Why is the Sun hotter in its core than on its surface?

The Sun’s core is hotter because the weight of the outer layers creates immense pressure at the center, squeezing hydrogen atoms tightly together. That pressure and temperature allow nuclear fusion, which converts hydrogen into helium and releases energy. The energy then works its way outward through the radiative and convective zones before escaping from the photosphere, the visible surface. Although the core is about 15 million °C, the surface is much cooler at roughly 5,500 °C because the Sun is not “burning” like a fire; it is radiating energy from fusion deep inside. This temperature drop is a normal result of how stars transfer energy.