It's been a while since I've posted anything astronomical, and when I saw this image of a galaxy called IC 2574, I couldn't resist!
IC 2574 Sigh. I just love this image! How beautiful it is is! How delightfully colorful, and how significant the colors are! You get such a wealth of information from them.
As you can see, the dominant color of this color is blue. The blue light comes from young, massive, hot, bright, blue stars, which live fast and die young. Some galaxies contain few or no young stars, and such galaxies are invariably yellow, dominated by stars like the Sun. Here is an example of such a galaxy, in this case NGC 2775:
NGC 2775 The fact that IC 2574 contains so many young blue star suggests that it is a dwarf galaxy, because most large galaxies today are dominated by old yellow stars. This is certainly true of our big bully of a neighbour, the Andromeda Galaxy, M 31 (did you know we are probably going to collide with it in about three billion years??
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M 31, the Andromeda Galaxy A dead giveaway which proves that M 31 is big and IC 2574 is small is the brightness of their centers. Note how bright the center of M 31 is, and how faint it is in IC 2574. In fact, it is hard to say exactly where the center of IC 2574 really is. It is located somewhere in the middle of the galaxy, of course, where there is a smooth yellowish 'oval' made up of relatively old stars. (Although there are some blue stars there as well.) But where inside this yellowish oval is the exact center of IC 2574, the nucleus of it? There is no obvious central brightening to be found in there. Compare that with M 31, where the central brightening is so obvious. The reason why M31 is so bright in the middle is precisely because it is such a big and massive galaxy. It creates a huge and very deep 'gravity well' in spacetime. Billions of stars in M 31 slide down the walls of this gravity well and gather near the bottom of it (where, by the way, there is a huge black hole). And because there are so many stars crowding near the center of M 31, its center is very bright. IC 2574, by contrast, contains few stars, makes a shallow gravity well in spacetime, and few stars slide down there. It may or may not have a central black hole at all, but if it does, this black hole is probably really tiny and lightweight compared with other black hole residents of galactic centers.
Well, like I said, the dominant color of IC 2574 is blue. But another striking color is red. There are large red areas in IC 2574. These are glowing clouds of hydrogen, which have been ionised by the ultraviolet light of hot newborn stars. Note how bright and obvious and also how large-looking these red clouds are. Compare them with the red clouds of M 31! The red clouds of M 31 look incredibly tiny by comparison. Actually, the biggest of the 'emission nebulae' of M 31 are smaller than the biggest ones of IC 2574, but not terribly so. IC 2574 is such a tiny galaxy compared with the giant M 31, which is why its large emission nebulae look so dominant and impressive.
But even though IC 2574 is a small galaxy, there are other galaxies which are much smaller! Just look at this one, a real runt of the galactic family, called Leo A:
Leo A As you can see, Leo A (which is a very nearby dwarf) is just a collection of stars with no obvious shape and no obvious 'color gradient' or anything. (All right, yes, there is a concentration of blue stars in the brighter central part of the galaxy and a halo of fainter mostly non-blue stars outside. But that's all.) And those blue stars aren't as blue as they look, by the way - they look so blue due to the filters that were used to make this image, which also explains the extremely red color of the bright foreground star - and the blue stars of Leo A aren't hugely bright at all. Blue stars come in many different brightness categories, and while many individual stars of IC 2574 blaze with the light of tens of thousands of stars like the Sun, the brightest of the bluish stars of Leo A are at best a few dozen times brighter than the Sun. Note the complete lack of red emission nebulae in Leo A.
So if we return to IC 2574, the spiral shape of it tells us that it is big and massive enough to give itself a distinctive shape. The dominant blue color of this galaxy suggests that it is small, however, and the strikingly large-looking red emission nebulae reinforce this impression. It is the faint center of this galaxy that is the dead giveaway, however: IC 2574 is a small galaxy.
But if you ask me, it sure is beautiful!
Ann
