Neutron-star equation of state

A relation linking pressure, density, and temperature in neutron-star matter. It determines how that matter responds to compression and helps set a star’s structure and maximum mass.

Tolman–Oppenheimer–Volkoff equation: A relativistic hydrostatic-equilibrium equation describing the pressure and density profiles of a spherical, nonrotating star. It uses an equation of state to calculate the interior structure of a neutron star.
Nuclear saturation density: The density at which nuclear matter is most tightly packed, about 0.16 nucleons per cubic femtometer. Neutron-star cores reach densities near and above this benchmark.
GW170817: A 2017 gravitational-wave event produced by the merger of two neutron stars, observed alongside electromagnetic signals. Its gravitational-wave signal constrained the tidal deformability of the merging stars.
Polytropic equation of state: A simplified relation in which pressure is a power of density, often used to model stellar matter. Its compact form is useful for calculations but does not specify neutron-star microphysics.
Neutron-star radius: The distance from a neutron star’s center to its surface, typically about ten to fifteen kilometers. At a given mass, the radius is especially sensitive to pressure at several times nuclear density.
Nuclear symmetry energy: The energy cost of an imbalance between neutron and proton numbers in nuclear matter. Its density dependence strongly affects the pressure of neutron-rich matter.
NICER: A NASA X-ray telescope on the International Space Station that measures neutron-star emission and pulse profiles. Its measurements of masses and radii help narrow the allowed equation of state.
MIT bag model: A phenomenological model of quark matter in which quarks are confined within a vacuum-energy bag. It provides a contrasting description when a neutron-star core contains deconfined quarks.
Maximum mass of a neutron star: The greatest mass a neutron star can support before collapsing into a black hole. The equation of state sets the pressure support that determines this upper limit.
Quantum chromodynamics: The fundamental theory of the strong interaction between quarks and gluons. It underlies dense-matter physics, though its equations are difficult to solve at neutron-star densities.

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