LGM-1: The First Pulsar

Summary

In 1967 a Cambridge research student, Jocelyn Bell, found radio pulses arriving every 1.337 seconds from a fixed point among the stars. The source was nicknamed LGM-1, for "little green men"; within months it was understood as a spinning neutron star.

It was the first pulsar. The discovery won a Nobel Prize in 1974, for Bell's supervisor, Antony Hewish, and the question of whether she should have shared it has been argued ever since. The possibility of an artificial signal was raised, tested and ruled out in a few weeks, which is how it is supposed to work.

What is documented

The telescope. In 1965 Antony Hewish designed a radio telescope to study interplanetary scintillation, the twinkling of compact radio sources as their waves pass through the solar wind. It covered 18,000 square metres, about four and a half acres, with more than 2,000 dipole aerials, and worked at a wavelength of 3.7 metres (about 81 MHz). It took two years to build and was complete and tested by July 1967. Bell, a research student of Hewish's, analysed the survey's paper chart records.

The scruff. Six to eight weeks into the survey Bell noticed a small patch of what she called "scruff" on the charts, from a part of the sky at right ascension 19h 19m. Hewish dates the first record she showed him to around the middle of August 1967. The scruff came and went, but it kept to sidereal time: it returned when that part of the sky returned, not at the same time each day, as local interference would.

The pulses. Faster chart recordings began at the end of October. On 28 November 1967 a fast recording showed that the signal was a series of regular pulses, about 1⅓ seconds apart; the period is 1.337 seconds. Hewish's first thought was that the pulses must be man-made.

The checks. J. D. H. Pilkington measured the dispersion of the pulses, the slight delay of lower radio frequencies behind higher ones, and found that the source lay well outside the solar system but inside the galaxy. P. F. Scott and R. A. Collins confirmed the signal with a separate telescope. Timing showed the pulses kept time to better than one part in a million, and that the only Doppler shift in their period was the one caused by the Earth's own motion. A transmitter on a planet circling another star would have shown that planet's motion too.

Little green men. The group considered, and had to exclude, the possibility that the pulses were artificial, sent by another civilisation. Bell later recalled going home one evening that December "very cross", because "some silly lot of little green men had to choose my aerial and my frequency to communicate with us". Back at the laboratory that night, shortly before Christmas 1967, she found a second source, at right ascension 11h 33m, pulsing every 1.2 seconds. The first source became known as CP 1919 (Cambridge pulsar, 19h 19m), and is now catalogued as PSR B1919+21.

Publication. The paper, "Observation of a Rapidly Pulsating Radio Source", by Hewish, Bell, Pilkington, Scott and Collins, was received by Nature on 9 February 1968 and published on 24 February. It proposed that the pulses "may be associated with oscillations of white dwarf or neutron stars", and, according to Bell, mentioned that the team had at one stage thought the signals might come from another civilisation.

The explanation. On 25 May 1968 Thomas Gold argued in Nature that the constancy of the pulses could be accounted for by the rotation of a neutron star, radiating in the pattern of a rotating beacon. In October 1968 M. I. Large, A. E. Vaughan and B. Y. Mills reported a pulsar with a period of 0.089 seconds at the position of a suspected supernova remnant, and early in 1969 J. M. Comella and colleagues reported on the pulsar in the Crab Nebula, the remnant of the supernova recorded by Chinese astronomers in 1054. Pulsars and exploded stars were linked, as the neutron-star picture required.

The Nobel Prize. On 15 October 1974 the Nobel Prize in Physics was awarded to Martin Ryle, for aperture synthesis, and to Antony Hewish "for his decisive role in the discovery of pulsars". The Academy's announcement did not mention Bell. In 1977 she said that "it would demean Nobel Prizes if they were awarded to research students, except in very exceptional cases", and that she did not think this was one. In 2018, fifty years after the discovery, she received a Breakthrough Prize in Fundamental Physics worth 3 million US dollars, and gave it to the Institute of Physics to fund graduate students from groups under-represented in physics.

Leading explanations

A rotating, magnetised neutron star. Established. Gold's model: the collapsed core of a massive star, about 20 kilometres across, spinning rapidly and sweeping a beam of radio waves past the Earth once per turn. Its regularity is the regularity of a spinning body. The link to supernova remnants, the Crab pulsar and the many pulsars found since all fit it.

Oscillating white dwarfs or neutron stars. Rejected. This was the discoverers' own first suggestion in February 1968. Rotation soon replaced it.

An artificial signal, terrestrial or extraterrestrial. Rejected within weeks. Local interference does not keep sidereal time. A transmitter on an orbiting planet would have shown the planet's orbital motion in the pulse timing, and none was there. The dispersion placed the source at interstellar distance, and a second source with a similar period turned up elsewhere in the sky.

Current status

Later explained. CP 1919 is a rotating neutron star, the first of a large class of objects now used as precise clocks. The identification has not been in serious doubt since 1968 and 1969.

The part of the record that is still argued is human, not astronomical: the 1974 prize. The registry records the documented positions. The Academy's announcement did not name Bell. Many have described the omission as a snub. Bell Burnell said in 1977 that prizes should not normally go to research students, and in 2019 that she "wasn't too bothered not to get it", adding that the Nobel committee did not normally recognise students.

Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.