What Is a Pulsar? A Lighthouse Made of a Dead Star

When a star of more than about eight times the Sun's mass runs out of fuel, its core collapses in under a second from the size of the Earth to the size of a city, and the electrons are crushed into the protons to make neutrons; the result is a neutron star, a ball of nuclear matter about twenty kilometres across with more mass than the Sun, so dense that a teaspoon of it would weigh a billion tonnes. The collapse spins it up, as a skater spins faster pulling in her arms, from a rotation of weeks to one of milliseconds, and it compresses the star's magnetic field into one a trillion times stronger than the Earth's. A pulsar is a neutron star whose magnetic axis is tilted from its spin axis, so that the beams of radiation streaming from its magnetic poles sweep round with the rotation, and if one of them crosses the Earth the star appears to flash.

The pulses are not the star flashing; they are the beam passing, once per rotation, like a lighthouse seen from a ship. The radio emission comes from charged particles accelerated along the magnetic field lines above the poles, and the beam is narrow, so most pulsars in the galaxy are invisible from Earth because their beams never cross it. The regularity is the rotation of a mass of a solar mass or more, which nothing much can disturb:

• •Ordinary pulsars: periods from a few tenths of a second to a few seconds, slowing gradually as the beam carries energy away, and switching off after ten million years or so
• •Millisecond pulsars: old pulsars spun back up to hundreds of turns a second by matter falling on them from a companion star, and the most stable clocks in the universe
• •Magnetars: neutron stars with fields a thousand times stronger still, which crack and flare in bursts of X-rays and gamma rays
• •The Crab pulsar: in the remains of the supernova that Chinese astronomers saw in 1054, spinning 30 times a second and visible in radio, light, X-rays and gamma rays

Bell was surveying the sky for quasars with a radio telescope she had helped build from wire and posts over four acres of Cambridgeshire, reading the output by eye on 120 metres of chart paper a week, and the scruff was too regular for a quasar and too fast for anything known. The 1974 Nobel Prize for the discovery went to her supervisor, Antony Hewish, and not to her, which has been argued about since; she has said that a prize to a student would have demeaned it and has been given nearly every other award in physics, including a three-million-dollar Breakthrough Prize in 2018 that she gave away to fund students from under-represented groups. The theoretical explanation, that the pulses were neutron stars, came within months from Thomas Gold, and the Crab pulsar, found in 1968 in a supernova remnant, confirmed it.

A millisecond pulsar's ticks are so regular, drifting by less than a microsecond a year, that they serve as instruments. Timing the pulses from a pair of neutron stars orbiting each other, found in 1974, showed the orbit shrinking at exactly the rate general relativity predicts for the emission of gravitational waves, the first evidence of them and a Nobel Prize in 1993. Arrays of pulsars timed across the sky are now used to detect the slow background hum of gravitational waves from supermassive black holes, announced in 2023. The first planets outside the solar system were found in 1992 orbiting a pulsar, by the wobble in its ticks. And the plaques on the Pioneer and Voyager spacecraft give the Sun's position by the periods of fourteen pulsars, on the reasoning that any civilisation that found the probe would know what a pulsar was and could work out when it was launched from how the periods had drifted.

A pulsar is the only place where matter at nuclear density can be studied, and the timing of its rotation, the glitches when its crust cracks, and the way its signal bends through the gas of the galaxy are probes of physics that no laboratory can reach. The three thousand known are a small fraction of the hundred million neutron stars thought to be in the Milky Way, most of them dark and silent, and the radio telescopes now being built in South Africa and Australia are expected to find tens of thousands more. The scruff on a chart recorder in 1967 turned out to be the ticking of dead stars, and half a century later the ticks are being used to hear the universe.

Pulsars also carry information about the space their signal crosses. Interstellar space contains a thin scattering of free electrons, and they slow radio waves very slightly, with lower frequencies delayed more than higher ones. A pulse that leaves the star as a single sharp flash therefore arrives at Earth smeared out, the high-frequency part first and the low-frequency part a fraction of a second later. The size of that delay, called the dispersion measure, reveals how many electrons lie along the line of sight, and with a model of how electrons are spread through the galaxy, astronomers use it to estimate a pulsar's distance.

Searching old pulsar survey data for dispersed signals led to an unexpected discovery. In 2007 astronomers going through archived observations from the Parkes radio telescope in Australia found a single, extremely bright burst lasting a few milliseconds whose dispersion was far too great for any source inside the Milky Way, implying it had crossed a vast stretch of intergalactic space. These fast radio bursts are now detected in large numbers, some of them repeat, and at least one has been traced to a magnetar within our own galaxy, suggesting that highly magnetised neutron stars are behind some of them, though perhaps not all.

The numbers behind the fastest pulsars are worth working through. A neutron star is roughly twenty kilometres across, so its equator is about 63 kilometres round. The pulsar that spins 716 times a second therefore carries a point on its equator about 45,000 kilometres every second, around fifteen percent of the speed of light. An ordinary star spinning like that would be torn apart at once; a neutron star holds together because its gravity is so intense. The maximum possible spin depends on the still uncertain properties of the matter inside, so finding faster pulsars would help narrow down what that matter is like.

The same regularity makes pulsars candidates for navigation far from Earth, where GPS satellites cannot help. The principle resembles a ship's navigator comparing a clock with known signals: a spacecraft that times the pulses from several known pulsars, and compares their arrival with predictions for a fixed reference point, can work out how far it is from that point in each direction. In 2017 an experiment on the International Space Station called SEXTANT used an X-ray telescope to time millisecond pulsars and calculated the station's position in orbit without help from the ground. Future probes to the outer solar system may navigate the same way.

One of the most recognisable images in rock music is a pulsar's signal. The cover of Joy Division's 1979 album Unknown Pleasures shows a stack of white wavy lines on black, which is a plot of successive radio pulses from PSR B1919+21, the first pulsar discovered by Jocelyn Bell Burnell and Antony Hewish in 1967.

The original plot was produced by an astronomer, Harold Craft, for his doctoral research, and appeared in a popular astronomy reference book, where the band found it and passed it to their designer, Peter Saville. The image has since been printed on countless T-shirts and posters, usually by people who have no idea that it shows the heartbeat of a dead star, which makes it one of the most widely reproduced pieces of scientific data ever.

A pulsar is a rapidly spinning neutron star, the city-sized collapsed core of a supernova, whose tilted magnetic field sends beams of radio waves from its poles sweeping across space like a lighthouse, so that from Earth it flashes once per rotation with a regularity rivalling atomic clocks. Found by Jocelyn Bell in 1967, pulsars have since revealed gravitational waves through the decay of a binary orbit, the first exoplanets, and a way to hear black holes merging across the universe.