A black hole is a region of space where gravity is so strong that nothing, not even light, can escape from inside a boundary called the event horizon. That sounds impossible, but it follows directly from Einstein's general relativity, which describes gravity as the curvature of spacetime by mass and energy. Pack enough mass into a small enough volume, and spacetime curves so steeply that all paths lead inward, which is why black holes are black: no light leaves them to reach our eyes.

Black holes are not cosmic vacuum cleaners. Far from one, gravity behaves normally, and objects can orbit a black hole safely just as planets orbit stars. The danger zone is close in, where tidal forces stretch anything approaching into a thin stream, a process astronomers cheerfully call spaghettification. Most black holes sit quietly for millions of years, detectable only through their effects on nearby stars and gas, which is why they remained theoretical for so long after they were first predicted.

Stellar-mass black holes form when massive stars die. A star many times heavier than the Sun burns through its fuel, its core collapses under its own gravity, and the outer layers explode as a supernova, leaving a collapsed core so dense it becomes a black hole. These typically weigh a few to a few dozen times the Sun's mass and are found across galaxies, sometimes in binary systems where they pull gas from a companion star, heating it until it glows in X-rays.

For decades, black holes were inferred from the motion of stars and gas around invisible massive objects. Then in 2015, gravitational wave detectors recorded ripples in spacetime from two merging black holes, the first direct detection of these objects and the opening of a new way to observe the universe. In 2019, the Event Horizon Telescope produced the first image of a black hole's shadow, a glowing ring of hot gas around the darkness of the event horizon in the galaxy M87, followed by an image of our own galaxy's central black hole.

Black holes are natural laboratories for extreme physics. Inside them, general relativity and quantum mechanics, our two best theories of nature, give conflicting answers, and resolving that conflict is one of the deepest problems in science. Hawking radiation, Stephen Hawking's prediction that black holes slowly evaporate by quantum effects at their horizons, remains unconfirmed but profoundly influential, because it connects gravity, quantum theory, and thermodynamics in a single object.