Black hole

A black hole is an object whose density is so great that the escape velocity at the surface is greater than the speed of light. According to general relativity, objects falling into a black hole will take an infinite amount of time, as observed by an outside observer, to reach the event horizon, the limit at which no light can escape. However, the amount of time as measured by the object falling into the black hole can be very short.

According to quantum mechanics theory, the location of the matter within a black hole is quantum uncertainty uncertain, and a very small amount of energy may end up outside the event horizon and radiating away; as a result, black holes are not truly "black", and have a surface temperature defined by their mass. The larger the mass of a black hole, the larger the diameter, and the lower the amount of energy which escapes, thus the lower the temperature, and the longer the time it takes for the black hole to "evaporate".

Black holes are thought to arise from the collapse of very massive stars. If the remnants of a star which has exhausted the energy available from nuclear fusion reactions are greater than about 3.4 times the mass of the sun, electron degeneracy and neutron degeneracy are insufficient to prevent the star from collapsing into a black hole.