Black holes are extremely compact objects.
The gravitational field of a black hole is so strong that even light would take an infinitely long time to reach us from its boundary. Astronomers have found evidence for the existence of black holes with masses several times larger than the mass of our Sun, and also for black holes with masses million and billion of times larger again. Such objects are referred to as massive and supermassive black holes. Massive black holes are thought to reside in the central nuclei of both ordinary and active galaxies. This picture provides evidence for one such system.