Harry > In stars deuterons formation begins with the fusion of two protons into a diproton.
http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction > Since the diproton is very unstable it usually fissions soon after by emitting a positron and a neutrino. This is not accurate. The diproton fissions back into two protons the vast majority of the time. The Wiki article is not well-worded on this point but later on it corrects the misunderstanding. It is only the rare occasion where the positron is emitted - otherwise the Sun would burn up its fuel too quickly. In RPF, Reversible Proton Fusion - the two protons which are immediately split from nascent He-2 are technically not the original two protons which fused, since there has been color charge alteration in the quarks during the brief instant when they were fused. > However, occasionally one of the protons transforms into a neutron by emitting a beta and a neutrino before fission occurs. This results in a stable deuteron. If this is correct, then a deuteron is stable because it is in a lower energy state than the diproton. Actually The neutron has mass slightly larger than that of a proton: 939.565378 MeV compared to 938.272046 MeV. Consequently, a deuteron has slightly more mass than a diproton. That is one of the many reasons why the reaction on the Sun, the one that results in a deuteron is extraordinarily rare. It is basically endothermic.
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