An illustration of the structure of the Sun

The radiation zone or radiative zone is a layer of a star's interior where energy is primarily transported toward the exterior by means of radiative diffusion, rather than by convection.1 Energy travels through the radiation zone in the form of electromagnetic radiation as photons. Within the Sun, the radiation zone is located in the intermediate zone between the solar core at .2 of the Sun's radius and the outer convection zone at .71 of the Sun's radius.1

Matter in a radiation zone is so dense that photons can travel only a short distance before they are absorbed or scattered by another particle, gradually shifting to longer wavelength as they do so. For this reason, it takes an average of 171,000 years for gamma rays from the core of the Sun to leave the radiation zone. Over this range, the temperature of the plasma drops from 15 million K near the core down to 1.5 million K at the base of the convection zone.2

Within a radiative zone, the temperature gradient—the change in temperature (T) as a function of radius (r)—is given by:

$\frac{\text{d}T(r)}{\text{d}r}\ =\ -\frac{3 \kappa(r) \rho(r) L(r)}{(4 \pi r^2)(16 \sigma) T^3(r)}$

where κ(r) is the opacity, ρ(r) is the matter density, L(r) is the luminosity, and σ is the Stefan–Boltzmann constant.1 Hence the opacity (κ) and radiation flux (L) within a given layer of a star are important factors in determining how effective radiative diffusion is at transporting energy. A high opacity or high luminosity can cause a high temperature gradient, which results from a slow flow of energy. Those layers where convection is more effective than radiative diffusion at transporting energy, thereby creating a lower temperature gradient, will become convection zones.3

For main sequence stars—those stars that are generating energy through the thermonuclear fusion of hydrogen at the core, the location of the radiative zone depends on the star's mass. Main sequence stars below about 0.3 solar masses are entirely convective, meaning they do not have a radiative zone. From 0.3 to 1.2 solar masses, the region around the stellar core is a radiation zone, separated from the overlying convection zone by the tachocline. The radius of the radiative zone increases monotonically with mass, with stars around 1.2 solar masses being almost entirely radiative. Above 1.2 solar masses, the core region becomes a convection zone and the overlying region is a radiation zone, with the amount of mass within the convective zone increasing with the mass of the star.4

## Notes and references

1. ^ a b c Ryan, Sean G.; Norton, Andrew J. (2010), Stellar Evolution and Nucleosynthesis, Cambridge University Press, p. 19, ISBN 0-521-19609-4
2. ^ Elkins-Tanton, Linda T. (2006), The Sun, Mercury, and Venus, Infobase Publishing, p. 24, ISBN 0-8160-5193-3
3. ^ LeBlanc, Francis (2011), An Introduction to Stellar Astrophysics (2nd ed.), John Wiley and Sons, p. 168, ISBN 1-119-96497-0
4. ^ Padmanabhan, Thanu (2001), Theoretical Astrophysics: Stars and stellar systems, Theoretical Astrophysics 2, Cambridge University Press, p. 80, ISBN 0-521-56631-2

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