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| © 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''.