by Ann » Mon Apr 04, 2011 1:59 am
Chris Peterson wrote:Richard Melton wrote:This giant galaxy (and many others seen at this site) has an enormous, bright white area in the middle, spanning several thousand lightyears. What is the composition of this area?
Stars.
The answer is correct, of course, but a bit short.
If you look at this NOAO image of the Andromeda Galaxy and check out the highest resolution, you'll get a good idea of how the stellar composition of a galaxy typically changes from the edges to the center:
http://www.noao.edu/image_gallery/html/im0638.html
You can see that the outermost parts are dominated by bright but widely scattered blue stars. As you work your way towards the center of the galaxy, the blue stars generally get a little fainter and less blue, and they get mixed with noticably yellow stars. Then the blue stars seem to disappear, and they are replaced with stars that are yellow, fainter and more and more numerous. Close to the center they are basically all yellow (except for, in some cases, nuclear rings of star formation), and the stellar density goes up a lot. Because the stars are packed so close together (relatively speaking: they are still far apart in absolute terms), most color photos that aim at portraying both the inner and the outer parts of a galaxy will overexpose the inner part, because it is so much brighter. Therefore this overexposed part will look white. In reality, though, it is yellow from the light of old yellow stars.
The yellow center of galaxy M81 is so bright that the center looks white. Even so, I'd say that the arms have been articificially brightened here to make them easier to see. In reality, the yellow bulge of M81 dominates over the arms even more than it appears to do here.
A Hubble image of M81, not at full resolution.
A remarkably different sort of galaxy, NGC 4395:
NGC 4395 all but lacks a yellow population. Almost all the light from it seems to come from blue stars.
The thing to remember is that bright blue stars are young and short-lived. They have formed recently and will disappear soon enough, unless they are replaced with new bright blue stars. Faint yellow stars, certainly when they are packed close together at the center of a galaxy, are old and very long-lived. The reason for this is mass. Massive stars burn bright and blue, but they also burn out quickly and die. Low-mass stars smoulder yellow and red and can hoard their meager energy supplies for billions and billions of years.
Ann
[quote="Chris Peterson"][quote="Richard Melton"]This giant galaxy (and many others seen at this site) has an enormous, bright white area in the middle, spanning several thousand lightyears. What is the composition of this area?[/quote]
Stars.[/quote]
The answer is correct, of course, but a bit short.
If you look at this NOAO image of the Andromeda Galaxy and check out the highest resolution, you'll get a good idea of how the stellar composition of a galaxy typically changes from the edges to the center: [url]http://www.noao.edu/image_gallery/html/im0638.html[/url]
You can see that the outermost parts are dominated by bright but widely scattered blue stars. As you work your way towards the center of the galaxy, the blue stars generally get a little fainter and less blue, and they get mixed with noticably yellow stars. Then the blue stars seem to disappear, and they are replaced with stars that are yellow, fainter and more and more numerous. Close to the center they are basically all yellow (except for, in some cases, nuclear rings of star formation), and the stellar density goes up a lot. Because the stars are packed so close together (relatively speaking: they are still far apart in absolute terms), most color photos that aim at portraying both the inner and the outer parts of a galaxy will overexpose the inner part, because it is so much brighter. Therefore this overexposed part will look white. In reality, though, it is yellow from the light of old yellow stars.
[img2]http://astro.neutral.org/images/20070214_m81_lrgb.jpg[/img2]
The yellow center of galaxy M81 is so bright that the center looks white. Even so, I'd say that the arms have been articificially brightened here to make them easier to see. In reality, the yellow bulge of M81 dominates over the arms even more than it appears to do here.
[img2]http://www.etsonly.com/images/deepspace/galaxy%20m81.jpg[/img2]
A Hubble image of M81, not at full resolution.
A remarkably different sort of galaxy, NGC 4395:
[img2]http://skepticalswedishscientists.files.wordpress.com/2010/07/lsbg-face-on-ngc4395.jpg[/img2]
NGC 4395 all but lacks a yellow population. Almost all the light from it seems to come from blue stars.
The thing to remember is that bright blue stars are young and short-lived. They have formed recently and will disappear soon enough, unless they are replaced with new bright blue stars. Faint yellow stars, certainly when they are packed close together at the center of a galaxy, are old and very long-lived. The reason for this is mass. Massive stars burn bright and blue, but they also burn out quickly and die. Low-mass stars smoulder yellow and red and can hoard their meager energy supplies for billions and billions of years.
Ann