A white dwarf is a stellar remnant, left after a larger star dies, composed mainly of electron-degenerate matter (matter in which electrons being forced into high-energy states produces an outwards pressure). A typical white dwarf contains the mass of the Sun compressed into a volume the size of the Earth, with a density of around 109 kg⋅m-3. They make up approximately 5% of luminous stars.
The upper limit of the mass of white dwarf stars is given by the Chandrasekhar limit, and is equal to 1.39 solar masses. Some white dwarf stars can exceed this limit temporarily by rotating rapidly or merging with other white dwarfs; the SN 2003fg supernova came from a white dwarf star of 2 solar masses.
The nearest white dwarf star to Earth is Sirius B, located 8.6 light years away in the Sirius system.
Over time, white dwarfs will slowly cool as it radiates away all its energy. Under natural circumstances, after hundreds of trillions of years, white dwarfs will become cold black dwarfs that emit a negligible amount of heat and light. No black dwarfs are thought to exist in the Universe.
Within the spectral superclass of Class D, there are six white dwarf spectral classes, depending on which spectral lines are present in the atmosphere of the star.
- Class DA stars: H lines are present; no He I or metal lines
- Class DC stars: No spectral lines are present
- Class DB stars: He I lines are present; no H or metal lines
- Class DO stars: He II lines are present, possibly accompanied by H and metal lines
- Class DZ stars: Metal lines are present; no He I or H lines
- Class DQ stars: Carbon lines are present
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