
Physical characteristics like size, mass, and internal structure of neutron stars.Neutron stars are the collapsed cores of massive stars that exploded as supernovae. They pack roughly 1.4 times the Sun's mass into a sphere about 20 kilometers in diameter, making them among the densest objects in the universe. Their properties — extreme density, rapid rotation, and intense magnetic fields — are governed by nuclear physics under conditions unattainable on Earth. Key characteristics include a solid crust of neutron-rich nuclei, a possible superfluid interior, and a radius that is remarkably insensitive to mass, a feature that constrains the equation of state of nuclear matter.
Neutron stars were first proposed by Walter Baade and Fritz Zwicky in 1934, shortly after the discovery of the neutron. They were observationally confirmed in 1967 with the discovery of pulsars by Jocelyn Bell Burnell and Antony Hewish. The first precise mass-radius measurements came from X-ray observations in the 2000s, and gravitational wave astronomy added new insights in 2017.
Neutron stars are supported against gravitational collapse by neutron degeneracy pressure and, at higher densities, by repulsive nuclear forces. Their structure is described by the Tolman–Oppenheimer–Volkoff equation, which relates pressure and density. Observations of neutron star masses (often near 1.4 solar masses, but up to 2.1) and radii (typically 10–14 km) help physicists determine the equation of state — the relationship between pressure and density — which is still uncertain at the highest densities. Gravitational wave signals from neutron star mergers, like GW170817, have provided new constraints on their deformability and maximum mass.
Neutron star studies are a natural laboratory for testing nuclear physics at extreme densities, informing models of matter in heavy-ion collisions and potentially guiding the design of dense materials.
A teaspoon of neutron star material would weigh about 4 billion tons on Earth — roughly the mass of all humans combined.







