Astrophysics Group

Cavendish Laboratory

Pulsars

Mullard Radio Astronomy Observatory
Pulsars

Serendipitous discovery – the chance discovery of some entity or phenomenon during the pursuit of a completely different objective – has played a considerable role in the history of radio astronomy. One of the most far-reaching instances of this was the discovery of pulsars here at the MRAO.

AHewish_smallIn 1967 a new array of aerials, designed by Antony Hewish to detect quasars – a class of radio source of very small angular size – came into operation at the Observatory. The array was designed to detect compact radio sources such as these by virtue of the fact that they would scintillate (twinkle) when observed from the Earth through the interplanetary medium – the moving plumes of charged particles emitted by the Sun. This is just like the familiar way stars are seen to twinkle when viewed through the moving non-uniform layers of the Earth’s atmosphere. In fact ‘quasar’ is an abbreviation for ‘quasi-stellar object’, emphasising this analogy. Larger emitters of radio waves or light do not twinkle as markedly as small ones.

 

JBell_array Output from the so-called 4-acre array was recorded on chart recorders, so that as the Earth rotated and the sky passed over the telescope, each day’s observations resulted in several long rolls of paper on which a pen trace recorded the signal as a function of time. One of Antony Hewish’s research students, Jocelyn Bell, was set the task of operating the telescope and inspecting the daily charts for the kind deflections in the trace which occurred when a scintillating source passed over the array. Such signatures might correspond to the quasars they were hoping to find. As always in radio astronomy, there were other problems to contend with such as man-made interference.

 

However, as time went on Jocelyn Bell noticed what she first described as a bit of ‘scruff’ on the records – and it was unlike either scintillation or man-made interference. What was more, it always appeared when the same bit of sky passed over. To investigate it in more detail she tried making faster chart recordings, to give more time resolution. Eventually, after dogged persistence when nothing seemed to show up, she captured the more detailed signal on a fast chart record in November 1967. The signal showed up as a series of pulses separated by 1.3 seconds.

When Jocelyn Bell alerted Antony Hewish, he first asserted that these 1.3-second fluctuations were too fast to be due to anything the size of a star, so must be man-made signals of some kind. But on checking further, he saw that their position was only consistent with an extra-terrestrial origin. Various solutions to this conundrum were posited and eliminated one-by-one on scientific grounds – from equipment malfunctions, to artificial satellites, to extraterrestrial aliens signalling from outer space (‘Little Green Men’ as they were jokingly dubbed). These latter were almost certainly dismissable on the grounds that a signal from an orbiting planet on which the ‘men’ could reside would display rises and falls in frequency by virtue of the Doppler effect, according to whether the planet was approaching or receding from the Earth at the time the signal was emitted. Antony Hewish found that the only such variations in frequency measured where explained by the orbital motion of the Earth itself.

Eventually other similar bursts of ‘scruff’ were observed in other parts of the sky, each displaying repeatable behaviour and each with its own characteristic frequency. It was too far-fetched to imagine that they could all be due to aliens, so the only possible conclusion was that a new class of object – ‘pulsars’ as they came to be known – had been found.

 

The explanation for the origin of pulsars – supernovae

supernova At the ends of their lives, many stars undergo a violent death: when the supply of nuclear fuel runs out in a massive star, the whole star can explode in a few seconds in an event called a supernova. The optical light from the star brightens for a month or so; but at radio wavelengths we can observe the aftermath of the explosion for up to ten thousand years. The material is ejected at up to 10,000 kilometres per second and produces strong radio waves as it expands into the space around the star. The image on the right is a Ryle Telescope radio map of just such a supernova which exploded 300 years ago. At the centre of a supernova, the remains of the star can collapse to form a compact, dense object called a neutron star which spins rapidly on its axis and emits pulses of radio waves as it spins. These objects – which became known as ‘pulsars’ – were the new class of object which Jocelyn Bell and Tony Hewish had discovered serendipitously in 1967, with the telescope that had originally been designed to search for quasars.

 

For a more detailed account of the discovery of pulsars, including some nice animations, visit the Cavendish Laboratory’s Pulsars section in the Cambridge Physics Educational outreach pages

You can also Listen to an interview with Jocelyn Bell Burnell, courtesy of the Jodrell Bank Jodcast for June 2007, recorded for the 40th anniversary year of the discovery – available in mp3 format.

For more information other aspects of about our work, follow the links on the left.