pulsar

pulsar in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a scintillation effect on radio sources caused by clouds of electrons in the solar wind. Because this telescope was specially designed to record rapid variations in signals, in 1967 it readily recorded a signal from a totally unexpected source. Jocelyn Bell Burnell noticed a strong scintillation effect opposite the sun, where the effect should have been weak. After an improved recorder was installed, the signals were received again as a series of sharp pulses with intervals of about a second. By the end of 1968 it was clear that the team had discovered a rapidly spinning neutron star , a remnant of a supernova .

In 1974 the first binary pulsar—two stars, at least one of which is a neutron star, that orbit each other—was discovered by Russell A. Hulse and Joseph H. Taylor, for which they shared the 1993 Nobel Prize in Physics. Using this binary system, they observed indirect evidence of gravitational waves and also tested the general theory of relativity. Several dozen binary pulsars are now known. In 1995 the orbiting Compton Gamma Ray Observatory detected the first object that bursts and pulses at the same time. This bursting pulsar, another class of pulsars, is currently the strongest source of X rays and gamma rays in the sky. Fewer than a dozen bursting pulsars are known to exist.

The intense magnetic field and plasma that are believed to surround a neutron star provide an effective source of radio waves. The high-energy electrons of the plasma spiral around the magnetic field and emit radio waves and other forms of electromagnetic radiation. This synchrotron radiation is highly directional, like a flashlight beam. If the neutron star is rotating, it will act like a revolving beacon and produce the observed pulses. The pulses recur at precise intervals, but successive pulses differ considerably in strength. Since 1968 more than 700 pulsars have been observed, with pulse rates from 4 seconds to 1.5 milliseconds; the very rapid ones are called millisecond pulsars. The interval between pulses decreases ever so slightly with the passage of time, and it is believed that the slower pulsers are the older stars while the rapid pulsers are the younger. Pulsars in the Crab Nebula and at the site of the Vela supernova can be detected optically as well as at X-ray and gamma-ray frequencies.

pulsar Object emitting radio waves in pulses of great regularity. English radio astronomer Jocelyn Bell first noticed pulsars in 1967. Pulsars are believed to be rapidly rotating neutron stars. A beam of radio waves emitted by the rotating pulsar sweeps past the Earth, and is received in the form of pulses. Pulsars are gradually slowing down, but some, such as the Vela Pulsar, occasionally increase their spin rate abruptly; such an event is called a glitch. More than 500 pulsars are now known, flashing at rates from c.4 seconds to 1 millisecond.

pulsar A radio source from which is received a highly regular train of pulses. More than 1600 pulsars have been catalogued since the first was discovered in 1967. Pulsars are rapidly spinning neutron stars, 20–30 km in diameter. The stars are highly magnetized (about 10⁸ tesla), with the magnetic axis inclined to the spin axis. The radio emission is believed to arise from the acceleration of charged particles above the magnetic poles. As the star rotates, a beam of radio waves sweeps across the Earth and a pulse is seen, much like the beam from a lighthouse. Pulse periods are typically 1s, but range from 1.4 ms (millisecond pulsars) to over 10 s. The pulse periods are lengthening gradually as the neutron stars lose rotational energy, but a few young pulsars are prone to abrupt disturbances known as glitches. Precise timing of pulses has revealed the existence of binary pulsars, and two pulsars, PSR 1257+12 and PSR B1620-26, have been shown to be accompanied by objects of planetary mass. Optical flashes have been detected from a few pulsars, notably the Crab and Vela Pulsars.

Most pulsars are believed to have been created in supernova explosions by the collapse of the core of a supergiant star, but there is now considerable evidence that at least some of them originate from white dwarfs that have collapsed into neutron stars following accretion of mass from a companion star (see recycled pulsar). The great majority of known pulsars are members of the Milky Way and are concentrated in the galactic plane. There are estimated to be about 100 000 pulsars in the Galaxy. Observations of interstellar dispersion (2) and the Faraday effect in pulsars provides information about the distribution of free electrons and magnetic fields in the Milky Way.

Pulsars are denoted by the prefix PSR followed by the approximate position in right ascension (4 digits) and declination (2 or 3 digits), usually for equinox 1950.0. The figures may be preceded by B if the coordinates are for epoch 1950.0, or J for epoch 2000.0.