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nuclei of galaxies too far away for any 
part of the galaxy other than the nu- 
cleus to show up on photographs. In 
short, quasars can be considered to be 
very distant Seyfert galaxies, and Sey- 
fert galaxies can be considered to be 
relatively nearby quasars. 
Quasars are generally more lumi- 
nous than Seyfert nuclei. As a class, 
however, the quasars are more distant 
than the Seyfert nuclei, which means 
that because of the finite speed of 
light, the energy generated by quasars 
reveals an earlier epoch in the history 
of the universe than does light coming 
from Seyfert galaxies. 
The rate at which energy is gen- 
erated in an active nucleus of a galaxy 
may depend on the age of the parent 
galaxy; perhaps the younger the gal- 
axy with an active nucleus, the more 
luminous the active nucleus. In ad- 
dition, since the quasars are so distant, 
the fainter ones are difficult to spot 
and the average luminosity of quasars 
might tend to be artificially enhanced 
by the observational selection of the 
brighter ones. In any case, the belief 
persists that quasars and Seyfert gal- 
axies are different in degree but not 
in kind. 
Theories attempting to account for 
the activity in the nucleus of a galaxy 
begin with the idea of a large mass 
concentrated at the center of the gal- 
axy. Observational evidence of both 
Seyfert galaxies and quasars points 
to the accumulation of such a mass 
in a region with a diameter no larger 
than one light-year across and the 
mass being between one million and 
one billion times the mass of the sun. 
That is a very small area for a mass 
of such dimensions. The three basic 
forms of mass concentration upon 
which the various theories proposed 
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