Electron degeneracy pressure

Electron degeneracy pressure is a particular manifestation of the more general phenomenon of Quantum degeneracy pressure. The Pauli Exclusion Principle disallows two half integer spin particles (fermions) from occupying the same quantum state at a given time. The resulting emergent repulsive force is manifested as a pressure against compression of matter into smaller volumes of space. Electron degeneracy pressure results from the same underlying mechanism which defines the electron orbital structure of elemental matter. Freeman Dyson showed that the imperviousness of solid matter is due to quantum degeneracy pressure rather than electrostatic repulsion as had been previously assumed. Furthermore electron degeneracy pressure prevents stars collapsing under their own weight once nuclear fusion has ceased. For stars which are sufficiently large electron degeneracy pressure is not sufficient to prevent the collapse of a star and a neutron star is formed. In this case neutron degeneracy pressure prevents the star collapsing further.

When electrons are squeezed too close together, 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 appears as a pressure.

Electron degeneracy pressure in a material can be computed as

$P = {h^2\over 20m_{\rm e}m_{\rm p}^{5/3}} \left({3\over\pi}\right)^{2/3} \left({\varrho\over\mu_e}\right)^{5/3},$

where h is Planck's constant, m_e is the mass of the electron, m_p is the mass of the proton, ρ is the density, and μ_e = N_e / N_p is the ratio of electron number to proton number. (When particle energies reach relativistic levels, a modified formula is required.)

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 (proportional to n / V) is high enough, and the temperature low enough, that the sum is dominated by the degeneracy pressure.

Also relevant to the understanding of electron degeneracy pressure is the Heisenberg uncertainty principle, which states that

$\Delta x \Delta p \ge \frac{\hbar}{2}$

where $\hbar$ is Planck's constant (h) divided by 2π, 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 become ever more compressed, and for electrons within it, the uncertainty in position measurements, Δx, becomes ever smaller. Thus, as dictated by the uncertainty principle, the uncertainty in the momenta of the electrons, Δp, becomes larger. 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 the "Heisenberg speed" 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 (1.38 solar masses). This is the pressure that prevents a white dwarf star from collapsing. A star exceeding this limit and without usable nuclear fuel 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.

See also

• Degenerate matter