Black holes form when massive stars collapse at the end of their lives. Gravity overwhelms every other force — the core compresses so extremely that it forms a singularity: a point where density and spacetime curvature become infinite and the known laws of physics cease to apply.
Surrounding the singularity is the event horizon — not a physical surface, but a geometric boundary. Whoever crosses it leaves the observable world forever. There, escape velocity exceeds the speed of light, and since nothing can travel faster than light, there is no way back — not for matter, not for information, not even for light itself.
In the extreme gravity near the horizon, time measurably slows down. An observer approaching the black hole would watch the outside world race through millions of years while barely aging themselves. Rotating black holes drag spacetime along like a whirlpool: within the ergosphere no object can stand still — even light is swept along.
Black holes themselves are invisible — but their effects make them visible. Surrounding gas heats to millions of degrees through friction and glows as an accretion disk. Their gravitational field bends the light of distant stars, making them appear at positions where they aren't. The first direct image of a black hole was captured by the Event Horizon Telescope in 2019.