Pulsars Discovered

Imagine you’re an astronomy student gathering data, and suddenly your radio telescope receives regular signals from deep space. It happened to Jocelyn Bell in 1967.

Hubble Space Telescope images of the tiny Crab Pulsar surrounded by vast nebula courtesy of NASA.

This is for Wednesday, November 28. On today’s date in 1967, a new kind of star was discovered by a graduate student at Cambridge University.

Jocelyn Bell was sifting through data needed for her doctoral dissertation. She noticed a strange signal emanating from the same part of the sky each night — a series of regular radio pulses — one and a third seconds apart.

At first, Bell and her professors at Cambridge thought she’d found signals beamed at Earth by aliens. After all, what in nature could produce such rapid and regular signals? Soon, though, Bell found a second pulsing signal, slightly faster, in another part of the sky. This second discovery implied a widespread natural phenomenon. Within two years, astronomers had spotted about 20 pulsing signals — and they had an explanation. Today, the signals are interpreted as coming from stars called pulsars — thought to be the remains of old, giant stars that’ve exploded. As they rotate, these stars cast twin beams of energy across space, like a lighthouse. When the beam sweeps past Earth, radio telescopes receive a burst of radio waves.

In 1998, astronomers located the 1,000th pulsar. And the number continues to climb.

A pulsar is a type of neutron star. So this was both the first pulsar discovered and the first neutron star discovered. A neutron star is the superdense remnant of an exploded star supernova.

For about six months after Cambridge University announced Bell’s pulsar discovery January 1968, astronomers remained unclear as to what kind of natural phenomenon could produce regular pulses. Two theories were put forward – some astronomers thought that pulsars were “vibrating” white dwarf stars, while others believed that pulsars were vibrating neutron stars.

White dwarfs are stellar corpses – the remains of medium-sized stars like our Sun. The nearest and most famous is Sirius B, only 8.6 light-years away, orbiting Sirius, the brightest star in Earth’s skies. White dwarfs are made of dense “degenerate” matter, and are typically about the size of the Earth.

Neutrons, the particles at the heart of atoms, were discovered by James Chadwick in 1933. The next year Walter Baade and Fritz Zwicky speculated on a possibility of stars composed of neutrons – neutron stars. In 1939, George Volkoff and Robert Oppenheimer of atomic bomb fame detailed the concept. Neutron stars, they wrote, should be made of superdense neutronium and measure about 30 kilometers across.

Unfortunately, pulsars pulsed too rapidly to be vibrating white dwarfs and too slowly to be vibrating neutron stars. But then Austrian-American astronomer Tommy Gold worked out that pulsars are spinning neutron stars. Stars rotate about an axis, just as does Earth. The Sun, for example, rotates in about 27 days. When a large star dies, its core collapses. As the core falls inward, the rate of spin increases. The common analogy – when a spinning ice skater pulls in his arms he spins faster.

When a large star millions of kilometers wide and needing days to complete a single rotation collapses into a neutron star 30 kilometers across, its spin rate soars. Bell’s first pulsar, which rotated once every 1.3 seconds, was a slow rotator compared to the pulsar in the Crab Nebula, which spins 30 times a second. Any kind of star less dense than a neutron star would fly apart at such speeds.

Spinning pulsars are giant dynamos pumping out energy. Much of the energy spills into space as two powerful energy beams. The beams radiate across a wide swath of the electromagnetic spectrum, including in visible light. The Crab Pulsar was the first to be imaged optically as its beam swept past Earth. The pulsar’s energy can only be seen as its beams sweep past Earth. Interestingly, if a pulsar’s beams do not intersect Earth, it will be virtually undetectable. That implies that many more pulsars exist than have been observed.

Pulsars slow down as they grow older, so Bell’s first pulsar probably once rotated much faster. The Crab Pulsar is young – the supernova explosion that marked its birth occurred in 1054. Astronomers calculate that the Crab Pulsar rotated 50 times per second at birth. Eventually the Crab Pulsar will spin as slowly as Bell’s first pulsar about 10 million years after birth it will slow so much that it will stop pulsing and become a quiet and almost undetectable neutron star.

Interestingly, astronomers have found that neutron stars can become pulsars a second time if they are part of a binary two-star system. As stars age they swell up. An aging, nearby companion star can swell up enough that its gas pours down on the neutron star. The gas can act like the arms of an ice skater, spinning the pulsar to life a second time again. Such rejuvenated pulsars can spin up to 1,000 times per second. A rejuvenated pulsar is typically ungrateful to its companion star for its new lease on life – it generates a powerful wind of particles which can evaporate the hapless star.

Jocelyn Bell received her doctorate in 1968, and later married, changing her name to Jocelyn Bell Burnell. The pulsar discovery was considered so revolutionary that it rated a Nobel Prize from the Swedish Academy in 1974. Frequently the Academy awards its prizes to multiple recipients for the same discovery. Amazingly, it awarded the Nobel for the pulsar discovery exclusively to Bell’s advisor, Anthony Hewish. The decision to exclude Bell Burnell remains controversial today. Bell Burnell has gone on to win many other medals and awards. She chairs the Physics Department of the Open University in the U.K, but this year 1999 is a Visiting Professor for Distinguished Teaching at Princeton University in the U.S.

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