Sunlight is produced when hydrogen nuclei fuse into helium deep in the solar core.

The longer version

Fusion in the core works because the temperature and pressure there are extreme enough to force hydrogen nuclei close together despite their mutual repulsion. When they combine into helium, a small amount of mass is converted into energy, and that energy is what makes the Sun shine. The process consumes an enormous amount of fuel each second.

The energy released works its way outward through radiation and convection, taking a very long time to reach the surface before escaping as light. Because fusion requires such conditions, it happens only in the deep interior, and the rate is self regulating: a hotter core expands and cools slightly.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.