Neutron Stars & Neutron Degeneracy: Observation, Measurement & Evidence

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.

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.

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 masses, X-ray pulse modeling constrains radii, glitches probe superfluid interiors, and gravitational-wave mergers measure tidal deformability. Observed two-solar-mass neutron stars exclude many overly soft equations of state.