What is nuclear fusion?

Two or more atomic nuclei are fused together to produce one or more new atomic nuclei and subatomic particles (protons or neutrons). Energy is released or absorbed depending on the mass differential between the reactants and products. After the process, the nuclei have a different atomic binding energy than before. A star's power comes from its ability to fusion, a process that releases enormous quantities of energy.

When nuclei lighter than iron-56 or nickel-62/62 are fused together, energy is released. Small masses per nucleon and high binding energies per nucleon characterise these elements. Exothermic reactions occur when nuclei smaller than these are fused, whereas endothermic reactions occur when nuclei larger than these are fused. Nuclear fission, the inverse process, is the exact opposite. According to the theory of nuclear fission, lighter elements like hydrogen and helium, as well as heavier elements like uranium and plutonium are more fusible than heavier elements like thorium and uranium. An event as intense as a supernova can fuse nuclei into heavier elements, such as uranium and plutonium.

Eddington proposed in 1920 that hydrogen and helium fusion may be the principal source of star energy. Atkinson and Houtermans utilised the measured masses of light elements to show that huge quantities of energy may be released by fusing tiny nuclei shortly after Friedrich Hund discovered quantum tunnelling. In 1932, Mark Oliphant achieved laboratory fusion of hydrogen isotopes, following in the footsteps of Patrick Blackett's early work in artificial nuclear transmutation. Hans Bethe figured out the primary cycle of nuclear fusion in stars over the rest of the decade. In the early 1940s, as part of the Manhattan Project, fusion research began for military applications. On November 1, 1952, the Ivy Mike hydrogen (thermonuclear) bomb test became the first to demonstrate self-sustaining nuclear fusion.

For more than 70 years, scientists have been working to perfect the science of controlled fusion in fusion reactors.

Stars' nuclear fusion

Stars, including the Sun, are powered by stellar nucleosynthesis, a fusion process. In the 20th century, it was discovered that star heat and light are sustained by the energy generated by nuclear fusion events. As the star's hydrogen and helium abundance increases, so does its ability to generate energy and create new nuclei. Each star has a unique set of reactions that are triggered by its mass (and therefore the pressure and temperature in its core).

In his work The Internal Constitution of the Stars, Arthur Eddington predicted the discovery and mechanism of nuclear fusion reactions in stars.

Astronomers were baffled as to the origins of stellar energy at the time, but Eddington accurately predicted that hydrogen fusion into helium would liberate vast amounts of energy. When fusion and thermonuclear energy were not yet discovered, or even that stars are mostly comprised of hydrogen, this was a particularly astonishing development (see metallicity). According to Eddington's paper,

• Because of the conservation of angular momentum, according to the contraction hypothesis, stars' revolution should speed faster. Cepheid variable stars, on the other hand, revealed that this wasn't the case.
• A tiny amount of matter might be converted to a great amount of kinetic energy, as Einstein had demonstrated some time before. This was the only other known conceivable source of energy.
• Aston also just discovered that the mass of a helium atom was around 0.8% less than the mass of the four hydrogen atoms that would combine to form a helium-atom atom (according to the then-prevailing theory of atomic structure which held atomic weight to be the distinguishing property between elements; work by Henry Moseley and Antonius van den Broek would later show that nucleic charge was the distinguishing property and that a helium nucleus, therefore, consisted of two hydrogen nuclei plus additional mass). A combination of these two would produce a lot of energy as a byproduct, according to this theory.
• In order to explain how stars obtain their energy, only 5% of a star's hydrogen should be fusible. Until recently, we had no idea that most "average" stars had more than 5% hydrogen.)

Other scientists had suggested that stars were the "crucible" in which light elements interacted to form heavy elements, but without more exact measurements of their atomic weights, nothing more could be known at the time.

In the ensuing decades, all of these theories came to fruition.

The fusion of hydrogen to helium (the proton–proton chain reaction) is the principal source of solar and similar-sized star energy, which occurs at a temperature of 14 million kelvin. Two neutrinos and two positrons are released in the process, which results in the fusing of four protons into a single alpha particle. A star's CNO cycle and other activities take on greater significance when it is more massive. Stars begin synthesis of heavier elements when they consume up a significant amount of hydrogen. A process called supernova nucleosynthesis produces the heaviest elements by fusing hydrogen and helium from a dying giant star during a cataclysmic supernova.