A pulsar is a neutron star left behind by a supernova, spinning extremely fast and carrying an intense magnetic field. As it rotates, it channels beams of radiation out from its magnetic poles, and because those poles rarely line up with its spin axis, the beam sweeps across space the way a lighthouse beam sweeps across the sea.
From Earth, that sweeping beam is picked up as a steady, precise pulse of radio waves each time it crosses our line of sight, some pulsars completing hundreds of rotations every second. Their timing is so exact that early pulsars were briefly mistaken for signals from another civilization.
Pulsars are now used as some of the most reliable natural clocks in the universe, precise enough that astronomers use networks of them to search for the faint ripple of gravitational waves passing through the galaxy. More on how they work is available from NASA’s Imagine the Universe.
Pulsars were discovered almost by accident in 1967, when graduate student Jocelyn Bell Burnell noticed a strange, impossibly regular radio signal repeating every 1.3 seconds in her telescope data. The signal was briefly nicknamed LGM-1, short for little green men, since nothing natural was known to produce something so precisely periodic, before astronomers realized they were looking at a rapidly spinning neutron star instead. The 1974 Nobel Prize in Physics for the discovery went to Bell Burnell’s supervisor, Antony Hewish, a decision that has remained controversial for decades given that she was the one who actually spotted the signal. Pulsars have since proven their worth many times over, and the first ever exoplanets discovered, in 1992, were found not around an ordinary star but orbiting a pulsar, identified through the tiny, telltale timing variations their gravity caused in the pulsar’s otherwise metronomic beat.