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.
In protostars
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 were 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 high 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 to 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.
The mass surrounding the radiative zone is still rich in
deuterium, and deuterium fusion proceeds in an increasingly thin shell
that gradually moves outwards as the radiative core of the star grows.
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 on the main
sequence. The total energy available by deuterium fusion is comparable to that released by gravitational contraction.
Due to the scarcity of deuterium in the Universe, a protostar's supply of it is limited. After a few million years, it will have effectively been completely consumed.
In substellar objects
Hydrogen fusion
requires much higher temperatures and pressures than does deuterium
fusion, hence, 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 before their deuterium supply is burned out.[6]
Objects above the deuterium-fusion minimum mass (deuterium
burning minimum mass, DBMM) will fuse all their deuterium in a very
short time (∼4–50 Myr), whereas objects below that will burn little, and
hence, preserve their original deuterium abundance. "The apparent
identification of free-floating objects, or rogue planets below the DBMM would suggest that the formation of star-like objects extends below the DBMM."
In planets
It
has been shown that deuterium fusion should also be possible in
planets. The mass threshold for the onset of deuterium fusion atop the
solid cores is also at roughly 13 Jupiter masses.
