the Nobel Prize in Physics in 1967. The proton–proton chain The first step in all the branches is the fusion of two protons into a deuteron. As the protons fuse, one of them undergoes beta plus decay, converting into a neutron by emitting a positron and an electron neutrino (though a small amount of deuterium nuclei is produced by the "pep" reaction, see below): {| border="0" |- style="height:2em;" |p ||+ ||p||→ || | +|| | + | || |} The positron will annihilate with an electron from the environment into two gamma rays. Including this annihilation and the energy of the neutrino, the net reaction {| border="0" |- style="height:2em;" |p ||+ ||p|| + → | + | ||+ || |} (which is the same as the PEP reaction, see below) has a Q value (released energy) of 1.442 MeV: The relative amounts of energy going to the neutrino and to the other products is variable. This is the rate-limiting reaction and is extremely slow due to it being initiated by the weak nuclear force. The average proton in the core of the Sun waits 9 billion years before it successfully fuses with another proton. It has not been possible to measure the cross-section of this reaction experimentally because it is so low but it can be calculated from theory. After it is formed, the deuteron produced in the first stage can fuse with another proton to produce the light isotope of helium, : :{| border="0" |- style="height:2em;" | ||+ || ||→ || ||+ || ||+ || |} This process, mediated by the strong nuclear force rather than the weak force, is extremely fast by comparison to the first step. It is estimated that, under the conditions in the Sun's core, each newly created deuterium nucleus exists for only about one second before it is converted into helium-3. In the Sun, each helium-3 nucleus produced in these reactions exists for only about 400 years before it is converted into helium-4. Once the helium-3 has been produced, there are four possible paths to generate . In , helium-4 is produced by fusing two helium-3 nuclei; the and branches fuse with pre-existing to form beryllium-7, which undergoes further reactions to produce two helium-4 nuclei. About 99% of the energy output of the sun comes from the various chains, with the other 1% coming from the CNO cycle. According to one model of the