Galaxies form from the collapse of primordial gas clouds. Astronomers do not yet understand why some clouds become spiral and others become elliptical galaxies. Perhaps the initial rotation speed and the rate at which the collapsing gas cloud forms stars play a role. If so, ellipticals may develop from slowly rotating clouds that create stars rapidly, whereas spirals may develop from more rapidly spinning clouds that flatten to a disk. Alternatively, random motions in the primordial clouds may determine a galaxy 's type.
Many galaxies have extremely small but very energetic and luminous nuclei.
These include radio and Seyfert galaxies as well as quasars . According to one currently popular theory, the activity results from matter falling in toward a giant black hole at the galactic center. For the theory to work, the hole must have a mass of about 100 million solar masses packed into a region not much larger than the Solar System. An accretion disk forms around the hole as matter falls in, and hot gas boils off the disk, forming jets and clouds of radio-emitting gas. The amount of energy generated may dwarf the light output from the galaxy's stars and, for a quasar, outshine the Milky Way by a factor of a thousand.