Electron degeneracy pressure
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Electron degeneracy pressure is a force caused by the Pauli exclusion principle, which states that two electrons cannot occupy the same quantum state at the same time.
Also relevant to the understanding of electron degeneracy pressure is the Heisenberg uncertainty principle, which states that
- <math>\Delta x \Delta p \ge \frac{\hbar}{2} </math>
where <math>\hbar</math> is Planck's constant (h) divided by 2π and Δx is the uncertainty of the position measurements and Δp is the uncertainty in the standard deviation of the momentum measurements.
As pressure increases ever more, 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 travelling 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 labeled as degenerate.
Electron degeneracy pressure is the pressure that keeps a white dwarf star from collapsing. When the electrons are degenerate, degenerate matter is formed. In the case of a gravitational collapse of a star, depending on the initial mass of the star, electron degeneracy pressure can affect the final evolutionary state of the collapsing star. For example, a star with initial mass exceeding the mass stated by the Chandrasekhar Limit will continue to collapse and may form neutron stars or black holes because the degeneracy pressure provided by the electrons are much weaker than the inward gravitational force.