Deuterium fusion

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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 a further proton, but can also proceed from primordial deuterium.

In protostars

Deuterium is the most easily fused nucleus available to accreting protostars,[1] and fusion in the center of protostars can proceed when temperatures exceed 106 K.[2] The reaction rate is so sensitive to temperature that the temperature does not rise very much above this.[2] Deuterium fusion drives convection, which carries the heat generated to the surface.[1]

If there were no deuterium fusion, 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 fusion would occur while the object would still be accreting matter.[2] Deuterium fusion prevents this by acting as a thermostat that stops the central temperature from rising above about one million degrees, a temperature not hot enough for hydrogen fusion.[3] Only after energy transport switches from convective to radiative, forming a radiative barrier around a deuterium exhausted core, does central deuterium fusion stop. Then the central temperature of the protostar can increase; at 107 K, hydrogen fusion will begin.

The rate of energy generation is proportional to (deuterium concentration)×(density)×(temperature)11.8. The core is in a stable state, so the energy generation should be constant. If one variable in the equation increases, the other two must decrease to keep energy generation constant. Due to the variable of temperature being to the power of 11.8, there would need to be very large changes in either the deuterium concentration or its density to make any small change in temperature. [2][3]

The matter surrounding the radiative zone is still rich in deuterium, and fusion 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.[2] The total energy available by deuterium fusion is comparable to that released by gravitational contraction.[3]

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.[4]

In substellar objects

Because hydrogen fusion requires much higher temperatures and pressures than deuterium fusion 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.[5] Brown dwarfs may shine for a hundred million years at most before their deuterium supply is burned out.[6]

Objects above the deuterium-fusion minimum mass fuse all deuterium in a very short amount of time (∼4–50 Myr), whereas objects below this mass 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.]"[7]

In planets

It has been shown that deuterium fusion should also be possible in objects forming around stars in circumstellar disks by the core-accretion paradigm, commonly called "planets". The mass threshold for the onset of deuterium fusion on top of the solid cores of these objects stays at roughly 13 Jupiter masses.[8][9]

Other reactions

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.[10]

References

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