 Carbon detonation generally occurs at the point when the accreted matter pushes the white dwarf's mass close to the Chandrasekhar limit of roughly 1.4 solar masses.This is the mass at which gravity can overcome the electron degeneracy pressure which had prevented the star from collapsing during its lifetime.The same also happens when two white dwarfs merge and the mass of the body formed is below the Chandrasekhar limit; if two white dwarves merge and the result is "over" the limit, a Type Ia supernova will occur.