A small number of "radio stars" (actually quasars, Chap. 26) were known in the early 1960s. Then at the Cavendish laboratory (University of Cambridge) Anthony Hewish developed a new method to find radio stars by using the scintillation. The ordinary stars twinkle because their light is passing through restless air layers. In the same manner, radio stars twinkle because radio waves have to traverse through the variable solar wind on the way to the Earth. Hewish filled a two hectare field with radio antennas and started to search the sky systematically for twinkling radio stars that could turn out to be quasars. The instrument produced 30 meters of paper tape every day. It was inspected by Hewish's student Jocelyn Bell who had the responsibility for operating the telescope and analyzing the data. She noticed that one of the radio sources twinkled in a special way. The peculiar thing was that there were pulses of radiation arriving at a constant interval of 1.3 s. First Hewish thought that the source is man made, but soon it became clear that the source was in the sky, not on Earth. The next, more exciting idea was that the pulses are generated by other intelligent beings living on a planet circling around their own sun.
However, Bell soon found another pulsing signal from a quite different part of the sky. Now, she reasoned, "it was very unlikely that two lots of little green men would both choose the same, improbable frequency, and at the same time, to try signaling the same planet Earth"! New similar sources were found all over the Milky Way and one had to consider a natural phenomenon (Fig. 19.5).
Before the results were published in Nature in early 1968, Hewish gave a seminar in Cambridge and suggested that the pulses come from white dwarf stars. Fred Hoyle, head of the Institute of Theoretical Astronomy, was in the audience, and he replied: "I don't believe they are white dwarfs, I think they are supernova remnants." Nobody could have been more correct after only a few minutes' brain work.
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