Proton-Proton Chain

Scheme of the proton–proton chain, branch I.

The only functioning natural fusion reactor in our solar system is the Sun. Our central star, weighing 1.9 × 10³⁰ kg (more than 330,000 Earth masses), fuses about 600 million metric tonnes of hydrogen in its core every second at temperatures of about 15.7 million kelvin. This produces a luminosity of about 383 yottawatts. The main process is the proton–proton chain, a series of reactions that ultimately converts four proton into a helium-4 nucleus. During the process, deuterium, positrons and neutrinos are produced, while other branches also involve lithium, beryllium or boron nuclei. The probability of the initial reaction is extremely low, and the Sun produces so much energy only because of the enormous number of protons in its core.

The Sun is a main-sequence star roughly halfway through its main-sequence lifetime. Other stars, and stars at different stages of their lives, can use many different types of fusion reactions.

CNO Cycle

Scheme of the CNO cycle.

Another stellar fusion chain is the CNO cycle, which becomes dominant in main-sequence stars more massive than about 1.3 times the mass of the Sun. This cycle converts hydrogen into helium, using carbon, nitrogen and oxygen as catalysts. These nuclei participate in individual reactions but are regenerated as the cycle proceeds. The CNO cycle becomes increasingly important at core temperatures of roughly 17–20 million kelvin.

Triple Alpha Process

When a star grows older, it eventually runs out of hydrogen fuel in its core, which is then rich in helium produced by the p–p chain or CNO cycle. The core contracts and heats up. If its temperature reaches about 100 million kelvin, alpha particles (helium nuclei) can fuse rapidly enough to produce significant amounts of stable carbon-12. This is called the triple-alpha process.

End of Stellar Life

Scheme of the triple-alpha process.

In sufficiently massive stars, the core temperature and density continue to increase, allowing successive fusion stages to produce heavier and heavier nuclei up to the iron group. Fusion beyond this region no longer releases energy, so once an iron-rich core develops, the star eventually exhausts its available sources of fusion energy.

Stars initially more massive than about eight times the Sun can then undergo a spectacular event called a core-collapse supernova. Without enough fusion energy to support it, the stellar core collapses under its own gravity. The resulting extreme conditions and shock wave trigger explosive nucleosynthesis and eject large amounts of stellar material into space. Many elements heavier than iron are formed through neutron-capture and other nuclear processes, some during stellar evolution and others in extreme astrophysical events.

The material dispersed by generations of stars enriches interstellar space and can later become part of new stars and planets. Many of the elements that make up the Earth and our bodies were forged in earlier generations of stars.

The extremely massive Eta Carinae binary system is expected to undergo a supernova explosion in the astronomically near future.