The Solar core of the Sun is considered to extend from the center to about 0.2 solar radius. The solar core is one of the hottest things in the universe. The solar core has a density of up to 150,000 kg/m3 (150 times the density of water on Earth) and a temperature of close to 15,000,000 Kelvins (by contrast, the surface of the Sun is close to 6,000 Kelvins). Energy is produced by exothermic thermonuclear reactions (nuclear fusion) that mainly convert Hydrogen into helium. The solar core is the only location in the Sun that produces an appreciable amount of Heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the solar core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.
About 3.6 ×1038 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.3 million tonnes per second, 380 yottawatts (3.8 ×1026 W) or 9.1 ×1010 megatons of TNT per second. The rate of nuclear fusion depends strongly on density, so 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, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.
The high-energy photons (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years. After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted 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 the effects of neutrino oscillation.
Fusion in the solar core.
The most important fusion process in nature is that which powers the stars. The net result is the fusion of four protons into one alpha particle, with the release of two positrons, two neutrinos, and energy. For stars the size of the sun, the proton-proton chain dominates.
At the temperatures and densities in the solar core the rates of fusion reactions are extremely slow. For example, at solar core temperature (T ~ 15 MK) and density (~150 g/cm3), the energy release rate is only ~11 W/m3 for the core (~0.25 W/m3 for the sun) - millions of times less than the rate of energy release of ordinary candles and hundreds of times less than the rate at which a human body generates heat. Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical.