|| © CAMBRIDGE UNIVERSITY PRESS 1997
In general the term ``active galactic nucleus'', or AGN, refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies which cannot be attributed clearly and directly to stars. The two largest subclasses of AGNs are Seyfert galaxies and quasars, and the distinction between them is to some degree a matter of semantics. The fundamental difference between these two subclasses is in the amount of radiation emitted by the compact central source; in the case of a typical Seyfert galaxy, the total energy emitted by the nuclear source at visible wavelengths is comparable to the energy emitted by all of the stars in the galaxy (i.e., ~ 1011 L), but in a typical quasar the nuclear source is brighter than the stars by a factor of 100 or more. Historically, the early failure to realize that Seyferts and quasars are probably related has to do with the different methods by which these two types of objects were first isolated, which left a large gap in luminosity between them. The appearance of quasars did not initially suggest identification with galaxies, which is a consequence of the basic fact that high-luminosity objects, like bright quasars, are rare. One is likely to find rare objects only at great distances, which is of course what happens with quasars. At very large distances, only the star-like nuclear source is seen in a quasar, and the light from the surrounding galaxy, because of its small angular size and relative faintness, is lost in the glare of the nucleus. Hence, the source looks ``quasi-stellar''.