Alpha process
The other class is a cycle of reactions called the triple-alpha process, which consumes only helium, and produces carbon.
[1] The alpha process most commonly occurs in massive stars and during supernovae.
After the triple-alpha process has produced enough carbon, the alpha-ladder begins and fusion reactions of increasingly heavy elements take place, in the order listed below.
Each step only consumes the product of the previous reaction and helium.
The energy produced by each reaction, E, is mainly in the form of gamma rays (γ), with a small amount taken by the byproduct element, as added momentum.
[2]) because it is the most tightly bound nuclide – i.e., the nuclide with the highest nuclear binding energy per nucleon – and production of heavier nuclei would consume energy (be endothermic) instead of release it (exothermic).
(Nickel-62) is actually the most tightly bound nuclide in terms of binding energy[3] (though
,[4] but nonetheless the sequence does effectively end at iron.
The sequence stops before producing elements heavier than nickel because conditions in stellar interiors cause the competition between photodisintegration and the alpha process to favor photodisintegration around iron.
They occur even less easily with elements heavier than neon (Z > 10) due to the increasing Coulomb barrier.
The status of oxygen (O) is contested – some authors[6] consider it an alpha element, while others do not.
O is surely an alpha element in low-metallicity Population II stars: It is produced in Type II supernovae, and its enhancement is well correlated with an enhancement of other alpha process elements.
Sometimes C and N are considered alpha process elements since, like O, they are synthesized in nuclear alpha-capture reactions, but their status is ambiguous: Each of the three elements is produced (and consumed) by the CNO cycle, which can proceed at temperatures far lower than those where the alpha-ladder processes start producing significant amounts of alpha elements (including C, N, & O).
[7] These stars contract as they age, increasing core temperature and density to high enough levels to enable the alpha process.
Requirements increase with atomic mass, especially in later stages – sometimes referred to as silicon burning – and thus most commonly occur in supernovae.
[8] Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne, Mg, Si, S, Ar, Ca, and Ti) while Type Ia supernovae mainly produce elements of the iron peak (Ti, V, Cr, Mn, Fe, Co, and Ni).
[7] Sufficiently massive stars can synthesize elements up to and including the iron peak solely from the hydrogen and helium that initially comprises the star.
[7] The second stage (neon burning) starts as helium is freed by the photodisintegration of one
atom, allowing another to continue up the alpha ladder.
The supernova shock wave produced by stellar collapse provides ideal conditions for these processes to briefly occur.
During this terminal heating involving photodisintegration and rearrangement, nuclear particles are converted to their most stable forms during the supernova and subsequent ejection through, in part, alpha processes.
and above, all the product elements are radioactive and will therefore decay into a more stable isotope; for instance,
[9] The abundance of total alpha elements in stars is usually expressed in terms of logarithms, with astronomers customarily using a square bracket notation: where
is the number of alpha elements per unit volume, and
is the number of iron nuclei per unit volume.
Theoretical galactic evolution models predict that early in the universe there were more alpha elements relative to iron.
