Neutron star

A neutron star is the compact remnant left when a massive star exhausts its fuel and its core collapses under gravity in a supernova. With a mass typically between about 1.1 and 2.3 times that of the Sun compressed into a sphere only about 20 kilometers across, it reaches densities comparable to an atomic nucleus — a teaspoon of its material would weigh billions of tonnes. The collapse forces electrons and protons to combine into neutrons, and the star is supported against further collapse mainly by neutron degeneracy pressure and the repulsive core of the strong nuclear force.

Neutron stars are laboratories for physics at the extremes. Their surface gravity is hundreds of billions of times Earth's, their magnetic fields can exceed a trillion gauss, and the spacetime around them is strongly curved, so general-relativistic effects on orbits, light, and clocks are large and measurable. Many neutron stars are observed as pulsars: rapidly rotating, magnetized stars whose radio (or X-ray) beams sweep past Earth like a lighthouse, producing pulses so regular they rival atomic clocks in stability.

When two neutron stars are bound in a tight binary, the system is a premier testbed for gravitation. The Hulse–Taylor binary PSR B1913+16 provided the first evidence that such systems radiate gravitational waves, through the slow decay of its orbit. Binary neutron-star mergers, like GW170817 detected in 2017, produce both gravitational waves and electromagnetic signals — a kilonova and a gamma-ray burst — and are thought to forge much of the universe's heavy elements such as gold and platinum.

Neutron stars were predicted by Walter Baade and Fritz Zwicky in 1934, shortly after the neutron's discovery, as the endpoint of supernova core collapse. They were confirmed observationally in 1967 with the discovery of the first pulsar by Jocelyn Bell Burnell.