Discovery of Pulsars (1967): Jocelyn Bell Burnell's Signal

The discovery of pulsars in 1967 revealed a new kind of star and gave the first direct evidence that neutron stars exist. Jocelyn Bell, a 24-year-old PhD student at the University of Cambridge, noticed an odd signal in her radio telescope data that pulsed with clockwork regularity. Explained as a rapidly spinning neutron star, it opened a new field of astrophysics and became one of the most famous discoveries of the 20th century.

Background

In the 1930s astronomers Walter Baade and Fritz Zwicky suggested that a supernova explosion could leave behind a neutron star, an object packing more mass than the Sun into a ball only about 20 kilometres across. For three decades such stars remained purely theoretical, because they were expected to be far too faint to see.

Meanwhile, radio astronomy was growing fast after the Second World War, with major centres at Cambridge and at Jodrell Bank, where the Lovell Telescope began work in 1957. In the 1960s astronomers were especially interested in quasars, very distant and powerful radio sources.

The telescope

At Cambridge, the radio astronomer Antony Hewish designed a new instrument to find quasars by studying interplanetary scintillation, the twinkling of compact radio sources caused by the solar wind. The Interplanetary Scintillation Array, at the Mullard Radio Astronomy Observatory west of Cambridge, covered about 4.5 acres (1.8 hectares) and used more than 2,000 simple dipole antennas strung on wooden posts.

Jocelyn Bell, born in 1943 in Northern Ireland, joined the project as a research student. She and other students and technicians spent around two years building the array, which began operating in July 1967. Its output was traced by pens onto paper charts, producing around 30 metres of chart every day, and Bell was responsible for analysing them by eye.

The discovery

In August 1967 Bell noticed a small, unusual mark on the charts, which she described as a bit of “scruff”. It did not look like scintillation or ordinary interference. She realised that it kept reappearing from the same part of the sky, and that it kept time with the stars rather than with the clock, suggesting a source far beyond Earth.

To study it in detail, she arranged faster chart recordings. On 28 November 1967 these showed a series of sharp pulses spaced about 1.3 seconds apart. Such precise timing from a natural object seemed so strange that the team half-jokingly labelled it LGM-1, for “Little Green Men”.

The group checked carefully for human sources such as satellites, radar or faulty equipment, and measured the signal with a second instrument. Shortly before Christmas 1967 Bell found a second pulsing source in a different part of the sky, and two more followed in January 1968. Several unrelated sources made aliens extremely unlikely and pointed to a new natural phenomenon.

The team also used careful timing to test the source. If the pulses came from a transmitter on a planet circling another star, their timing should shift slightly as the planet moved in its orbit. The measurements showed only the shift caused by Earth’s own motion around the Sun, which suggested a natural object. The pulses were also spread out in arrival time across different radio frequencies, a delay caused by passing through thin gas between the stars, which showed that the source lay far outside the Solar System but within our own galaxy.

Hewish kept the discovery confidential within the group for some weeks while these checks were made. Some critics later felt the secrecy delayed other astronomers, but the team argued that it needed to be sure before announcing such an unusual result.

Publication and explanation

The team announced the discovery in the journal Nature on 24 February 1968, in a paper by Hewish, Bell and three colleagues. They suggested that the source might be a white dwarf or a neutron star. The newspapers quickly adopted the name “pulsar”, short for pulsating star.

Later in 1968 the astrophysicist Thomas Gold argued that pulsars were rotating neutron stars with intense magnetic fields, sending out beams of radio waves that swept past Earth like a lighthouse beam. Franco Pacini had proposed in 1967 that such a star could power the Crab Nebula. The idea was strongly supported when a pulsar spinning about 30 times a second was found in the Crab Nebula, the remains of a supernova seen in 1054, and another in the Vela supernova remnant. Only a tiny, extremely dense star could spin that fast without flying apart.

How a pulsar works

A neutron star forms when the core of a massive star collapses during a supernova. Protons and electrons are squeezed together into neutrons, leaving a sphere perhaps 20 kilometres across that contains more mass than the Sun. A teaspoon of its material would weigh in the region of a billion tonnes or more on Earth.

As the core collapses, it spins faster, much as an ice skater spins faster when pulling in her arms, and its magnetic field is concentrated to enormous strength. Charged particles accelerated near the magnetic poles produce narrow beams of radiation. If the magnetic axis is tilted from the spin axis, the beams sweep around the sky, and an observer in their path sees a pulse once per rotation.

By the numbers

The Nobel Prize controversy

In 1974 the Nobel Prize in Physics was awarded to Martin Ryle and Antony Hewish for pioneering research in radio astrophysics, with Hewish cited for his decisive role in the discovery of pulsars. Bell, by then Jocelyn Bell Burnell after her marriage, was not included.

Some scientists, most prominently the astronomer Fred Hoyle, publicly criticised the decision. The case became a widely discussed example of how junior researchers and women could be overlooked. Bell Burnell herself has generally said that she did not resent the decision and that prizes should not usually go to students.

She went on to a distinguished career in astronomy and became president of both the Royal Astronomical Society and the Institute of Physics. In 2018 she received a Special Breakthrough Prize in Fundamental Physics worth 3 million US dollars, which she gave to fund scholarships for physics students from under-represented groups.

Timeline

• 1934: Walter Baade and Fritz Zwicky propose that supernovae produce neutron stars.
• 1967, July: The Interplanetary Scintillation Array near Cambridge begins observations.
• 1967, August: Jocelyn Bell notices the unusual signal on the chart recordings.
• 1967, 28 November: Fast recordings reveal regular pulses about 1.3 seconds apart.
• 1967, December: Bell finds a second pulsating source shortly before Christmas.
• 1968, 24 February: The discovery is published in Nature.
• 1968: Thomas Gold explains pulsars as rotating neutron stars; pulsars are found in the Crab and Vela supernova remnants.
• 1974: The Nobel Prize in Physics is awarded to Hewish and Ryle.

Legacy

More than 3,000 pulsars are now known. They have become precise natural clocks for testing physics. In 1974 Russell Hulse and Joseph Taylor discovered a pulsar orbiting another neutron star, and its slowly shrinking orbit provided the first indirect evidence for gravitational waves, earning them the 1993 Nobel Prize. The first planets found outside the Solar System, in 1992, were orbiting a pulsar.

The first pulsar also entered popular culture. A plot of its stacked radio pulses, made from research data around 1970, became the cover of Joy Division’s 1979 album Unknown Pleasures. Space telescopes such as Hubble later photographed the Crab Nebula around its pulsar in remarkable detail.

Why it matters

The discovery of pulsars confirmed that neutron stars are real and showed what careful attention to unexpected data can achieve. Jocelyn Bell’s persistence turned a smudge on a paper chart into a new branch of astronomy, and her story continues to shape debates about recognition in science.