The pulsar story can be traced back to the mid- 1960s, when a pioneering survey in the quest for new radio sources was conducted at Cambridge University in England, Some of the newly discovered sources seemed to change brightness from minute to minute, but only when they were observed close to the direction of the sun. This phenomenon is called scintillation and is produced when the radio waves pass through a patchy cloud of electrons. Such clouds will cause the radio waves to alter their path slightly, jiggling back and forth from minute to minute, which results in the scintillation of the radio source.
The smallest-diameter radio sources scintillate when their beams pass through electron clouds blowing out of the sun in the solar wind. Larger diameter radio sources, however, glow steadily because many beams of radiation from such sources suffer scintillation on the way to the telescope, and when they are added together the average signal is steady. Observation of radio source scintillation thus contains information about both the properties of the particle clouds streaming from the sun and the angular size of the radio sources themselves.
After the discovery of radio source scintillation, the Cambridge radio astronomers realized that a good, cheap radio telescope could be built that would allow persistent monitoring of this phenomenon so that radio source diameters, largely unknown at the time, could be estimated.
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