- Tags: fusion, stellar, thermonuclear
- Difficulty: Moderate
Nuclear fusion combines light nuclei into heavier ones, releasing energy. The proton-proton chain in the Sun: 4 H-1 -> He-4 + 2 + 2 neutrino + 26.7 MeV. Energy per nucleon (~6.7 MeV) far exceeds fission (~0.9 MeV). Fusion requires temperatures of ~10^7 K to give nuclei enough kinetic energy (thermal energy ~1 keV) to overcome Coulomb repulsion and get within nuclear force range (~1 fm). At these temperatures, matter is a plasma. The Sun's core is at ~1.5 x 10^7 K. On Earth, fusion has been achieved in hydrogen bombs (uncontrolled) but sustained controlled fusion remains elusive. Approaches: magnetic confinement (tokamak) and inertial confinement (laser). The CNO cycle is another fusion process dominant in stars heavier than the Sun: carbon acts as a catalyst, with the net result being the same 4H -> He conversion. Fusion produces no long-lived radioactive waste, making it an ideal energy source if controlled.