Jocelyn Bell Burnell and the Discovery of Pulsars

Some of the most important discoveries in astronomy began as an unexplained smudge on a chart. In 1967 a graduate student named Jocelyn Bell (now Dame Jocelyn Bell Burnell) noticed a signal that “did not look exactly like a scintillating source, and yet did not look exactly like man-made interference either” (Bell Burnell 1977^[1]). It was the first known pulsar. This post is part of our physicist portraits and sits alongside Who Really Discovered It? Physics Credit Myths. Her portrait is our Jocelyn Bell Burnell sticker.

How did Jocelyn Bell discover pulsars?

As a PhD student in Antony Hewish’s group at Cambridge, Bell helped build a new radio telescope designed to study the twinkling (scintillation) of compact radio sources. In her 1977 after-dinner speech, published as “Petit Four,” she described an array covering 4½ acres: “we put up over a thousand posts, and strung more than 2000 dipoles between them.” The telescope’s output went onto pen recorders that produced “96 feet of chart paper every day,” and the charts “were analyzed by hand by me” (Bell Burnell 1977^[1]).

Six or eight weeks into the survey she noticed a bit of “scruff” from one part of the sky, which she had seen before from the same position. After weeks of trying to catch it with a faster recorder, she got it at the end of November 1967: “As the chart flowed under the pen I could see that the signal was a series of pulses … They were 1 1/3 seconds apart” (Bell Burnell 1977^[1]).

Were the signals from aliens?

Hewish’s first reaction was that the pulses “must be man-made.” Bell called this “a very sensible response in the circumstances.” But the source kept to sidereal time, the time set by the stars, which ruled out anything on Earth. The team took seriously, if briefly, the thought that the pulses might be from another civilization, and Hewish measured the pulse period precisely to look for a Doppler shift as “the little green men on their planet orbited their sun.” None appeared beyond the Earth’s own orbit (Bell Burnell 1977^[1]).

The alien idea faded when Bell found a second source just before Christmas 1967, in a different part of the sky, pulsing about 1.2 seconds apart. In her words, “It was very unlikely that two lots of little green men would both choose the same, improbable frequency, and the same time, to try signalling to the same planet Earth.” Two more sources followed in the new year (Bell Burnell 1977^[1]).

When was the pulsar discovery published?

The paper “Observation of a Rapidly Pulsating Radio Source” appeared in Nature on 24 February 1968, by A. Hewish, S. J. Bell, J. D. H. Pilkington, P. F. Scott and R. A. Collins. It reported unusual pulsating signals recorded at the Mullard Radio Astronomy Observatory, apparently from objects within our Galaxy, that “may be associated with oscillations of white dwarf or neutron stars” (Hewish et al. 1968^[2]). Bell recalled that it was submitted at the end of January, “based on a total of only 3 hours’ observation of the source” (Bell Burnell 1977^[1]).

What is a pulsar?

A pulsar is a rapidly rotating neutron star, the collapsed core left after a massive star explodes. A few months after the discovery, Thomas Gold argued in Nature that the pulses “can be accounted for by the rotation of a neutron star.” Strong magnetic fields and fast rotation drive charged particles near the star to relativistic speeds, producing radiation “in the pattern of a rotating beacon” (Gold 1968^[3]). Like a lighthouse, the star sweeps a beam across the Earth once per turn, so each pulse marks one rotation.

The pulse period P is the rotation period. For the first pulsar, P ≈ 1 1/3 s, so its rotation frequency f = 1/P is about 0.75 turns per second. At the time, as Bell recalled, 1 1/3 seconds seemed “far too fast a pulsation rate for anything as large as a star.” A neutron star, only a few tens of kilometres across, solves that puzzle (Bell Burnell 1977^[1]).

Why didn’t Jocelyn Bell get the Nobel Prize?

The 1974 Nobel Prize in Physics was awarded jointly to Martin Ryle and Antony Hewish “for their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular of the aperture synthesis technique, and Hewish for his decisive role in the discovery of pulsars” (NobelPrize.org^[4]). The Nobel Foundation’s own page on Hewish states that “In 1967 Antony Hewish and Jocelyn Bell discovered” the first pulsar (NobelPrize.org^[5]).

The prize rules were not the obstacle: the statutes say “In no case may a prize amount be divided between more than three persons” (NobelPrize.org^[6]), and only two shared the 1974 prize. Her omission has been debated ever since. Bell Burnell’s own view, given in 1977, is worth quoting in full rather than paraphrasing:

“It has been suggested that I should have had a part in the Nobel Prize awarded to Tony Hewish for the discovery of pulsars. There are several comments that I would like to make on this: First, demarcation disputes between supervisor and student are always difficult, probably impossible to resolve. Secondly, it is the supervisor who has the final responsibility for the success or failure of the project. … It seems only fair to me that he should benefit from the successes, too. Thirdly, I believe it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases, and I do not believe this is one of them. Finally, I am not myself upset about it — after all, I am in good company, am I not!” (Bell Burnell 1977^[1])

Whether the committee should have included her is a matter of opinion, and we leave it open. What is not in doubt is the record: she spotted the signal, chased it, found the second source and is the second author on the discovery paper.

What happened after?

Bell Burnell went on to a long career in astronomy. In September 2018 she received a Special Breakthrough Prize in Fundamental Physics for the discovery of radio pulsars, recognising also her scientific leadership. She chose to donate the $3 million prize to the Institute of Physics to fund graduate studentships “for people from under-represented groups in physics research” (Royal Society 2018^[7]).

Her story pairs naturally with that of Cecilia Payne-Gaposchkin, who showed what stars are made of (see Cecilia Payne and What Stars Are Made Of) and has her own Cecilia Payne-Gaposchkin sticker. Both sit in our sticker collection.

FAQ

Who discovered the first pulsar?

Jocelyn Bell, then a PhD student, first noticed the signal in 1967 while analysing data from the radio telescope built by Antony Hewish’s group at Cambridge. The discovery paper was published in February 1968 with Hewish as first author and Bell as second.

What was the first pulsar’s period?

About 1 1/3 seconds between pulses, according to Bell Burnell’s account.

Why were pulsars nicknamed “little green men”?

The regular pulses briefly raised the idea of an artificial signal. Finding a second, similar source elsewhere in the sky made that explanation very unlikely.

Did Jocelyn Bell Burnell think she deserved the Nobel Prize?

In her 1977 speech she argued that it was fair for the supervisor to receive it and said she was “not myself upset about it.”

References

1. S. J. Bell Burnell (1977), “Petit Four,” Annals of the New York Academy of Sciences 302, 685–689 (DOI 10.1111/j.1749-6632.1977.tb37085.x); reprinted as “Little Green Men, White Dwarfs or Pulsars?” in Cosmic Search 1(1), 1979. https://www.bigear.org/CSMO/HTML/CS01/cs01p16.htm
2. A. Hewish, S. J. Bell, J. D. H. Pilkington, P. F. Scott & R. A. Collins (1968), “Observation of a Rapidly Pulsating Radio Source,” Nature 217, 709–713 (DOI 10.1038/217709a0). https://www.nature.com/articles/217709a0
3. T. Gold (1968), “Rotating Neutron Stars as the Origin of the Pulsating Radio Sources,” Nature 218, 731–732 (DOI 10.1038/218731a0). https://www.nature.com/articles/218731a0
4. NobelPrize.org, The Nobel Prize in Physics 1974 (Martin Ryle and Antony Hewish). https://www.nobelprize.org/prizes/physics/1974/summary/
5. NobelPrize.org, Antony Hewish – Facts (Nobel Prize in Physics 1974). https://www.nobelprize.org/prizes/physics/1974/hewish/facts/
6. NobelPrize.org, “Nobel Prize facts” (statutes of the Nobel Foundation). https://www.nobelprize.org/prizes/facts/nobel-prize-facts/
7. Royal Society (6 September 2018), “Dame Jocelyn Bell-Burnell awarded Breakthrough Prize”. https://royalsociety.org/news/2018/09/dame-jocelyn-bell-burnell/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.