Solar core

The core of the Sun is considered to extend from the center to about 0.2 of the solar radius (139,000 km; 86,000 mi).

[2] The core is made of hot, dense plasma (ions and electrons), at a pressure estimated at 26.5 million gigapascals (3.84×1012 psi) at the center.

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.

In the photosphere, it is about 73–74% hydrogen by mass, the rest being primarily helium, which is the same composition as the atmosphere of Jupiter, and the primordial composition of gases at the earliest star formation after the Big Bang.

However, as depth into the Sun increases, fusion decreases the fraction of hydrogen.

[5] Approximately 3.7×1038 protons (hydrogen nuclei)[failed verification], or roughly 600 million tonnes of hydrogen, are converted into helium nuclei every second, releasing energy at a rate of 3.86×1026 joules per second.

The energy produced by fusion in the core, except a small part carried out by neutrinos, must travel through many successive layers to the solar photosphere before it escapes into space as sunlight, or else as kinetic or thermal energy of massive particles.

The energy conversion per unit time (power) of fusion in the core varies with distance from the solar center.

At the center of the Sun, fusion power is estimated by models to be about 276.5 watts/m3.

[7] Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap, and is lower than the power density produced by the metabolism of an adult human.

[8] The low power outputs occurring inside the fusion core of the Sun may also be surprising, considering the large power which might be predicted by a simple application of the Stefan–Boltzmann law for temperatures of 10–15 million kelvins.

Within 24% of the radius (the outer "core" by some definitions), 99% of the Sun's power is produced.

Beyond 30% of the solar radius, where temperature is 7 million K and density has fallen to 10 g/cm3 the rate of fusion is almost nil.

This reaction sequence is thought to be the most important one in the solar core.

The characteristic time for the first reaction is about one billion years even at the high densities and temperatures of the core, due to the necessity for the weak force to cause beta decay before the nucleons can adhere (which rarely happens in the time they tunnel toward each other, to be close enough to do so).

The second reaction sequence, in which 4 H nuclei may eventually result in one He nucleus, is called the CNO cycle and generates less than 10% of the total solar energy.

This process can be further understood by the picture on the right, starting from the top in clockwise direction.

The rate of nuclear fusion depends strongly on density.

[citation needed] Therefore, 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.

[citation needed] However, the Sun gradually becomes hotter during its time on the main sequence, because the helium atoms in the core are denser than the hydrogen atoms they were fused from.

This process speeds up over time as the core gradually becomes denser.

[13] The high-energy photons (gamma rays) released in fusion reactions take indirect paths to the Sun's surface.

According to current models, random scattering from free electrons in the solar radiative zone (the zone within 75% of the solar radius, where heat transfer is by radiation) sets the photon diffusion time scale (or "photon travel time") from the core to the outer edge of the radiative zone at about 170,000 years.

From there they cross into the convective zone (the remaining 25% of distance from the Sun's center), where the dominant transfer process changes to convection, and the speed at which heat moves outward becomes considerably faster.

[14] In the process of heat transfer from core to photosphere, each gamma photon in the Sun's core is converted during scattering 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 very 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 much lower than theories predicted, a problem which was recently resolved through a better understanding of neutrino oscillation.