nuclear fusion

Take two light nuclei and force them close enough that the short-range nuclear attraction can grab them, and they will merge into a single heavier nucleus, releasing energy. Nuclear fusion is this joining of light nuclei. It is the process that lights the stars and, in principle, the cleanest and most abundant energy source we could ever tap, running on isotopes of hydrogen.

The obstacle is the Coulomb barrier: both nuclei are positively charged and repel fiercely until they are within a couple of femtometres, so they must approach with enormous relative energy, meaning enormous temperature. Even then, classically they would not quite make it; quantum tunnelling through the residual barrier is what lets fusion proceed at stellar temperatures far below the classical requirement. Once fused, the product sits higher on the binding-energy-per-nucleon curve (climbing toward iron), and the mass difference emerges as kinetic energy of the products, an example being D + T -> He-4 + n + 17.6 MeV. Fusion releases even more energy per unit mass than fission, because the light nuclei are the steepest part of the binding-energy climb.

In the Sun, the proton-proton chain nets 4 protons -> helium-4 + 2 positrons + 2 neutrinos, releasing about 26.7 MeV, with heavier stars also using the CNO cycle. On Earth, achieving net energy gain is extraordinarily hard: you must confine a plasma hot enough and dense enough for long enough, quantified by the Lawson criterion, whether by magnetic confinement (tokamaks) or inertial confinement (laser implosion). The honest state of affairs is that controlled fusion has produced energy but sustained, economical net-gain power remains an unsolved engineering challenge.