They have an event horizon.
The masses of these objects are related to velocity dispersion in the M-sigma relation, and their abrupt spin flip may be a result of galaxy mergers. They can be described by a metric named for Kerr when they possess angular momentum, and that metric allows angular momentum to be extracted in a process named for Penrose. The simplest metric describing these objects gives a namesake radius equal to two G M over c squared or two thirds of the radius of the photon sphere; that radius is named for Schwarzschild. They have an event horizon. For 10 points, name these objects with a singularity in the center from which not even light can escape.
The masses of these objects are related to velocity dispersion in the M-sigma relation, and their abrupt spin flip may be a result of galaxy mergers.
black holes