APOD: The Once and Future Stars of Andromeda (2011 Jan 20)

[APOD Robot]

The Once and Future Stars of Andromeda

Explanation: The big, beautiful Andromeda Galaxy, aka M31, is a spiral galaxy a mere 2.5 million light-years away. Two space-based observatories have combined to produce this intriguing composite image of Andromeda, at wavelengths outside the visible spectrum. The remarkable view follows the locations of this galaxy's once and future stars. In reddish hues, image data from the large Herschel infrared observatory traces enormous lanes of dust, warmed by stars, sweeping along Andromeda's spiral arms. The dust, in conjunction with the galaxy's interstellar gas, comprises the raw material for future star formation. X-ray data from the XMM-Newton observatory in blue pinpoint Andromeda's X-ray binary star systems. These systems likely contain neutron stars or stellar mass black holes that represent final stages in stellar evolution. More than twice the size of our own Milky Way, the Andromeda Galaxy is over 200,000 light-years across.

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[Beyond]

WOW!!

[bystander]

http://asterisk.apod.com/vie ... 29&t=22494

[orin stepanek]

http://www.thetechherald.com/article.ph ... y-bulks-up

[drollere]

the half dozen or so unusually bright (large) xray points, excluding the population at the nucleus of the galaxy, are probably multiple, overlapping xray binary signals from inside globular clusters.

[Leon Green]

Hm. So this makes me wonder about our Milky Way: a) How many blue point binary X-ray sources are there in "our" galaxy? If each of them hides a black hole, which is the nearest one to us in the Orion arm? Clearly I am still a very amateur star-gazer that I did not know that black holes weren't only at the center of galaxies. Curious to learn more!

[Jay20120]

Not exactly reality, but beautiful nonetheless. Unfortunately photos like this are never corrected for the time it takes light to get to us from different elements of the photo. The light from the more distant parts of the Andromeda Galaxy takes about 150,000 years longer to reach our eyes than the closer parts (this would be more like 200,000 years if the galaxy were viewed edge-on). It takes our sun about 230,000,000 years to orbit the black hole in the center of our milky way. 15/23000 = approx .065% change in position relative to the closer parts of the Andromeda Galaxy. Actually probably more like .03%, since the outer elements of the galaxy are significantly more distant from the Andromeda center than is our sun and, judging from the shape of the galaxy (spiral) their angular velocity is less. This may not seem like much, but we make lesser corrections in astronomical photgraphs in an effort to see more clearly. It could actually make a difference in clarity of the photos if any distant part is sharing a near-the-same light path as any closer part; so this effect is more significant in edge-on galaxies.

[bystander]

If you take the estimated width of M31 (141 Kly) and divide by the estimated distance to M31 (2.54 Mly),
the difference in the time the light took to reach us is less than 0.006%. Do you really think that is significant?

[Chris Peterson]

Not exactly reality, but beautiful nonetheless. Unfortunately photos like this are never corrected for the time it takes light to get to us from different elements of the photo. The light from the more distant parts of the Andromeda Galaxy takes about 150,000 years longer to reach our eyes than the closer parts (this would be more like 200,000 years if the galaxy were viewed edge-on). It takes our sun about 230,000,000 years to orbit the black hole in the center of our milky way. 15/23000 = approx .065% change in position relative to the closer parts of the Andromeda Galaxy. Actually probably more like .03%, since the outer elements of the galaxy are significantly more distant from the Andromeda center than is our sun and, judging from the shape of the galaxy (spiral) their angular velocity is less. This may not seem like much, but we make lesser corrections in astronomical photgraphs in an effort to see more clearly. It could actually make a difference in clarity of the photos if any distant part is sharing a near-the-same light path as any closer part; so this effect is more significant in edge-on galaxies.

In practice, there is no way to correct for the light aberration. And in reality, it doesn't matter in the slightest, since the corrected and uncorrected images would be virtually identical. Even a 0.03% correction amounts to less than one pixel width in the image.

Also, the angular velocity is much more constant with radius than you might think, because of the effects of the dark matter halo. The shape of the spiral arms does not relate in a simple way to the orbital speed of stars at different radii.

[flash]

The shape of the spiral arms does not relate in a simple way to the orbital speed of stars at different radii.

What does the shape of the spiral arms relate to? Are not the stars closer to the center of the galaxy orbiting faster than those further out? I have often wondered about the relationship between the spiral arm curvature and the rotational direction of the galaxy. I presume that the spiral arms are more visible due to the increased density of stars in them, and that most if not all of the stars in a spiral galaxy are orbiting the galaxy in the same direction (the direction the entire galaxy turns). I have heard that the spiral arms are wavefronts of such increased star density. Do these waves turn in the same direction as the stars? In the same direction as the galaxy as a whole? Given a spiral galaxy undisturbed by outside influences, I wonder how does the shape of the spiral arms change over time?

[neufer]

Memorable quotes for The Andromeda Strain (1971)
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Dr. Rudolph Karp: Fools! They refuse to believe life exists in meteorites. I showed them at the astrophysics conference what I just showed you. But no! Even with a microscope they are blind! What do I have to do? Hit them over the head?
----------------------------------------
Dr. Jeremy Stone: [handing out suppositories] Umm... stop by your rooms and insert these before taking the elevator.

Dr. Ruth Leavitt: I have risked drowning in that foul bath! I have been par-boiled, irradiated and xenon-flashed! and now you suggest I...
[pushing suppository upward in the air]

Dr. Jeremy Stone: I HAVE to! We haven't done a thing about the G.I. tract yet. On level five we must be as nearly germ-free as possible.

Dr. Ruth Leavitt: [eyeing suppository sheepishly] Anyone care to join me for a "smoke"?
----------------------------------------

[neufer]

[size=200]170 × 10[sup]6[/sup] M☉ Andromeda black hole (Buckingham Fountain, Chicago)[/size]

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[size=115]4.2 × 10[sup]6[/sup] M☉ Milky Way black hole[/size]

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Chandra X-ray telescope image of the center of M31. A number of X-ray sources, likely X-ray binary stars, within Andromeda's central region appear as yellowish dots. The blue source at the center is at the position of the supermassive black hole.

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<<M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.

In 1991 Tod R. Lauer used WFPC, then on board the Hubble Space Telescope, to image Andromeda's inner nucleus. The nucleus consists of two concentrations separated by 1.5 parsecs. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 1.1–2.3 × 10⁸ M☉ in 2005.

Scott Tremaine has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. The eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy "cannibalized" by Andromeda, this is no longer considered to be a viable explanation. The primary reason is that such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.>>

[Chris Peterson]

What does the shape of the spiral arms relate to?

The spiral arms are a density fluctuation in the overall pattern of rotation. Stars move into the arms at the tail and then back out of them from the front edge. They are not produced because of the different rates of stellar rotation. If they were, the spiral would wind itself up over time. That doesn't happen.

Are not the stars closer to the center of the galaxy orbiting faster than those further out?

Only very close to the center- the inner few percent of the radius. Across almost the entire disc, the stars have the virtually the same angular velocity. The reason for this curious behavior is that the great majority of the galaxy's mass is in the dark matter halo surrounding it. This means that the gravitational effect of the central bulge and disc itself is small.

A is the predicted curve without dark matter, B is the observed curve

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I have often wondered about the relationship between the spiral arm curvature and the rotational direction of the galaxy. I presume that the spiral arms are more visible due to the increased density of stars in them, and that most if not all of the stars in a spiral galaxy are orbiting the galaxy in the same direction (the direction the entire galaxy turns). I have heard that the spiral arms are wavefronts of such increased star density. Do these waves turn in the same direction as the stars? In the same direction as the galaxy as a whole?

It depends on your frame of reference. The spiral arms move backwards compared with the stars. If you define the galaxy rotation in term of the stars, you'd have to say the spiral arms rotate in the opposite direction from the galaxy. But if you define the galaxy rotation in terms of the arms with respect to some outside frame, everything is rotating the same way, just at different rates.

Given a spiral galaxy undisturbed by outside influences, I wonder how does the shape of the spiral arms change over time?

That's not well understood, but in general, spiral arms appear stable over very long periods (billions of years). So it is likely that you'd see no substantive change in the shape or number of arms.

[flash]

Chris, thanks for the detailed explanation.

All this has got me thinking about the angular momentum of galaxies. Given your reponse that in a spiral galaxy all the stars have "virtually the same angular velocity", I wonder if anyone has plotted the angular momentum of galaxies (due to the stars, not necessarily considering the dark matter halo)? I believe that the angular momentum of spherical galaxies must be significantly less than spiral galaxies, if only because the stars in them move more randomly, subject to frequent perturbations from each other, and if the stars shared the same or similar angular momentum, wouldn't they evolve into a disk-shaped galaxy? So how does the angular monentum of galaxies vary (between galaxies)? What would the shape be of a plot of galactic angular momentum vs galactic mass? How about vs galactic diameter? vs galaxy type? What about elliptical galaxies angular momentum? Would elliptical galaxies have greater angular momentum than a similar sized spherical galaxy, but smaller than a similar sized spiral galaxy?

On a separate note, are the angular momentum vectors of galaxies aligned at all with each other, (perhaps just locally), or are they random as the orientation of their disks appear?

Thanks again.

[Chris Peterson]

On a separate note, are the angular momentum vectors of galaxies aligned at all with each other, (perhaps just locally), or are they random as the orientation of their disks appear?

I'm not aware of any comparison of the magnitudes of the angular momentum in different galaxy populations. But I haven't looked for any studies. However, there are several analyses of the orientation of galactic angular momentum vectors, and these suggest correlations with the large scale structure of the Universe. In particular, galaxies that lie along the "bubble walls" of material surrounding voids may tend to have their angular momentum vectors parallel to those walls. So there is some sort of connection between galaxy orientation and large scale structure, which is predicted by most models. (There has also been at least one analysis that concluded the orientations of galaxies on void walls is random, so this is still an open question. But as regards this discussion, the point is that people are actively looking at the orientation of galactic angular momentum vectors.)

On a local scale, however, I don't think there is any correlation in the orientation of galaxies in clusters, except in the obvious case where you have significant interaction.

[jkeller21]

This APOD looks like there is a blue light at the center of golden fire revolving around in a circle. I like how you can still see the stars amongst all of the blue and golden light surrounding them. It seems that the light comes out of nowhere because the light is not attached to anything and it seems like the light just stays with those specific stars.

[jkeller21]

This picture looks like golden fire revolving around a blue light. It seems like the light is not connected to anything, but it also seems like the light is staying with those specific stars. This light looks like fire to me because it looks like the light is just floating there as if the stars were on fire. This is a little complex to me as well because its wierd that there are two different colors just floating there.