Nuclear reactions in the Sun’s core produce light and other types of radiation. As light photons leave the core, they run into electrons and atomic nuclei, scattering off each one. These interactions cause photons to take, on average, 200,000 years to move from the Sun’s core to its surface. Astronomy: Roen Kelly The speed of … Continue reading I heard that the light from nuclear fusion reactions in the Sun’s core takes about 200,000 years to reach its surface. I thought the speed of light is constant. Why does it take so long?