|| |- style="height:2em;" | || || ||→ || ||+ || ||+ || || |- style="height:2em;" | || || ||→ ||2 |} The last three stages of this chain, plus the positron annihilation, contribute a total of 18.209 MeV, though much of this is lost to the neutrino. The chain is dominant if the temperature exceeds . The chain is not a major source of energy in the Sun, but it was very important in the solar neutrino problem because it generates very high energy neutrinos (up to ). The (Hep) branch This reaction is predicted theoretically, but it has never been observed due to its rarity (about in the Sun). In this reaction, helium-3 captures a proton directly to give helium-4, with an even higher possible neutrino energy (up to ). :{| border="0" |- style="height:2em;" | ||+ || ||→ || ||+ || ||+ || |} The mass–energy relationship gives for the energy released by this reaction plus the ensuing annihilation, some of which is lost to the neutrino. Energy release Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.7 percent of the mass of the original protons has been lost. This mass has been converted into energy, in the form of kinetic energy of produced particles, gamma rays, and neutrinos released during each of the individual reactions. The total energy yield of one whole chain is . Energy released as gamma rays will interact with electrons and protons and heat the interior of the Sun. Also kinetic energy of fusion products (e.g. of the two protons and the from the reaction) adds energy to the plasma in the Sun. This heating keeps the core of the Sun hot and prevents it from collapsing under its own weight as it would if the sun were to cool down. Neutrinos do not interact significantly with matter and therefore do not heat the interior and thereby help support the Sun against gravitational collapse. Their energy is lost: the neutrinos in the , , and chains carry away 2.0%, 4.0%, and 28.3% of the energy in those reactions, respectively. The following table calculates the amount of energy lost to neutrinos and the amount of "solar luminosity" coming from the three branches. "Luminosity" here means the amount of energy given off by the Sun as electromagnetic radiation rather than as neutrinos. The starting figures used are