An Etymological Dictionary of Astronomy and Astrophysics

A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active galaxies: → Seyfert galaxies, → quasars, and → blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object,
because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one → light-year in diameter cannot vary significantly in brightness over a period of less than one year.

See also: → active; → galaxy.