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A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping.

Albert Einstein's theory of general relativity, which describes gravitation as the curvature of spacetime, predicts that any sufficiently compact mass will form a black hole. General relativity also predicts that every black hole should have a central singularity, where the curvature of spacetime is infinite. The first widely accepted black hole was Cygnus X-1, an x-ray source proposed as a black hole binary through several studies between 1971 and 1974. Supermassive black holes of millions of solar masses may form by absorbing stars and merging with other black holes, or via direct collapse of gas clouds. This causes the black hole to lose mass very slowly, provided it is not accreting matter. However, even the smallest class of black holes observed, stellar black holes, are gaining mass from the cosmic microwave background faster than they are losing mass via Hawking radiation. Matter falling toward a black hole can form an accretion disk of infalling plasma, heated by friction and emitting light. If stars are orbiting a black hole, their motions can be used to determine the black hole's mass and location. 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.

While Cygnus X-1, a stellar-mass black hole, was generally accepted by the scientific community as a black hole by the end of 1973, it would be decades before a supermassive black hole would gain the same broad recognition.

However, verifying an object as a black hole by this definition would require waiting for an infinite time and at an infinite distance from the black hole to verify that nothing has escaped, and thus cannot be used to identify a physical black hole.

The simplest equilibrium black hole model with only mass but neither electric charge nor angular momentum is called a Schwarzschild black hole. The most general stationary black hole solution known is the Kerr–Newman metric, which describes a black hole with both charge and angular momentum.

Theoretical models predict that the accretion disc that feeds black holes will be unstable once a black hole reaches 50×109–100×109 M☉, setting a rough upper limit to black hole mass.

For a photon orbiting prograde, the photon sphere will be 0.5–1.5 Schwarzschild radii from the centre of the black hole, while for a photon orbiting retrograde, the photon sphere will be between 3–4 Schwarzschild radii from the centre of the black hole. For a charged, nonrotating black hole, there will only be one photon sphere, and the radius of the photon sphere will decrease for increasing black hole charge.

Since the volume within the Schwarzschild radius increases with the cube of the radius, average density of a black hole inside its Schwarzschild radius is inversely proportional to the square of its mass: supermassive black holes are much less dense than stellar black holes.

The defining feature of a black hole is the existence of an event horizon, a boundary in spacetime through which matter and light can pass only inward towards the centre of the black hole.

Black holes can result from the merger of two neutron stars or a neutron star and a black hole.

When a black hole accretes matter, the gas in the inner accretion disk orbits at very high speeds because of its proximity to the black hole.

For a black hole that is not still forming or accreting matter, the zeroth law of black hole mechanics states the black hole's surface gravity is constant across the event horizon.

Quick Facts

• The first widely accepted black hole was Cygnus X-1, an x-ray source proposed as a black hole binary through several studies between 1971 and 1974.
• If stars are orbiting a black hole, their motions can be used to determine the black hole's mass and location.
• For a black hole that is not still forming or accreting matter, the zeroth law of black hole mechanics states the black hole's surface gravity is constant across the event horizon.
• While Cygnus X-1, a stellar-mass black hole, was generally accepted by the scientific community as a black hole by the end of 1973, it would be decades before a supermassive black hole would gain the same broad recognition.
• Supermassive black holes of millions of solar masses may form by absorbing stars and merging with other black holes, or via direct collapse of gas clouds.

Source material: Wikipedia - "Black hole". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.