Pulsar is any of a class of cosmic objects that emit extremely regular pulses of radio waves; a few such objects are known to give off short rhythmic bursts of visible light, X rays, and gamma radiation as well.

Pulsars are thought to be rapidly spinning neutron stars, extremely dense stars composed almost entirely of neutrons and having a diameter of only 20 km (12 miles) or less. A neutron star is formed when the core of a violently exploding star called a supernova collapses inward and becomes compressed together. Neutrons at the surface of the star decay into protons and electrons. As these charged particles are released from the surface, they enter an intense magnetic field that surrounds the star and rotates along with it. Accelerated to speeds approaching that of light, the particles give off electromagnetic radiation by synchrotron emission. This radiation is released as intense beams from the pulsar's magnetic poles.

These magnetic poles do not coincide with the rotational poles, and so the rotation of the pulsar swings the radiation beams around. As the beams sweep regularly past the Earth with each complete rotation, an evenly spaced series of pulses is detected by ground-based telescopes.

Antony Hewish and Jocelyn Bell, astronomers working at the University of Cambridge, first discovered pulsars in 1967 with the aid of a radio telescope specially designed to record very rapid fluctuations in radio sources. Subsequent searches have resulted in the detection of more than 300 pulsars. A significant percentage of these objects are concentrated toward the plane of the Milky Way Galaxy, the enormous galactic system in which the Earth is located.

Although all known pulsars exhibit similar behaviour, they show considerable variation in the length of their periods--i.e., the intervals between successive pulses. The periods of the slowest pulsars so far observed are about four seconds in duration. The fastest, the Millisecond Pulsar (discovered in 1982), has a period of 0.00155 second, or 1.55 milliseconds, which is considerably shorter than that of any known pulsar. It has been determined that the Millisecond Pulsar is spinning 642 times per second. This is close to the theoretical limit for a pulsar because a neutron star rotating only four times faster would fly apart as a result of "centrifugal force" at its equator, notwithstanding a gravitational pull so strong that the star's escape velocity is about half the speed of light.

Careful timing of radio pulsars shows that they are slowing down very gradually at a rate of typically a millionth of a second per year. The ratio of a pulsar's present period to the average slow-down rate gives some indication of its age. This so-called timing age is in close agreement with the actual age of the one pulsar whose time of birth is known independently: the Crab Pulsar, which was formed during a supernova explosion observed in AD 1054. The Crab Pulsar is the youngest known, followed by the Vela Pulsar that has a projected timing age of 11,000 years.

The Crab and Vela pulsars are losing rotational energy so precipitously that they also emit radiation of shorter wavelength. The Crab Pulsar appears in optical photographs as a moderately bright (magnitude 16) star in the centre of the Crab Nebula. Soon after the detection of its radio pulses in 1968, astronomers at the Steward Observatory in Arizona found that visible light from the Crab Pulsar flashes at exactly the same rate. The star also produces regular pulses of X rays and gamma rays. The Vela Pulsar is much fainter at optical wavelengths (average magnitude 24) and was observed in 1977 during a particularly sensitive search with the large Anglo-Australian Telescope situated at Parkes, Australia. It also emits X rays though it does not seem to pulse at those wavelengths. The Vela Pulsar does, however, give off gamma rays in regular pulses and is the most intense source of such radiation in the sky.

Older radio-emitting pulsars are slowing down at a lesser rate than the young ones and have long periods. Moreover, it has been determined on the basis of timing ages that pulsars "switch off" after about 10 million years when their magnetic fields have weakened appreciably. The number of pulsars detected relatively close to the Sun indicates that there must be some one million active pulsars in the Milky Way Galaxy. These two figures taken together suggest that an object of this kind must be born every 10 years, a rate which is inexplicably five times greater than the rate of occurrence of supernovae generally thought to produce neutron stars.

Excerpt from the Encyclopedia Britannica without permission.