Annu. Rev. Astron. Astrophys. 1977. 15: 69-95 Copyright © 1977 by Annual Reviews. All rights reserved

8. CONCLUSIONS

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 production that can arise in a small volume is called for. The various possibilities for a theoretical explanation of the energy source(s) are reviewed by Saslaw (1974) and Burbidge (1970). To the extent that there is now a developing consensus, it seems to be in the direction of using gravity to explain things. This is not so much because of improvements in the theory of galactic nuclei, but is because observations of stellar X-ray sources have demonstrated empirically that gravitational accretion onto compact objects is a potent energy source (e.g. Blumenthal & Tucker 1974). Admittedly, such accretion is on a much smaller scale than needed in Seyferts. However, if a 10⁷ M black hole or even 10⁷ neutron stars were inserted into a galactic nucleus, most of the activity in Seyfert nuclei could be made to happen as a consequence of accretion onto these objects. But an explanation of how the required compact objects get into the nuclei requires comprehensive knowledge of galactic evolution, and that's another story.

I thank Vanderbilt University and the National Science Foundation for continued support of research on the problems reviewed herein. As it is customary to acknowledge one's working environment, I wish to report that this review was prepared at the Dyer Observatory, in the quiet and pleasant Tennessee forest.