Deuteron fusion is a nuclear fusion reaction where deuteron ($D \equiv \ce{^{2}_{1}H^{+}}$) and a proton ($p$) combine to form a helium-3 ($\ce{^{3}He}$) nucleus. *(this reaction occurs in the second step of the [[Proton-Proton Chain]])* Because deuteron fusion can occur when temperatures exceed $10^{6} \; K$, this reaction can occur in a variety of locations, including: - Stars - [[Stellar Classes#Protostar|Protostars]] - [[Stellar Classes#Brown Dwarf|Brown Dwarfs]] - [[Planetary Classes#Gas Giants|Gas Giants]] / [[Planetary Classes#Hot Jupiters|Hot Jupiters]] with masses $M \gtrsim 13 \; {\rm M_{Jup}}$ (effectively becoming a [[Stellar Classes#Brown Dwarf|Brown Dwarf]]) > [!temperature] Deuteron Thermostat > > The reaction rate for deuteron fusion is very sensitive to temperature. As such, if deuteron fusion is is taking place in a [[Stellar Classes#Protostar|protostar]], it can act as a thermostat and regulate the temperature from rise too much above $T \sim 10^{6} \; {\rm K}$