Simply speaking, a black hole is what’s left after a large star dies. You’re already aware that a star is an energy producer, a nuclear fusion reactor — its core is a gigantic nuclear fusion bomb that’s trying to explode—but its mass of surrounding gases is so large that its gravity contains the explosion, and the balance that exists between the gravity and the fusion is what determines the star’s size. However, as a star gets older, as it ages, its fuel gets used up and its nuclear reactor slows down. And then, its gravity gets the the upper hand. The star implodes. Gravity pulls inward and compresses the stellar material into the star’s center. As it’s compressed, the core heats up tremendously—and then, at some point, a supernova, a great explosion, occurs, and the stellar material and a lot of radiation are blasted out into space. Only the extremely dense, extremely massive core is left. Its gravitational field is so strong that nothing can escape it, not even light. So it disappears from view: it’s black. It's now a black hole. Now, the idea of a “black hole” — an object with so much gravity that it won’t let light escape——was first proposed more than two hundred years ago, in 1795, by a French mathematician, Pierre LaPlace. He used Newton’s gravitational theory to calculate that if an object was compressed small enough, it would require an escape velocity of almost 300,000 kilometers per second——the speed of light. More recently, the name of Stephen Hawking, the great British physicist, has become synonymous with black hole theory.