Pulsar (redirected from Pulsars)

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 Univ. 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.

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pulsar

 in full pulsating radio star

A pulsar emits two beams of electromagnetic radiation along its magnetic axis. If the magnetic axis …

(credit: © Merriam-Webster Inc.)

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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pulsar

any of a number of very small extremely dense objects first observed in 1967, which rotate very rapidly and emit very regular pulses of polarized radiation, esp radio waves. They are thought to be neutron stars formed following supernova explosions

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pulsar [′pəl‚sär]

(astrophysics)

A celestial radio source, emitting intense short bursts of radio emission; the periods of known pulsars range between 33 milliseconds and 3.75 seconds, and pulse durations range from 2 to about 150 milliseconds with longer-period pulsars generally having a longer pulse duration.

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Pulsar 

a weak source of cosmic radio emissions, bursts of which repeat with a period that changes extremely slowly. The first pulsar was discovered in 1967 by British astronomers; by 1975 approximately 100 such objects had been detected. Pulsars differ in their radio emission from all previously known cosmic sources, which are characterized either by constant intensity, as are galaxies or radio galaxies, or by irregular bursts, as are the sun and certain flare stars.

The periods of known pulsars, that is, the intervals of time between successive bursts of radiation, range from 0.033 to 3.75 sec. The first observations of pulsars indicated that their periods were constant to an extraordinarily high degree. Subsequent observations, however, established that pulsars have periods that increase extremely slowly. The time required for the periods of most pulsars to double is of the same order of magnitude as the age of the pulsar, amounting to millions and tens of millions of years. However, there are two pulsars whose periods double in substantially less time: the pulsar in the Crab Nebula—a remnant of the Supernova of 1054—whose period doubles every 2,400 years, and the pulsar in the supernova in the constellation Vela, whose period doubles every 24,000 years. These are the youngest pulsars and have the shortest periods. The fact that these pulsars have envelopes characteristic of supernovas favors the hypothesis that they were formed as a result of the explosion of supernovas. Other, older, pulsars lack such envelopes, apparently because the envelopes were able to disperse into space. An interesting feature of young pulsars is the sudden and irregular shortening of the period as a result of violent internal processes. Nearly all pulsars are observed only in the radio band of electromagnetic radiation; the only exception is the pulsar in the Crab Nebula, which can also be detected in the optical, X-ray, and gamma-ray bands.

Studies of the radio emission from pulsars in the radio-wavelength band between 10 cm and 10 m have established that the maximum emission is usually in the meter-wavelength band. It has also been determined that the same pulse is not simultaneously detected in observations at different wavelengths; that is, emissions at the shorter wavelengths reach the earth before emissions at longer wavelengths. This separation of a radio burst occurs because radio emissions with shorter wavelengths propagate through the plasma of interstellar space with a velocity close to that of light in a vacuum, while those with longer wavelengths travel noticeably more slowly. Thus, the time lag of a pulse observed at two distinct wavelengths is proportional to the distance from the observer to the pulsar and the average line-of-sight concentration of electrons. Since this concentration is known, it is possible to calculate the distance to the pulsar and to estimate the strength of the radio emission by measuring the radio emission flux in the direction of the earth and determining the lag time. The distances to currently known pulsars have been found to vary between tens of parsecs and several thousand parsecs; the radio-emission strength of a pulsar is millions of times greater than that of the sun, even during periods of violent solar activity.

The most likely explanation of pulsars is provided by the “lighthouse” theory. According to this theory, a pulsar is a rotating star that emits a narrow beam of radio waves. An observer in the path of the beam detects periodically repeating radio pulses. The theory holds that the pulse period of a pulsar is equal to the rotational period, which explains why the pulse periods are very nearly constant. The lighthouse model also explains many other observed data; in particular, it suggests that the slow increase in the pulse period is a consequence of a slowdown in the rotation of the star.

However, serious difficulties arise in selecting the class of stars that could be responsible for the observed phenomena. The very high angular velocity of rotation characteristic of pulsars is possible only for extremely dense and small stars. White and red dwarfs (dense stars) cannot have such angular velocities of rotation because centrifugal forces would rapidly break them up. Neutron stars, whose existence is postulated only on the grounds of theoretical research, represent the only possible relevant class of stars. Observations of pulsars have thus provided confirmation of the existence of neutron stars.

Neutron stars are characterized by extremely small dimensions; the diameter of a neutron star with a mass approximately that of the sun is only several tens of kilometers. The density of matter within such a star reaches 10^l4–10¹⁵ g/cm³, that is, approximately the density of matter within atomic nuclei. A neutron star is apparently an immense atomic nucleus consisting chiefly of neutrons. Its kinetic rotational energy is assumed to be the source of the energy emitted by a pulsar. The mechanism by which a pulsar emits energy is associated with strong magnetic fields on the surface of the pulsar, with strengths reaching thousands of billions of oersteds. The transformation of the kinetic rotational energy of a star into emissions is apparently the result of the fact that a rotating magnetic star induces an electric field about itself, which accelerates particles of the surrounding plasma to high energy levels. These accelerated particles produce the observed emissions.

In the 1970’s, pulsars that emit chiefly in the X-ray band were discovered. These pulsars are apparently neutron stars that are part of a binary system; the second component of such systems is a normal star. Gas from the envelope of the normal star flows toward the neutron star, twists around it, and finally falls onto its surface following magnetic lines of force of the neutron star’s field. As a result, directional X-radiation is generated, which is responsible for the pulsations detected by an observer in the path of the directional-radiation beam.

REFERENCE

Dyson, F. and D. Ter Haar. Neitronnye zvezdy i pulsary. Moscow, 1973. (Translated from English.)

V. V. USOV