When a Star Collapses

Stars above roughly eight solar masses end their lives in a core-collapse supernova. Once the core runs out of nuclear fuel, gravity wins. In less than a second, the iron core implodes from Earth-size to a city-size sphere just 20 kilometers across.

The outer layers rebound off this ultra-dense core and blast outward as a supernova. What remains is a neutron star: more mass than our Sun packed into that city-size sphere. A single teaspoon of neutron star material weighs about one billion tons.