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 Rs = GM/c2, where Rs 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.