Neutron Stars & Neutron Degeneracy: Structures & Vocabulary

A neutron star is the compact remnant produced when a massive stellar core collapses to densities comparable to atomic nuclei. With roughly one to two solar masses compressed into a sphere only tens of kilometers across, neutron stars probe matter at densities unattainable in terrestrial laboratories.

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.

The outer crust contains nuclei and electrons, deeper crust becomes increasingly neutron-rich, and the core may contain superfluid neutrons, superconducting protons, and possibly more exotic phases. The exact interior depends on the dense-matter equation of state.