A neutron star is what remains after a massive star, roughly eight to twenty times the mass of the sun, runs out of fuel and collapses in a supernova explosion. The core that is left behind gets crushed so violently that protons and electrons are forced together into neutrons, packing an entire star’s worth of mass into a sphere only about twenty kilometers across.
A single teaspoon of neutron star material would weigh roughly as much as a mountain here on Earth. Many neutron stars spin extremely fast, some completing hundreds of rotations per second, and sweep beams of radiation across space like a lighthouse, an effect that lets us detect them as pulsars.
When two neutron stars eventually collide, they release a burst of gravitational waves along with a flash of light called a kilonova, an event first directly observed in 2017 that also confirmed neutron star collisions forge heavy elements like gold and platinum.
Some neutron stars take the extremes even further. Magnetars are neutron stars with magnetic fields trillions of times stronger than anything producible on Earth, powerful enough that, at close range, they could theoretically disrupt the atomic structure of ordinary matter. Occasionally a magnetar’s crust cracks under the strain of its own field, releasing a sudden burst of gamma rays called a starquake, energetic enough to be detected from tens of thousands of light-years away. Despite these extremes, neutron stars remain relatively small targets for astronomers, and most of what is known about their interior structure, whether it contains exotic states of matter beyond ordinary neutrons, comes from indirect evidence, like how their mass and radius relate to each other, rather than any direct observation of what lies beneath their surface.
Neutron stars were proposed as a theoretical possibility in 1934 by astronomers Walter Baade and Fritz Zwicky, more than three decades before anyone actually detected one. It took until 1967, when Jocelyn Bell Burnell picked up the rapid, regular radio pulses of a spinning neutron star, now called a pulsar, for the prediction to finally be confirmed observationally.