An object with such a strong gravitational field that its escape velocity exceeds the velocity of light. One way in which black holes are believed to form is when massive stars collapse at the end of their lives. A collapsing object becomes a black hole when its radius has shrunk to a critical size, known as the Schwarzschild radius, and light can no longer escape from it. The surface having this critical radius is referred to as the event horizon, and marks the boundary inside which all information is trapped. Hence events within the black hole cannot be observed from outside. Theory indicates that both space and time become distorted inside the event horizon and that an object collapses to a single point, a singularity, at the centre of a black hole. Black holes may have any mass. Supermassive black holes (105 solar masses) may exist at the centres of active galaxies. At the other extreme, mini black holes of radii 10−10 m and masses similar to that of an asteroid may have been formed in the extreme conditions following the Big Bang.
No black hole has ever been observed directly. However, an accretion disk may form around a black hole when matter falls towards it from a nearby companion star or other source. Energy predominantly at X-ray wavelengths is produced as matter in the accretion disk loses momentum and spirals in; these X-rays can be detected by satellites in orbit. Several black-hole candidates have been located in our Galaxy, most famously Cygnus X-1. In addition, there is good evidence for a supermassive black hole of 3–4 million solar masses at the centre of our Galaxy.
There are several theoretically possible forms of black hole. A non-rotating black hole without electrical charge is known (after K. Schwarzschild) as a Schwarzschild black hole. A non-rotating black hole with electrical charge is termed a Reissner–Nordström black hole after the German physicist Hans Jacob Reissner (1874–1967) and the Finn Gunnar Nordström (1881–1923). In practice, black holes are likely to be rotating and uncharged, a form known as a Kerr black hole. Black holes are not entirely black; theory suggests that they can emit energy in the form of Hawking radiation.
Subjects: Astronomy and Astrophysics.