Back to: Astronomy & the Cosmos
A black hole is a region where matter has been crushed so densely that gravity overwhelms everything else; within a boundary called the event horizon, not even light moves fast enough to escape, which is why the object is ‘black.’ Stellar-mass black holes form when the collapsing cores of the most massive stars have nothing left to stop them. Despite the sci-fi reputation, black holes are not cosmic vacuum cleaners: if the Sun were somehow swapped for a black hole of equal mass, Earth’s orbit wouldn’t change at all. Distance is safety; only things that venture close are captured.
Black holes also warp time itself. Einstein’s general relativity, our extraordinarily well-tested theory of gravity, predicts that clocks run slower in strong gravity, so time near a black hole genuinely passes more slowly relative to distant observers. And nearly every large galaxy, including our Milky Way, hosts a supermassive black hole at its center, millions to billions of times the Sun’s mass; ours is called Sagittarius A*.
Once purely theoretical, black holes are now observed directly. Astronomers have detected gravitational waves, ripples in spacetime, from black holes colliding, and in 2019 the Event Horizon Telescope produced the first actual image of a black hole’s glowing shadow, in galaxy M87, followed by an image of our own Sagittarius A*. The most extreme prediction of twentieth-century physics turned out to be simply, photographably real.