Celebrating the 50th anniversary of the discovery of the pulsar, the pulsating star that gave rise to numerous insights into the structure of the universe. The Nobel Prize for the discovery was not awarded to the scientist who found it, but only to her PhD supervisor
“The excitement was because this was a totally unexpected, totally new kind of object, behaving in a way that astronomers never expected, never dreamt of,” said Prof. Jocelyne Bell-Burnell in a BBC special. This was no unfounded claim; the discovery made by the young British PhD student opened up a new window to previously unknown worlds, endowing physicists with innovative tools for studying the universe.
Little green men
Jocelyne Bell was born in 1943 in Northern Ireland. Her father was a book-loving architect, and among the many books they had at home, the ones that really caught his daughter’s attention were about astronomy – the books he had brought home when he was involved in planning a local astronomical observatory. “It was reading those books that made me realize what an exciting and interesting subject astronomy was,” said Bell on the BBC program. When she was in elementary school, her parents, together with parents of other girls, contested the girls’ curriculum, fighting to change it so that they could study science like the boys did, instead of learning only “practical” subjects like embroidery.
For high school, Bell was sent to England, to a Quaker girls’ boarding school, where she met a physics teacher that would change her life. “Mr. Tillet was distinctly elderly, he'd come out of retirement a second time to teach us,” she recalled in an interview with Kate Weatherall. He explained that in physics, “you don't have to learn lots and lots and lots of facts; you just learn a few key things, and if you really got hold on them, then you can apply and build and develop from those.” In effect, he showed Bell how easy a subject physics can be.
After high school, Bell enrolled in the physics program at Glasgow University, where she encountered a very different attitude. She was the only woman in her class, and had to deal with the disrespectful and condescending attitude of her fellow classmates. It was worth it, though. Graduating from Glasgow in 1965, she was accepted to the PhD program at Cambridge University, under the supervision of Anthony Hewish. At Cambridge, those were the early days of one of the world’s most advanced radio telescopes – an array of antennas spanning an area of 57 tennis fields.
Like an optical telescope, the radio telescope also receives electromagnetic radiation from space. But instead of the visible light wave frequencies that the optical telescope receives, the radio telescope picks up radio waves, which are much longer and have a wider range. These telescopes enable us to identify celestial objects that emit radio waves, such as different kinds of stars. Bell's task was to focus on such stars – the Quasars (short for Quasi Stellar Object), discovered just a short time before. She studied the changes in Quasar-emitted radiation, which stems from the waves passing through the particles of solar winds. This phenomenon is similar to the one we observe through an optical telescope, in which some stars seem to be twinkling or blinking, due to distortions in the light they emit towards us.
November 28, 1967, was just another cold night for Bell in the large telescope's control room, scanning different areas of the sky and analyzing the charts of the radio signals. Suddenly, she noticed a strange phenomenon: a source that was emitting a repeating radio signal, appearing, like clockwork, every 1.337 seconds. The source of such the signal detected by the telescope could have been a man-made interference, such as a signal from a passing plane’s radio transmitter, but because it persisted without change, this was unlikely. Bell followed the signal throughout the night, and realized it was moving across the sky, together with the motion of other stars, which means it was coming from outside the solar system. This raised the possibility that the radio signal came from extraterrestrial life forms. Bell thought this was improbable, but still, for her own amusement, she called the wave source LGM, which is short for Little Green Men.
A dispute over aliens and a discussion of credit. Hewish and Bell-Burnell at the radio telescope site in Cambridge | Photograph: Science Photo Library
Hewish and the head of the department, world–renowned astrophysicist Martin Ryle, were apprehensive of publishing these results, mainly of the public reaction to the possibility of discovering extraterrestrial life forms. Bell, who was somewhat excluded from the meetings, continued work on ruling out the possibility that the signal came from aliens.
One of the ways to check whether radiation originates in an extraterrestrial life form is based on the assumption that aliens would reside on a planet, and not on its star (sun). A planet orbits around its sun, so it would appear to move interchangeably towards us and away from us. In radio waves, a phenomenon like this can be measured using the Doppler Effect, but Bell's measurements did not indicate such fluctuations.
One night, while analyzing the output from a previous observation, she noticed another repeating pattern, but this time, from a different region in the sky. “That bit of the sky [was] due to go through the telescope beam at 2 o'clock, 3 o'clock in the morning…and I switched on the high-speed recorder and it came blip, blip, blip, blip, clearly the same family,” she told the BBC. “And that was great, that was really sweet. It finally scotched the 'little green men hypothesis', because it's highly unlikely [that] there're two lots of little green men, [at] opposite sides of the universe, both deciding to signal to a rather inconspicuous planet, Earth, at the same time, using a daft technique and at around the same frequency.”
If these radio signals were not aliens’ communications, then where do they come from? “It has to be some new kind of star, not seen before,” said Bell. In the first paper about the discovery in early 1968, Bell, Hewish and their colleagues suggested that the source of the strange waves was fast-rotating stars, hypothesizing that these might be neutron stars – small and very dense stars that form following a star explosion, called a supernova, which causes the remains to implode into a dense core. Even though Bell had made the discovery and was pivotal in drawing the conclusions from it, Hewish appeared as the first author of this important paper.
Additional studies corroborated this hypothesis. The type of star discovered by Bell received the name Pulsar, short for pulsating star. Certain neutron stars emit radiation from their poles due to their high rotation velocity. At times, the radiation is emitted in radio frequency, but other types of pulsars emit stronger radiation. If the star is tilted on its side, so that its poles do not point “up” and “down”, an observer at the appropriate location would be able to see, or receive, the beam of radiation in a constant cycle, depending on the size of the star, its rotation velocity and its rotation axis.
The supervisor listed as first author on the discovery, as it was published in Nature in February 1968 | Source: the research paper
In 1969, Bell received her doctorate in astrophysics. The pulsars were only mentioned in an appendix of her dissertation. That same year, she married Martin Burnell, a government employee that was stationed each time in a different location. She moved with him throughout the UK, usually settling for temporary positions that were offered to her at different universities. Most of her scientific work to follow was not focused on radio telescopy, but on observations of gamma radiation and X-rays.
In 1974, the Nobel Committee for Physics decided to award the Nobel Prize in Physics to Martin Ryle and Anthony Hewish. Ryle received the honor for his contribution and developments in the field of radio telescopy. Hewish was awarded “for his decisive role in the discovery of pulsars.” Bell-Burnell was not mentioned at all, even though she was an active participant in designing and building the telescope, the actual discoverer of the radio wave emissions, and had played a pivotal role in analyzing the data. Furthermore, she was the one who convinced Hewish to publish it, even though he initially objected it, being sure it was a man-made radio interference that they could not pin down.
World-renowned physicist Fred Hoyle was among those objecting the decision not to award the prize to Bell, blaming Hewish for taking credit for his student's work. Bell-Burnell remained indifferent to the whole matter, and even justified the committee's decision. In a speech she gave in 1979, she said “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… 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.” Nevertheless, it should be noted that many Nobel Prize laureates were awarded for work they did during their PhD, and in many cases, they shared the award with their supervisor.
Map to the stars
Over the years, with the discovery of many additional types of pulsars, their importance in understanding the universe emerged. Scientists have found different types of such stars, which emit not only radio waves, but also X-rays and gamma rays. Pulsars with an even higher rotation velocity were identified, with their signal repeating every few milliseconds. Discoveries were made of pulsars in binary star systems, with an additional star, and also of pulsars that absorb matter from their neighboring star, thus accelerating even more.
Studying the activity of pulsars, especially in binary systems, has contributed to locating possible sources for gravitational waves, and in fact, scientists have recently detected gravitational waves that were created by a neutron star merger. The utilization of a radiation source with a known and constant frequency opens more opportunities for studying the interstellar space and the composition of the universe by measuring the effect of different factors on pulsar-emitted radiation. The high accuracy of the pulsar frequency makes them excellent tools for designing watches accurate than atomic watches.
Another advantage of pulsars is that they are very prominent throughout the universe. The Pioneer and Voyager spacecrafts launched in the early 1970s to study the solar system carried a metal plate and a map in the event that aliens find it. The map explained the location of our solar system based on the locations of 14 prominent pulsars, noting each one's frequency. But actually, there’s no need to wait for aliens to navigate by pulsars: Plans are already underway for using pulsars for spacecraft navigation, for clock synchronization of GPS satellites, whose time in space moves faster than on Earth.
Bell's statement that the pulsar was something astronomers never even dreamt of, has been validated time and again, as they enable us to push the limits of our understanding of the universe. We can only regret that this groundbreaking scientist – and their discoverer – did not receive the honor and recognition she deserved.
A lighthouse in the sky – watch NASA's short video about pulsars:
Translated by Elee Shimshoni