<P> Various authors have considered the existence of a gradient in the isotopic compositions of solar and planetary noble gases, e.g. correlations between isotopic compositions of neon and xenon in the Sun and on the planets . </P> <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>

Where is most of the sun's energy produced