Solar core

An illustration of the structure of the Sun:
1. Core
2. Radiative zone
3. Convective zone
4. Photosphere
5. Chromosphere
6. Corona
7. Sunspot
8. Granules
9. Prominence

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 (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. The core, inside 0.24 solar radius, generates 99% of the fusion power of the Sun.

Energy production

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 energy that is transferred outward 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) produced by fusion in the core varies with distance from the solar center. At the center of the sun, fusion power is estimated by model to be about 276.5 watts/m³, a power production density which more nearly approximates reptile metabolic heat generation than it does a thermonuclear bomb. Peak power production in the Sun's center, per volume, has been compared to the volumetric heats generated in an active compost heap. The tremendous power output of the Sun is not due to its high power per volume, but instead due to its gigantic size.

The low power outputs encountered inside the fusion core of the Sun may also be surprising in terms of the large powers which might be predicted by a simple application of the Stefan–Boltzmann law for temperatures of 10 to 15 million kelvins. 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.

By the time 19% of the solar radius is reached (near the edge of the conventional core), temperatures have dropped to about 10 million kelvins and fusion power densities to 6.9 watts/m³ (about 2.5% of maximum). 91% of solar energy is produced within this zone. At 24% of the radius (the outer "core" by some definitions), 99% of the Sun's power has been produced. By the time 30% of the radius is reached, fusion has stopped almost entirely.

Equilibrium

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.

Energy transfer

The high-energy photons (gamma rays 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. However, the concept of photon travel is not a well-defined one, since photons are not conserved, and one photon at a high temperature normally turns into many photons at a lower temperature, during passage of heat out of the solar core to the Sun's photosphere. The long periods of time (tens of millions of years) refer to the characteristic time for the entire solar tempererature distribution to change, as a result of changing heat generation rate in the core. This is far longer than the average time for transport of heat through the Sun because most of the Sun's heat capacity is in the kinetic energy of the particles in its plasma, not in the electromagnetic radiation present within it. The shorter estimates of "photon travel time" (tens of thousands of years) refer to the (relatively rapid) mean time needed for radiation to travel from the center of the Sun to the photosphere, even though the Sun's heat cannot pass from core to surface at this rate, due to the large heat capacity needed to be heated or cooled in the process, as mentioned above.

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.

External links

• Animated explanation of the core of the Sun (University of Glamorgan).
• Animated explanation of the temperature and density of the core of the Sun (University of Glamorgan).