White Dwarf Magnetism
Almost all stars end their life as white dwarfs: dense objects about the size of the Earth that are held together by the Pauli exclusion principle of their electrons.​
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The magnetic fields of many white dwarfs are more than a million times stronger than the Earth's (MG = Mega Gauss). The origin of these fields is still a puzzle; a possible solution is the crystallization dynamo theory.
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Most white dwarfs are made of carbon and oxygen. As a white dwarf cools down, its core begins to crystallize (solidify). The crystal phase is enriched in oxygen, generating an unstable composition gradient in the liquid above (a heavier element on top of a lighter one), which drives convection. This convection in the electrically conducting liquid can generate a magnetic field through the dynamo effect.
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First appearance of crystallization-dynamo magnetic fields as a function of the white dwarf's mass. Magnetism may be used to distinguish between carbon-oxygen (CO) and oxygen-neon (ONe) white dwarfs at high masses, where the electrons are relativistic.