The Sun produces light and heat through nuclear processes deep within its interior. This colossal star is structured into distinct layers, each defined by unique physical conditions and methods of energy transport. The Radiative Zone is a vast, dense shell that acts as a thermal transit layer for the immense power created at the star’s center. This layer is where the intense energy from the core begins its long journey outward toward the solar surface.
Location and Boundaries of the Radiative Zone
The Radiative Zone is situated directly above the Sun’s core, the site of nuclear fusion. Its internal boundary begins at approximately 0.25 solar radii and extends outward to roughly 0.70 solar radii. This makes the zone an enormous spherical shell, occupying the largest volume of the Sun’s interior by radius.
The outer boundary meets the Convective Zone, the outermost interior layer, at a transition region known as the tachocline. This boundary marks a significant change in how energy moves through the plasma. The Radiative Zone rotates uniformly, while the Convective Zone exhibits differential rotation; this shear layer is thought to generate the Sun’s magnetic field.
The Temperature Range and Physical State
The temperature within the Radiative Zone is high, decreasing substantially with distance from the core. At its inner boundary, adjacent to the core, the temperature is approximately 7 million Kelvin (K). This heat is residual energy flowing directly from the core. The temperature falls steadily to about 2 million K at the zone’s outer edge, where it meets the Convective Zone.
The matter in this zone exists as fully ionized plasma, a superheated gas of atomic nuclei and free electrons. Under these thermal conditions, hydrogen and helium atoms are stripped of their electrons, creating a dense medium. Density drops from around 20 g/cm³ near the core to about 0.2 g/cm³ at the top of the layer. This combination of high temperature and density dictates the process by which energy is transported through the zone.
How Energy Moves Through the Radiative Zone
Energy is transported through the Radiative Zone primarily by electromagnetic radiation, specifically the movement of photons. High-energy gamma rays produced by fusion in the core enter this layer and encounter the dense plasma. These photons travel only a few millimeters before they are absorbed by an ion and then re-emitted in a random direction, or scattered by an electron.
This repeated process of absorption, scattering, and re-emission is called a “random walk,” causing the energy to diffuse outward slowly. With each interaction, photons lose energy, shifting from high-energy gamma rays and X-rays toward lower-energy wavelengths. The density of the plasma makes this progression slow, contrasting sharply with the speed of light at which photons travel between collisions. Estimates suggest a photon may take tens of thousands to over a hundred thousand years to finally escape the Radiative Zone and enter the Convective Zone.