Transcript: The radius corresponding to the event horizon is called the Schwarzschild radius after the first theorist who solved Einstein’s equations of General Relativity for the situation of a collapsed object. Mathematically, the Schwarzschild radius is given by twice the gravitational constant times the mass of the star divided by the velocity of light squared. This is a fairly simple relationship which means that the Schwarzschild radius scales proportionally to the mass of the star. If the Sun were turned into a black hole it would have to be compressed to three kilometers in radius, the size of a small town, but a more realistic situation is a star with a core mass of three times the mass of the Sun where the Schwarzschild radius is nine kilometers. In the theory of General Relativity any star that ends its life with a core mass of three solar masses or larger must become a black hole because no known force can prevent its collapse to within the event horizon.