Initially, the nuclei in the white dwarf slowly cool and the white dwarf loses a little bit of pressure support (because the pressure due to the nuclei is not completely negligible). This causes the white dwarf to contract and generate heat through gravitation.

The white dwarf must thus rid itself of the heat with which it was born and the heat generated by the slow (and small) contraction.

• During the initial phase of white dwarf cooling, L greater than around 0.1 L(Sun), the dominant losses are in neutrinos. They are much more important than photon losses. At lower L (and hence T), the photon losses eventually take over, however.

• The cooling time scale for white dwarfs:

  lifetime ~ 10,000,000 [M/M(sun)]^0.6 [L/L(sun)]^-0.6 years ====> 1,000,000,000 years for a 0.001 L(Sun), 1 M(Sun) white dwarf.

  For less luminous white dwarfs, the lifetimes can be as large as 10 billion years and white dwarfs can also serve to place a lower limit on the age of our Galaxy (and hence the Universe).

  The key point of the above is that the lifetime of a white dwarf is long. Further, they only stop being white dwarfs because we arbitrarily say that they are not white dwarfs once they become cold and faint.