..... Click the link for more information. . Astronomers Baade Baade, 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.
..... Click the link for more information. and Zwicky Zwicky, 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
..... Click the link for more information. predicted the existence of neutron stars in 1933. In the central core of a neutron star there are no stable atoms or nuclei; only elementary particles elementary 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.
..... Click the link for more information. can survive the extreme conditions of pressure and temperature. Surrounding the core is a fluid composed primarily of neutrons neutron, 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.
..... Click the link for more information. squeezed in close contact. The fluid is encased in a rigid crystalline crust a few hundred meters thick. The outer gaseous atmosphere is probably only a few centimeters thick. The neutron star resembles a single giant nucleus nucleus, 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.
..... Click the link for more information. 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: 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.
..... Click the link for more information. 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
Any of a class of extremely dense, compact stars thought to be composed mainly of neutrons with a thin outer atmosphere of primarily iron atoms and electrons and protons. Though typically about 12 mi (20 km) in diameter, they have a mass roughly twice the Sun's and thus extremely high densities (about 100 trillion times that of water). Neutron stars have very strong magnetic fields. A solid surface differentiates them from black holes. Below the surface, the pressure is much too high for individual atoms to exist; protons and electrons are compacted together into neutrons. The discovery of pulsars in 1967 provided the first evidence of the existence of neutron stars, predicted in the early 1930s and believed by most investigators to be formed in supernova explosions. See also white dwarf star.
neutron star [′nü‚trän ‚stär]
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 
∽ 2 × 1033; that is, the mass is equal to the sun’s mass 
⊙; 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 
of neutron stars falls within the limits 0.05 
⊙ < 
< max, where 
max = (1.6-2.4) 
⊙; here the spread in the computed values of 
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
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