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About 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9×1056 total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (3.83×1026 W) or 9.15×1010 megatons of TNT per second. This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 W/m³ (watts per cubic meter). This is less power than generated by a candle. Power density is about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, roughly a million times greater per unit mass. The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW fusion power plant wo

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  • Structure of the Sun
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  • About 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9×1056 total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (3.83×1026 W) or 9.15×1010 megatons of TNT per second. This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 W/m³ (watts per cubic meter). This is less power than generated by a candle. Power density is about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, roughly a million times greater per unit mass. The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW fusion power plant wo
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abstract
  • About 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9×1056 total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (3.83×1026 W) or 9.15×1010 megatons of TNT per second. This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 W/m³ (watts per cubic meter). This is less power than generated by a candle. Power density is about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, roughly a million times greater per unit mass. The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW fusion power plant would require about 170 billion tonnes of plasma occupying almost one cubic mile. Hence, terrestrial fusion reactors utilize far higher plasma temperatures than those in Sun's interior. The rate of nuclear fusion depends strongly on density and temperature, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level. The high-energy photons (gamma rays) released in fusion reactions are absorbed in only few millimetres of solar plasma and then re-emitted again in random direction (and at slightly lower energy)—so it takes a long time for radiation to reach the Sun's surface. Estimates of the "photon travel time" range between 10,000 and 170,000 years. After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was recently resolved through the discovery of the effects of neutrino oscillation: the Sun in fact emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 2/3 of them because the neutrinos had changed flavor.
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