A star cools by emitting light from its photospheres, which gives the star a simple structure. The heat generated at the star's core, where the temperature is highest, flows to the cooler photosphere and is radiated into space. This simple structure is found also in the giant gaseous planets and the brown dwarfs. The degenerate (white) dwarfs, however, can have a very unusual temperature structure: they can be cooler at their cores than in their outer layers. In this state, the heat flows from the outer layers to the core. The reason this peculiar situation arises is that a very effective mechanism of removing heat from a degenerate dwarf's core exists. The degenerate dwarf can generate neutrinos—a type of fundamental particle—that freely escape from the core into space, carrying away the energy at the core.
A degenerate dwarf generates neutrinos through an exotic process; within the high-density core of the degenerate dwarf, photons are unstable and decay into neutrinos and their antiparticles. This cooling enables the degenerate dwarf to cool much faster than the brown dwarfs and the more-massive giant gaseous planets. The degenerate dwarfs rapidly become fully degenerate, acquiring radii that are close to the minimum possible values.
The page added this issue discusses the details of how a degenerate dwarf is cooled by neutrinos.
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The Neutrino Cooling of Degenerate Dwarfs. The principal way that a degenerate dwarf cools is through the emission of neutrinos. Unlike the main-sequence stars, which generate neutrinos as part of their thermonuclear generation of power, degenerate dwarfs generate neutrinos from photons. This process allows degenerate stars to radiate away all of the energy in their cores, giving them an inverted temperature structure of a cold core surrounded by a hot outer layer. (continue)