galactic nucleus

1. The center of a galaxy, usually small, very bright, and containing a high density of stars and other objects.
2. ◆ An active galactic nucleus is a nucleus that emits more radiation than can be explained in terms of stars alone. Blazars and quasars are thought to be active galactic nuclei; the nuclei of Seyfert galaxies are also active, and are thought to contain an extremely massive black hole .