or gravitational radius
Radius inside which the gravitational attraction between a body's particles must cause its irreversible gravitational collapse, named for Karl Schwarzschild .
This is thought to be the final fate of the most massive stars (see black hole ). The gravitational radius (R g ) of an object of mass M is given by R g = 2 G M / c 2 , where G is the universal gravitational constant and c the speed of light. For a star like the Sun, the Schwarzschild radius would be about 1.8 mi (3 km).