The previous chapters described the general properties of normal galaxies in detail. The vast majority of galaxies are normal in the sense that they have general properties in common. In particular, their present rate of evolution, as measured for example by their rate of star formation, is consistent with their age as indicated by their stellar populations.

A relatively small number of galaxies, however, show levels of energetic activity that could not be sustained for more than a small fraction of the total expected lifetime of a galaxy. Despite the considerable variety of activity exhibited, these objects are usually grouped under the general heading of active galaxies. Active galaxies are of considerable interest to astronomers since they are objects which represent the extremes of galactic structure. The activity in these galaxies is accessible and therefore more easily measured than in normal galaxies. Such observations can give valuable clues to phenomena that are present to a much lesser extent in many galaxies but that are nevertheless of great importance to our understanding of them all. It is important to appreciate that most classes of active galaxies have been discovered largely by accident, as a by-product of work on relatively normal objects. Only in the case of the radio galaxies have the properties of active galaxies been surveyed systematically and comprehensively. It may well be that it is the haphazard way in which they were discovered that is responsible for their apparently great diversity. Because of this, we shall concentrate on the various forms that this activity can take and then look at a few of the better-studied objects in some detail. As the strong extragalactic radio sources and quasars-have been studied fairly systematically, they will be described separately.

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