Black dwarf

A black dwarf is a theoretical stellar remnant representing the final evolutionary stage of a white dwarf that has cooled to the point where it emits virtually no measurable heat or light. Because cooling to this state requires timescales vastly exceeding the current age of the universe (approximately 13.79 billion years), astrophysicists conclude that no black dwarfs yet exist. The concept therefore remains a prediction based on established models of stellar evolution and thermodynamics.
White dwarfs are exceptionally long-lived objects. Their observed temperatures, including those of the coolest known specimens, provide an empirical lower limit on the age of the universe. Since even the coldest white dwarfs found—estimated at about 11–12 billion years old—still radiate faintly, full cooling to black-dwarf status lies far in the cosmological future.

Formation

Black dwarfs originate as white dwarfs, the dense electron-degenerate remnants of stars with initial masses below roughly 9–10 solar masses. After such stars exhaust their nuclear fuel and expel their outer layers as planetary nebulae, the remaining core contracts into a hot, dense white dwarf. With no further fusion processes to replenish their energy, white dwarfs cool slowly through thermal radiation.
Over extraordinarily long intervals, the internal energy dissipates until the star becomes a cold, dark, and inert object—a black dwarf. This transition is governed by:

• The rate of radiative cooling
• The star’s mass and composition
• Potential exotic processes, such as interactions with dark-matter particles or proton decay

Although black dwarfs would be effectively invisible electromagnetically, they would retain mass and remain detectable through their gravitational influence.

Timescales and Physical Uncertainty

Because the physics of the far future is still partly speculative, estimates of black-dwarf formation times vary:

• Classical thermodynamic models suggest cooling times of roughly 10¹⁵ years, already far beyond current cosmic age.
• If weakly interacting massive particles (WIMPs) exist and scatter within white dwarfs, they may supply additional heating lasting up to 10²⁵ years, delaying cooling.
• If proton decay occurs (a hypothetical process), energy released by decaying baryons would keep white dwarfs warmer for extraordinarily long periods—potentially up to 10³⁷ years.

Some researchers propose that decay processes involving spacetime curvature–induced pair production could drastically shorten the lifetime of compact remnants, but this remains heavily debated, and several studies argue that such mechanisms cannot operate in white dwarfs.

Possible Black-Dwarf Supernovae

A remarkable hypothesis suggests that in the distant future, black dwarfs of sufficiently high mass may undergo pycnonuclear reactions—density-driven fusion occurring even at extremely low temperatures. Over immense timescales, such reactions could convert significant portions of the dwarf’s material into nickel-56, which decays into iron.
If enough mass converts, the star’s effective mass relative to the Chandrasekhar limit may change, triggering runaway collapse and a supernova. Predictions include:

• Most massive black dwarfs (~1.41 solar masses) exploding after ~10¹⁰⁰ years
• Least massive (~1.16 solar masses) exploding after ~10¹¹⁰ years
• Only about 1% of all black dwarfs would reach this fate

A major caveat is that proton decay, if real, would reduce mass faster than pycnonuclear reactions could build toward instability, preventing these explosions.

Future of the Sun

In around 8 billion years, the Sun will exhaust its helium fuel and shed its outer layers as a planetary nebula, leaving behind a white dwarf. Over trillions of years, it will cool gradually. Its transition to a black dwarf is estimated to require at least 10¹⁵ years, though this timeframe increases dramatically under scenarios involving WIMPs or proton decay.
Once fully cooled, the Sun would be invisible to the naked eye, though its gravitational influence would persist. Observing the faint thermal signatures of cooling white dwarfs in the far future may provide indirect evidence for exotic particles and processes that influence black-dwarf temperatures.