Discovering pulsars from a Cambridge field

Susan Jocelyn Bell Burnell (b. 1943)
The measurement of radio source diameters using a diffraction method
Cambridge, 1969

Building a dipole telescope to her supervisor Anthony Hewish’s design at Lord’s Bridge, west of Cambridge, Bell Burnell was to use the phenomenon of interplanetary scintillation, identified by Hewish in 1964, as a means of measuring the size of quasars. As output from the telescope, she used a pen-recorder tracing on moving paper. Reviewing one set of traces Bell Burnell noticed a ‘scruff’ of sudden activity recorded. Following this up, she increased the speed at which the paper moved until the scruff showed a highly regular pulse: the discovery of pulsars, one of the most significant in the history of astronomy.

PhD.6567, p. 214, reproduced by kind permission of Professor Jocelyn Bell Burnell

Gravity is in the final analysis the dominant force in the Universe and will eventually overcome all other forces. We observe our local galaxy and its denizens and can trace the history and infer the future of stars such as our Sun that, thankfully, exist emitting approximately constant amounts of light and heat over billions of year. We believe from the evidence we have that the Sun is about half-way through its normal, or main sequence, life of ten billion years where the principal fuel of the thermonuclear fusion giving the Sun’s emitted energy is hydrogen. When hydrogen is finally depleted to a great extent, heavier elements will begin to fuse. We observe that stars of about our Sun’s mass expand enormously during the later phases of their lives to become red giant stars – we believe that the Sun might one day have a radius as far out as the Earth’s present orbit. The dawns of that distant era will we think be much as H.G. Wells presciently envisaged for his time traveller.

Our star will reach a point where the energy flooding from its core will not support such a huge object and the Sun will collapse to form a white dwarf. The two great pillars of wisdom in modern physics are gravitational relativity and quantum mechanics, two ends of the scale of the Universe from the vast down to the minutely small, and it is the considerations of quantum physics that keep white dwarf stars for the Sun’s approximate mass from collapsing further. Naturally there are larger stars that evolve and eventually collapse; if they have about 50% greater mass than the Sun stars may collapse further, sometimes in the furnace of a huge supernova (such as those observed by both Tycho Brahe and Johannes Kepler) which today we believe form neutron stars, wherein ordinary matter is compacted to the limit possible, to such as density that the whole of the Earth’s mass might be compressed to a sphere 200 metres across. However, just 50 years ago the idea of neutron stars was beyond our scientific speculation. During the star’s collapse the enormous energy evolved would in some cases cause this star to spin at very high rates; vast quantities of radiation emitted in the direction of the intense magnetic fields of such an object would sweep round as if it were some titanic galactic lighthouse. There are many conditional clauses in this narrative but again it could be that the sweep might be in the direction of our planet – and perhaps towards a few acres of a muddy field off to the west of Cambridge.

Soon after graduating in natural philosophy from the University of Glasgow in 1965, Jocelyn Bell began studying for a Cambridge doctorate with Professor Anthony Hewish as her supervisor. As a practical method of determining the size of highly energetic objects in the Universe named quasars, together with colleagues in the Mullard Radio Astronomy Observatory she built a radio telescope to his design to detect the radio-wave equivalent of the sparkling of stars that Hewish had identified in 1964, interplanetary scintillation. The telescope consisted of some 2,000 wooden poles erected in a regular array on which copper wire was looped; the radio output from this array was fed to an analogue paper chart pen-recorder, the whole being designed to detect scintillation effects over very short times. The operation of the array having been achieved by July 1967 Bell began to study the output amounting to some 30 metres of paper per day, very soon noticing a ‘scruff’ of an ink trace about 10mm wide occurring regularly each day. Increasing the speed at which the paper traversed the pen-recorder she found that the scruff resolved itself into a regularly timed pulse every 1.3 seconds, the source of which moved in the sky by the Earth’s rotation as would a star so that any regular radio pulse generated on Earth causing the signal could be discounted. Though nothing known to astronomers could account for such a signal, Bell persisted – the first signal disappeared, to make the phenomenon seem transient but then returned and then another similar pulsing signal was found and because of the stability of their position in the sky they were shown to be far outside the solar system.

So was made one of the most significant observational discoveries of astronomy in the 20th century, that of stellar sources pulsating in the electromagnetic spectrum, a class of objects known as the pulsars, a class now containing many hundreds. Our current understanding of the creation of pulsars is given above; in fact it was the discovery of these exotic objects that lead to our current ideas about the very existence of neutron stars.

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