Deuterium burning
Deuterium burning is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus. It occurs as the second stage of the proton–proton chain reaction, in which a deuterium nucleus formed from two protons fuses with a further proton, but can also proceed from primordial deuterium.
In protostars
Deuterium is the most easily fused nucleus available to accreting protostars, and burning in the center of protostars can proceed when temperatures exceed 106 K. The reaction rate is so sensitive to temperature that the temperature does not rise very much above this. Deuterium burning drives convection, which carries the heat generated to the surface.
If there were no deuterium burning, then there should be no stars with masses more than about two or three times the mass of the Sun in the pre-main-sequence phase because hydrogen burning would occur while the object was still accreting matter. Deuterium burning prevents this by acting as a thermostat that stops the central temperature rising above about one million degrees, which is not hot enough for hydrogen burning. Only after energy transport switches from convective to radiative, forming a radiative barrier around a deuterium exhausted core, does central deuterium burning stop. Then the central temperature of the protostar can increase. While there is deuterium in the star the temperature is kept at 7006100000000000000♠106 K because the deuterium prevents the star from any further collapsing or contracting therefore the star's temperature will stay at 7006100000000000000♠106 K until the deuterium has been completely consumed, once the star is void of deuterium it will begin to contract and collapse, the temperature increasing with contraction, and at 7007100000000000000♠107 K hydrogen burning will begin.
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