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Astronomer Jocelyn Bell Burnell recounts her discovery of pulsars

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The Sky Is for Everyone (Image credit: Princeton University Press)

The story of Jocelyn Bell Burnell's early-career discovery of pulsars and the accolades that flowed to her male supervisor is often told as emblematic of astronomy's ongoing struggles with gender discrimination.

Bell Burnell and a host of other female astronomy luminaries recall their struggle with gender discrimination in the male-dominated world of physics in a new book, "The Sky is for Everyone: Women Astronomers in Their Own Words (opens in new tab)" (Princeton University Press, 2022). The book includes essays from exoplanet scientist Sara Seager and astrophysicist Priyamvada Natarajan, among many others.

In her essay, Bell Burnell follows her history all the way from her birth in Northern Ireland to her role in the reclassification of Pluto to a dwarf planet. The excerpt below recounts her graduate work at Cambridge University in the United Kingdom and her discovery of pulsars, which came in data gathered by a telescope she helped build.

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Princeton University Press, 2022 | $29.95 on Amazon

Kites Rise against the Wind

After completing my undergraduate work in 1965, I moved to Cambridge, England, to start a Ph.D. I did not expect to get into Cambridge but put in an application just in case. I was very surprised to be accepted. 

Up till then I had lived in the north and west, the upland areas of the United Kingdom. Cambridge was close to London in the affluent, posh, southeast, and when I got there I was overawed. The students, predominantly male, were poised, well-networked, confident in their ability and their right to be there. I was female and from outside the normal catchment areas. I noted that the few women faculty referred to themselves as Miss or Mrs., not Dr. or Professor. (Was this so as not to frighten the men?) Clearly Cambridge had made a mistake admitting me; they would discover their mistake and throw me out. I decided my best policy was to work my very hardest, so that when they threw me out I would not feel guilty — I would know I had done my best and just was not good enough for Cambridge. It is a strategy I would recommend to anyone feeling overawed in a new position.

The typical Ph.D. program in the U.K. has funding for three years. I would spend two years "in the field" (with five or six others) building a radio telescope, six months as the first user debugging it and taking data, and six months completing the data analysis and writing the thesis. A few years previously quasars had been recognized as bright radio sources at considerable distances. Only about twenty were known and not much was understood about them. 

My project was to find many more quasars. My Ph.D. supervisor, Tony Hewish, building on previous work by Margaret Clarke, who showed that quasars scintillated (twinkled), obtained grant funding to build a large radio telescope to search for more scintillating quasars. The telescope was built in-house. My responsibility in the construction was for all the cables, connectors, and transformers. I was spared most (but not all) of the sledgehammering of posts into the ground and became very strong and weather-beaten!

I was the first user of the telescope. After two years building, the radio telescope worked the first time it was switched on — that is probably a record! It looked due south and the beam could be swung in a north-south direction (declination) but was dependent on the Earth's rotation to scan in the other direction (right ascension). With sixteen east-west rows the telescope had, in theory, sixteen beams, but we could only afford receivers for four. To detect the scintillation a short time constant had to be used.

Computers were rare — the University of Cambridge had just one (which took up a large room) and with memory comparable to that of a laptop today. Very few people had access to it and I did not! My data came out on rolls of paper chart — almost 100 feet per day, and it was my job to analyze these charts, picking out the scintillating quasars. In the six months that I observed I accumulated (and analyzed!) 3.3 miles of chart paper. I quickly became used to recognizing the quasars and to recognizing the inevitable radio interference that the sensitive telescope also picked up.

An artist's depiction of a pulsar. (Image credit: NASA’s Goddard Space Flight Center)

I continued to work hard and work thoroughly — still feeling the need to justify my place in Cambridge. Along with the radio interference and the scintillating quasars there was occasionally a small signal that I could not identify. It occupied about one-quarter inch on the chart paper but was so small it was not always there when observing that bit of sky. However, because I did not know what it was, it stuck in my brain. Having established that it was always from the same position in the sky, I showed the trace to my supervisor, Tony Hewish. 

He (rightly) observed that since it only covered one-quarter of an inch of the roll of chart paper, it was hard to see what it was — we needed an enlargement. With chart recordings that is easy — one runs the chart paper faster under the pen and all gets spread out! However, we could not leave the chart recorders permanently on that setting so I had to go out to the observatory shortly before that part of the sky was due to transit and switch to a high-speed recording. I did that for a month and got only high-speed recordings of receiver noise. The source had disappeared! This (of course) was the grad student's fault. 

Finally, I caught it — a string of equally spaced pulses — about 1.3 seconds apart. I called Tony with the news and he became even more suspicious that he had a stupid grad student. However, he came out to the observatory at the appropriate time the next day and stood watching over my shoulder as I prepared for the recording and saw the pulsed signal with his own eyes. We established that the pulse period was the same as on the previous day.

We asked a colleague (Paul Scott) and his grad student (Robin Collins) who also had a telescope and receiver operating at 81.5MHz if they could see this signal, and they did! John Pilkington took two of my receivers, retuned them, and established that the signal swept down in frequency — a signature of dispersion by free electrons, most likely in interstellar space. Guessing at that electron density he came up with a distance which put the source out in the galaxy. We were wondering how we could publish this result and be believed, when I found a second one. It pulsed at a slightly different period and was in a totally different part of the sky. A few weeks later, in early 1968, I found numbers three and four.

We could now proceed with publication of the first source — the other three would follow in a second paper. There were two likely scenarios to explain this kind of emission — a star vibrating or a star rotating. Tony, who had worked on solar flares, preferred a white dwarf star vibrating but accepted it could be a rotating object. 

He gave a colloquium in Cambridge a day or two before the Nature paper announcing the discovery came out. Every astronomer in Cambridge attended, it seemed, and Fred Hoyle sat in the front row. Tony explained what we had found and mentioned he thought it might be an oscillating star like a white dwarf. Fred spoke first at the end. Noting that this was the first he had heard of these things, he said he did not think it was a white dwarf but a neutron star. I was impressed that he could digest so much information so fast and come up with what was ultimately shown to be the right answer.

The press took much interest in the discovery, and Tony and I were interviewed many times. Tony would be asked questions about the astrophysical significance of this discovery. I was asked what my bust, waist and hip measurements were and how many boyfriends I had! Photographers asked me to undo more blouse buttons. I hated it and would have loved to have been very rude to those journalists and photographers, but I had still to get my Ph.D. and did not feel I could rock the boat too much.

Adapted from "Jocelyn Bell Burnell (PhD, 1968): Kites Rise against the Wind" from The Sky Is for Everyone: Women Astronomers in Their Own Words, edited by Virginia Trimble and David Weintraub. Copyright © 2022 by Jocelyn Bell Burnell. Reprinted by permission of Princeton University Press.

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