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[[Image:Iu8969hu.jpg|right|thumb|300px|Artist's conception of a binary system consisting of a black hole and a main sequence star.  The black hole is drawing matter from the main sequence star via an [[accretion disk]] around it, and some of this matter forms a gas jet.]]

[[Image:Iu8969hu.jpg|right|thumb|300px|Artist's conception of a binary system consisting of a black hole and a main sequence star.  The black hole is drawing matter from the main sequence star via an [[accretion disk]] around it, and some of this matter forms a gas jet.]]

A '''black hole''' is a theoretical mass with an [[escape velocity]] greater than the speed of light.  They cannot be observed directly using visible light spectrums, but the nature and properties of a black hole mean that they can be discovered through the detection of specific kinds of radiation, as well as other methods.  Several black holes have already been discovered, <ref>http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971124b.html</ref><ref>http://dsc.discovery.com/news/2008/04/01/smallest-black-hole.html</ref> with the largest black hole so far discovered being named Chandra <ref>http://www.nasa.gov/mission_pages/chandra/news/07-112.html</ref>. While matter and energy, even light, may not escape a black hole, [[Stephen Hawking]] has shown that they should emit [[Hawking radiation]], which absent of an influx of mass-energy would lead to the evaporation of the black hole. This theory however is unable to be tested.

A '''black hole''' is a theoretical mass with an [[escape velocity]] greater than the speed of light.  There is no direct evidence that they actually exist.  Some scientists claim to have identified black holes based on circumstantial evidence.<ref>http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971124b.html</ref><ref>http://dsc.discovery.com/news/2008/04/01/smallest-black-hole.html</ref>  While matter and energy, even light, may not escape a black hole, [[Stephen Hawking]] has shown that they should emit [[Hawking radiation]], which absent of an influx of mass-energy would lead to the evaporation of the black hole. This theory however is unable to be tested.

The outer surface of a black hole is an [[event horizon]], a non-material dimensional boundary, beyond which nothing can escape.

The outer surface of a black hole is an [[event horizon]], a non-material dimensional boundary, beyond which nothing can escape.