The Schwarzschild radius is the radius of the event horizon surrounding a non-rotating black hole. Any object with a physical radius smaller than its Schwarzschild radius will be a black hole. This quantity was first derived by Karl Schwarzschild in 1916:

where R S is the Schwarzschild radius, G is the gravitational constant, M is the mass of the object and c is the speed of light.

The following table gives the Schwarzschild radii of some familiar astronomical objects: