A black hole is a region of spacetime in which gravity prevents anything, including light, from escaping across a boundary called the event horizon. Black holes are predicted by general relativity and identified through their effects on nearby matter, radiation, and spacetime. Although their interiors cannot be observed directly, orbiting stars, luminous gas, gravitational-wave signals, and images of their surroundings provide evidence for their existence. (science.nasa.gov)
Relativistic description
In the general theory of relativity, developed by Albert Einstein, gravity is described through spacetime curvature. Black holes arise when this geometry produces a region from which no outward signal can reach distant observers. They are therefore more than unusually dense objects: their defining property is a causal boundary separating the interior from the exterior universe. (nobelprize.org)
For a nonrotating, electrically neutral black hole, the horizon lies at the Schwarzschild radius,
[ r_{\mathrm{s}}=\frac{2GM}{c^{2}}, ]
where (G) is the gravitational constant, (M) is the black hole’s mass, and (c) is the speed of light. The radius is approximately three kilometres per solar mass. This formula describes the simplest black-hole geometry; rotation changes the horizon’s structure. (nasa.gov)
In classical general relativity, an isolated, stationary black hole is characterized by mass, electric charge, and angular momentum. Its horizon is not a solid material surface. Classical solutions also contain a gravitational singularity, where the mathematical description becomes incomplete. This does not establish that a physically observable point of infinite density exists; it indicates a regime where a more complete theory, potentially involving quantum gravity, is needed. (hawking.org.uk)
Formation and mass categories
Astronomers distinguish stellar-mass, intermediate-mass, and supermassive black holes, although their mass boundaries are approximate. Stellar-mass black holes can form when the core of a massive star collapses after exhausting its fuel. Depending on the conditions, collapse may accompany a supernova or leave a neutron star rather than a black hole. Black holes subsequently grow by capturing matter and merging with other black holes. (science.nasa.gov)
Supermassive black holes contain hundreds of thousands to billions of solar masses and occur at the centres of most large galaxies. Their initial formation remains uncertain. Proposed pathways include the collapse of very massive early stars or gas clouds, followed by accretion and mergers. The existence of massive black holes early in cosmic history places constraints on these growth mechanisms. Intermediate-mass objects occupy the range between stellar and supermassive populations and are important for investigating possible evolutionary connections. (science.nasa.gov)
Primordial black holes are hypothetical objects that could have formed from dense regions in the early universe rather than stellar collapse. They have not been conclusively identified and constitute a distinct formation hypothesis, not an established population. (science.nasa.gov)
Surrounding matter and radiation
Matter approaching a black hole may collect into an accretion disk, where orbital motion and internal interactions heat the gas. Such material can emit intense electromagnetic radiation, including X-rays, before crossing the horizon. The observed brightness therefore comes from the surroundings, not from light escaping the interior. Some accreting systems also launch oppositely directed jets at nearly the speed of light; these originate outside the horizon. (science.nasa.gov)
Strong gravitational lensing bends light around the black hole and distorts the appearance of nearby gas. Light capture and bending produce a dark “shadow” against surrounding emission. This shadow is larger than the event horizon and should not be confused with a photograph of a material surface. (science.nasa.gov)
Differences in gravitational acceleration across an object produce tidal stretching, often called spaghettification. Its severity near the horizon depends on the black hole’s mass: small black holes can produce stronger horizon-scale tidal forces than supermassive ones. Crossing the horizon and being torn apart are consequently distinct physical events. (science.nasa.gov)
Observational evidence
Stellar orbits reveal the mass and compactness of otherwise unseen objects. At the Milky Way’s centre, observations of rapidly orbiting stars established a compact object of approximately four million solar masses associated with Sagittarius A*. Accreting stellar-mass black holes can likewise be investigated through companion-star motion and variable X-ray emission. (nobelprize.org)
A separate line of evidence comes from gravitational waves. On September 14, 2015, the two LIGO detectors observed GW150914, a signal produced by the merger of two black holes. Its evolving waveform traced their inspiral, merger, and the settling of the remnant, enabling tests of relativistic predictions in strong gravity. (arxiv.org)
The Event Horizon Telescope uses radio interferometry across widely separated observatories. On April 10, 2019, its collaboration released an image of the shadow and surrounding emission of M87*. On May 12, 2022, it released an image of Sagittarius A*. Both results reconstruct radio emission near the horizon rather than revealing the black-hole interior. (hq.eso.org)
Quantum physics and thermodynamics
Black holes connect gravitation with quantum mechanics and thermodynamics. Stephen Hawking’s calculations predict Hawking radiation, approximately thermal emission associated with quantum effects in black-hole spacetime. For a neutral, nonrotating black hole, the predicted temperature decreases as mass increases. Radiation carries away energy and can cause evaporation, although the expected timescales for stellar and supermassive black holes greatly exceed the universe’s present age. (hawking.org.uk)
The Bekenstein–Hawking relation assigns black holes an entropy proportional to horizon area rather than volume. Evaporation also raises the black-hole information paradox: how information about an initial quantum state is represented if a black hole disappears. Research on this problem investigates the compatibility of quantum evolution, semiclassical radiation calculations, and the microscopic description of gravity. (journals.aps.org)