Figure 16-4: The Sun’s Internal Structure Thermonuclear reactions occur in the Sun’s core, which extends out to a distance of 0.25 R_⊙ from the center. Energy is transported outward, via radiative diffusion, to a distance of about 0.71 R_⊙. In the outer layers between 0.71 R_⊙ and 1.00 R_⊙, energy flows outward by convection.