Normal galaxies are dynamically stable: they will change their large-scale structure only very gradually. Certain active galaxies exhibit structures that must be short-lived. The jets or tails seen attached to some galaxies can only last until the gravitational attraction of the parent galaxy pulls them back towards the galaxy. Indeed their very presence suggests that they must have been ejected from their parent galaxies m the fairly recent past.

It is clear from the above survey that there is tremendous variety in the forms and violence of the activity in galaxies. In the next section we look at the variety found in active galaxies and search for features common to a number of different classes

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