In 1967, radio astronomers at Cambridge, England, were astounded to find strange radio signals coming from certain places in space. The signals came in short rapid bursts, or pulses. Each lasted only a few hundredths of a second. However, the pulses repeated with extraordinary regularity. Scientists realized these radio signals were coming from objects sending streams of radiation into space. From Earth, this radiation is seen as short bursts, or pulses. These objects became known as pulsars.

The pulses that astronomers receive from these objects are very sharp. As a result, astronomers conclude that pulsars are very tiny and have a radius of only about 10 miles (16 kilometers). When an object emits a burst of radio-wave radiation, the waves from different parts of the object arrive at the Earth at different times. This causes the original burst of radiation to become blurred. The smaller the object, however, the sharper the burst, or pulse.
Only one other object in the universe—a neutron star—is known to have such a small size yet is able to emit such bursts of radiation. Neutron stars mark the final stage in the life of many giant stars. When a giant star has used up most of its fuel and its nuclear reactions begin to lessen, the star collapses. As it collapses, gases and other materials are compressed. Then, the hot, compressed gases explode. This creates what is known as asupernova. The remaining materials in the dying star continue collapsing inward. This produces enormous pressures that crush the nuclei of atoms together, forming particles called neutrons. The resulting ball of neutrons, called a neutron star, is very small and dense. The density of matter in a neutron star is about one billion tons per cubic inch.

A pulsar is a neutron star that spins rapidly and has a strong magnetic field because of its great density. It acts like a radio transmitter, beaming a narrow radio wave into space. As a pulsar spins, this radio wave sweeps through space like the light from a revolving lighthouse beacon. Astronomers on Earth can only detect the radio wave when it is pointed directly at our planet. It appears to blink on and off as it arrives as a series of short pulses. As neutron stars age, they tend to rotate more slowly and no longer act as pulsars. In fact, most neutron stars in the universe are not pulsars.
Astronomers tried to link neutron stars and pulsars by studying the Crab Nebula. This is a cloud of gas produced by a supernova that occurred in A.D.1054. They reasoned that if neutron stars are the remains of supernova explosions and pulsars are neutron stars, then they should find a pulsar in the remains of a supernova explosion. In 1968, astronomers found a pulsar at the center of the Crab Nebula. The pulsar was located precisely where the neutron star from the supernova should have been.

Since then, more than 1,000 pulsars have been found. Many emit not just radio energy, but energy across the spectrum. This energy includes X-rays and visible light. In 2003, the first two-pulsar binary was found. (Binaries are systems in which two stars orbit each other.) Pulsars are so dense they can exert extreme gravitational forces on their binary companions. Scientists study the effects of these forces to test theories about gravity.