Neutron Star
An incredibly dense stellar remnant left after a core-collapse supernova. About 20 km wide but containing more mass than the Sun.
A neutron star is the collapsed core of a massive star that has gone supernova. Its mass is typically 1.4-2.5 times that of the Sun, but compressed into a sphere only ~20 km across. A teaspoon of neutron-star material would weigh roughly a billion tonnes on Earth.
Neutron stars are held up against gravity by neutron degeneracy pressure — a quantum-mechanical effect resulting from the Pauli exclusion principle. If a remnant exceeds about 2.5-3 solar masses, even this pressure cannot resist gravity, and the result is a black hole instead.
Many neutron stars rotate hundreds of times per second and emit powerful beams of radiation; if these beams sweep across Earth, we detect them as pulsars. The 2017 detection of gravitational waves from a neutron star merger (GW170817) confirmed that such mergers are a major source of heavy elements like gold and platinum.