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

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

A '''black hole''' is a formation in space with a density so large that its gravitational force causes the escape velocity to exceed the speed of light. Thus, "black" refers to the impossibility of any light ([[photon]]s) escaping, thereby making them completely dark.<ref>They are assumed to come into existence from extremely large stars that collapse into a state of high density when they run out of fusion fuel.  An object becomes a black hole when it lies entirely inside the Schwarzschild radius (see below) determined by its mass.  For most objects, the Schwarzschild radius is very tiny compared with its size (for Earth it is about 1 centimeter), so the object could not lie inside that radius.</ref>

A '''black hole''' is a formation in space with a density so large that its gravitational force causes the escape velocity to exceed the speed of light. Thus "black" refers to the impossibility of any light ([[photon]]s) escaping, thereby making them completely dark.<ref>They are assumed to come into existence from extremely large stars that collapse into a state of high density when they run out of fusion fuel.  An object becomes a black hole when it lies entirely inside the Schwarzschild radius (see below) determined by its mass.  For most objects, the Schwarzschild radius is very tiny compared with its size (for Earth it is about 1 centimeter), so the object could not lie inside that radius.</ref>

Black holes have become extremely common topics of conversation (as well as scientific discussion, of course) over the last few decades.  These discussions are common in such things as the science pages of the ''[[New York Times]]'' and other magazines, and movies including ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014).

Black holes have become extremely common topics of conversation (as well as scientific discussion, of course) over the last few decades.  These discussions are common in such things as the science pages of the ''[[New York Times]]'' and other magazines, and movies including ''Event Horizon'' (1997), ''The Black Hole'' (TV, 2006), and ''Interstellar'' (2014).

Black holes have taken on a very common everyday meaning, as a place from which nothing emerges, as in the ''New York Times'' referring to Justice [[David Souter]]'s "chambers as a black hole, from which nothing emerges," <ref>Linda Greenhouse, ''New York Times'' (May 16, 1991).</ref>

Black holes have taken on a very common everyday meaning, as a place from which nothing emerges, as in the ''New York Times'' referring to Justice [[David Souter]]'s "chambers as a black hole, from which nothing emerges,"<ref>Linda Greenhouse, ''New York Times'' (May 16, 1991).</ref> After that disparagement by the [[liberal media]], Souter flipped from the [[conservative]] to the [[liberal]] side on [[abortion]] and was never criticized like that again by [[liberal]]s.

Black holes had never been '''directly''' observed, just as electrons, pions, and gamma rays haven't.  But, like those other things, evidence for their existence is now incontrovertible. In early 2019 astronomers released an image of a super-massive black hole located in the galaxy Messier 87 - some 55 million light-years away<ref>https://www.washingtonpost.com/science/2019/04/10/see-black-hole-first-time-images-event-horizon-telescope/</ref><ref>https://news.harvard.edu/gazette/story/2019/04/harvard-scientists-lead-team-revealing-black-hole/</ref>. This required gathering data from nine observatories around the world and combining them, using atomic clocks.  Like all radio astronomy observations, extensive computer processing was used to produce a visible image.  That black hole has since been informally named "Powehi".<ref>http://time.com/5569262/black-hole-hawaii-powehi/</ref>

Black holes had never been '''directly''' observed, just as electrons, pions, and gamma rays haven't.  But, like those other things, evidence for their existence is now incontrovertible. In early 2019 astronomers released an image of a super-massive black hole located in the galaxy Messier 87 - some 55 million light-years away<ref>https://www.washingtonpost.com/science/2019/04/10/see-black-hole-first-time-images-event-horizon-telescope/</ref><ref>https://news.harvard.edu/gazette/story/2019/04/harvard-scientists-lead-team-revealing-black-hole/</ref>. This required gathering data from nine observatories around the world and combining them, using atomic clocks.  Like all radio astronomy observations, extensive computer processing was used to produce a visible image.  That black hole has since been informally named "Powehi".<ref>http://time.com/5569262/black-hole-hawaii-powehi/</ref>

Evidence for the existence of black holes relies on the observations of radiation from accretion disks, motion of stars near the center of the galaxy, and "gravitational lensing," around places where there are no visible objects.

Evidence for the existence of black holes relies on the observations of radiation from accretion disks, the motion of stars near the center of the galaxy, and "gravitational lensing," around places where there are no visible objects.

Because of [[quantum mechanics|quantum-mechanical]] phenomena involving Hawking Radiation, discussed later in the article, black holes must emit radiation and thus are not ''perfectly'' black.  Nevertheless, they are called "black holes".  Quibbling about this is similar to quibbling about the statement "The Earth is round".

Because of [[quantum mechanics|quantum-mechanical]] phenomena involving Hawking Radiation, discussed later in the article, black holes must emit radiation and thus are not ''perfectly'' black.  Nevertheless, they are called "black holes".  Quibbling about this is similar to quibbling about the statement "The Earth is round".