How does hydrogen fusion power a main-sequence star?

Main-sequence stars are powered by the nuclear fusion of hydrogen into helium within their cores. This process releases energy because the mass of the resulting helium nucleus is approximately 0.7 percent less than the mass of the four hydrogen nuclei (protons) consumed to create it; this "lost" mass is converted into energy, primarily as radiation, in accordance with Einstein's equation $E=m{c}^2$.123

Depending on the star's mass and core temperature, this occurs via two primary reaction chains:

• Proton-proton cycle: The dominant energy source for cooler, lower-mass stars like the Sun. It involves a series of collisions where protons combine to form deuterium, then helium-3, and finally helium-4.132
• CNO cycle: The primary mechanism for hotter, more massive main-sequence stars. This cycle uses carbon, nitrogen, and oxygen as catalysts to fuse hydrogen into helium.132

These fusion reactions generate the outward pressure necessary to balance the inward pull of gravity, maintaining the star in a stable state known as hydrostatic equilibrium.42