<P> Even so, it was unclear how proton--proton fusion might proceed, because the most obvious product, helium - 2 (diproton), is unstable and almost instantly dissociates back into two protons . In 1939, Hans Bethe proposed that one of the protons could decay by beta emission into a neutron via the weak interaction during the brief moment of fusion, making deuterium a vital product in the chain . This idea was part of the body of work in stellar nucleosynthesis for which Bethe won the Nobel Prize in Physics in 1967 . </P> <P> The first step involves the fusion of two 1 nuclei (protons) into deuterium, releasing a positron and a neutrino as one proton changes into a neutron . It is a two - stage process; first, two protons fuse to form a diproton: </P> <Dl> <Dd> <Table> <Tr> <Td> 1 </Td> <Td> + </Td> <Td> </Td> <Td> → </Td> <Td> 2He </Td> <Td> </Td> </Tr> </Table> </Dd> </Dl> <Dd> <Table> <Tr> <Td> 1 </Td> <Td> + </Td> <Td> </Td> <Td> → </Td> <Td> 2He </Td> <Td> </Td> </Tr> </Table> </Dd>

What is the overall nuclear fusion reaction in the sun briefly describe the proton-proton chain