The Discovery of Pulsars

Overview

The discovery of pulsars in 1967 can be said to have been almost accidental. Pulsars were discovered by Jocelyn Bell Burnell (1934-), then a graduate student at the University of Cambridge who was using her advisor's radio telescope to search for quasars. Her discovery had considerable impact, both for astronomers in general and women scientists in particular. Since their discovery, pulsars have been recognized by astronomers as crucial to understanding the nature of stars, especially exotic stars like black holes. For women scientists, Bell Burnell's discovery was to be an inspiration. Rarely had a female scientist gained so much fame for a scientific discovery. And although she did not share the Nobel Prize in Physics given to her advisor for the discovery of pulsars, she has since come to be recognized for something perhaps more significant: for helping to pave the way for women in all fields of science.

Background

The discovery of pulsars has as its background the whole of radio astronomy, and the discovery of quasars in particular. This is because the use of radio telescopes to search for quasars led to the discovery of pulsars. The history of radio astronomy and the development of radio telescopes is important to the discovery of both quasars and pulsars.

Radio telescopes receive radio waves, not light. Therefore they are not like the optical telescopes we normally associate with astronomy. Radio telescopes do not have lenses and they are not shaped like tubes. Instead, radio telescopes typically consist of radar dishes, or of very large arrays of wires suspended above the ground. These "telescopes" receive radio waves from space. Unlike optical telescopes they can operate night and day, and also during cloudy weather. They can amplify the signals they receive so that they are made stronger, and these signals can then be transformed into both audio and video signals that are interpreted by astronomers. One problem that radio telescopes have is that they often pick up human-made radio signals from Earth. This can cause considerable confusion. Such confusion was part of the story of the discovery of pulsars, and will be discussed below. First, however, we must consider the discovery of quasars.

Quasars were discovered in 1960 with a type of radio telescope called an interferometer. The radio astronomer Thomas Matthews was using this telescope to get an accurate position for an object referred to as "3C 48." Earlier this object had been observed as a blue-colored star. Matthews demonstrated that this star was a source of large quantities of radio waves. In the next few years, other such radio wave-emitting objects were discovered. One of these objects, called "3C 273," was closely studied in 1962. It was demonstrated to be both very distant and very bright. So bright, in fact, that astronomers estimated this single object to be as bright as 100 galaxies, the equivalent of one trillion stars. Further studies of these objects revealed that they all shared the traits of being extremely bright, large (each being roughly the size of our solar system), and radiating vast quantities of energy in the form of radio waves. They were called quasi-stellar radio objects, or quasars.

The best way to detect quasars was to use a technique called "interplanetary scintillation." Radio waves coming to the Earth from objects in space, like quasars, will be slightly disrupted by the solar wind (ionized gas) that "blows" off of our Sun. While radio signals from space are affected by the solar wind, radio signals from Earth are not. The technique of "interplanetary scintillation" detects radio signals from space by looking for the disruption of these signals by the solar wind; this disruption is detected as a twinkling or "scintillation." In order to detect such scintillations, unique radio telescopes had to be built.

In July 1967 radio astronomers at the university of Cambridge in England finished building such a radio telescope. The director of this project was Antony Hewish (1924- ). He was aided by Jocelyn Bell Burnell, who was then a graduate student, and other volunteers. This radio telescope took two years to build and consisted of 120 miles (193 km) of cable suspended on 128 pairs of poles. The entire telescope covered roughly 4.5 acres of ground. As part of her Ph.D. work, Bell Burnell analyzed the charts of data produced by the telescope's computer. Her job was simply to review the numerous data charts, find scintillations like those produced by quasars, and then plot their positions on maps of the heavens. She could not have known that this seemingly mundane task would lead to a most remarkable discovery.

Impact

Jocelyn Bell Burnell's work with the radio telescope was routine for about two months, until August of 1967. On 6 August the telescope picked up a radio source whose signals came in pulses. At first Bell Burnell thought the pulses were just "scruff," as they did not appear to be the quasars for which she was looking. After a while she realized that these pulses of "scruff" came with extreme regularity. Initially, neither Bell Burnell nor her advisor Hewish thought they had discovered anything new. They believed it was a human-made radio signal, perhaps reflected back to their telescope off of the Moon or a satellite, or even a nearby building. But by November they realized this was not the case, that their mysterious signal did in fact come from a location outside of our solar system. Astoundingly, its radio-wave pulses came with such rapid regularity—once every 1-1/3 seconds—that Bell Burnell and Hewish thought the source might not be natural. As a joke, they said the signal must come from "Little Green Men," and so they called the pulsing radio source LGM1.

In the next month, December of 1967, Bell Burnell was analyzing data from a different part of the sky and found another regularly pulsing radio source with a slightly shorter period of 1-1/5 seconds. And then, over the Christmas holiday, she discovered two more such pulsing sources. So by January 1968, Bell Burnell and Hewish knew they had discovered a new class of objects in space. They announced their discovery in February of 1968 in a paper in the journal Nature. The announcement was sensational, and soon afterwards the objects were given the name pulsars.

But what sort of objects were these pulsars? A few months before Bell Burnell's discovery, the astronomer Franco Pacini, then at Cornell University in New York, published a paper arguing that a rapidly rotating neutron star, if one were found to exist, would have a very strong magnetic field and would therefore be a powerful source of radiation. In June of 1968, soon after the discovery of pulsars was announced, Thomas Gold (1920- ), also at Cornell University, published a paper in Nature in which he identified the pulsars discovered by Bell Burnell with the theoretical rotating neutron stars indicated by Pacini. Thus it was shown that the pulsars were rapidly rotating neutron stars. They emitted high-intensity radio waves from their magnetic poles. Because of their rapid rotation, pulsars' radio waves are detected as "pulses" much like the way light is seen "pulsing" from a light-house.

Perhaps one of the most interesting results of the discovery of pulsars was the controversy over who actually discovered them. In 1974, Antony Hewish and Sir Martin Ryle (1918-1984) received the Nobel Prize in physics for their work in radio astronomy. Hewish was recognized for his role in the discovery of pulsars. Jocelyn Bell Burnell did not share the prize. She was not considered the discoverer of pulsars; at the time she had merely been a graduate student and the Nobel Prize committee felt that the award should go to a scientist with a long and established record of research. Her exclusion from the Nobel Prize led many distinguished astronomers, including Thomas Gold, to complain that Bell Burnell was in fact the discoverer of pulsars and therefore should have shared the prize.

In all of this Bell Burnell did not complain. She said "Nobel Prizes are based on long-standing research, not on a flash-in-the-pan observation of a research student." She did win many other prizes, medals, and honors for her discovery of pulsars and became an inspiration for women scientists. Living in England, she considers herself "a role model, a spokeswoman, a representative, and a promoter of women in science in the United Kingdom." And she has undoubtedly inspired women scientists throughout the world.

The discovery of pulsars has impacted science and society in two significant ways. First, it was an incredible discovery for astronomers. It not only confirmed the existence of the theoretical neutron star, but it also enabled scientists to make advances in astrophysics, particularly in their theories of stellar collapse and the formation of black holes. Furthermore, pulsars are the most regular "clocks" in the universe. They have enabled scientists to make important tests of Albert Einstein's theory of general relativity.

Second, the discovery of pulsars shed light on the important role of women in science. Perhaps more surprising than the fact that a new type of star was discovered was that a woman had discovered it. In 1967 there were relatively few established women in science. Jocelyn Bell Burnell was then and continues to be an important example for women scientists. In 1991 she was made a Professor of Physics at the Open University in England. Soon after her appointment, the number of women physics professors in the United Kingdom doubled.

STEVE RUSKIN