An Etymological Dictionary of Astronomy and Astrophysics

The critical radius at which a massive body becomes a → black hole, i.e., at which light is unable to escape to infinity:
R_s = 2GM / c², where G is the → gravitational constant, M is the mass, and c the → speed of light. The fomula can be approximated to R_s≅ 3 x (M/Msun), in km. Therefore, the Schwarzschild radius for Sun is about 3 km and for Earth about 1 cm.

See also: → Schwarzschild black hole; → radius.