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A neutron star is a type of stellar remnant resulting from the gravitational collapse of a massive supergiant star after depleting the fuel in its nucleus and exploding like a type II, Ib type or Ic type supernova. As its name suggests, these stars are mainly composed of neutrons, plus other types of particles both in their solid iron crust and inside, which can contain both protons and electrons, as pions and Kaones. Neutron stars are very hot and are supported against a greater collapse by pressure of quantum degeneration, due to the phenomenon described by the Pauli exclusion principle. This principle establishes that two neutrons (or any other fermiónica particle) can not occupy the same quantum space and state simultaneously.

A typical neutron star has a mass between 1.35 and 2.1 solar masses, 1 2 3 A with a corresponding radius of approximately 12 km. 4 b instead, the radius of the sun is about 60 000 times that number. Neutron stars have total densities of 3.7 × 1017 at 5.9 × 1017 kg/m3 (2.6 × 1014 to 4.1 × 1014 times the density of the sun), c comparable with the approximate density of an atomic nucleus of 3 × 1017 kg/m 3.5 The density of a neutron star varies from less than 1 × 109 kg/m3 in the crust, increasing with depth to more than 6 × 1017 or 8 × 1017 kg/m3 still further in (more dense than an atomic nucleus). 6 This density is roughly equivalent to the mass of a Boeing 747 compressed in the size of a small grain of sand.

In general, compact stars of less than 1.44 solar masses — the limit of Chandrasekhar — are white dwarfs, and above 2 to 3 solar masses — the limit of Tolman-Oppenheimer-Volkoff — a star of quarks can be created; But this is uncertain. Gravitational collapse usually occurs in any compact star of between 10 and 25 solar masses, and will produce a black hole. Some neutron stars turn quickly and emit electromagnetic radiation rays as pulsars.

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Any star of the main sequence with an initial mass of more than 8 solar masses can become a neutron star. Thus, in this type of stars, at the end of the primary phase of hydrogen fusion with its consequent separation of the main sequence, there is a warm-up of the nucleus, which enables other types of fusions, due to which produces a nucleus rich in Iron. When all the nuclear fuel has been used, the nucleus becomes unstable, having to withstand the pressure of degeneration alone. At the same time, heavy materials are still deposited in the nucleus, causing the Chandrasekhar limit to be exceeded. The degenerate pressure of the electrons increases and the nucleus collapses faster, increasing the temperature up to 3 x 109 K. At these temperatures, Photodecay occurs (rupture of the iron core in alpha particles due to high energy gamma rays). Thus, the alpha particles, having less load, absorb more easily the electrons that are put inside the nuclei, combining with the protons. Also the resulting helium is susceptible to be disintegrating, so that huge quantities of free protons will be generated.

This results in an even greater increase in temperature, resulting in the formation of neutrons of the proton and Electron union, by means of a process known as electronic capture, emitting neutrinos. In principle, the density needed to give the neutronización (recombination of electrons with protons to give neutrons) is 2.4 × 107 g/cm³. As in the degenerate stars there are no free protons, the necessary density is, in fact, higher, since the electrons have to overcome a rather greater coulombiana barrier, requiring approximately about 109 g/cm³.

Iron Photodecay: {\displaystyle \gamma + {} ^ {56} \mathrm {Fe} \rightarrow 13 \ Alpha + 4n} \gamma + {} ^ {56} \mathrm{Fe} \rightarrow 13 \alpha + 4n

Helium Photodecay: {\displaystyle \gamma + {} ^ {4} \mathrm {He} \rightarrow 2p + 2n} \gamma + {} ^ {4} \mathrm{He} \rightarrow 2p + 2n

This cycle follows its effect until it reaches a nuclear density of 4 x 1017 kg/m3; K, when the nuclear degenerated pressure stops the contraction. The outer atmosphere of the star is expelled creating a supernova type II or Ib, while the rest becomes a neutron star, whose mass will be less than 5 solar masses (if its mass was greater would end up becoming a black hole as the pressure D and insufficient neutron degeneration to stabilize the process. Neutron stars can also be produced from binary systems. Its nucleus will be formed by Hyperdense Iron, along with other heavy metals, and will continue to compact, being its mass too large and the degenerated electrons are not able to stop the collapse.

The Photodecay cools the compact star, because it is an endothermic reaction that absorbs some of the internal heat of the same. On the other hand, the concentration of electrons decreases when absorbed by the nuclei, causing a plummeting pressure of the degeneration, accelerating even more the collapse. The overloaded neutron nuclei lose them, leaving them free, where they become part of a compact mass of neutrons called neutronium.

The process continues to reach the neutron degeneration density, approximately around 1014 G/cm³, at which point almost the entire mass of the star will have been transformed into neutrons. The nucleus of degenerate neutrons must have a mass of less than three solar masses, called limit of Tolman-Oppenheimer-Volkoff. If you have a higher mass, the collapse of the neutron star cannot be stopped but is believed to form a black hole. Some scientists speculate on the possible existence of an intermediate state between neutron star and black hole; It would be the star of quarks, but such an object has not yet been observed. 9 However, there are several candidates for Quark star, such as RJX J185635-375.

Features

The main characteristic of neutron stars is that they resist the gravitational collapse by the degeneration pressure of the neutrons, added to the pressure generated by the repulsive part of the strong nuclear interaction between the barium. This contrasts with the main sequence stars, which balance the force of gravity with the thermal pressure originated in the thermonuclear reactions inside.

It is not currently known whether the nucleus of a neutron star has the same structure as its outer layers or whether it is formed by quark-gluons plasma. The truth is that the high densities that occur in the central area of these objects are so high that do not allow valid predictions with computer models or experimental observations.