Pulsar, a celestial body that emits radio signals in short, very evenly spaced bursts, or pulses. The pulse period (the time from the beginning of one pulse to the beginning of the next) ranges from about a thousandth of a second for some pulsars to more than four seconds for others. Pulsars with a period close to a thousandth of a second are known as millisecond pulsars. The pulses from some of these pulsars are so regular they can be used to keep time as accurately as the best atomic clocks.

According to a widely accepted theory, a pulsar is a rapidly rotating neutron stara star composed of subatomic particles called neutrons. A neutron star is thought to have a mass somewhat greater than that of the sun but a diameter of less than 20 miles (32 km). According to this theory, a pulsar emits radio-frequency radiation from a small area that faces the earth only briefly with each rotation.

A neutron star is thought to form from the core of a supernova (exploding star). A number of pulsars show evidence of being remnants of supernovae. One such pulsar is located in the center of the Crab Nebula, a cloud of glowing gases thrown off by a supernova seen in 1054 A.D. This pulsar emits regular bursts of light and X rays as well as radio waves.

The first pulsar was discovered in 1967 by radio astronomers at Cambridge University, England. Since that time astronomers have detected hundreds of pulsars, including binary pulsarssystems in which a pulsar and a star are in orbit around each other. In 1992, astronomers announced the discovery of a pulsar with a varying pulse period that indicates it may have planet-sized bodies in orbit around it.