Carbon detonation is the violent re-ignition of thermonuclear fusion in a white dwarf, which produces a Type Ia supernova. A white dwarf undergoes carbon detonation only if it has a normal binary companion which is close enough for the dwarf star to siphon sufficient amounts of matter onto the dwarf, expelled during the process of the companion's own late stage evolution.
If the companion supplies enough matter to the dead star, the white dwarf's internal pressure and temperature will rise high enough to fuse the previously unfusable carbon in the white dwarf's core. Carbon detonation generally occurs when the accreted matter pushes the white dwarf's mass close to the Chandrasekhar limit of roughly 1.4 solar masses.
Because the temperature increases from the fusion, but volume is controlled by degenerate electrons and thus does not increase, pressure and thus the rate of fusion increase uncontrollably. The resumption of fusion spreads outward in a series of uneven, expanding "bubbles" exhibiting Rayleigh–Taylor instability. Within the fusion area, the increase in heat with unchanged volume results in an exponentially rapid increase in the rate of fusion – a sort of supercritical event as pressure increases boundlessly. With no hydrostatic equilibrium possible, this triggers a "thermonuclear flame," and an explosive eruption through the dwarf star's surface, seen as a Ia supernova.
This pattern, of a volume supported by electron degeneracy instead of fusion-heat gradually reaching conditions capable of fusion ignition that cascades rapidly, is also found in a less dramatic form in helium flash in sufficiently large stars.