neutronstar

A neutron star, sometimes written as neutronstar, is a compact stellar remnant formed when a massive star collapses in a core-collapse supernova. With masses around 1.1 to 2.3 solar masses compressed into a radius of about 10 to 12 kilometers, a neutron star has densities comparable to that of atomic nuclei. The gravity at the surface is extreme, and the matter is held up primarily by neutron degeneracy pressure and strong interactions.

Structure and composition: The outer layers form a thin atmosphere and a solid crust of nuclei and electrons. Deeper, free neutrons permeate a neutron superfluid; the core may contain superfluid neutrons, superconducting protons, and possibly more exotic phases such as deconfined quark matter or nuclear pasta, depending on the equation of state. The exact composition is not well constrained.

Magnetic fields and rotation: Many neutron stars are observed as pulsars, emitting beams of radio waves (and sometimes X-ray/gamma-ray) radiation that sweep across Earth as the star rotates. Rotation periods span milliseconds to several seconds; magnetic fields typically range from 10^8 to 10^15 gauss.

Observational properties and applications: Neutron stars can be detected through radio pulsations, X-ray emission from hot surfaces or accretion, and gravitational waves from binary mergers. Masses are measured in binary systems; radii are inferred from X-ray observations and, more recently, gravitational-wave and multi-messenger data. The Tolman–Oppenheimer–Volkoff limit sets an upper bound on mass, depending on the equation of state, above which collapse to a black hole occurs.

Formation and significance: Neutron stars form from the remnants of massive stars after supernova explosions. Mergers of neutron stars are a source of gravitational waves and sites of rapid neutron capture (r-process) nucleosynthesis, contributing to the production of heavy elements and kilonova transients.