Schwarzschild black hole
Main features of a Schwarzschild black hole. Credit: N. Rumiano.
A Schwarzschild black hole is the simplest kind of black hole in theory but one that could never occur in the real universe. A Schwarschild black hole doesn't rotate (i.e. has zero angular momentum), has no electric charge, and exists in a spacetime with no other masses. It arises as an exact solution to Einstein's equations of general relativity worked out by Karl Schwarzschild in 1916.
There are three important features of a Schwarzschild black hole:
• A photon sphere 1.5 times bigger than the Schwarzschild radius
• An event horizon: effectively, the outer surface of the black hole; its distance from the singularity is the Schwarzschild radius
• A point singularity where space and time have infinite curvature
Compare with Kerr black hole and Reissner-Nordstrom black hole.