According to its mass, density, temperature, etc., every star follows a particular life cycle at the end of which some will become neutron stars—very small bodies with collapsed atoms, low luminosity, and a density so high as to be scarcely believable: a thimbleful of neutron star material would weigh over 10 million tons. Neutron stars are believed to form in supernovae such as the one that formed the Crab Nebula. The stars that eventually become neutron stars are thought to start out with about 15 to 30 times the mass of our sun. The star’s core accumulates iron through fusion reaction until it gets to about 1.4 solar, at which point the electron degeneracy pressure that had been supporting it against gravity gives up the ghost and collapses inward. At the very high pressures involved in this collapse, it is energetically favorable to combine protons and electrons to form neutrons plus neutrinos. The neutrinos escape after scattering a bit and helping the supernova happen, and the neutrons settle down to become a neutron star, with neutron degeneracy managing to oppose gravity.

Vela Neutron Star, Chandra, courtesy NASA/PSU