In this chapter we look at galaxies with unusual activity within and around them. For many years astronomers thought of these various types of activity as being distinct. We now realize that many of them have similar origins, but differ in the specific conditions within the galaxy or its environment. We realize that all of this activity takes place in the nucleus of the galaxy, or is driven by activity in the nucleus. We say that these phenomena are associated with active galactic nuclei (AGN).
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