The distance from the Sun's core to its surface is approximately 696,000 kilometers (432,000 miles). The Sun is a massive ball of hot, glowing gases, and its core is the central region where nuclear fusion reactions occur, releasing enormous amounts of energy in the form of light and heat. From the core, energy gradually makes its way outward through a process called radiative diffusion, taking millions of years to reach the Sun's surface, known as the photosphere.