|Annu. Rev. Astron. Astrophys. 1977. 15: Copyright © 1977 by .
All rights reserved Mark Twain said that the nice thing about
science is that one gets such wholesale returns of conjecture
from such a trifling investment of fact. Amazingly, he said it
without even being exposed to modern astrophysics. The purpose
of this review has been to summarize the facts now known about
Seyfert galaxies, but the reason for their existence remains one
of the most pressing astrophysical mysteries. The primary
difficulty in explaining Seyfert nuclei is in accounting for
their energies. Many have bolometric luminosities as great as
1045 ergs sec-1. Even this is 1012 solar luminosities, which has
to arise in a volume about a parsec in diameter. Such a nucleus
can be visualized by imagining the Crab Nebula radiating 107 as
much energy as it does, since the absolute size, nonthermal
radiation, and filamentary structure of the Crab likely resemble
that of Seyfert nuclei. Obviously, some very efficient mechanism
of energy productio