What is a Pulsar?
A pulsar is a rapidly rotating, strongly magnetised neutron star that fires beams of radiation from its magnetic poles. Because those poles are usually tilted away from the rotation axis, the beams sweep through space like a lighthouse — and if one crosses Earth, we see a pulse of astonishing regularity.
The spin is inherited. When a stellar core collapses from something the size of Earth to a ball 22 kilometres across, conservation of angular momentum spins it up by a factor of tens of thousands, exactly as a skater pulls in their arms. Known pulsars range from a few seconds per rotation down to PSR J1748-2446ad, which turns 716 times every second — its equator is moving at a quarter the speed of light.
Jocelyn Bell Burnell found the first one in 1967: a signal repeating every 1.337 seconds, so precise that it was half-jokingly catalogued as LGM-1, for "little green men". The natural explanation turned out to be stranger than the artificial one. The intense magnetic field, up to a trillion times Earth's, rips charged particles off the surface and accelerates them along field lines, producing tightly collimated beams from radio through gamma rays.
That regularity makes pulsars instruments. The fastest millisecond pulsars rival atomic clocks in long-term stability, and by watching an array of them across the sky for tiny correlated timing shifts, astronomers detected the gravitational-wave background from merging supermassive black holes in 2023. Pulsar timing also gave the first confirmed exoplanets, in 1992, and the first indirect proof that gravitational waves exist at all.