|MadSci Network: Astronomy|
Your question is a good one and, in fact, I study the very types of black holes you are referring to. You are right that light cannot escape from a black hole, but what you don't realize is that black holes, even the supermassive ones lurking at the centers of galaxies, are extremely small. The supermassive ones are not much bigger than one Astronomical Unit (AU) in radius which is the distance from the Sun to the Earth. This leaves lots of space for other objects, like stars, to shine brightly and give the galactic center the glow we see. The centers of all spiral galaxies like the Milky Way have a puffed-up cloud of stars called a bulge. However, don't be fooled by our own Galactic Center. The bright patch of stars you see toward the center of our galaxy, which is near the constellation Sagittarius, is not the true center. The bright patch is a region called Baade's window. The true center lies to the side of the bright patch but is much fainter due to all the gas and dust along the plane of our galaxy which absorbs the light from the Galactic Center. So in conclusion, the centers of galaxies are bright because there are a lot of stars in the galactic bulge. Black holes are not big enough to gobble up all the light from the many stars in the bulge.
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