Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli exclusion principle disallows two identical half-integer spin particles (electrons and all other fermions) from simultaneously occupying the same quantum state. The result is an emergent pressure against compression of matter into smaller volumes of space. Electron degeneracy pressure results from the same underlying mechanism that defines the electron orbital structure of elemental matter. For bulk matter with no net electric charge, the attraction between electrons and nuclei exceeds (at any scale) the mutual repulsion of electrons plus the mutual repulsion of nuclei; so absent electron degeneracy pressure, the matter would collapse into a single nucleus. In 1967, Freeman Dyson showed that solid matter is stabilized by quantum degeneracy pressure rather than electrostatic repulsion. Because of this, electron degeneracy creates a barrier to the gravitational collapse of dying stars and is responsible for the formation of white dwarfs.
When electrons are squeezed together too closely, the exclusion principle requires them to have different energy levels. To add another electron to a given volume requires raising an electron's energy level to make room, and this requirement for energy to compress the material manifests as a pressure.
Electron degeneracy pressure in a material can be computed as
where ħ is the reduced Planck constant, me is the mass of the electron, and ρN is the free electron density (the number of free electrons per unit volume).
When particle energies reach relativistic levels, a modified formula is required.
This pressure is derived from the energy of each electron with wave number k = 2π/λ, having
and every possible momentum state of an electron within this volume up to the Fermi energy being occupied.
This degeneracy pressure is omnipresent and is in addition to the normal gas pressure P = NkT/V. At commonly encountered densities, this pressure is so low that it can be neglected. Matter is electron degenerate when the density (n/V) is high enough, and the temperature low enough, that the sum is dominated by the degeneracy pressure.
Perhaps useful in appreciating electron degeneracy pressure is the Heisenberg uncertainty principle, which states that
where Δx is the uncertainty of the position measurements and Δp is the uncertainty (standard deviation) of the momentum measurements. A material subjected to ever-increasing pressure will compact more, and, for electrons within it, their delocalization, Δx, will decrease. Thus, as dictated by the uncertainty principle, the spread in the momenta of the electrons, Δp, will grow. Thus, no matter how low the temperature drops, the electrons must be traveling at this "Heisenberg speed", contributing to the pressure. When the pressure due to this "Heisenberg motion" exceeds that of the pressure from the thermal motions of the electrons, the electrons are referred to as degenerate, and the material is termed degenerate matter.
Electron degeneracy pressure will halt the gravitational collapse of a star if its mass is below the Chandrasekhar limit (1.39 solar masses). This is the pressure that prevents a white dwarf star from collapsing. A star exceeding this limit and without significant thermally generated pressure will continue to collapse to form either a neutron star or black hole, because the degeneracy pressure provided by the electrons is weaker than the inward pull of gravity.