Nuclear Fusion in Stars

Why are stars hot and bright?

Nuclear fusion and nucleosynthesis

Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright.

Nuclear fusion is an atomic reaction that fuels stars. In fusion, many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). The result of this process is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy by the equation E=mc²).

Stars are powered by nuclear fusion in their cores, mostly converting hydrogen into helium.

The production of new elements via nuclear reactions is called nucleosynthesis. A star’s mass determines what other type of nucleosynthesis occurs in its core (or during explosive changes in its life cycle). Each of us is made from atoms that were produced in stars and went through a supernova.

Small stars

The smallest stars only convert hydrogen into helium.

Medium-sized stars (like our Sun)

Late in their lives, when the hydrogen becomes depleted, stars like our Sun can convert helium into oxygen and carbon.

Massive stars

(greater than five times the mass of the Sun): When their hydrogen becomes depleted, high mass stars convert helium atoms into carbon and oxygen, followed by the fusion of carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Later reactions transform these elements into calcium, iron, nickel, chromium, copper and others. When these old, large stars with depleted cores supernova, they create heavy elements (all the natural elements heavier than iron) and spew them into space, forming the basis for life.