White Dwarf

Sirius b
Image from Steve Bowers
Sirius B, a white dwarf star, is difficult to observe in the glare from the A-type star Sirius A

White dwarfs are remnants of stars with insufficient mass to naturally go supernova. They are formed when the radiation pressure within a star fails, resulting in the star shrinking under its immense gravity. They are denoted by the spectral type D.

Unlike main sequence stars whose outward pressure to balance against gravity is provided by nuclear fusion reactions, white dwarfs are held up by electron degeneracy pressure, the result of the Pauli exclusion principle which forbids electrons from occupying the same quantum state. This imposes a strict mass limit for white dwarfs of around 1.4 solar masses, known as Chandrasekhar limit; above this limit, electron degeneracy pressure is overcome and the white dwarf continues to collapse, resulting in a supernova.

The radius of a white dwarf depends on its mass and composition. More massive white dwarfs are smaller than less massive ones due to greater electron degeneracy pressure. Bright Diamond (40 Eridani B), with a mass of 0.573 solar masses, is around 9,100 kilometers in radius. In contrast, Two-to-One Miracle (ZTF 1901+1458), with a mass of 1.34 solar masses, has a radius of just 2,140 kilometers.

Black dwarfs

White dwarfs have no internal means of energy production; as they age, the heat retained from their fusing phase is radiated away into space, and the star cools down, eventually resulting in a black dwarf. Barring external interventions (including mass loss to a denser companion or sophont manipulation), the coolest white dwarfs in the present-day universe have temperatures of around 3,000 K, generally considered too high to qualify as a black dwarf. As such, any black dwarfs known or discovered in the present day are likely artifacts of xenosophont intelligence.

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Development Notes
Text by The Astronomer 2023
Original text by Chris Clowes and M. Alan Kazlev
Initially published on 12 December 2001.

Text update (2023-02-14, by The Astronomer)