In A.D. 964 a Persian astronomer listed a hazy patch in the constellation Andromeda as a fixed star. What he had just managed to discern with his naked eye was in fact a whole galaxy 2.2 million light years away—the Andromeda Galaxy, or M31. Andromeda, shaped like a beautiful spiral, is in fact the largest of our own local group of galaxies of which our own, the Milky Way, is a very close second in size. It is now known that there exists a supermassive black hole in the nucleus of the galaxy. This black hole contains the mass of 30 million suns. Numerous powerful X-ray sources are also apparent. Most of these are probably due to X-ray binary systems, in which a neutron star or black hole is in a close orbit around a normal star.

M31, Kitt Peak, courtesy NOAQ/AURA/NSF