Introduction

Quasars are unique among objects of the universe in the observable span of their continuous spectra. In some cases, the same quasar can be seen with existing instruments at wavelengths from X-rays to radio, including everything in between. The quasar continuous spectrum is deceptive. Order-of-magnitude agreement over all wavelengths, from tens of centimeters to fractions of an angstrom, covering a range of >1011 in frequency, can be obtained by fitting a single power law spectrum, of form fν∞να, where a is ˜ – 1. It is tempting in the face of such a result to attribute all parts of the spectrum to related mechanisms. As has become very clear from more careful examination of spectra, that is not valid. Different components of the continuous spectra are produced by drastically different mechanisms, and there are sometimes no physical relations among these mechanisms. It is nevertheless assumed that all of these mechanisms are basically set in motion by a single underlying engine, such as gravitational accretion, but the radiation which comes out represents many ways of transforming gravitational to radiative energy. The greatest success of the intensive observational effort has been to show the exceptional similarities among spectroscopic properties for quasars covering a factor approaching 107 in luminosity. This is the single key fact to be explained by theoretical models of quasars. Whatever processes control the radiation must be capable of scaling over this range of energy release without fundamentally changing character.