Published in Re. Prog. Phys. 54 (1991) 579-633. Printed in the UK


D. E. Osterbrock

Institute for Advanced Study, Princeton, New Jersey 08540, USA

Abstract. Our current knowledge of active nuclei is reviewed. The importance of observational data taken over a wide range of frequencies, from radio and infrared through optical and ultraviolet to x-rays and γ-rays, is emphasized. Important overall principles include the continuity from quasars and QSOs through Seyfert and radio galaxies to low-luminosity LINERs, the importance of considering roughly cylindrically symmetric (rather than spherically symmetric) structures, and that the various regions generally have different axes and planes of symmetry, and are often warped.

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