1 The collapse of a star's core, as in the formation of a neutron star. The core collapses when it has evolved to the point where it consists entirely of iron, which cannot be burnt in nuclear reactions and hence produces no energy to support itself against gravity. The core then contracts, releasing gravitational potential energy which tears the nuclei of iron atoms into their constituent protons and neutrons. As the density rises, protons combine with electrons to form neutrons. Collapse is halted when the pressure from the degeneracy of the neutron gas balances the inward pull of gravity. The entire process takes less than a second.
2 The collapse of the central part of a star cluster after energy is transferred to its outer parts through encounters between its constituent stars. The age of the cluster at which this should theoretically occur is, however, less than the true age of many globular clusters. The effects of encounters involving binary stars are thought to be important in averting core collapse.