1967

Pulsars

Antony Hewish (b. 1924), Samuel Okoye (1939–2009), Jocelyn Bell (b. 1943)

The astrophysicists Walter Baade and Fritz Zwicky proposed the concept of neutron stars—highly dense, compact stellar remnants from supernova explosions—back in 1933. However, it wasn’t until 1965 that radio astronomers Antony Hewish and Samuel Okoye discovered the first observational evidence for a neutron star—a powerful but very small source of intense radio energy coming from the center of the Crab Nebula, the explosive remains of the famous “Daytime Star” supernova of 1054.

Hewish and colleagues at the University of Cambridge continued searching for new neutron stars and other radio sources. Just two years later, using the new, more sensitive four-acre radio telescope west of Cambridge, Hewish’s student Jocelyn Bell discovered the first rapidly pulsating radio star (“pulsar”) in the constellation Vulpecula, with a constant pulse rate of every 1.3373 seconds.

Bell and Hewish considered the possibility that the pulsar’s eerily regular radio signal might be a sign of extraterrestrial intelligence (they had jokingly named the source LGM-1, for “Little Green Men-1”). However, by 1968 they and other astronomers had come up with a more plausible explanation, partly because the neutron star in the center of the Crab Nebula had also been discovered to be a radio pulsar, with a pulse rate of every 33 milliseconds. Pulsars were found to be rapidly spinning neutron stars with strong magnetic fields that “beam” some of their energy in specific directions (usually along or close to their rotation axis). If the beamed electromagnetic radiation from the spinning pulsar is aligned so that it sweeps past the Earth, it can “light up” radio telescopes like the spinning beacon of a lighthouse.

Several thousand pulsars have since been discovered, including several hundred-millisecond pulsars like the one in the Crab Nebula. Amazingly, variations in the timing of signals from the pulsar named PSR B1257+12 were interpreted in 1992 to be caused by the presence of planets orbiting the pulsar—the first examples of extrasolar planets.

A high-resolution composite Hubble Space Telescope (red) and Chandra X-ray Observatory (blue) image of the central region of the Crab Nebula (Messier 1), a remnant from a supernova explosion in 1054. The central energy source is a pulsar—a rapidly rotating neutron star—with a 33-millisecond rotation period.