A black hole is a region where mass has been packed so densely that its gravitational pull becomes inescapable beyond a certain boundary, called the event horizon. Nothing that crosses that boundary, not gas, not light, not even information, can ever get back out.
Most black holes form when a massive star collapses at the end of its life, but far larger ones, called supermassive black holes, sit at the centers of most galaxies, including our own Milky Way, with masses millions or billions of times that of the sun.
Despite their name, black holes are not empty voids that swallow everything nearby indiscriminately, objects can safely orbit one at a distance, the same way planets orbit the sun. In 2019, the Event Horizon Telescope captured the first ever direct image of a black hole’s silhouette, confirming decades of theoretical predictions with an actual picture.
Black holes span an enormous range of sizes. Stellar mass black holes, formed from collapsing stars, typically weigh a few to a few dozen times the mass of the sun and measure only kilometers across. Supermassive black holes are a different beast entirely, Sagittarius A*, the one at the center of the Milky Way, weighs about four million times the mass of the sun and was directly imaged by the Event Horizon Telescope in 2022. How supermassive black holes grew so large so early in the universe’s history is still an open question, since simply swallowing gas and merging with other black holes does not obviously explain masses that large appearing as early as observations show them existing.
Black holes are not entirely eternal either. Stephen Hawking showed in 1974 that quantum effects near the event horizon should let a black hole slowly leak radiation and lose mass over time, a process now called Hawking radiation. For any black hole formed from a collapsing star, that evaporation is so slow it would take far longer than the current age of the universe to matter, but the prediction remains one of the most important attempts yet to connect gravity with quantum mechanics.