An unusual array and an unexpected signal
In 1967, the University of Cambridge’s Mullard Radio Astronomy Observatory had a new instrument: a large field of wires and posts, spread over several acres, designed to pick up low-frequency radio waves from space. Antony Hewish, who led the project, wanted to study how the radio signals from distant, compact sources called quasars flicker as they pass through the thin gas streaming out from the Sun. Quasars had only been recognised a few years earlier, and the flickering, known as scintillation, offered a way to pick out the most compact ones.
Jocelyn Bell, then a postgraduate student working under Hewish, had helped to build the array and was responsible for operating it and analysing the output. The data arrived as long paper chart recordings, which she checked by eye, day after day. This slow, manual work is part of the story, since a computer-based analysis might have discarded the strange signal as noise.
The “scruff”
In the summer of 1967, Bell noticed a small, odd mark on the recordings, which she described as “scruff”. It appeared from a patch of sky that did not match what she expected from a scintillating quasar, and it was seen again at the same position in the sky when she looked at later recordings. That repetition was an important clue: interference from human sources, such as electrical equipment or nearby transmitters, would not normally follow the sky, keeping to the same celestial position, from day to day.
The team then used faster recording to look more closely. In late November 1967, the mark resolved into a series of extremely regular pulses, arriving about every 1.34 seconds. The regularity was far greater than anything then known from natural radio sources. The signal was designated CP 1919, for Cambridge Pulsar, and it is now catalogued as PSR B1919+21.
Ruling out the obvious explanations
The team at first could not tell what was causing the pulses, so they treated the problem as an exercise in elimination. Several explanations were considered and tested:
- Interference from Earth. The pulses kept to sidereal time, following the stars rather than the clock, which argued against terrestrial sources.
- A fault in the equipment. Observations with a second receiver and with other instruments showed the signal was not an artefact of one piece of hardware.
- A nearby source. Measurements of how the pulse arrival times varied with radio frequency indicated the source lay far outside the solar system, within our own galaxy.
The sheer regularity of the signal led to a half-serious nickname within the group, “LGM”, for Little Green Men, since an artificial origin could not be dismissed at the outset. It was a joke rather than a serious conclusion. When Bell found a second pulsing source in a different part of the sky, with a different period, shortly before Christmas, the idea of an alien beacon lost most of its appeal: it would be a remarkable coincidence for two distant civilisations to be signalling at the same time at the same frequency. Further sources followed in early 1968, giving the team a new class of object.
Publication and explanation
The discovery was published in the journal Nature in February 1968, with Hewish listed as first author and Bell as second, alongside other colleagues. The paper described the pulses and suggested that they might come from oscillations in white dwarfs or neutron stars. A theoretical explanation soon emerged. Thomas Gold proposed that pulsars were rapidly rotating neutron stars, the extremely dense remnants left when a massive star collapses, with a beam of radio emission sweeping across space like a lighthouse. The idea of neutron stars had been suggested in the 1930s by Walter Baade and Fritz Zwicky, but no one had seen one. Pulsars supplied the first strong evidence that they exist, and the discovery of a pulsar in the Crab Nebula in 1968, at the site of a supernova observed in 1054, strengthened the case.
The Nobel Prize and the question of credit
In 1974, the Nobel Prize in Physics was awarded to Antony Hewish and Martin Ryle, another Cambridge radio astronomer, for work in radio astronomy. Hewish was cited for his decisive role in the discovery of pulsars. Bell, by then known by her married name, Burnell, was not included, and the omission has been much discussed ever since. Prominent astronomers criticised it at the time, with Fred Hoyle among the best known. Bell herself has been measured in public comments, noting that the system of the period tended to credit supervisors, and has since received many honours, including the 2018 Special Breakthrough Prize in Fundamental Physics, for which she gave the prize money to support physics students from under-represented groups.
Why it mattered
Pulsars gave astronomers a new set of tools. Their pulses are so regular that they function as natural clocks, and some of the most precise are used to test theories of gravity and to search for faint gravitational effects. The discovery also showed a general lesson about research: an instrument built for one purpose can reveal something quite different, but only if someone notices an anomaly and pursues it.
The year was a busy one for science and technology. In the same months that Bell was puzzling over her chart recordings, rockets and spacecraft dominated the news during the space race, yet the pulsar was a discovery from the ground, made with wire, posts and patience.