A Mystery of Space: S T A R S- [The Formation of Black Holes]

Black holes are believed to be formed in the process of stellar evolution. When a star starts to run out of nuclear "fuel", the pressure that was caused by the nuclear reactions is no longer there. Then, the core of the star collapses, making it very dense. When the density of the core is increased, and the solar mass of the star is more than 1.7, then the star inevitably turns into a black hole. When the star is not quite is big, it forms a compact white dwarf or a neutron star.
The black hole was first theorized by Karl Schwarzschild, a German astronomer, in 1916. He made the theory based on Albert Einstein's theory of relativity.