Black dwarf

Hertzsprung–Russell diagram

Spectral type

Brown dwarfs

White dwarfs

Red dwarfs

Subdwarfs

Main sequence
("dwarfs")

Subgiants

Giants

Bright giants

Supergiants

Hypergiants

absolute

magni-

tude

(M_V)

A black dwarf is a white dwarf that has sufficiently cooled that it no longer emits significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe (13.8 billion years), no black dwarfs are expected to exist in the universe yet, and the temperature of the coolest white dwarfs is one observational limit on the age of the universe. A white dwarf is what remains of a main-sequence star of low or medium mass (below approximately 9 to 10 solar masses (M_☉)), after it has either expelled or fused all the elements for which it has sufficient temperature to fuse.^[1] What is left is then a dense ball of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf.^[2][3] If black dwarfs were to exist, they would be extremely difficult to detect, because, by definition, they would emit very little radiation. They would, however, be detectable through their gravitational influence.^[4] Various white dwarfs cooled below 3900 K (M0 spectral class) have been found recently by astronomers using MDM Observatory's 2.4-meter telescope. The age estimations for these dwarfs are 11 to 12 billion years.^[5]

Because the far-future evolution of stars depends on physical questions, such as the nature of dark matter and the possibility and rate of proton decay, which are poorly understood, it is not known precisely how long it will take white dwarfs to cool to blackness.^{[6], § IIIE, IVA.} Barrow and Tipler estimate that it would take 10¹⁵ years for a white dwarf to cool to 5 K;^[7] however, if weakly interacting massive particles exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 10²⁵ years.^{[6], § IIIE.} If protons are not stable, white dwarfs will also be kept warm by energy released from proton decay. For a hypothetical proton lifetime of 10³⁷ years, Adams and Laughlin calculate that proton decay will raise the effective surface temperature of an old one-solar-mass white dwarf to approximately 0.06 K. Although cold, this is thought to be hotter than the cosmic background radiation temperature 10³⁷ years in the future.^{[6], §IVB.}

The name black dwarf has also been applied to substellar objects that do not have sufficient mass, less than approximately 0.08 M_☉, to maintain hydrogen-burning nuclear fusion.^[8][9] These objects are now generally called brown dwarfs, a term coined in the 1970s.^[10][11] Black dwarfs should not be confused with black holes or neutron stars.

Future of the Sun

Once the Sun stops fusing helium in its core and ejects its layers in a planetary nebula in about 5 billion years, it will become a white dwarf and, over billions of years' time, eventually will no longer emit any light. After that, the Sun will not be visible to the naked human eye, removing it from view. The estimated time for the Sun to cool enough to become a black dwarf is about 10¹⁵ (1 quadrillion) years, though it could take much longer than this, if weakly interacting massive particles (WIMPs) exist.

See also

Look up black dwarf in Wiktionary, the free dictionary.

• Degenerate matter
• Heat death of the universe

References

Star Evolution Formation Pre-main-sequence star Main sequence Horizontal branch Asymptotic giant branch Dredge-up Instability strip Red clump PG1159 star Mira variable Planetary nebula Protoplanetary nebula Luminous red nova Luminous blue variable Wolf–Rayet star Supernova impostor Supernova Hypernova Hertzsprung–Russell diagram Color–color diagram Protostars Molecular cloud H II region Bok globule Young stellar object Herbig–Haro object Hayashi track Hayashi limit Henyey track Orion T Tauri FU Orionis Herbig Ae/Be Luminosity class Subdwarf Dwarf Blue Red White Yellow Black Brown Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Blue straggler Spectral classification O B A F G K M Be OB Subdwarf O Subdwarf B Late-type Peculiar Am Ap/Bp Oscillating Barium Carbon CH Extreme helium Lambda Boötis Lead Mercury-manganese S Shell Technetium Remnants White dwarf Black dwarf Helium planet Neutron star Pulsar Magnetar Stellar black hole Compact star Quark Exotic Stellar core: EF Eridani B Theoretical stars Dark-matter star Frozen star Q star Quasi-star Thorne–Żytkow object Iron star Nucleosynthesis Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Carbon burning Neon burning Oxygen burning Alpha process Silicon burning S-process R-process Fusor Nova Remnants Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Asteroseismology Eddington luminosity Kelvin–Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Magnitude Absolute Mass Metallicity Rotation UBV color Variability Star systems Binary Contact Common envelope Multiple Accretion disc Star cluster Association Open cluster Globular cluster Super star cluster Planetary system Earth's Solar System Earth-centric observation of Pole star Circumpolar star Magnitude Apparent Extinction Photographic Color Radial velocity Proper motion Parallax Photometric-standard star Lists Star names Arabic Chinese Extremes Most massive Least massive Largest volume Smallest volume Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs Collapsars Milky Way novae Notable supernovae Supernova candidates Supernova remnants Planetary nebulae Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Supercluster Helioseismology Guest star Constellation Asterism Gravity Intergalactic star Infrared dark cloud Category:Stars · Star portal