neutron star

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neutron star,

extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolutionstellar evolution,
life history of a star, beginning with its condensation out of the interstellar gas (see interstellar matter) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable configuration.
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. Astronomers BaadeBaade, Walter
, 1893–1960, German-born American astronomer. From 1919 to 1931 he was on the staff of the Hamburg observatory; from 1931 to 1958, at the Mt. Wilson observatory.
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 and ZwickyZwicky, Fritz
, 1898–1974, Swiss-American astrophysicist, b. Bulgaria, educated at Zürich. Associated with the California Institute of Technology after his arrival in the United States in 1925, he became professor of astrophysics in 1942 and emeritus professor in 1972.
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 predicted the existence of neutron stars in 1933. The central core of a neutron star is composed of neutrons or possibly a quark-gluon plasma (see elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter

Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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); there are no stable atoms or nuclei because these cannot survive the extreme conditions of pressure and temperature. Surrounding the core is a fluid composed primarily of neutronsneutron,
uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons.
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 squeezed in close contact. The fluid is encased in a rigid crystalline crust a mile or two thick. The outer gaseous atmosphere is probably only a few feet thick. The neutron star resembles a single giant nucleusnucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Composition

Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
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 because the density everywhere except in the outer shell is as high as the density in the nuclei of ordinary matter. There is observational evidence of the existence of several classes of neutron stars: pulsarspulsar,
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
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 are periodic sources of radio frequency, X ray, or gamma ray radiation that fluctuate in intensity and are considered to be rotating neutron stars. A neutron star may also be the smaller of the two components in an X-ray binary star.

neutron star

An extremely dense compact star that has undergone gravitational collapse to such a degree that most of its material has been compressed into neutrons. Neutron stars were postulated in the 1930s by a number of astronomers including Landau and Zwicky. They are thought to form when the mass of the stellar core remaining after a supernova explosion exceeds the Chandrasekhar limit, i.e. about 1.4 solar masses. Such a core is not stable as a white dwarf star since even the pressure of degenerate electrons cannot withstand the strong gravity. Collapse continues until the mean density reaches 1017 kg m–3 and the protons and electrons coalesce into neutrons, supporting the star against further contraction. Accretion of gas on to an existing white dwarf, raising its mass above the Chandrasekhar limit, may also lead to its collapse to become a neutron star.

With diameters of only 10–15 km, intense magnetic fields (108 tesla), and extremely rapid spin, young neutron stars are believed to be responsible for the pulsar phenomenon. Their strong gravity is also thought to give rise to rapid heating of material observed in some X-ray binary systems. Older pulsars, ‘spun up’ by accretion are detected as millisecond pulsars, the fastest rotating neutron stars. Gamma-ray bursts probably have neutron star origins.

Models of the structure of neutron stars have been derived from sudden changes in pulsar spin rates – so-called glitches. A typical neutron star may have an atmosphere only a few centimeters thick, under which is a crystalline crust about one kilometer in depth. Beneath the crust the material is thought to act like a superfluid of neutrons (having zero viscosity) all the way through to a solid crystalline core.

Neutron Star

 

one of the possible final states in the evolution of a star of large mass; a neutron star is primarily composed of neutrons, with a small fraction of electrons, protons, and heavier nuclei. The possibility of the existence of neutron stars was first proposed by L. D. Landau in 1932, shortly after the discovery of the neutron by J. Chadwick earlier that year.

In 1934 the American astronomers W. Baade and F. Zwicky suggested that neutron stars might be formed during supernova outbursts. It follows from the theory of stellar evolution that when massive stars have almost completely “burned up” the nuclear fuel in their central regions, they can undergo a catastrophically rapid gravitational compression, which is called gravitational collapse (seeGRAVITATIONAL COLLAPSE). During a star’s collapse, the density of the matter of the star increases so much that a state in which neutrons are more stable than protons is attained. Under these conditions, protons and stable atomic nuclei are transformed into neutrons and atomic nuclei that have an excess of neutrons (the neutronization of matter). This process requires a density ρ > 1010 g/cm3. At densities ρ ≥ 10 12

g/cm3 and temperatures T < 1010 ≤K, which are characteristic for neutron stars, the matter is in the form of a degenerate neutron gas (seeDEGENERATE GAS).

The mechanical equilibrium of neutron stars is related to the compensation of the force of gravity by the pressure of the degenerate neutron gas. The following are typical average values for parameters of a neutron star in a state of stable equilibrium: mass Neutron Star ∽ 2 × 1033; that is, the mass is equal to the sun’s mass Neutron Star ⊙; radius R ∽ 2 × 106 cm = 20 km (R⊙ = 7 × 1010cm); density ρ ∽ 2 × 1014 g/cm3 (p⊙ = 1.4 g/cm3); pressure ρ ∽ 1033 -1034 dynes/cm2; and minimum period of rotation 10-3 sec. The magnetic field of a neutron star can reach about 1012 gauss (the average magnetic field of the sun is about 1 gauss).

The average density of a neutron star is close to the nuclear density of matter, or even greater. Therefore, the structure and properties of neutron stars depend, to a significant extent, on nuclear forces. Moreover, neutron stars are characterized by a large amount of gravitational binding energy (approximately 1053 ergs); this energy leads to the appearance of substantial corrections to Newtonian gravitational theory, these corrections following from the general theory of relativity. It is especially important to take into account the nuclear forces and the gravitational binding energy in the calculation of the internal structure of neutron stars. From calculations, the theoretically expected mass Neutron Star of neutron stars falls within the limits 0.05 Neutron Star ⊙ < Neutron Star < Neutron Starmax, where Neutron Starmax = (1.6-2.4) Neutron Star ⊙; here the spread in the computed values of Neutron Star is due to difficulties in taking into account the effect of nuclear forces.

Most existing theories relate the formation of neutron stars to supernova outbursts, since the gravitational collapse of a star under specific conditions is accompanied by a powerful explosion that ejects the star’s outer layers into space. Neutron stars were discovered in 1967 by detecting pulsations in the radio emission from certain stars (these stars are called pulsars); moreover, a number of pulsars are definitely associated with supernova remnants (in particular, the pulsar PSR 0532 in the Crab Nebula).

REFERENCES

Dyson, F., and D. ter Haar. Neitronnye zvezdy i pul’sary. Moscow, 1973.
(Translated from English.) Tayler, R. J. Stroenie i evoliutsiia zvezd. Moscow, 1973. (Translated from
English.)
Zel’dovich, Ia. B., and I. D. Novikov. Teoriia tiagoteniia i evoliutsiia zvezd. Moscow, 1971.

V. S. IMSHENNIK

neutron star

[′nü‚trän ‚stär]
(astronomy)
A star that is supposed to occur in the final stage of stellar evolution; it consists of a superdense mass mainly of neutrons, and has a strong gravitational attraction from which only neutrinos and high-energy photons could escape so that the star is invisible.
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