Editing Degenerate matter (section)

{{Short description|Type of dense exotic matter in physics}}
{{About|a state of matter|multiple states with equal energy|Degenerate energy levels|other uses|Degeneracy (disambiguation){{!}}Degeneracy}}

'''Degenerate matter''' occurs when the [[Pauli exclusion principle]] significantly alters a [[state of matter]] at low temperature. The term is used in [[astrophysics]] to refer to dense stellar objects such as [[white dwarf]]s and [[neutron star]]s, where thermal pressure alone is not enough to prevent [[gravitational collapse]]. The term also applies to metals in the [[Fermi gas]] approximation.

Degenerate matter is usually modelled as an ideal [[Fermi gas]], an ensemble of non-interacting fermions. In a quantum mechanical description, particles limited to a finite volume may take only a discrete set of energies, called [[quantum state]]s. The [[Pauli exclusion principle]] prevents identical fermions from occupying the same quantum state. At lowest total energy (when the thermal energy of the particles is negligible), all the lowest energy quantum states are filled. This state is referred to as full degeneracy. This '''degeneracy pressure''' remains non-zero even at absolute zero temperature.<ref name="apod.nasa.gov">see http://apod.nasa.gov/apod/ap100228.html</ref><ref name="ReferenceA">Andrew G. Truscott, Kevin E. Strecker, William I. McAlexander, Guthrie Partridge, and Randall G. Hulet, "Observation of Fermi Pressure in a Gas of Trapped Atoms", Science, 2 March 2001</ref> Adding particles or reducing the volume forces the particles into higher-energy quantum states. In this situation, a compression force is required, and is made manifest as a resisting pressure. The key feature is that this degeneracy pressure does not depend on the temperature but only on the density of the fermions. Degeneracy pressure keeps dense stars in equilibrium, independent of the thermal structure of the star.

A degenerate mass whose fermions have velocities close to the speed of light (particle kinetic energy larger than its [[rest mass energy]]) is called '''relativistic degenerate matter'''.
The concept of '''degenerate stars''', stellar objects composed of degenerate matter, was originally developed in a joint effort between [[Arthur Eddington]], [[Ralph Fowler]] and [[Arthur Milne]].