Astronomers know that a giant black hole sits in the center of each large galaxy. This black hole can weigh anywhere between a few million times our Sun’s mass to tens of billions of times our Sun’s mass.
A black hole’s extreme mass means it has a huge gravitational force that pulls in all material nearby — like, from a passing gas cloud or star — to form a disk of gas and dust around the black hole. Friction in this accretion disk causes it to glow, and that light energizes more nearby material. Thus, the area surrounding the supermassive black hole glows in visible, ultraviolet, and X-ray light. When a galaxy’s central black hole is pulling in material and glowing, astronomers call it an active galactic nucleus, or AGN. The brightest and most energetic type of AGN is a quasar.