pulsating star

Pulsar is a portmanteau of pulsating and star, coined in 1968 by science journalist Antony Hewish in a press release describing the discovery made by his doctoral student Jocelyn Bell Burnell the previous year. The word pulsating derives from Latin pulsare (to beat, to throb, to strike repeatedly), the frequentative of pellere (to push, to drive, to beat), from the Proto-Indo-European root *pel-, meaning to strike or push. The root pellere also gives English pulse, propel, compel, repel, and expel — all verbs of pushing and striking. The pulsar beats with preternatural regularity: CP 1919, the first pulsar discovered, emitted a pulse every 1.3373011 seconds, a precision that exceeded the best atomic clocks of 1967 and initially defied any natural explanation.

Jocelyn Bell, then a graduate student at Cambridge, detected the first pulsar signal in August 1967 while analyzing miles of chart recorder paper from the Mullard Radio Astronomy Observatory. The signal was so regular — a repeating pulse with a period constant to better than one part in ten million — that it seemed more likely to be interference from a television transmitter or reflected radar than a natural astronomical source. Bell's supervisor Antony Hewish initially suspected terrestrial interference. When systematic checks ruled out earthly causes and revealed that the source tracked the stars rather than the Sun, the team considered the uncomfortable possibility that the signal was artificial — hence the half-joking designation LGM-1, for Little Green Men. A second similar source in a different part of the sky made an extraterrestrial intelligence explanation untenable; the signals were natural.

The physical explanation for pulsars arrived quickly after the discovery. In 1968, Thomas Gold at Cornell proposed that pulsars were rapidly rotating neutron stars — the dense remnants of massive stars that had exploded as supernovae. A neutron star compresses roughly one and a half solar masses into a sphere about 20 kilometers in diameter, creating a density so extreme that atomic electrons and protons are crushed together into neutrons. The rotation, which the parent star had before collapse, is preserved and dramatically speeded up by conservation of angular momentum as the star contracts — just as a spinning figure skater spins faster when drawing in their arms. The pulsar's powerful magnetic field channels radiation into narrow beams that sweep through space with each rotation. Earth intercepts the beam once per revolution.

The fastest pulsars, called millisecond pulsars, rotate hundreds of times per second — one, PSR J1748-2446ad, rotates 716 times per second, its equator moving at about 24 percent of the speed of light. Millisecond pulsars are spun up by accreting material from a companion star in a binary system. Their rotational stability is so extreme that they function as natural clocks more precise than any human instrument: timing arrays of millisecond pulsars can detect the subtle spacetime distortions caused by gravitational waves passing through the galaxy. In 2023, the North American Nanohertz Observatory for Gravitational Waves announced evidence of a gravitational wave background detected through pulsar timing — the beating neutron stars serving as the galaxy's own gravitational wave detector. The Little Green Men signal turned out to be among the most useful tools in fundamental physics.