The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radius. It is the hottest part of the Sun and of the Solar System. It has a density of up to 150 g/cm³ (150 times the density of liquid water) and a temperature of close to 15,000,000 kelvin, or about 15,000,000 degrees Celsius; by contrast, the surface of the Sun is close to 6,000 kelvin. The core is made of hot, dense gas in the plasmic state, at a pressure estimated at 265 billion bar (26.5 quadrillion pascals or 3.84 trillion psi). The core, inside 0.24 solar radius, generates 99% of the fusion power of the Sun.
About 3.6×1038 protons (hydrogen nuclei) are converted into helium nuclei every second, releasing mass and energy at the mass-energy equivalence rate of 4.3 million tonnes per second, 380 yottawatts (3.8×1026 watts), equivalent to 9.1 ×1010 megatons of TNT per second.
The core produces almost all of the Sun's heat via fusion: the rest of the star is heated by the outward transfer of heat from the core. 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 kinetic energy of particles.
The energy production 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.
The power production density of the core overall is similar to the metabolic production density of a reptile. The peak power production in the Sun's center, per volume, has been compared to the volumetric heat generated in an active compost heap.
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 to 15 million kelvin. However, layers of the Sun are radiating to outer layers only slightly lower in temperature, and it is this difference in radiation powers between layers which determines net power production and transfer in the solar core.
At 19% of the solar radius, near the edge of the core, temperatures are about 10 million kelvin and fusion power density is 6.9 watts/m3, which is about 2.5% of the maximum value at the solar center. Some 91% of the solar energy is produced within this radius. 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, the rate of fusion is almost nil.
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 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 scattering from free electrons in the solar radiative zone (the zone inside 0.75 radii where heat transfer is by radiation). Current models put the photon diffusion time scale, or "photon travel time" from the core to the inner edge of the radiative zone, at about 170,000 years. This is the time it would take a photon, randomly scattering, to travel from the core of the Sun to the inner edge of the convective zone. If photons traveled by pure diffusion through the convective zone (the remaining 25% of distance from the core) they would require a total of 250,000 years to reach the photosphere. In practice, since heat transport through the convective zone is faster than by pure diffusion, the transfer of heat energy from the Sun's center to photosphere is considerably shorter than the previous figure.
In the process of heat transfer from core to photosphere, each gamma ray 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.
- García, Ra; Turck-Chièze, S; Jiménez-Reyes, Sj; Ballot, J; Pallé, Pl; Eff-Darwich, A; Mathur, S; Provost, J (Jun 2007). "Tracking solar gravity modes: the dynamics of the solar core.". Science 316 (5831): 1591–3. Bibcode:2007Sci...316.1591G. doi:10.1126/science.1140598. ISSN 0036-8075. PMID 17478682.
- Table of temperatures, power densities, luminosities by radius in the sun
- A 50 kg adult human has a volume of about 0.05 m3, which would correspond to 13.8 watts at the volumetric power of the solar center. This is 285 Kcal = Cal/day, about 10% of the actual average caloric intake and output for humans in non-stressful conditions.
- Karl S. Kruszelnicki (17 April 2012). "Dr Karl's Great Moments In Science: Lazy Sun is less energetic than compost". Australian Broadcasting Corporation. Retrieved 25 February 2014.
- Mitalas, R. & Sills, K. R. "On the photon diffusion time scale for the sun" http://adsabs.harvard.edu/full/1992ApJ...401..759M
- Animated explanation of the core of the Sun (University of South Wales).
- Animated explanation of the temperature and density of the core of the Sun (University of South Wales).