General Astronomy/Active Galactic Nuclei and Quasars

In the 1970s, astronomers discovered an object so bright that it shone with the light of a trillion or more suns. Oddly, this object appeared small and starlike in the images, and became known as a "quasi-stellar radio source," which was shortened to "quasar." Since then many more have been discovered. Quasars also are extremely far away and long ago in time, and apparently represent a very early stage of galaxy evolution.

Other similar objects exists with very energetic cores, although not as energetic as the quasars. These include Seyfert Galaxies, BL Lacerta objects and variations. Such galaxies are said to have "Active Galactic Nuclei" or AGN for short. Interestingly, they are generally closer to us in both space and time than the quasars, and appear to form part of a time evolution sequence for galaxies.

The galaxies nearest the Milky Way (us) are quiescent or quiet.

So to recap, very distant objects such as quasars, which also represent objects very far back in time, are very energetic. Those that are mid-range from us (in both space and time) are less energetic, but more so than the Milky Way. Those that are closest to us (again, in space and time) are quiet.

Evidence suggests that a supermassive black hole (SMBH) forms at the core of most, if not all, galaxies (including quasars). In the early stages of galaxy formation, material continues to fall into the black hole. Although the black hole itself emits no (thermal) radiation, material falling into the black hole emits prodigious amounts of energy. This represents the quasars.

As time goes on, less and less material is available to fall into the SMBH, and correspondingly the energy radiating from the vicinity of the black hole diminishes. This represents the mid-range galaxies with Active Galactic Nuclei.

Eventually, the SMBH becomes quiescent (quiet) when no significant material is available to be "devoured." These represent the Milky Way and nearby galaxies.