# Schwarzschild radius

The Schwarzschild radius is the critical radius, according to the general
theory of relativity, at which a body under the influence of its own
gravitation becomes a black hole. Alternatively,
it can be defined as the radius of the event horizon of
a spherical black hole, from within which the strength of gravity is so
strong that light cannot escape. Named after Karl Schwarzschild,
it is given by *R*_{s} = 2*GM*/*c*^{2}, where *M* is the mass of the body, *G* is the universal constant of
gravitation, and *c* is the speed of light. For the Sun, *R*_{s} = 2.5 kilometers; for Earth *R*_{s} = 0.9 centimeter.