Neutron Stars & Neutron Degeneracy: Physical Mechanisms

A neutron star has a thin atmosphere, a solid crust of nuclei and electrons, deeper layers containing increasingly neutron-rich matter, and an uncertain high-density core whose composition may include superfluid neutrons, superconducting protons, hyperons, or other phases depending on the equation of state.

During core collapse, electrons combine with protons and produce neutrons and neutrinos. At nuclear density, short-range nuclear interactions and neutron degeneracy contribute pressure that can halt collapse if the remnant mass is not too large. The exact maximum neutron-star mass depends on the dense-matter equation of state.

Pulsar timing measures neutron-star masses with great precision, X-ray observations constrain radii, gravitational waves from neutron-star mergers probe tidal deformability, and observed masses above two solar masses rule out many overly soft equations of state.

Core collapse drives electrons into protons, producing neutrons and neutrinos. At extreme density, neutron degeneracy and nuclear interactions provide pressure that can halt collapse below the maximum stable neutron-star mass.