
The internal composition and properties determining neutron star mass and radius.Neutron stars are the collapsed cores of massive stars that exploded as supernovae. They pack more mass than the Sun into a sphere only about 20 kilometers across, making them the densest visible objects in the universe. Their internal structure — the arrangement of neutrons, protons, and possibly exotic particles like hyperons or quark matter — determines how mass and radius relate, which is a key observable constraint for nuclear physics at extreme densities.
The concept of neutron stars was proposed by Walter Baade and Fritz Zwicky in 1934, shortly after the discovery of the neutron. The first confirmed detection came in 1967 with the discovery of pulsars by Jocelyn Bell Burnell and Antony Hewish.
The structure is governed by the equation of state (EoS), which describes how pressure and density relate inside the star. Gravity compresses the star until neutron degeneracy pressure and strong nuclear forces halt collapse. Measuring the mass and radius of neutron stars — for example, via X-ray observations or gravitational waves — allows astronomers to infer the EoS and test models of dense matter.
Neutron star structure studies inform nuclear physics models used in heavy-ion collision experiments and help calibrate theories of dense matter that apply to atomic nuclei.
A teaspoon of neutron star material would weigh about a billion tons on Earth.















