White dwarf

: For the magazine, see White Dwarf (magazine).

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A white dwarf is an astronomical object which is produced when a low or medium mass star dies. These stars are not heavy enough to generate the core temperatures required to fuse carbon in nucleosynthesis reactions, and after they have become a red giant during their helium-burning phase, they will shed their outer layers to form a planetary nebula, leaving behind an inert core consisting mostly of carbon and oxygen.

Related Topics:
Astronomical object - Star - Carbon - Nucleosynthesis - Red giant - Helium - Planetary nebula - Oxygen

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This core has no further source of energy, and so will gradually radiate away its energy and cool down. The core, no longer supported against gravitational collapse by fusion reactions, becomes extremely dense, with a typical mass of about half that of the sun contained in a volume about equal to that of the Earth. The white dwarf is supported only by electron degeneracy pressure. The maximum mass of a white dwarf, beyond which degeneracy pressure can no longer support it, is about 1.4 solar masses. A white dwarf which exceeds this limit (known as the Chandrasekhar limit), typically by mass transfer from a companion star, may explode as a Type Ia supernova via a process known as "carbon detonation".

Related Topics:
Earth - Electron degeneracy pressure - Solar mass - Chandrasekhar limit - Supernova

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Eventually, over hundreds of billions of years, white dwarfs cool to temperatures at which they are no longer visible. However, over the universe's lifetime to the present (about 13.7 billion years) even the oldest white dwarfs still radiate temperatures of a few thousand kelvins.

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