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While degeneracy pressure usually dominates at extremely high densities, it is the ratio between degenerate pressure and thermal pressure which determines degeneracy. Given a sufficiently drastic increase in temperature (such as during a red giant star's helium flash ), matter can become non-degenerate without reducing its density.
Using the Fermi gas as a model, it is possible to calculate the Chandrasekhar limit, i.e. the maximum mass any star may acquire (without significant thermally generated pressure) before collapsing into a black hole or a neutron star. The latter, is a star mainly composed of neutrons, where the collapse is also avoided by neutron degeneracy ...
The boundaries of this valley are the neutron drip line on the neutron-rich side, and the proton drip line on the proton-rich side. [2] These limits exist because of particle decay, whereby an exothermic nuclear transition can occur by the emission of one or more nucleons (not to be confused with particle decay in particle physics).
In the nonrelativistic case, electron degeneracy pressure gives rise to an equation of state of the form P = K 1 ρ 5/3, where P is the pressure, ρ is the mass density, and K 1 is a constant. Solving the hydrostatic equation leads to a model white dwarf that is a polytrope of index 3 / 2 – and therefore has radius inversely ...
During the formation of neutron stars, or in radioactive isotopes capable of electron capture, neutrons are created by electron capture: p + e − → n + ν e. This is similar to the inverse beta reaction in that a proton is changed to a neutron, but is induced by the capture of an electron instead of an antineutrino.
Reactions with neutrons are important in nuclear reactors and nuclear weapons. While the best-known neutron reactions are neutron scattering, neutron capture, and nuclear fission, for some light nuclei (especially odd-odd nuclei) the most probable reaction with a thermal neutron is a transfer reaction:
For a typical neutron star of 1.4 solar masses (M ☉) and 12 km radius, the nuclear pasta layer in the crust can be about 100 m thick and have a mass of about 0.01 M ☉. In terms of mass, this is a significant portion of the crust of a neutron star.
The above processes reduce the core energy and its lepton density. Hence, the electron degeneracy pressure is unable to stabilize the stellar core against the gravitational force, and the star collapses. [15] When the density of the central region of collapse exceeds 10 12 g/cm 3, the diffusion time of neutrinos exceeds the collapse time ...