The combination of small nuclei like hydrogen to produce larger ones such as helium is called nuclear fusion. Because nuclei have to overcome electrostatic repulsion, fusion reactions require temperatures of 40 million kelvins or more and so are known as thermonuclear reactions.

Nuclides with mass numbers between 40 and 100 have high binding energies per nucleon and are generally stable. Thus, lighter nuclei with low nuclear binding energies per nucleon tend to combine, yielding heavier nuclei with higher binding energies. 

The difference between the nuclear binding energies of product and reactant nuclides generates a huge amount of energy. Notably, the energy released during the formation of one gram of helium-4 is significantly larger than that of the fission of one gram of uranium-235.

So, is fusion used to produce electricity? Well, not yet! At the high temperatures required for fusion, all molecules dissociate into atoms, which ionize, forming a plasma. For such reactions, a strong, torus-shaped magnetic field serves as a reactor. However, its efficient use is still a technical challenge. 

Indeed, the fusion of hydrogen to helium is one of the major hydrogen-burning processes in main-sequence stars like the sun.

Once stars begin helium fusion, two helium nuclei combine into beryllium-8. Unlike helium-4, beryllium-8 is highly unstable, making this an endothermic, easily reversed fusion reaction. 

As helium fusion accelerates, beryllium-8 becomes more abundant and fuses with helium-4, producing excited-state carbon-12, which occasionally relaxes to stable carbon-12.

In massive stars, a chain of fusion reactions initiated by the combination of carbon-12 and helium-4 produces a sequence of elements up to magnesium-24.

As further fusion reactions create heavier nuclides, the decreasing difference in binding energies between reactants and products results in less energy being produced from these reactions. 

The sequence ends at nickel-56, which has one of the highest binding energies per nucleon. Heavier elements are instead produced by multiple neutron- or proton-capture events just before and during the unique explosions of stars, or supernovae.