Celebrating 50 years since the discovery of the pulsar, the pulsating star that gave rise to many insights on 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 Jocelyne Bell-Burnell for a BBC special. This claim was not completely inconceivable. The discovery made by the young British PhD student opened up a totally 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, his daughter was most attracted to the astronomy books that he 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, fought for changing the curriculum for girls, so that they could learn science like the boys do, and not only learn more "practical" subjects like embroidery.   

Bell was sent to England for high school, to a Quaker boarding school, where she became acquainted with the physics teacher that would change her life. "Mr. Tillet was quite an old man, coming out of retirement a second time to teach us," she recalled in an interview with Kate Wetherall. He explained that in physics, you do not have to memorize fact on top of fact by heart. You just learn a few basic principles, and if you understand them well enough, you can build and develop things on top of that foundation. He actually showed Bell how easy a subject physics really is.

After high school, Bell enrolled in the physics program at Glasgow University, where she encountered quite a different attitude towards her. 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. In 1965, she graduated from Glasgow and was accepted for the PhD program at Cambridge University, under the supervision of Anthony Hewish. At Cambridge, one of the most advanced radio telescopes was being employed – it was an array of antennas spanning an area of 57 tennis fields.

Much like the optical telescope, the radio telescope receives electromagnetic radiation from space. However, unlike the optical telescope, it does not receive wave frequencies of visible light, but of radio waves, which are much longer and have a larger 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 that were called Quasars (short for Quasi Stellar Object), which were 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.

On November 28th 1967, Bell was just spending another cold night 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 every 1.337 seconds, like clockwork. The source of such a signal could have been a man-made interference – for instance, signals from a radio transmitter of a plane passing by that were detected in the telescope – but since this signal was steady and persistent, this was not likely. Bell followed the signal all night long, and realized it was moving across the sky, together with the motion of the other stars, which means it was coming from outside of our solar system. This raised the possibility that this radio signal came from extraterrestrial life forms. Bell thought this was unlikely, but still, for her own amusement, she called the wave source LGM, which is short for Little Green Men.     

מחלוקת על חייזרים ודיון על קרדיט. יואיש ובל-ברנל באתר טלסקופ הרדיו בקיימברידג' | צילום: Science Photo Library
The argument about aliens and discussing credit. Hewish and Bell-Burnell at the radio telescope site in Cambridge | Photograph: Science Photo Library 

Pulsating star

Hewish and the head of the department, world–renowned astrophysicist Martin Ryle, were afraid to publish the results, mainly due to the public reaction that discovering extraterrestrial life forms would lead to. Bell, who was somewhat discluded from the meetings, continues working on ruling out the possibility that the signal came from aliens.

One of the ways to check whether radiation comes from an extraterrestrial life form is based on the assumption that aliens would reside on a planet, and not on a star (sun). A planet orbits around its sun, so it would appear to move towards and away from us, interchangeably.  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's 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". 

So, if these radio signals did not come from aliens, 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 was the one to discover this phenomenon and was pivotal in drawing the conclusions from it, Hewish was the first author of this important paper.

Additional studies corroborated this hypothesis. The type of star discovered by Bell received the name Pulsar, which is short for Pulsating Star. Certain neutron stars emit radiation from their poles due to their high rotation velocity. At times, the radiation is in the form of radio waves, 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.

המנחה היה המחבר הראשון. כותרת המאמר על הגילוי שפורסם ב-Nature בפברואר 1968 | מקור: מאמר המחקר
The supervisor was the first author on the discovery, as it was published in Nature in February 1968 | Source: the research paper 

Decisive role

In 1969, Bell received her doctorate in Astrophysics. The pulsars were only mentioned in an appendix of her dissertation. In that same year, she married her partner, Martin Burnell, a government worker that was stationed each time in a different location. She traveled after 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 mostly on gamma radiation and X-rays.

In 1974, the Nobel Prize committee decided to award the Prize in Physics to Martin Ryle and Anthony Hewish. Ryle had 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 one who actually discovered the radio waves and 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.

One of the advocators against the decision not to award the prize to Bell was the world-renowned physicist Fred Hoyle, who blamed Hewish for taking credit for his student's work. Bell-Burnell, of all people, was actually pretty 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. 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". Nevertheless, it is worth mentioning 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, many more types of pulsars were discovered, and their importance in assisting our understanding of the universe has come to light. Scientists have discovered different types of such stars, which do not only emit radio waves, but also X-rays and gamma rays. Pulsars with an even higher rotation velocity were discovered, with their signal repeating every few milliseconds. Some pulsars were found to be in a binary star system, which sometimes absorb matter from their neighboring star, making them accelerate 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 via measuring the effect different factors have on the radiation coming from the pulsars. The high accuracy of the pulsar frequency makes them good tools for designing watches that are more accurate than atomic watches.

Another advantage of pulsars is that they are very prominent throughout the universe. The Pioneer and Voyager spacecraft that were launched in the early 1970's for studying 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. Actually, there is no need to wait for aliens in order to use pulsars for navigation. There are already plans for using pulsars for spacecraft navigation, for clock synchronization of GPS satellites, since time moves faster for them than it does 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 breakthrough scientist, who found this discovery, did not receive the honor and appreciation she deserved.

Like a lighthouse in the sky – watch NASA's short video about pulsars:


Translated by Elee Shimshoni