<P> Prior to 1983, it was thought that the whole Sun has the same composition as the solar atmosphere . In 1983, it was claimed that it was fractionation in the Sun itself that caused the isotopic - composition relationship between the planetary and solar - wind - implanted noble gases . </P> <P> The core of the Sun extends from the center to about 20--25% of the solar radius . It has a density of up to 7005150000000000000 ♠ 150 g / cm (about 150 times the density of water) and a temperature of close to 15.7 million kelvins (K). By contrast, the Sun's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the radiative zone above . Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through a series of steps called the p--p (proton--proton) chain; this process converts hydrogen into helium . Only 0.8% of the energy generated in the Sun comes from the CNO cycle, though this proportion is expected to increase as the Sun becomes older . </P> <P> The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely . The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space as sunlight or the kinetic energy of particles . </P> <P> The proton--proton chain occurs around 7037919999999999999 ♠ 9.2 × 10 times each second in the core, converting about 3.7 × 10 protons into alpha particles (helium nuclei) every second (out of a total of ~ 8.9 × 10 free protons in the Sun), or about 6.2 × 10 kg / s . Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass--energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen), for 384.6 yottawatts (7026384600000000000 ♠ 3.846 × 10 W), or 9.192 × 10 megatons of TNT per second . Theoretical models of the Sun's interior indicate a power density of approximately 276.5 W / m, a value that more nearly approximates that of reptile metabolism or a compost pile than of a thermonuclear bomb . </P>

The long term source of energy that powers the sun is
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