The Tolman-Oppenheimer-Volkoff Limit

Neutron stars cannot grow without limit. Above roughly 2.2 solar masses, not even neutron degeneracy pressure can resist gravity. The star collapses further, this time into a black hole.

Recent observations of very massive neutron stars, around two solar masses, have already ruled out several proposed equations of state. Each new measurement narrows the possibilities for what matter does under these extreme conditions.