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.
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.
The matter surrounding the radiative zone is still rich in deuterium and burning proceeds in a shell that gradually moves outwards as the star becomes more and more radiative. The generation of nuclear energy in these low-density outer regions causes the protostar to swell, delaying the gravitational contraction of the object and postponing its arrival onto the main sequence. The total energy available by deuterium burning is comparable to that released by gravitational contraction.
In substellar objects
Since hydrogen burning requires much higher temperatures and pressures than deuterium burning does, there are objects massive enough to burn deuterium but not massive enough to burn hydrogen. These objects are called brown dwarfs, and have masses between about 13 and 80 times the mass of Jupiter. Brown dwarfs may shine for a hundred million years at most before their deuterium supply is gone.
Though fusion with a proton is the dominant method of consuming deuterium, other reactions are possible. These include fusion with another deuterium nucleus to form helium-3, tritium, or (more rarely) helium-4, or with helium to form various isotopes of lithium.
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