Active Galactic Nuclei

When a supermassive black hole actively consumes material, it powers an Active Galactic Nucleus (AGN). The accretion disk reaches temperatures of millions of degrees, and jets of plasma shoot from the poles at nearly the speed of light, stretching thousands of light-years beyond the galaxy.

Quasars are the most luminous AGN, visible across billions of light-years. They were common in the early universe, when galaxies had more fuel to feed their central black holes. Today most supermassive black holes, including the Milky Way's Sagittarius A*, are relatively quiet, flaring up only when stray gas drifts too close.