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.