Black holes with masses of a million to a few billion times the mass of the Sun are believed to be the engines that power nuclear activity in galaxies. Active nuclei range from faint, compact radio sources like that in M31 to quasars like 3C 273 that are brighter than the whole galaxy in which they live. Some nuclei fire jets of energetic particles millions of light years into space. Almost all astronomers believe that this enormous outpouring of energy comes from the death throes of stars and gas that are falling into the central black hole. This is a very successful explanation of the observations, but until recently, it was seriously incomplete: we had no direct evidence that supermassive black holes exist.
For the past twenty years, astronomers have looked for supermassive black holes by measuring rotation and random velocities of stars and gas near galactic centers. If the velocities are large enough, as in the Sombrero Galaxy, then they imply more mass than we see in stars. The most probable explanation is a black hole. About 37 have been found as of 2001 March. Their masses are in the range expected for nuclear engines, and their numbers are consistent with predictions based on the energy output of quasars.