I know that white dwarfs are composed primarily of carbon but are supported by electron degeneracy pressure. Is the electron degeneracy pressure produced by carbon atoms, or is the core of the white dwarf made of electron degenerate Fermi gas while the outer part is atomic carbon? I am trying to reconcile the fact that white dwarfs are made of both carbon and electron degenerate matter. Can carbon be electron degenerate?


1 Answer 1


Most white dwarfs consist of fully ionised carbon and oxygen atoms$^1$. Each carbon nucleus must be accompanied by 6 electrons and each oxygen nucleus by 8 electrons. The electrons are essentially a free Fermi gas around the nuclei. All of this can be referred to as "electron-degenerate matter".

It is however the free electrons that are degenerate and which exert the degeneracy pressure that supports a white dwarf. The carbon and oxygen nuclei are far too massive to become a degenerate gas (and they are bosons too!).

$^1$ We are talking here about white dwarf interiors, not their atmospheric layers which can contain hydrogen, helium and other metals and are not degenerate.

  • $\begingroup$ One can assume a thin (probably mm-scale) "atmosphere" where non-degenerate matter exists. This is an important feature of the white dwarf because it is opaque (in contrast to the degenerate matter, which is transparent). $\endgroup$
    – fraxinus
    May 3, 2022 at 9:47
  • $\begingroup$ @fraxinus The non-degenerate later is about 1% of the white dwarf radius (depending on temperature). Electron-degenerate matter would not be transparent - it would be an extremely good conductor and therefore highly reflective. There are examples you can test in the lab - copper for example! $\endgroup$
    – ProfRob
    May 3, 2022 at 10:05
  • $\begingroup$ Copper is somewhat different because of the different Fermi levels involved. $\endgroup$
    – fraxinus
    May 3, 2022 at 10:55
  • $\begingroup$ I just read up on Bosons, to which the Pauli exclusion principle does not apply. As you say, nuclei with even numbers of nucleons are Bosons, so neither Oxygen-16 nor Carbon-12 can become degenerate matter. I assume that a star made of Fermions like Be-9 would be very different!? It's just that odd-mass nuclei are not abundant because nucleosynthesis pathways/probabilities are not in their favor. But the gedankenexperiment of such a star may be interesting :-). $\endgroup$ Feb 22, 2023 at 12:22

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