Deuterium burning
Deuterium fusion, also called 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 another proton, but can also proceed from primordial deuterium.
Deuterium is the most easily fused nucleus available to accreting protostars, and such fusion 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. The energy generated by fusion drives convection, which carries the heat generated to the surface.
If there was no deuterium fusion, there would be no stars with masses more than about two or three times the mass of the Sun in the pre-main-sequence phase as the more intense hydrogen fusion would occur and prevent the object from accreting matter. Deuterium fusion allows further accretion of mass by acting as a thermostat that temporarily stops the central temperature from rising above about one million degrees, a temperature not hot enough for hydrogen fusion, but allowing time for the accumulation of more mass. When the energy transport mechanism switches from convective to radiative, energy transport slows, allowing the temperature to rise and hydrogen fusion take over in a stable and sustained way. Hydrogen fusion will begin at 107 K.
The rate of energy generation is proportional to (deuterium concentration)×(density)×(temperature)11.8. If the core is in a stable state, the energy generation will be constant. If one variable in the equation increases, the other two must decrease to keep energy generation constant. As the temperature is raised to the power of 11.8, it would require very large changes in either the deuterium concentration or its density to result in even a small change in temperature. The deuterium concentration reflects the fact that the gasses are a mixture of ordinary hydrogen and helium and deuterium.
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