Karl Schwarzschild predicted that a Black Hole would be formed when a massive star stopped shining and collapsed in on itself. The Gravity would be so large that the material would become more and more dense, making the gravity still stronger. Gravity would eventually become so great that the star would keep collapsing past its gravitational field! In 1939, American astrophysicists Robert Oppenheimer and Hartland Snyder used mathematics to show that this collapse would happen.