Degenerate matter is matter which has sufficiently high density that the dominant contribution to its pressure arises from the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle forbids the constituent particles from occupying identical quantum states. Therefore, reducing the volume requires forcing the particles into higher-energy quantum states. The species of fermion is sometimes identified, so that we may speak of electron degeneracy pressure, neutron degeneracy pressure, and so forth.
Unlike gases, degenerate matter is difficult to compress and the volume of degenerate matter does not change much in response to changes in temperature or pressure.
Exotic examples of degenerate matter include neutronium, strange matter, metallic hydrogen and white dwarf matter. Degeneracy pressure contributes to the pressure of conventional solids, but these are not usually considered to be degenerate matter as a significant contribution to their pressure is provided by the interplay between the electrical repulsion of atomic nuclei and the screening of nuclei from each other by electrons allocated among the quantum states determined by the nuclear electrical potentials. In metals it is useful to treat the conduction electrons alone as a degenerate, free electron gas while the majority of the electrons are regarded as occupying bound quantum states. This contrasts with the case of the degenerate matter that forms the body of a white dwarf where all the electrons would be treated as occupying free particle momentum states.
The pressure maintained by a body of degenerate matter is called the degeneracypressure, and arises because the Pauli principle forbids the constituent particles from occupying identical quantum states.
As protons and electrons occur in roughly equal numbers in most forms of matter, proton degeneracy is usually modelled as a correction to the equations of state of electron-degenerate matter, as opposed to the dominant source of degeneracypressure (which would require proton-degenerate matter that was free of electrons).
Neutrondegeneracy is analogous to electrondegeneracy and is demonstrated in neutron stars, which are supported by the pressure from a degenerate neutrongas.
Neutrondegeneracypressure arises when neutrons are so close that their quantum states begin to overlap.
The combined pressure from neutrondegeneracypressure and the strong nuclear force prevent further gravitational collapse of a neutron star if the remaining supernova core (neutron star) is less than 2-3 solar masses.
Neutron stars rotate more rapidly than ordinary stars because the rate of spin increased as the stars collapsed to a small size due to conservation of angular momentum (like the ice skater who pulls her mass in closer to her rotation axis and spins faster or my demonstration in class on the stool using masses).
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