On the nature of quasars and active galactic nuclei

Attractive features and difficulties of the most widely held concepts on the energy sources in quasars and active galactic nuclei (a supermassive rotating magnetoplasmic body, an accreting black hole, a compact star cluster) are reviewed and discussed. In the light of the available data (particularly of recent results on the character of optical variability in a number of objects) models of a compact star cluster seem improbable. The concept of a magnetoid is less vulnerable, but the final choice between magnetoid and accreting black hole will be possible only after the details of these models have been thoroughly elaborated, and when new observational data become available.An extended version of the paper read at the Joint Meeting of Commissions 40 and 48 at the XVI General Assembly of the IAU (Grenoble, 27 August, 1976).