At the opposite end of the binding curve, two light nuclei can join to form a heavier nucleus — also closer to the iron maximum — releasing energy. This is fusion, the process that powers the Sun and the stars. The solar reaction, which occurs in several steps (the proton-proton cycle), can be summarised as:
Fusing two protons, however, requires bringing them as close as , overcoming the Coulomb repulsion that grows as they approach. A kinetic energy of is needed, which, translated into temperature (via the scale ), would correspond to . But in the Sun the central temperatures are “only” , a thousand times lower. How, then, does fusion happen?
The answer lies in quantum tunnelling: the small tails of the velocity distribution carry some protons past the Coulomb barrier even though classically they would not have enough energy. It is, however, a rare phenomenon: an average proton in the solar core waits on average billions of years before fusing. It is this extreme slowness that guarantees the Sun a lifetime of ten billion years: if fusion were easy, it would burn out in a flash.
Context — fusion on Earth
For almost years we have been trying to reproduce fusion on Earth: it would be clean, with abundant fuel (deuterium) in seawater and no long-lived waste. Experiments such as JET (England), ITER (France, under construction until ) and SPARC (MIT/CFS) aim at a net gain , that is, releasing more energy than is spent heating the plasma. In December 2022 the NIF (National Ignition Facility, USA) briefly achieved with an inertial-confinement experiment: the first net fusion in a laboratory in history.
Collegamenti
Argomenti: Fisica nucleare Concetti: Fusione nucleare · Energia di legame nucleare
Esercizi collegati: Curva di Aston e direzione dell’energia · Consumo di idrogeno del Sole · Perché la fissione conviene