Unknown pleasures: pulsars and the first data revolution

The scientific story behind the cover of Joy Division’s treasured debut


Image: Nasa (cropped), CC BY 2.0.

Many of the 20th century’s most famous albums are as celebrated for their artwork as they are for their music. This is particularly true of albums from the 1970’s: think of Bowie’s Aladdin Sane,  Fleetwood Mac’s Rumours, or Pink Floyd’s Dark side of the moon. The 1970’s were a golden era for concept albums in popular music, meaning that the world’s biggest artists spent a lot of time and money producing beautiful covers to fit the sound and message of the songs within.

A recurring theme in artistic images from that decade is space. In the age of the moon landing, when much of technological development was focused on pushing the human frontier, cosmic images were everywhere in popular culture. The original Star Trek series was first broadcast in 1966, the first Star Wars film was released in 1977, and E.T. followed five years later. Similarly, Parliament’s Mothership connection (1975) and Supertramp’s Crime of the century (1974) both feature space-inspired covers. With space comes science, so it’s no surprise that many of the most famous album covers have an interesting scientific tale to tell.

One spacey image with a particularly rich backstory comes from Joy Division’s 1979 debut, Unknown pleasures. The cover of the album features an array of wavy, organic-looking lines on a black background, with no information suggesting where they might come from. (Neither the title of the album nor the name of the artist appears on the front cover.) In fact the image is a ‘stacked plot’ depicting radiation emitted by a pulsar, a type of star that had been discovered 12 years earlier, and gives a glimpse into the transformative effect that early computers had on scientific research.

The cover of Unknown pleasures.

Candidate (for an album cover)

But first, a short bit of background. In many ways, Unknown pleasures has enjoyed an unusual level of fame. Joy Division did not release any singles from the album, it did not chart, and does not include the band’s most commercially successful song, Love will tear us apart. Yet, despite all this, the album is named as one of the most influential of the decade and the cover image has been copied and parodied many times; having been referenced by the Simpsons, Disney, and (my favourite) cartoonist Moose Allain.

Jocelyn Bell in 2009. CC BY-SA 3.0.

The first pulsar was discovered by Jocelyn Bell, a PhD student at the University of Cambridge. Bell was working with a radio telescope that she had helped to build known as the Interplanetary scintillation array, which was designed to measure variations in radio waves emitted by astronomical bodies. During the latter part of her PhD, Bell was studying the readout from the telescope when she noticed a regular, recurring signal. In fact, the signal was so regular that she jokingly dubbed it LGM-1 (Little green man 1) in the half-serious thought that it could be a sign of extraterrestrial life. In a paper published in 1968, Bell and her co-authors announced that this signal actually came from a new type of star that they dubbed a ‘pulsar’. There is some controversy about the fact that Bell’s supervisor, Antony Hewish, was awarded the Nobel prize for physics in 1974 for his “decisive role in the discovery of pulsars”, while Bell was not. (For what it’s worth, Bell says she does not believe she should have been given the award.)

When a star reaches the end of its life, three things can happen. For a relatively small star, like our sun, once it has used up all of its fuel it collapses in on itself, leaving behind a ‘white dwarf’ that gradually cools and fades. A very big star, more than twenty times the size of the sun, collapses abruptly when it dies in an event so violent that it can create a black hole. Stars that lie in between these two extremes turn into very dense objects called neutron stars, one type of which is a pulsar. Pulsars have strong magnetic fields and rotate very quickly, which causes them to emit a narrow beam of radio waves. This beam sweeps through space like a lighthouse beacon, rotating with the star. If the pole of the star is oriented in the right way, an observer on Earth will be able to detect the radio beam once every revolution of the pulsar. It was exactly this regular, high-frequency signal, that Bell’s telescope was picking up.

Pulsars emit a narrow beam of radiation (purple) that spins as the star rotates, and can be detected from Earth. Image: NASA.

But what about the image itself? Well, this fantastic article by Jen Christiansen in Scientific American has traced the origins of the image back to the PhD thesis of Harold Craft, Jr, an astronomer at Cornell University. In the 1960’s, Craft was working at the Arecibo radio observatory in Puerto Rico. Pulsars had just been discovered, and Arecibo was “the best instrument in the world” to study them, so Craft and his colleagues were busy recording the pulses that they gave off, and trying to understand their frequency and regularity. One of the images that Craft produced during this research was included in the Cambridge encyclopedia of astronomy, which Joy Division guitarist Bernard Sumner happened upon on a visit to Manchester Central Library. Sumner had spent a lot of time watching 2001: A space odyssey, and felt that Craft’s image somehow reminded him of the film. He showed the image to designer Peter Saville, and, as far as the visual history of the cover goes, that’s that. But there’s also an important scientific story to be told.


An analogue readout from the ISA, showing the first time that Bell saw a pulsar. The peaks at 19:20 show the signal received every time the pulsar’s radio beam hit the telescope, about once a second for one minute. Image: Wikipedia, CC BY-SA 4.0

When Bell discovered pulsars she did so using an analogue instrument known as a chart recorder, which measures the signal received by the radio telescope and draws it onto paper with an ink cartridge, printing the results in real time. Bell’s chart recorder produced up to 30m of paper every night, which she then had to analyse by hand. The pulsar showed up as an intense burst of activity lasting around one minute, re-appearing night after night in the same part of the sky at about 19:20, the time when the telescope’s field of vision included the pulsar. For Bell to manipulate the data at all, or even extract numerical values from it, would have involved tedious, painstaking measurements of the chart recorder’s output. I actually find it quite amazing that she was able to detect the signal at all, when you think that it involved physically scanning through huge rolls of paper looking for a tiny spike in activity, and then noticing that it was at the same point every night.

Things were a little different at Arecibo. Craft and his fellow PhD students had access to a computer, and they set about using it to better visualise the data that they were collecting. One type of graph that Craft was fond of was the ‘stacked plot’, where the part of the data containing the pulsars signal was extracted, and each sweep of the radio beam was presented on a different row. This meant that different pulses, taken over several days, could be compared easily, and Craft didn’t have to trawl through all the data in order to do so. The stacked plots are meant to be read from bottom to top: each pulse is plotted above the previous one, and only the bits of the readout that contain the radio beam are shown (for the pulsar that Bell discovered, the beam crossed the telescope about once every 1.3 seconds, but lasted only four hundredths of a second).

Craft even wrote a program that would present the data in 3D, obscuring bits of the pulse if the ‘hill’ in front was too high and offsetting the different pulses so that those behind looked like they were further away. It was one of these stacked plots that made it into the Encyclopedia of astronomy and, eventually, onto the cover of Unknown pleasures.


So, stacked plots were an aesthetically-pleasing way of looking at the data that would have been much harder to produce if Craft didn’t have access to a computer. But they were also very useful too. The radio beam that a pulsar emits is created by its magnetic field, and it was thought that variations in the signal could be due to variations in the field, which could then be analysed to learn more about the structure of pulsars. It was in order to analyse these variations that Craft began to use stacked plots: by plotting each pulse literally on top of each other, he hoped to pick out consistent patterns in the signal that could then be related to the magnetic field. However, the plots showed that the variations in the signal did not have much structure, and so were likely due to background noise, or interference from the solar wind. Although this was not what Craft was looking for, it was only by producing stacked plots from many different pulsars that allowed him to assert this with confidence.

A stacked plot showing maximum daily temperature in Strabane, Ireland. Visualisation created by Andrew Lehm, using data from the Met office.

Above is a stacked plot in the style of Craft that shows the maximum daily temperature in Strabane, Ireland, between 1980 and 2016, with each year on a different row. Higher temperatures in the summer are clearly visible.

It’s pretty obvious that computers have had a huge impact on scientific research, and that they have revolutionised how scientists can gather and analyse data. In this context, the story of Craft’s stacked plots might seem like a small one, but its relationship to Unknown pleasures has seen it gain cult status among the data community. In recent years, the field of data visualisation has gone through a revolution at least as big as it did during Craft’s time. But while the first data revolution was driven by the natural sciences, this time round things have largely been led by social scientists such as Hans Rosling, who was famous for creating beautiful, animated graphics that revealed long-term trends in global health statistics. Rosling’s philosophy is now being adopted by businesses, politicians and journalists who want to present narratives in a form that is clear and concise, but that appears scientific and unbiased. In this new revolution, stacked plots have undergone a bit of a resurgence, and for much the same reasons as Craft was originally drawn to them: because they allow one to spot long-term trends in a repeated signal or data.

Coda: Day of the Dame

Today, Bell (now Dame Jocelyn Bell Burnell) is widely credited for her role in discovering pulsars and has had a long and distinguished academic career. She is also a prominent voice in the fight for representation in science. In 2018, she was awarded the Special breakthrough prize in fundamental physics, and chose to use the three-million dollar prize to set up a fund that provides financial support for under-represented groups in physics: including not only women, but also people from ethnic minorities and refugees. Forty years after a computer graphic inspired by her discovery featured on the cover of Unknown pleasures, it seems that Bell is once again at the centre of a transformative movement in science.

This article follows a previous post from 2017 about the mathematics behind Pink Floyd’s Dark side of the moon cover.

Sean is a PhD student researching geophysical fluid dynamics at UCL. He studies coastal outflows, but so far has been unable to persuade the department to send him on a research trip to the beach.

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