When a stellar core is no longer supported against gravity, it collapses in on itself with velocities reaching 70,000 km/s (0.23 c), resulting in a rapid increase in temperature and density. What follows depends on the mass and structure of the collapsing core, with low-mass degenerate cores forming neutron stars, higher-mass degenerate cores mostly collapsing completely to black holes, and non-degenerate cores undergoing runaway fusion.