In the summer of 1967 Jocelyn Bell, a PhD student at Cambridge, was checking long paper chart recordings from a new radio telescope when she noticed an odd scrap of signal. Her supervisor Antony Hewish first thought it was interference. By 28 November they had resolved it into pulses arriving every 1.337 seconds, as regular as a clock. Half-jokingly, the source was labelled LGM-1, for “little green men”. When Bell found a second one in another part of the sky in December, it was clear this was something natural. The first, now called PSR B1919+21, was the first pulsar.
Lighthouses
A pulsar is a neutron star, the collapsed core of a dead star, spinning rapidly and carrying an enormous magnetic field. Beams of radiation stream out from its magnetic poles. Because those poles are tilted relative to the spin axis, the beams sweep around like a lighthouse, and we see a pulse each time one crosses Earth.
The Crab Pulsar, at the heart of the remains of the supernova seen in 1054, spins about 30 times a second. The fastest known, PSR J1748−2446ad, spins 716 times a second. These “millisecond pulsars” are spun up by gas pulled from a companion star.
Tools for physics
Pulsars turned out to be some of the most precise clocks in nature. In 1974 Russell Hulse and Joseph Taylor found a pulsar orbiting another neutron star every eight hours. Its orbit slowly shrinks exactly as Einstein’s theory predicts if the pair is losing energy as gravitational waves, and they shared the 1993 Nobel Prize for it. In 1992 Aleksander Wolszczan found planets around the pulsar PSR B1257+12, the first planets ever confirmed outside the Solar System. Today networks of pulsars are timed across the sky to search for the slow ripple of gravitational waves passing through the galaxy.
The prize she didn’t share
The 1974 Nobel Prize in Physics went to Hewish and Martin Ryle, the first ever given for astronomy. Bell was left out, which caused considerable controversy. She has said she holds no grudge.