Electron Degeneracy & White Dwarfs: Observation, Measurement & Evidence

Typical white dwarfs contain roughly a stellar mass compressed into a volume comparable to Earth. Their interiors are commonly dominated by carbon and oxygen, though other compositions occur, and they possess a thin nondegenerate outer atmosphere over a dense degenerate interior. More massive white dwarfs are smaller because stronger gravity compresses the electron gas further.

Masses and radii measured in binaries match the predicted inverse mass-radius trend, and spectra reveal hot compact stars whose luminosities decline as they cool. White-dwarf populations in star clusters follow cooling sequences consistent with degenerate-remnant models.

The Pauli exclusion principle prevents identical electrons from occupying the same quantum state. As matter is compressed, electrons must occupy progressively higher momentum states, producing degeneracy pressure even when the gas is cold by stellar standards. This pressure is largely independent of ordinary thermal pressure and can support a remnant after nuclear burning ends.

Binary masses, radii, gravitational redshifts, pulsations, and cooling sequences test white-dwarf models. The observed inverse mass-radius relation is especially distinctive evidence for degenerate support.