Definition
A black hole is a region of spacetime where gravity is so intense that no matter or radiation inside its boundary can escape to the outside universe.
The boundary itself is the event horizon. The interior contains a singularity — a region of formally infinite curvature predicted by general relativity. Black holes form when massive stars exhaust their nuclear fuel and collapse, and they grow by accreting matter and merging with each other.
Why it matters
How it works
A non-rotating black hole is described by the 1916 Schwarzschild solution to Einstein's equations: outside the event horizon, spacetime is curved but well-behaved; at the horizon (radius r_s = 2GM/c²), the escape velocity equals the speed of light; inside, all timelike futures point toward the central singularity. A rotating, charged black hole is described more generally by the Kerr–Newman solution.
Astrophysical black holes come in three observed populations: stellar-mass (a few to ~100 solar masses, from collapsed stars), intermediate-mass (10² to 10⁵ solar masses, recently confirmed), and supermassive (10⁵ to 10¹⁰ solar masses, in galactic cores — Sagittarius A* at the center of the Milky Way is 4.3 million solar masses).
Direct evidence accumulated rapidly in the 21st century: LIGO detected merging stellar-mass black holes in 2015; the Event Horizon Telescope imaged the silhouette of M87* in 2019 and Sagittarius A* in 2022; X-ray observations track accretion disks; orbital mechanics in galactic centers map central masses.
Hawking radiation (1974) means black holes are not eternally black — they emit a faint thermal spectrum and slowly evaporate, with a temperature inversely proportional to mass. A solar-mass black hole would take 10⁶⁷ years to evaporate; only primordial mini-holes could plausibly be evaporating today.