<Ul> <Li> Granules </Li> <Li> Sunspot </Li> <Li> Photosphere </Li> <Li> Chromosphere </Li> <Li> Convection zone </Li> <Li> Radiation zone </Li> <Li> Tachocline </Li> <Li> Solar core </Li> <Li> Corona </Li> <Li> Flare </Li> <Li> Prominence </Li> <Li> Solar wind </Li> </Ul> <P> The core of the Sun is considered to extend from the center to about 0.2 to 0.25 of solar radius . It is the hottest part of the Sun and of the Solar System . It has a density of 150 g / cm (150 times the density of liquid water) at the center, and a temperature of 15 million kelvins (15 million degrees Celsius, 27 million degrees Fahrenheit). The core is made of hot, dense plasma (ions and electrons), at a pressure estimated at 265 billion bar (3.84 trillion psi or 26.5 peta pascals (PPa)) at the center . Due to fusion, the composition of the solar plasma drops from 68 - 70% hydrogen by mass at the outer core, to 33% hydrogen at the core / Sun center . </P> <P> The core inside 0.20 of the solar radius contains 34% of the Sun's mass, but only 0.8% of the Sun's volume . Inside 0.24 solar radius, the core generates 99% of the fusion power of the Sun . There are two distinct reactions in which four hydrogen nuclei may eventually result in one helium nucleus: the proton - proton chain reaction--which is responsible for most of the Sun's released energy--and the CNO cycle . </P>

What is the approximate temperature at the center of the sun where nuclear fusion occurs