Exploring the X-ray Universe

Discovery and nature of neutron stars

Until the discovery of radio pulsars by Jocelyn Bell and Antony Hewish in 1967 (Hewish et al., 1968), neutron stars had existed only in the minds of theoretical physicists. First proposed as an end state of stellar evolution by Robert Oppenheimer and George Volkoff (1939), they are now accepted as the only explanation for radio pulsars. The discovery was serendipitous. No one had conjectured or even dreamed that this sort of signal might be generated.

Bell, then a student, had just spent months wiring antennas for a new radio telescope. In the course of testing, she noticed ‘a bit of scruff’ in the recorded signal. This ‘scruff’ was found to repeat, not every 24 hours (the solar day), but every 23 hours and 56 minutes (the sidereal day), showing that the source was anchored in the sky, not to the rotating Earth. Furthermore, when present, the signal was periodic with a remarkably stable period of 1.337 s. Thus CP 1919, the first of the radio pulsars, was discovered. The stable period could only be explained by rotation, and only a small object with strong gravity could rotate this fast without breaking up. A neutron star was the only reasonable explanation (Gold, 1968).