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DocOzz
2011-Feb-22, 12:21 AM
Perhaps this question is silly, as once you think of the premise, it seems only logical that astronomers would compensate their models to account, but I have never heard this mentioned in my casual studies of Astronomy.

So here is the premise: granting that the speed of light is constant, and therefore the further away a source of lumination is from the Earth the longer it takes to reach us, do the traditional models we all learned about the shapes of galaxies (spiral etc.) take into account that the observed positions of stars relative to each other are actually from different periods of time?
In other words, do we take into account that we are seeing the recent position of nearby stars, but ancient positions of distant stars and then extrapolate the positions of these stars relative to each other, or do we just base the model on observed shapes?
I would assume that astronomers thought about this a long time ago, but the idea that it may have been overlooked is too fascinating not to ask about.

antoniseb
2011-Feb-22, 12:55 AM
Welcome to the forum DocOzz,

People with deeper knowledge than me will hopefully post soon, but yes they do take time into account for some kinds of studies. In other studies, looking at galaxies within about a billion light years from here, it doesn't matter so much, because on the galactic evolution scale of things 'now' covers a billion years or more.

Van Rijn
2011-Feb-22, 01:24 AM
Perhaps this question is silly, as once you think of the premise, it seems only logical that astronomers would compensate their models to account, but I have never heard this mentioned in my casual studies of Astronomy.

So here is the premise: granting that the speed of light is constant, and therefore the further away a source of lumination is from the Earth the longer it takes to reach us, do the traditional models we all learned about the shapes of galaxies (spiral etc.) take into account that the observed positions of stars relative to each other are actually from different periods of time?
In other words, do we take into account that we are seeing the recent position of nearby stars, but ancient positions of distant stars and then extrapolate the positions of these stars relative to each other, or do we just base the model on observed shapes?


If you're thinking about the effect on the shape of our own galaxy, most stars move quite slowly compared to the speed of light, so there isn't much of an issue there: A star 50,000 light years away just isn't going to move that far in 50,000 years.

On the other hand, images of very distant galaxies, thought to be images of galaxies as they were in the early universe, do tend to look different, but that's because of long term issues of how galaxies interact and change over time.

Jens
2011-Feb-22, 01:35 AM
If you're thinking about the effect on the shape of our own galaxy, most stars move quite slowly compared to the speed of light, so there isn't much of an issue there: A star 50,000 light years away just isn't going to move that far in 50,000 years.


Just to expand a tiny bit, the velocity of the sun around our galaxy is said to be about 220 km/sec, and the speed of light is like 300,000 km/sec, so it's less than 1,000th the speed of light. So a star is not going to have moved very much at all by the time the light reaches us.

forrest noble
2011-Feb-22, 01:46 AM
Perhaps this question is silly, as once you think of the premise, it seems only logical that astronomers would compensate their models to account, but I have never heard this mentioned in my casual studies of Astronomy.

So here is the premise: granting that the speed of light is constant, and therefore the further away a source of lumination is from the Earth the longer it takes to reach us, do the traditional models we all learned about the shapes of galaxies (spiral etc.) take into account that the observed positions of stars relative to each other are actually from different periods of time?

In other words, do we take into account that we are seeing the recent position of nearby stars, but ancient positions of distant stars and then extrapolate the positions of these stars relative to each other, or do we just base the model on observed shapes? I would assume that astronomers thought about this a long time ago, but the idea that it may have been overlooked is too fascinating not to ask about.

There are problems concerning interpretations of galactic visual appearances vs. time. Taking the Milky Way as an example, it is thought to be about 12 billion years old. Similar appearing galaxies in our vicinity are thought to be about the same age. Large elliptical galaxies at the center of galaxy clusters appear to have a larger population of older looking stars within them, so these galaxies would seem to be older still. So to see both large Spiral and Elliptical galaxies 10 billion light years away is a big problem concerning explanations of galaxy size and appearances vs. time. A number of papers have made such claims concerning very distant large appearing galaxies both spiral and elliptical. Here's one of them:

http://outreach.jach.hawaii.edu/pressroom/2008_nam2008b/index.html

According to your question, it is presently believed that all galaxies that appear to be large at great distances, would appear to be very different if we could observe them now. How different is a matter of conjecture.

WayneFrancis
2011-Feb-22, 02:07 AM
To give you an idea of scale it takes 250 million years for a star to orbit the galaxy.
http://www.users.on.net/~waynefrancis/MilkyWaySmall.jpg (http://www.users.on.net/~waynefrancis/MilkyWay.jpg)
click for larger image.

The diameter is only 100,000 light years. We are about 26,000 ly from the centre which means if we could see the furthest away star it would be about 75,000 light years away. Back of the napkin calculation ...
75,000 / 250,000,000 = 0.0003 .. That is just over 1/10th of a degree that star would have moved around the centre of the galaxy during the light travel time. So if we didn't take that light travel time into account our worst case would be a star out by 1/10th of a degree in its position. For stars that we can actually see it is a lot less.

To boot we only care about relative positions of stars as a group. Like we don't really care about the star that is on the other side of the galaxy in relation to us. We don't even care about a star close to the centre in relation to us. Some of those stars orbit our central black hole in just years. The work that is done doesn't really care about their location in relation to ours.

Locally the stars move VERY little in relation to us so Betelgeuse is 640 ly away but its position will change very little in 640 years. In fact it will take orders of magnitude longer before its position would change to the casual naked eye measurement.

transreality
2011-Feb-22, 05:34 AM
Large elliptical galaxies at the center of galaxy clusters appear to have a larger population of older looking stars within them, so these galaxies would seem to be older still. So to see both large Spiral and Elliptical galaxies 10 billion light years away is a big problem

Actually, elliptical galaxies have a population mostly made of older stars, simply because they have largely stopped producing newer stars. The same can be said for the central region of a spiral. Look at the image put up by WayneFrancis, the same effect can be seen, yet the core and the arms are not of different ages.

DocOzz
2011-Feb-22, 11:18 AM
So, if I might sum up what I have gathered from the thread: The fact that the orbital periods of stars are so long means that the amount of distortion caused by an image comprised of images of stars from different points in time is negligible.

Here is the problem I am having wrapping my head around this one. Say we were making a star chart of our galaxy, as seen from earth. The location of stars is stable in relation to each other, and the distance from earth is stable. It would seem to follow that a star 10 light years away, seen to have a certain position relative to a star 20 light years away, actually holds that position relative to where the second star was 10 years prior. Extending this thesis out, if we begin considering multiple stars, clusters or galaxies, the relative positions from our point of view become less and less meaningful.

Perhaps I'm just looking at it incorrectly, or this is picking at philosophical threads which don't really mean anything, but I keep getting the feeling that the shapes of the galaxies as we see them are optical illusions caused by the effect of distance.

antoniseb
2011-Feb-22, 01:53 PM
... Say we were making a star chart of our galaxy, as seen from earth. ...

So far, we don't have a need for the positions of stars that far away more accurately than we already have. Yes, if this were Star Wars, and we had FTL spaceships, you might need those things, but for the moment, there's no science in charting the difference.

forrest noble
2011-Feb-22, 06:39 PM
Actually, elliptical galaxies have a population mostly made of older stars, simply because they have largely stopped producing newer stars. The same can be said for the central region of a spiral. Look at the image put up by WayneFrancis, the same effect can be seen, yet the core and the arms are not of different ages.

There have been a number of papers and articles that assert that galaxies of the highest observed metalicities, mostly elliptical galaxies at the center of galaxy clusters, may be older than the mainly spiral galaxies that surround them. This might also apply to the cores of some spiral galaxies as you suggest.

http://www.astro.virginia.edu/~rwo/Eage93-pp.pdf
http://adsabs.harvard.edu/abs/1996astro.ph..2032B
http://www.dailygalaxy.com/my_weblog/2011/02/do-new-discoveries-challenge-age-of-the-universe.html
http://www.space.com/9610-galaxy-giving-birth-baby-stars.html
http://www.dailygalaxy.com/my_weblog/2010/11/new-star-birth-found-in-ancient-ellipical-galaxy.html

Jens
2011-Feb-23, 04:35 AM
Here is the problem I am having wrapping my head around this one. Say we were making a star chart of our galaxy, as seen from earth. The location of stars is stable in relation to each other, and the distance from earth is stable. It would seem to follow that a star 10 light years away, seen to have a certain position relative to a star 20 light years away, actually holds that position relative to where the second star was 10 years prior.

You're absolutely right, but the thing is that in 10 years, the star isn't going to have moved a meaningful amount (relatively speaking).

wd40
2011-Feb-23, 04:48 AM
If it takes 200 million years for a galaxy to rotate on its axis, are there are any photos yet of galaxies with their spiral arms wrapped round themselves like a catherine wheel c5 times in 1 billion years?

antoniseb
2011-Feb-23, 04:56 AM
If it takes 200 million years for a galaxy to rotate on its axis, are there are any photos yet of galaxies with their spiral arms wrapped round themselves like a catherine wheel c5 times in 1 billion years?

I'm not sure I follow you. Are you suggesting a time-lapse photography project and a moratorium on galaxy rotation discussion until the project is complete (in a billion years)?

loglo
2011-Feb-23, 10:11 AM
If it takes 200 million years for a galaxy to rotate on its axis, are there are any photos yet of galaxies with their spiral arms wrapped round themselves like a catherine wheel c5 times in 1 billion years?

Spiral galaxies do not rotate on their axes, all of the stars are orbiting independently and the arms do not rotate in a fixed structure. Remember that galaxies have flat rotation curves so the stars are all travelling around the same speed. Because of this we do not see spiral arms wrapped tightly. The lack of such observations is called the Winding Problem. It is resolved by the theory of Lindblad Resonance (http://en.wikipedia.org/wiki/Lindblad_resonance), which explains the structure of the arms as being formed by resonances between stellar orbital eccentricity, the gravity of the gas disk and the galactic gravitational potential. Somehow this causes a density wave compressing interstellar gas and creating new stars, the largest and brightest of which are what we see as the arms.

Last I looked Lindblad Resonance still had quite a few details to work out but had no other major competing theories contending with it. The classic paper from Lin and Shu is here (http://articles.adsabs.harvard.edu//full/1964ApJ...140..646L/0000648.000.html).

wd40
2011-Feb-24, 01:02 PM
The spiral arms rotating is only apparent and is actually an illusion? http://www.sciencedaily.com/releases/2011/02/110214204052.htm

slang
2011-Feb-24, 01:35 PM
The spiral arms rotating is only apparent and is actually an illusion? http://www.sciencedaily.com/releases/2011/02/110214204052.htm

I struggle to understand how you could come to that conclusion from that story. All it says is that there are stars in the galaxy that are not in in the central bulge or in the spiral arms, and that those are found to make up a "thicker" disc.

Cougar
2011-Feb-24, 02:17 PM
...do the traditional models we all learned about the shapes of galaxies (spiral etc.) take into account that the observed positions of stars relative to each other are actually from different periods of time?.... I would assume that astronomers thought about this a long time ago, but the idea that it may have been overlooked is too fascinating not to ask about.

In my studies, I have found that astronomers and astrophysicists are very clever.

Galaxy formation and evolution is one of the most active research areas in astrophysics.

But if you're looking at the shape of a galaxy billions of lightyears away, the difference in light travel time between the near and far sides of the galaxy will not distort the image significantly.

With respect to galaxy morphological classification, Jeff Kanipe, author of Chasing Hubble's Shadows, The Search for galaxies at the Edge of Time tells us:







Madore: "Who goes in and classifies what turbulent eeddies look like? No physicist in their right mind would classify that sort of thing. And maybe that's all we're doing [in the classification of galaxies]. We have this frozen picture in time of things that don't look like what they used to look like, probably won't look the same in the future, and don't even look like what they do look like when you look at them in another wavelength! It may well be that every galaxy's structure is so contingent on the last event or environment that it found itself in, that what you see now in this fine little slice in time and this other tiny slice in wavelength have nothing to do with its global history... to reduce a whole galaxy down to two letter and a number seems to trivialize the whole thing."

For Simon Driver, a galaxy specialist at Australia's Mount Stromlo Observatory, the Hubble tuning fork is just butterfly counting. "The name of the game is to find something to replace it with."

Middenrat
2011-Feb-25, 02:22 AM
Fascinating, thanks to contributors.

wd40
2011-Feb-26, 08:02 PM
Discounting the Creationist explanation of a recent creation as to why the arms are not wound up more than one would expect http://pseudoastro.wordpress.com/2008/12/21/creationist-claim-spiral-galaxies-wind-up-too-fast-for-an-old-universe/ would we expect eg M101 to look any different than it does now, in 200-500 million years time?

loglo
2011-Mar-02, 05:29 AM
Discounting the Creationist explanation of a recent creation as to why the arms are not wound up more than one would expect http://pseudoastro.wordpress.com/2008/12/21/creationist-claim-spiral-galaxies-wind-up-too-fast-for-an-old-universe/ would we expect eg M101 to look any different than it does now, in 200-500 million years time?

My guess is that it would look qualitatively the same, perhaps a tad redder. The position of the arms may be different.