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 [dyo͞oˈtirēəm](symbol D, or ²H, also known as heavy Hydrogen), from the Greek Deuteros meaning "second" (in reference to the two particles that make up the atom), is a stable isotope of Hydrogen with a mass approximately double that of the usual isotope. It consists of one proton and one neutron, and since it contains a neutron it is heavier than Protium, which is a common isotope of Hydrogen. It is often called heavy Hydrogen due to being more massive and heavier than Protium.
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 10^6K because the Deuterium prevents the star from any further collapsing or contracting therefore the star's temperature will stay at 10^6K 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 10^7K Hydrogen burning will begin.
The energy generation rate is proportional to (deuterium concentration)x(density)x(temperature)^11.8, the core is in a stable state therefore 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 to either the deuterium concentration, and density to make any small change in temperature. 
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 burned out.
Objects above the DBMM(deuterium-burning minimum mass) destroy Deuterium in a very short amount of time(∼4–50 Myr), respectively, 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.]"
It has been shown that deuterium burning 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 burning on top of the solid cores of these objects stays at roughly 13 jupiter masses.
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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