What are Active Galactic Nuclei?
As astrophysics approached the close of the 20th century, we learned that many of the galaxies that we can observe in detail seem to have very massive black holes in their cores. For instance, our galaxy has a black hole of mass approximately 2 x 10^6 solar masses at its core, and studies of gas and stellar motion in other galaxies have revealed other supermassive (with masses greater than 10^6 solar masses) black holes in their cores. Most of these galaxies don’t exhibit any clue to their central black holes besides the stellar or gas motion used to find their masses. However, some galaxies (for reasons we don’t quite yet understand) have black holes that draw much more attention to themselves by generating extraordinary amounts of energy in their host-galaxy cores: much more than ordinary galaxies. We call these galaxies Active Galactic Nuclei, or AGNs, for short. So, at the most basic level, AGNs are simply the cores of galaxies that are not quiescent like normal galaxy cores: for i
