Carbon detonation | Wikipedia audio article | Wikipedia audio article
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Date: 2018-11-18

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SUMMARY
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Carbon detonation or Carbon deflagration is the violent reignition of thermonuclear fusion in a white dwarf star that was previously slowly cooling. It involves a runaway thermonuclear process which spreads through the white dwarf in a matter of seconds, producing a Type Ia supernova which releases an immense amount of energy as the star is blown apart. The carbon detonation/deflagration process leads to a supernova by a different route from the better known Type II (core-collapse) supernova (the type II is caused by the cataclysmic explosion of the outer layers of a massive star as its core implodes).A white dwarf is the remnant of a small to medium size star (our sun is an example of these). At the end of its life, the star has burned its hydrogen and helium fuel, and thermonuclear fusion processes cease. The star does not have enough mass to either burn much heavier elements, or to implode into a neutron star or type II supernova as a larger star can, from the force of its own gravity, so it gradually shrinks and becomes very dense as it cools, glowing white and then red, for a period many times longer than the present age of the Universe.
Occasionally, a white dwarf gains mass from another source – for example, a binary star companion that is close enough for the dwarf star to siphon sufficient amounts of matter onto itself; or a collision with other stars, the siphoned matter having been expelled during the process of the companion's own late stage stellar evolution. If the white dwarf gains enough matter, its internal pressure and temperature will rise enough for carbon to begin fusing in its core. 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.
A main sequence star supported by thermal pressure would expand and cool which automatically counterbalances an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the fusion process in the manner of normal stars, so it is vulnerable to a runaway fusion reaction.
In the case of a white dwarf, the restarted fusion reactions releases heat, but the outward pressure that exists in the star and supports it against further collapse is initially due almost entirely to degeneracy pressure, not fusion processes or heat. Therefore, even when fusion recommences the outward pressure that is key to the star's thermal balance does not increase much. One result is that the star does not expand  much to balance its fusion and heat processes with gravity and electron pressure, as it did when burning hydrogen (until too late). This increase of heat production without a means of cooling by expansion raises the internal temperature dramatically, and therefore the rate of fusion also increases extremely fast as well, a form of positive feedback known as thermal runaway.
A 2004 analysis of such a process states that:

A deflagration flame burning from the center of the white dwarf star outward leaves hot and light burnt material behind. The fuel in front of it is, however, cold and dense. This results in a density stratification inverse to the gravitational field of the star, which is therefore unstable. Thus, blobs of burning material form and ascend into the fuel. At their interfaces shear flows emerge. These effects lead to strong swirls. The resulting turbulent motions deform the flame and thus enlarge its surface. This increases the net burning rate of the flame and leads to the energetic explosion ...