Our parent star. The structure of Sun’s interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region.

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The core is the hot, dense central region in which the nuclear reactions that power the Sun take place, temperatures range from 8-15 x 106 K with densities from 10-160 g/cm3. It comprises about 25% of the interior radius. The core is the region where the energy of the Sun is produced. Density and temperature are high enough to cause nuclear fusion reactions. These reactions release energy both in the form of γ-rays and particles (in particular neutrinos).

The radiative zone extends from ~25% to 85% of the solar radius. Here, and in the core, the primary transport of energy is by the movement of photons; temperatures range from 0.5-8 x 106 K with densities from 0.01-10 g/cm3. The radiative zone is not transparent. On average, photons only ~2 cm before being scattered in a random direction by an encounter with an electron. The resulting "drunkard’s walk" is very inefficient and it typically takes ~170,000 years for energy generated in the core to escape to the surface.

The convective zone starts at ~85% of the solar radius and extends to just below its surface; its density is <0.01 g/cm3. In this region, the change in temperature with increasing radius is so rapid (0.1-5 x 105 K) that the interior becomes unstable and undergoes convection (rapid up and down motion of large packets of gas). Convective movement is responsible for the granulation pattern seen on the surface.

Basic data for the Sun: RSun = 7.0 x 105 km; MSun = 2.0 x 1030 kg;
ρSun = 1.4 g/cm3; Composition: 74% H, 25% He, 1% other elements; Tsurface = 5800 K; LSun = 3.9 x 1026 W.

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