarantee that they could be seen pulsing from another angle within
the galaxy.
Yousuf Khan >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Sep 08, 2005
Posts: 24
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(Msg. 13) Posted: Wed Jan 03, 2007 8:50 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
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Yousuf Khan kirjutas:
> Erik Max Francis wrote:
> > Correct. The exact distances over which it would be useful are kind of
> > hard to say. A good guess would be not much farther than our local
> > stellar neighborhood. Also, the identification is the period of the
> > pulsars, but they slowly despin, and probably despin at slightly
> > different rates, so the further away it goes, the less accurate the
> > pulsar data would be -- even if pulsars were omnidirectional.
>
>
> Do the pulsars' axes also precess, like the Earth's does? Or are they
> just too massive and spinning too fast for that?
>
Earth precesses because there are some massive objects nearby, but away
from equator of Earth - like Moon and Sun.
A pulsar which has massive objects nearby, like another neutron star,
black hole or perhaps white dwarf, should likewise precess. >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Jan 02, 2007
Posts: 4
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(Msg. 14) Posted: Wed Jan 03, 2007 10:10 am
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
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Erik Max Francis wrote:
> Correct. The exact distances over which it would be useful are kind of
> hard to say. A good guess would be not much farther than our local
> stellar neighborhood. Also, the identification is the period of the
> pulsars, but they slowly despin, and probably despin at slightly
> different rates, so the further away it goes, the less accurate the
> pulsar data would be -- even if pulsars were omnidirectional.
Do the pulsars' axes also precess, like the Earth's does? Or are they
just too massive and spinning too fast for that?
Yousuf Khan >> Stay informed about: Identifying galaxies within 1 MPc |
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External
Since: Sep 08, 2005
Posts: 4
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(Msg. 15) Posted: Wed Jan 03, 2007 4:30 pm
Post subject: Re: Identifying galaxies within 1 MPc [Login to view extended thread Info.]
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Erik Max Francis wrote:
> Brian Tung wrote:
> > Erik Max Francis wrote:
> >> That doesn't really follow. It certainly is true that the rotation axis
> >> and emission cone is not aligned, but all that means is that the
> >> emission region is not confined to a cone, but rather a locus of cones.
I'm not sure what you mean exactly by "locus" of cones.
Correct me if I'm wrong, but as I understand it pulsars
emit with essentially constant strength like a flashlight.
These flashlights are aligned with the magnetic poles,
and the light appears to pulse specifically because
they aren't lined up with the rotation axis. Thus, like
a lighthouse the star seems to pulse as the beam
momentarily lines up with the viewer.
Right?
> > I didn't intend what I wrote to give all the reasoning...
> > It was my impression that the cones of emission are of significant
> > angular extent; is that not so? Do you happen to have a figure for how
> > big they are?
> The longitude is less than about 10 degrees (_Black Holes, White Dwarfs,
> and Neutron Stars_, Shapiro and Teukolsky, p. 289). Presuming that ends
> up being a conical beam of apex angle 10 degrees, converting to a solid
> angle gives only about 24 msr. All told, that covers less than 1/500 of
> the sky.
But surely the cone it sweeps out covers more than that. For
example, if it were misaligned with the rotation axis by 5
degrees, then the total coverage would be a cone 20 degrees
wide. And of course, it's matched by another equal and opposite
cone.
Isaac Kuo >> Stay informed about: Identifying galaxies within 1 MPc |
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