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*is the Schwarzschild radius,_{S}*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:Object Mass *R*_{S}Sun 2.0 × 10 ^{30}kg3.0 × 10 ^{3}m= 3 km Earth 6.0 × 10 ^{24}kg8.7 × 10 ^{-3}m= 8.7 mm Moon 7.3 × 10 ^{22}kg1.1 × 10 ^{-4}m= 0.11 mm Jupiter 1.9 × 10 ^{27}kg2.2 m = 2.2 m Neutron Star 2.8 × 10 ^{30}kg4.2 × 10 ^{3}m= 4.2 km