What mechanism does an isolated white dwarf use to generate energy?

Question

1. Proton-proton chain

2. CNO cycle

3. Triple alpha process

4. White dwarfs don't generate their own energy.

Answer 1

4.

White dwarfs are stars have used up all their fuel. They are the remnants of once much brighter stars.  They are still hot and they shine by radiating this heat away.

No new energy is being generated.

Answer 2

The CNO cycle is for stars at about the weight of the sun or heavier.  The sun is currently right at the borderline with only about 1.7% of its energy being generated by a CNO reaction.

The other type of reaction is for stars that are the sun's mass or less and this is the proton-proton reaction which generates the majority of the sun's energy.

White dwarf's, however - do not get their energy from either one of these types of reactions.  Since they have already fused most of their mass into iron or higher, fusion becomes an energy sink rather than an energy gain.  As such, they get their relatively low energy from something called the triple-alpha process caused by electron degeneracy pressure.

The energy created by the triple-alpha process puts an upper bound on how large a white dwarf can get; called the Chandrasekhar limit—approximately 1.4 solar masses.

If a star goes beyond this limit in mass, it heads down the path of becoming a black hole, where white dwarfs continue emitting radition from the triple-alpha process until they have completely collapsed.  At which point they are a neutron star (otherwise known as a black dwarf, since it emits no radiation).

Theoretically speaking, no white dwarf has had time to stop emitting radiation yet, so even the oldest white dwarf will still be emitting radiation as a few thousand kelvin.

If memory serves correctly, the first discovered white dwarf was 40 Eridani B; part of a triple star system.

Answer 3

Electron Proton Cycle

Answer 4

A dwarf generator.

Answer 5

Hydro-electric power.

(ironic, no?)