andyg wrote: ↑
Wed Feb 24, 2021 9:17 pm
Dumb question? Is all the bright light at the center reflected from haze or a heck of a lot of stars? I'm assuming the latter, but then why are they so much smaller than the new blue stars? Or are these old, small "yellow dwarf" stars that just stick around a long time?
Young blue star cluster NGC 6231. Photo: Sergio Eguivar.
The "big blue stars" in M66 are actually clusters of young bright stars, similar to cluster NGC 6231 in the picture at left.
The large blue patch just below center is a large field of young stars, with some star clusters mixed into it. It is similar to the large blue patch or field of young stars in Sagittarius, called the Small Sagittarius Star Cloud.
The Large Sagittarius Star Cloud, made up of small old red stars.
Photo: Michael Stecker.
In the picture at left you can see the Large Sagittarius Star Cloud of the Milky Way. In the picture at right you can see the Andromeda Galaxy. As you can see, the yellow center (or bulge) of Andromeda is very much brighter than its bluish disk.
The Large Sagittarius Star Cloud is part of the bulge of the Milky Way. Basically, the stellar population in the bulge of the Milky Way (and in the bulge of Andromeda) is made up of modest (not overly bright) red giant stars like Pollux, Arcturus and Dubhe, as well as truly huge numbers of stars like the Sun, and, even more, stars fainter and redder than the Sun.
Most stars that are born are low-mass stars, lower in mass than the Sun. They are called red dwarfs, and they are redder and much fainter than the Sun, and they have extraordinarily long life spans. Not a single red dwarf that was ever born in the history of the Universe has ever died of old age! That is because they fuse their meager supply of gas so exceedingly slowly!
So when more and more stars are born in the Universe, the "supply" of small red stars just grows larger and larger. More and more of them are born, and none of them die. The sheer overwhelming numbers of them make their total light bright, even if the vast majority of the individual stars are faint. (Admittedly though, most of the light from galactic bulges comes from the red giant stars. They are also numerous in there.)
Why do they gather in the center of galaxies? The way I understand it, star formation starts in the center of galaxies, and a lot of stars are born there. But after some time the molecular gas, which is necessary for new stars to be born, is "used up" or "blown away" from the center of spiral galaxies (like the Milky Way and Andromeda), and instead, what remains of the gas gathers in the disk and in the spiral arms, where relatively small numbers of bright, blue and short-lived stars are born.
But in some former spiral galaxies, all the gas has been used up, all the bright blue stars have died, and only the small red and yellow stars remain. These galaxies are "all yellow".
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