Which stellar process did Hans Bethe explain, earning the 1967 Nobel Prize in Physics?

Hans Bethe explained energy production in stars, earning the 1967 Nobel Prize in Physics.

Bethe showed how nuclear reactions convert hydrogen into helium inside stars and release the energy that powers starlight. For stars like the Sun, the proton–proton chain is especially important. In hotter, more massive stars, the carbon–nitrogen–oxygen cycle can dominate while using carbon, nitrogen and oxygen as catalysts.

The Nobel citation honored his contributions to the theory of nuclear reactions, especially his discoveries concerning energy production in stars. These reactions occur in stellar cores under immense pressure and temperature, where positively charged nuclei can overcome their electrical repulsion through quantum tunneling.

Bethe did not discover that stars shine by burning ordinary chemical fuel. Stellar energy comes from nuclear fusion, whose mass-to-energy conversion is far more powerful than chemical combustion. His work connected nuclear physics with the life cycles of stars.