Black holes form when massive stars collapse under their own gravity at the end of their life cycles.

When a giant star runs out of fuel, the outward pressure of nuclear fusion stops. Gravity wins. The star's core collapses inward, packing immense mass into a tiny volume. This collapse triggers a supernova explosion, leaving behind a dense stellar corpse. If the remaining core is heavy enough, no known force can stop the collapse. It shrinks down past its own event horizon, creating a stellar-mass black hole.