A pulsar is an astronomical source of radio waves that pulsate with periods ranging from about four seconds to about one millisecond. A pulsar’s signals are regular to within 1 part in 10 billion, making pulsars some of the most accurate timekeepers in the universe. A pulsar is almost certainly a small, rapidly rotating body, probably a neutron star. A neutron star has a mass comparable with that of the Sun, collapsed into a sphere about 20 km (12 mi) across. It results from the explosion of a very massive star—a supernova. More than 500 pulsars are known, most of them concentrated towards the disc of the Milky Way, our own galaxy, where supernovae commonly occur.

A neutron star has intensely strong magnetic fields. Radiation is given out in two beams along the neutron star’s magnetic axis. This axis is tilted in relation to the star’s rotational axis, so the beams sweep round in space with every rotation—much like those of a lighthouse. If one of the beams points towards the Earth at some point in its rotation, astronomers detect it as a rapidly pulsating signal. The fastest flash every few thousandths of a second, and are known as "millisecond" pulsars.

Pulsars slow down with increasing age. Occasionally, there is a "glitch" in the highly regular flashing. This is believed to be caused by a "starquake", a sudden cracking in the outer crust of a neutron star that causes the pulsar to shrink by about 1 mm and to spin slightly faster in consequence.

Pulsars emit electromagnetic radiation in all regions of the spectrum, visible as well as radio. One of the most famous optical pulsars lies deep within the supernova remnant known as the Crab Nebula in the constellation Taurus. A pulsar that is one of a close pair of stars can pull gas from its companion star on to its magnetic poles, creating high temperatures that result in the emission of rapid pulses of x-rays and gamma-rays, in addition to radio waves and visible light.

Most pulsars are thought to be more than a million years old, since the supernova remnants that once shrouded them have long since dissipated. The periods of all the known pulsars are increasing, and their emissions of visible light and gamma radiation are decreasing. Astronomers can measure these rates of change to calculate pulsar ages.

The first pulsar (now called PSR 1919+21) was found in 1967 by a then young postgraduate student named Jocelyn Bell (now Jocelyn Bell Burnell) and her adviser Antony Hewish at the Mullard Radio Astronomy Observatory in Cambridge. It had a period of 1.337 seconds.

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