Black dwarf

[1] What is left is then a dense sphere of electron-degenerate matter that cools slowly by thermal radiation, eventually becoming a black dwarf.

[8] Various white dwarfs cooled below 3,900 K (3,630 °C; 6,560 °F) (equivalent to M0 spectral class) were found in 2012 by astronomers using MDM Observatory's 2.4 meter telescope.

[9] Because the far-future evolution of stars depends on physical questions which are poorly understood, such as the nature of dark matter and the possibility and rate of proton decay (which is yet to be proven to exist), it is not known precisely how long it would take white dwarfs to cool to blackness.

One major caveat is that proton decay would decrease the mass of a black dwarf far more rapidly than pycnonuclear processes occur, preventing any supernova explosions.

The estimated time for the Sun to cool enough to become a black dwarf is at least 1015 (1 quadrillion) years, though it could take much longer than this, if weakly interacting massive particles (WIMPs) exist, as described above.