Stellar core

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/cm³ (150 times the density of liquid water) at the center, and a temperature of 15 million degrees Celsius. The core is made of hot, dense gas in the plasmic state (ions and electrons), at a pressure estimated at 265 billion bar (3.84 trillion psi or 26.5 petapascals (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.

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.

The Sun at the photosphere is about 73-74% by mass hydrogen, which is the same composition as the atmosphere of Jupiter, and the primordial composition of hydrogen and helium at the earliest star formation after the Big Bang. However, as depth into the Sun increases, fusion decreases the fraction of hydrogen. Traveling inward, hydrogen mass fraction starts to decrease rapidly after the core radius has been reached (it is still about 70% at a radius 25% of the Sun's radius) and inside this, the hydrogen fraction drops rapidly as the core is traversed, until it reaches a low of about 33% hydrogen, at the Sun's center (radius zero). All but 2% of the remaining plasma mass (i.e., 65%) is helium, at the center of the Sun.

Approximately 3.6×10³⁸protons (hydrogen nuclei), or roughly 299 million metric tons of hydrogen, are converted into helium nuclei every second releasing energy at a rate of 3.86×10²⁶ joules per second.

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Wikipedia