pulsar

/pul"sahr/, n.

Astron. one of several hundred known celestial objects, generally believed to be rapidly rotating neutron stars, that emit pulses of radiation, esp. radio waves, with a high degree of regularity.

[1965-70; puls(ating st)ar, on the model of QUASAR]

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in full pulsating radio star

Any of a class of cosmic objects that appear to emit extremely regular pulses of radio waves.

A few give off short rhythmic bursts of visible light, X rays, and gamma radiation as well. Thought to be rapidly spinning neutron stars, they were discovered by Antony Hewish and Jocelyn Bell Burnell in 1967 with a specially designed radio telescope. More than 550 have been detected since. All behave similarly, but the intervals between pulses (and thus their rotation periods) range from one-thousandth of a second to four seconds. Charged particles from the surface enter the star's magnetic field, which accelerates them so that they give off radiation, released as intense beams from the magnetic poles. These do not coincide with the pulsar's own axis of rotation, so as the star spins, the radiation beams swing around like lighthouse beams and are seen as pulses. Pulsars have been shown to be slowing down, typically by a millionth of a second per year. It has been calculated that pulsars "switch off" after about 10 million years, when their magnetic fields weaken enough.

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▪ cosmic body

in full  pulsating radio star 

 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 (Hewish, Antony) and Jocelyn Bell (Bell Burnell, Jocelyn), 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 pulsar designated PSR J1939+2134 was the fastest known for more than two decades. Discovered in 1982, it has a period of 0.00155 second, or 1.55 milliseconds, which means it is spinning 642 times per second. In 2006 an even faster one was reported—known as Ter5ad, it has a period of 1.396 milliseconds, which corresponds to a spin rate of 716 times per second. These spin rates are close to the theoretical limit for a pulsar because a neutron star rotating only about 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, which 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 about 100,000 active pulsars in the Milky Way Galaxy. These two figures taken together suggest that an object of this kind must be born every 20 years, a rate which is inexplicably about twice as great as the rate of occurrence of supernovae generally thought to produce neutron stars.

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