What is a black hole?

  A black hole is a region of space-time from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform space time to form a black hole. Around a black hole there is a mathematically defined surface called an event horizont that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater. Black holes of stellar mass are expected to form when massive stars collapse in a supernova the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may be formed.