 Various white dwarfs cooled below 3900 K (M0 spectral class) were found in 2012 by astronomers using MDM Observatory's 2.4 meter telescope.They are estimated to be 11 to 12 billion years old.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, it is not known precisely how long it will take white dwarfs to cool to blackness.