The common feature of all the forms of energetic galactic activity is that all are relatively short-lived. We have a fairly good idea of the ages of stars and galaxies as well as of the way in which they evolve. ACTIVE GALAXIES are those that appear to be evolving in a manner that they could not sustain for more than a small part of
their total lifetime without the presence of some source of energy much more powerful than that needed to account for the properties of normal galaxies. It is in the nuclei of these galaxies that we often see the many and varied signs of activity.

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