A black hole is a region of spacetime wherein gravity is so strong that no matter
or electromagnetic energy (e.g. light) can escape it.[2] Albert Einstein's theory
of general relativity predicts that a sufficiently compact mass can deform
spacetime to form a black hole.[3][4] The boundary of no escape is called the event
horizon. A black hole has a great effect on the fate and circumstances of an object
crossing it, but it has no locally detectable features according to general
relativity.[5] In many ways, a black hole acts like an ideal black body, as it
reflects no light.[6][7] Quantum field theory in curved spacetime predicts that
event horizons emit Hawking radiation, with the same spectrum as a black body of a
temperature inversely proportional to its mass. This temperature is of the order of
billionths of a kelvin for stellar black holes, making it essentially impossible to
observe directly.
Objects whose gravitational fields are too strong for light to escape were first
considered in the 18th century by John Michell and Pierre-Simon Laplace.[8] In
1916, Karl Schwarzschild found the first modern solution of general relativity that
would characterise a black hole. Due to his influential research, the Schwarzschild
metric is named after him. David Finkelstein, in 1958, first published the
interpretation of "black hole" as a region of space from which nothing can escape.
Black holes were long considered a mathematical curiosity; it was not until the
1960s that theoretical work showed they were a generic prediction of general
relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked
interest in gravitationally collapsed compact objects as a possible astrophysical
reality. The first black hole known was Cygnus X-1, identified by several
researchers independently in 1971.[9][10]
Black holes of stellar mass form when massive stars collapse at the end of their
life cycle. After a black hole has formed, it can grow by absorbing mass from its
surroundings. Supermassive black holes of millions of solar masses (M☉) may form by
absorbing other stars and merging with other black holes, or via direct collapse of
gas clouds. There is consensus that supermassive black holes exist in the centres
of most galaxies.
The presence of a black hole can be inferred through its interaction with other
matter and with electromagnetic radiation such as visible light. Any matter that
falls toward a black hole can form an external accretion disk heated by friction,
forming quasars, some of the brightest objects in the universe. Stars passing too
close to a supermassive black hole can be shredded into streamers that shine very
brightly before being "swallowed."[11] If other stars are orbiting a black hole,
their orbits can be used to determine the black hole's mass and location. Such
observations can be used to exclude possible alternatives such as neutron stars. In
this way, astronomers have identified numerous stellar black hole candidates in
binary systems and established that the radio source known as Sagittarius A*, at
the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3
million solar masses.
