A black hole is a region of space from which nothing, including light, can escape. According to the general theory of relativity, it is the result of the curving of spacetime caused by a very dense mass. Black Holes were once the cores of very massive stars that were compressed into a fraction of its original size prior to the star exploding as a supernova or hypernova. Anything that is compressed to a size smaller than its Schwarzschild radius will become a black hole (e.g. the Sun compressed to the size of a city). The Schwarzschild radius depends on the object's mass. The «actual boundary» of a black hole is called the event horizon. Anything that passes the event horizon will be sucked into the black hole, never to return.
Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of most galaxies. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.
- How It Works, Imagine Publishing (22)