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Fusion powers stars and produces virtually all elements in a process called nucleosynthesis. The Sun is a main-sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen and makes 616 million metric tons of helium each second.
As a result, there is little mixing of fresh hydrogen into the core or fusion products outward. In higher-mass stars, the dominant energy production process is the CNO cycle , which is a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries and in the end produces a helium nucleus as with the proton–proton chain ...
In astrophysics, the carbon–nitrogen–oxygen (CNO) cycle, sometimes called Bethe–Weizsäcker cycle, after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker, is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction (p–p cycle), which is more ...
The proton–proton chain, also commonly referred to as the p–p chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun , [ 2 ] whereas the CNO cycle , the other known reaction, is suggested by theoretical models to dominate ...
Fusion forces together atoms of very light, stable elements like isotopes of hydrogen, creating slightly heavier elements like helium and producing as much as four times as much energy, per unit ...
In the years immediately before World War II, Hans Bethe first elucidated those nuclear mechanisms by which hydrogen is fused into helium. Fred Hoyle's original work on nucleosynthesis of heavier elements in stars, occurred just after World War II. [4] His work explained the production of all heavier elements, starting from hydrogen.
Once a star has converted all the hydrogen in its core into helium, the core is no longer able to support itself and begins to collapse. It heats up and becomes hot enough for hydrogen in a shell outside the core to start fusion. The core continues to collapse and the outer layers of the star expand. At this stage, the star is a subgiant. Very ...
As a side effect of the process, some carbon nuclei fuse with additional helium to produce a stable isotope of oxygen and energy: 12 6 C + 4 2 He → 16 8 O + γ (+7.162 MeV) Nuclear fusion reactions of helium with hydrogen produces lithium-5, which also is highly unstable, and decays back into smaller nuclei with a half-life of 3.7 × 10 −22 s.