In 1967, a graduate student named Jocelyn Bell found something she had never seen before. Data from a very large radio telescope monitoring some of the most distant things in the universe showed an unexpected type of pulse. At first, Bell thought the strange readings were "scruff," meaningless and probably caused by local interference. But the readings showed a very rapid and regular beat. Bell knew that rapid pulses indicated something small, while regular pulses indicated something large, so she had trouble understanding how a beat could be both rapid and regular. How could something be both small and large at the same time?

Bell and her fellow researchers jokingly referred to the unconfirmed source of the pulses as LGM, for "Little Green Men," a British expression for space aliens. After confirming that the telescope was operating correctly and eliminating man-made objects, such as orbiting satellites, or local interference, for example from transistor radios, as the source, Bell and her faculty supervisor, Anthony Hewish, determined the pulses were coming from a previously unidentified kind of star. They named it a "pulsar," short for "pulsating star."

Pulsars are a kind of neutron star -- a small remnant of an exploding star. For a star much larger than our Sun, death can be violent. No longer able to withstand the force of its own gravity, it collapses in on itself. Pressure generated by the collapse builds until the star's outer gas layers are released in a huge explosion called a supernova. The star's inner core, however, implodes, squeezing protons and electrons even further together to form an incredibly dense neutron core. The resulting object, a neutron star, is only about 6 miles (10 km) in diameter yet contains even more mass than our Sun!

A neutron star is surrounded by an extremely powerful magnetic field. This field is so strong that it causes most of the light and energy that the star emits to be concentrated into beams at the magnetic poles. As the neutron star rotates on its axis, the emitted radiation, like beams from a lighthouse, can appear to switch on and off as each beam sweeps past Earth. This creates the characteristic pulses of a pulsar. Using a giant radio telescope, an astronomer can detect a pulse of radio waves -- with wavelengths up to a million times longer than visible light -- each time the beam sweeps past. Although pulsars are most commonly detected with radio telescopes, radio waves are not the only waves emitted by pulsars. Pulsars also emit X-rays, gamma rays, and even visible light.

So, what can be learned from studying pulsars? For one thing, pulsars rotate in such a predictable manner that their pulses can be used to tell time. The discovery of pulsars has also provided insight into the death of stars. To date, astronomers have identified close to 1500 pulsars, most of them in our Milky Way Galaxy.

Print Background Essay