In astrophysics, the radius of the event horizon surrounding a black hole within which light cannot escape its gravitational pull.
For a black hole of mass m, the Schwarzschild radius Rs is given by Rs = 2gm/c2, where g is the gravitational constant and c is the speed of light. The Schwarzschild radius for a black hole of solar mass is about 3 km. It is named after Karl Schwarzschild, the German astronomer who deduced the possibility of black holes from Einstein's general theory of relativity in 1916.