Pulsars are older stars that give out regular
pulses of electromagnetic radiation, mainly radio waves (see radio astronomy). They were discovered in 1967 by the radio astronomers Jocelyn Bell Burnell and Antony Hewish at Cambridge, England. The name is an abridgment of the term "pulsating radio star," because a pulsar was first taken to be a new form of pulsating variable star. Hundreds of
Pulsars are now known. Their "pulse" periods range from a few milliseconds to about 4 seconds. The periods are accurately maintained but show a slight slowing down in most cases. Pulsars are believed to be rapidly rotating neutron stars, the "pulse" period actually being the rotation period, and the slowing down being the result of loss of rotational energy. The pulses themselves are highly variable. For example, strong pulses may be followed by weak ones, and a pulsar may occasionally spend some time not exhibiting pulses and then resume. When many individual pulses from a given pulsar are averaged, they show a characteristic pulse profile. The best-known pulsar is situated in the Crab nebula, where the loss of energy as the pulsar slows down provides the means of keeping the nebula radiating. The velocity of pulsar radio waves as they travel through the interstellar medium is a function of the frequency, with high frequencies arriving sooner than low frequencies because of refraction effects. This phenomenon enables the pulsar's distance to be determined. Pulsars appear to be located anywhere in our Galaxy within a few hundred parsecs of the galactic plane. In 1984 the first pulsar discovered outside our Galaxy was found in the Large Magellanic Cloud. The Crab and Vela pulsars show pulses at optical and gamma-
ray frequencies, and X-ray pulses have been detected from the Crab pulsar.
Binary radio pulsars have also been discovered. Another kind of pulsar, the binary X-ray pulsar, is thought to consist of a neutron star and a
companion star revolving around one another. Their periods are longer than those of radio pulsars, and because of the binary nature of the system, the pulses show Doppler effects as the pulsar orbits around the companion star. Some X-ray pulsars show eclipses. The X rays are believed to arise because of material being ejected from the companion star and landing on the neutron star. Unlike radio pulsars, the X-ray pulsars are speeding up, an effect caused by the mass transfer from the companion. The best known X-ray pulsar is Hercules X-1. A few pulsars have been observed almost certainly to possess planetary
systems (see planets and planetary systems). The possible nature of such systems, however, is not yet known.
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