Bell Discovers Pulsars

Date February 24, 1968

Jocelyn Bell discovered pulsars, a new class of star that provided the key to understanding supernovas and neutron stars and gave clues to the origins and nature of the universe.

Locale Cambridge, England

Key Figures

• Jocelyn Bell (b. 1943), Irish graduate student in radio astronomy at the University of Cambridge
• Antony Hewish (b. 1924), English astronomer

Summary of Event

The history of science is replete with serendipitous discoveries that eclipse the original intentions of researchers and their experiments. Jocelyn Bell’s discovery of pulsars is illustrative of this phenomenon. The path of discovery leads to the Mullard Radio Astronomy Observatory of the University of Cambridge, where in 1965 astronomer Antony Hewish was constructing a new kind of radio telescope. Although the telescope was designed for quasar detection, quasars became the least significant part of the research. By luck, the observational parameters for quasar detection were very close to those of something they were not looking for: pulsars. (The word “pulsar” comes from “pulsating star.”)

Bell, Hewish’s graduate student, was the first person to identify a pulsar. In an effort to bring the new telescope on line and work out the bugs, she could not account for an anomaly in the radio data she called “scruff.” The source of this unwanted signal noise was annoying, elusive, and invisible. As with any new instrument, Hewish’s first thoughts were that the pulses were electrical noise within the instrument or, perhaps, some type of local noise such as ham operators, automobile ignitions, or other electrical interference. Bell was determined to isolate and filter the pulses from the radio signature. Then, curiously, they disappeared.

Bell continued searching, checking the telescope for “scruff.” In late November of 1967, it reappeared. Hewish thought the signals were freakish because of their shift in position and their fluctuating appearance. An attempt to measure any parallax failed, which suggested that the source was beyond the solar system. Hewish then hypothesized the pulses might be the radio signature of a flare star. To resolve the question, they used a high-speed chart recorder. Since the telescope was not steerable, Bell had to synchronize her observations with the daily, or nightly, sidereal passage of the object overhead.

It was clear that Bell discovered something unusual, but the source was not clear. These signals did not demonstrate the radio signatures of other known sources— stars, galaxies, or solar wind—which suggested something entirely different. Other explanations pointed to some terrestrial human-made interference with the telescope. The question of how to distinguish the two came from a simple but unique phenomenon: the difference in Earth time and star (sidereal) time. Bell observed the pulsating “scruff” over time and realized that it was not in synchronization with Earth time but was coordinated with sidereal time. This suggested an extraterrestrial origin. It also raised the question of what could create extraterrestrial pulses with such preciseness, with pulses arriving at intervals of 1.3373011 seconds. Regularity of this precision is rarely found in nature, which raises the question of whether it is natural.

Hewish and Bell had to consider the possibility of the LGM (Little Green Men) phenomenon and in good humor identified the source as LGM 1. These regular pulses could be tangible evidence of alien intelligence. They were presented with a fascinating dilemma: what to do with this kind of knowledge. If they announced the discovery without all the evidence and were proved wrong later, it would be a textbook example of how not to conduct a scientific investigation. If these pulses were evidence of alien intelligence, however, they would represent the beginning of a momentous discovery. Hewish and Bell attacked the problem in the spirit and method of good science.

The LGM hypothesis faded; they renamed the source CP 1919 (for Cambridge pulsar and its sky position) and turned their attention to describing the phenomenon. Hewish maintained the survey over Christmas in 1967 and placed the raw data on Bell’s desk. Upon her return, Bell began to analyze the charts and found another source of pulses. Then, sources number three and four appeared. In the next two weeks, Bell was able to confirm that these were, indeed, independent sources. Perhaps the sky was teeming with pulsating sources. Bell reviewed an endless number of charts to determine if there were other sources she might have overlooked. This research resulted in a number of candidates, but nothing as definitive as the first four sources. The nature of the source was still eluding Hewish, Bell, and other astronomers, such as John Pilkington, P. F. Scott, and R. A. Collins, who had joined the search at Mullard Observatory.

They announced their discovery (without resolving the nature of the pulsating source) in Nature, February 24, 1968, with the following statement: “A tentative explanation of these unusual sources in terms of the stable oscillations of white dwarf or neutron stars is proposed.” In publications later that year, Hewish seemed to favor the white dwarf hypothesis. The editors of Nature seem to favor the other option, because on the cover of that issue were the words “Possible Neutron Star.” At this point, the problem of the nature of the pulsating sources passed to the world community of scientists. The final connection of the pulsar with a rapidly rotating neutron star, rather than a pulsating white dwarf, came with the combined work of Franco Pacini and Thomas Gold in 1968.

Significance

Jocelyn Bell did not anticipate the instant celebrity status the news of the discovery of pulsars brought, especially in the popular press. Bell quietly ended her observations, wrote her dissertation, and accepted a job in another field of research in another part of the country. The story of the pulsars became an appendix in her dissertation.

The announcement by Hewish and Bell triggered a flood of observational and theoretical papers on pulsars. In the following year, the list of pulsar locations grew to more than 24; in 1976, there were more than 150 identified, and the current list is more than 400 pulsar locations. Pulsars were not discovered sooner because radio astronomers were using centimeter wavelengths to look at the sky, as opposed to Hewish’s meter wavelengths and the adding of signals over long periods of time in order to resolve weak signals. This was the reason why the Hewish telescope was successful and Bell was able to resolve the pulses. For this and other outstanding work, Hewish shared the 1974 Nobel Prize in Physics with Sir Martin Ryle. It was the first awarded to astronomers. Hewish’s award was based on his role in the detection of pulsars. Interesting, Bell—the acknowledged discoverer—was not included in the Nobel recognition.

The discovery carried the teasing phrase “possible neutron star,” and the timing mechanism that regulated the radio pulses remained a mystery. The 1968 discovery of the Vela pulsar within a supernova remnant established the link between pulsars and neutron stars. This was strengthened by the discovery of a pulsar in the Crab nebula, another supernova remnant. There was a direct relationship between the age of the supernova remnant, diffusion of the nebulosity, and pulse rate with neutron stars as the product of the supernova event. The history of pulsars appears to follow this sequence: First, a star explodes (supernova); then, the core collapses, forming a neutron star. Finally, as the nebulosity expands, the pulses slow in proportion to the rotation of the neutron star. The “scruff” Bell identified on the radio telescope was the radio signature of rotating neutron stars.

The important consequence of the pulsar-neutron star discovery is that astronomers have an opportunity to examine the behavior of matter under incredible and extreme conditions of pressure and temperature not obtainable in terrestrial laboratories and at densities so great that they involve general relativistic considerations and micro physics. Pulsars stimulated further research on stellar evolution and its products such as white dwarfs, neutron stars, collapsars, frozen stars, and black holes.

Observing binary neutron stars gives confirming evidence for Albert Einstein’s theory of general relativity, the distortion of space-time near massive objects, and the existence of gravity waves. Further, pulsar-neutron stars represent matter on the verge of total gravitational collapse; their existence implies that what was once a theoretical convenience like Wolfgang Pauli’s neutrino particle may be a physical reality. The paradigms of the nature of the universe have changed because Jocelyn Bell persisted in understanding the nature of the “scruff” on her recording chart.

Bibliography

Astronomical Society of the Pacific. The Discovery of Pulsars. San Francisco: Author, 1989. An audiotape interview with Hewish and Bell. Available from A.S.P., 390 Ashton Avenue, San Francisco, CA 94112. Perhaps the best primary source. Targeted to the interested reader as an example of how science works.

Greenstein, George. Frozen Star. New York: Freundlich Books, 1983. A thorough work on the nature of pulsars, supernovas, and neutron stars. This work is geared for the interested amateur or astronomy student.

‗‗‗‗‗‗‗. “Neutron Stars and the Discovery of Pulsars.” Mercury 34 (March/April, 1985): 34-39, 66-73. Captures the spirit and detail of the discovery. A good starting point for the interested reader.

Hewish, A. “Pulsars, After Twenty Years.” Mercury 38 (January/February, 1989): 12-15. A summary from the Nobel Prize winner on the discovery of pulsars and the knowledge of these strange radio sources. Written for nonspecialists, with illustrations of the telescope and a model of a neutron star.

‗‗‗‗‗‗‗, et al. “Observation of a Rapidly Pulsating Radio Source.” Nature 217 (February 24, 1968): 709-713. This is the primary source for the serious reader. Interesting understatement illustrating the cautious nature of announcing discoveries whose scope is not fully understood.

Lyne, Andrew G., and Francis Graham-Smith. Pulsar Astronomy. 3d ed. New York: Cambridge University Press, 2006. Comprehensive, definitive work on the study and significance of pulsars. Part of the Cambridge Astrophysics series. Bibliographic references and index.