Electron Degeneracy Pressure

When an electron is confined in a very small space – as in a plasma under very high pressure – it "flies about its tiny cell at high speed, kicking with great force against adjacent electrons in their cells. This degenerate motion ... cannot be stopped by cooling the matter. Nothing can stop it; it is forced on the electron by the laws of quantum mechanics, even when the matter is at absolute zero temperature" (Thorne 1994, p. 146).
“According to quantum mechanics, the simultaneous position and momentum of an electron (or any other particle) cannot be known more precisely than allowed by the Heisenberg uncertainty principle. More specifically, if a particle’s location is known to be within a volume element DV and its momentum is within an element D³p in the three-dimensional momentum space, then DV and D³p are constrained by the condition
DV D³p ł h³.	(1)
Prialnik 2000, p. 41.

References

Prialnik, Dina 2000: An introduction to the theory of stellar structure and evolution. Cambridge, 261 pp.