compact object

Any astronomical object that is substantially denser or more compact (typically by a factor of a hundred) than most objects in its class. There is no precise definition of this rather general term. White dwarf stars, for example, are compact relative to most stars because they are a million times denser than main-sequence stars. An unusually bright galactic nucleus might also be referred to as a compact object if its light comes from a much smaller volume than is generally the case for the observed luminosity. The Swiss-American astronomer Fritz Zwicky made an important catalogue (1971) of compact galaxies that at the time were thought to be intermediate in nature between ordinary galaxies and quasars.