A black hole is a region in space where gravity pulls so much that even particles and electromagnetic radiation such as light cannot pass through it.
In the whole life of a star there is war between gravity pulling in and pressure pulling out. For most of the life of its life gravity and pressure balance each other exactly, and therefore the star is stable. However, when a star reaches end of his life and its nuclear fuel runs out, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of star, the greater the force of gravity that compress the material, collapsing it under its own weight.
For a small stars, when its nuclear fuel runs out and there are no more nuclear reaction to fight gravity the repulsive force among electrons within star eventually create enough pressure to halt further gravitational collapse. The stars then cools and dies peacefully. This type of star is called a “White dwarf”.
It is the core remain after the supernova. Supernova is a astronomical event which occur during the last stage of life of a massive star. It is an explosion of a massive super giant star. It may shine with brightness 10 billionth of our sun. The total energy released may be as much as produce by the sun in during its 10 billion year of lifetime.
They are very massive (more than 2.5 times mass of the sun) no any known repulsive force inside star can bush hard enough to prevent gravity from completely collapsing the core into black hole.
Albert Einstein was the first predicted who black holes in 1916 with his general theory of relativity. The term “black hole” was coined in 1967 by American astronomer John Wheeler, and the first one was discovered in 1971.