Defined by German astronomer, Karl Schwarzschild the
gravitational radius is the distance that defines the size at which a
spherical astronomical object such as a star becomes a black hole. A black hole
is an object so dense that not even light can escape the pull of its
gravitational force. If an object collapses to within its Schwarzschild radius,
it becomes a black hole. Karl Schwarzschild derived the first model of a black
hole in 1916.

Nothing, not even a particle moving at the speed of light,
can escape the gravitational pull of a black hole. Therefore, the Schwarzschild
radius is the largest radius that a body with a specific mass can have and
still keep light from escaping. The formula for the Schwarzschild radius of a
body is R

_{s}= GM/c^{2}, where R_{s}is the Schwarzschild radius of the body, G is a constant known as the universal constant of gravitation, M is the mass of the object, and c is the speed of light.