black hole
A black hole is a place where gravity has won so completely that nothing can climb back out — not a rocket, not a beam of light, not even the fastest thing in the universe. Picture a funnel so steep that whatever rolls in can never roll back up. That "point of no return" is a real, invisible boundary called the event horizon: cross it, and there is simply no road back.
Most black holes are the corpses of giant stars. When a star far heavier than our Sun runs out of fuel, its core collapses under its own weight and crushes down to an unimaginable density — so much mass squeezed into so little space that gravity near it becomes overwhelming. We can't see the hole itself, because no light leaves it; we spot it by how it bends starlight, whips nearby stars around, or heats the gas spiralling in until it blazes.
A common myth is that black holes are cosmic vacuum cleaners, roaming space and sucking everything in. They don't. From a safe distance a black hole pulls on you exactly as an ordinary star of the same mass would — if the Sun were swapped for a black hole of equal weight, Earth's orbit wouldn't change at all. You only meet your doom if you wander past the edge.
The event horizon of a black hole as heavy as the Sun would have a radius of only about 3 km — small enough to tuck inside a city.
Crush the entire Sun small enough, and its gravity would trap its own light.
The vivid name "black hole" was popularised by physicist John Wheeler in 1967, though the idea of a star whose gravity traps its own light goes back to the 1780s. The boundary itself — the event horizon — was a prediction of Einstein's general relativity, and in 2019 the Event Horizon Telescope released the first actual image of one, a glowing ring of gas around the dark heart of galaxy M87.