The Discovery of Pulsars and the Aftermath1

Article excerpt

THE DISCOVERY OF PULSARS in 1967 is associated with the names of Anthony Hewish and Jocelyn Bell-Burnell, but the seeds of their achievement were sown long before during the exciting era when radio astronomy developed from a specialist pursuit of physicists and electrical engineers into a key area of contemporary astronomy.

After the Second World War, a number of university groups began the investigation of the nature of the cosmic radio emission, which had been discovered by Karl Jansky in 1933. The principal radio groups involved were those at Cambridge, Manchester, and Sydney. The Cambridge efforts were led by Martin RyIe, who assembled a brilliant team of young physicists, including Graham Smith, Tony Hewish, and Peter Scheuer, to attack these problems. The Cambridge efforts were largely devoted to the development of the technique of aperture synthesis as a means of obtaining high angular resolution and sensitivity by combining coherently the radio signals received by arrays of telescopes.

HEWISH AND THE SCINTILLATION OF RADIO SOURCES

As part of that effort, Hewish's research involved understanding the nature of the fluctuations, or scintillations, of the intensities of radio sources because of intervening moving plasma clouds. This research followed in the tradition of the Cavendish Radio Group, which, after the war, was led by Jack Ratcliffe, building on the pioneering ionospheric researches of Appleton. Just as stars twinkle even on the clearest nights, so point sources of radio emission are observed to scintillate, particularly at the long radio wavelengths, which were the focus of research in the early days of radio astronomy. The cause of the radio scintillations is the deflections of radio rays when they pass through irregularities in the ionospheric plasma, as illustrated schematically in the diagram (fig. 1).

The theory of the process of scintillation was worked out in detail by Hewish in 1951 in a paper entitled "The Diffraction of Radio Waves in Passing through a Phase-Changing Ionosphere" (A. Hewish, Proc. Roy. Soc. 209 [1951]: 81-96). The paper sets out the theoretical background needed to understand the short-term fluctuations in the intensities of radio sources due to irregularities in an ionised plasma. The same concepts and techniques could be applied to the physics of fluctuations due to ionospheric, interplanetary, and interstellar electron density fluctuations. This theoretical paper was followed in 1952 by another entitled "The Diffraction of Galactic Radio Waves as a Method of Investigating the Irregular Structure of the Ionosphere" (A. Hewish, Proc. Roy. Soc. IIA [1952]: 494-514). Applying these concepts to observations of the fluctuating radio signals, Hewish showed that the scale of the irregularities ranged from 2 to 10 km, that the variation of electron content was about 5 × 10^sup 9^ electrons per cm-2, and that the irregularities are at a height of about 400 km. These irregularities moved with a steady wind-like motion at a velocity of the order 100 to 300 m s^sup -1^.

The same technique could be used to study the solar corona, the region of hot plasma surrounding the Sun. The radio source Taurus A (the Crab Nebula) was observed at varying angular distances from the Sun, and the variability of the signal could be accounted for by scattering because of the presence of fluctuations of the electron density in the solar corona. In his paper of 1955 "The Irregular Structure of the Outer Regions of the Solar Corona" (A. Hewish, Proc. Roy. Soc. 228 [1955]: 238-51), Hewish derived the sizes and electron densities of coronal irregularities in the distance range 5 to 15 solar radii.

THE CONTROVERSY OVER THE RADIO SOURCE COUNTS

During the 1950s, the first Cambridge catalogues of radio sources were produced, giving rise to the famous controversy concerning the excess of faint radio sources that was observed. Martin RyIe interpreted these data as evidence for an evolving Universe and inconsistent with expectations of steady state cosmology. …