Thealpha process, also known asalpha capture or thealpha ladder, is one of two classes ofnuclear fusion reactions by which stars converthelium into heavierelements. The other class is a cycle of reactions called thetriple-alpha process, which consumes only helium, and producescarbon.[1] The alpha process most commonly occurs in massive stars and duringsupernovae.
Both processes are preceded byhydrogen fusion, which produces thehelium that fuels both the triple-alpha process and the alpha ladder processes. After thetriple-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 later-stage reactions which are able to begin in any particular star, do so while the prior stage reactions are still under way in outer layers of the star.
The energy produced by each reaction,E, is mainly in the form ofgamma rays (γ), with a small amount taken by thebyproduct element, as addedmomentum.
It is a common misconception that the above sequence ends at (or, which is a decay product of[2]) because it is the most tightly boundnuclide – i.e., the nuclide with the highestnuclear binding energy pernucleon – and production of heavier nuclei would consume energy (beendothermic) instead of release it (exothermic). (Nickel-62) is actually the most tightly bound nuclide in terms of binding energy[3] (though has a lower energy or mass per nucleon). The reaction is actually exothermic, and indeed adding alphas continues to be exothermic all the way to,[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 betweenphotodisintegration and the alpha process to favor photodisintegration aroundiron.[2][5] This leads to more being produced than
All these reactions have a very low rate at the temperatures and densities in stars and therefore do not contribute significant energy to a star's total output. They occur even less easily with elements heavier thanneon (Z > 10) due to the increasingCoulomb barrier.
Alpha process elements (oralpha elements) are so-called since their most abundant isotopes are integer multiples of four – the mass of the helium nucleus (thealpha particle). These isotopes are calledalpha nuclides.
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-metallicityPopulation II stars: It is produced inType II supernovae, and its enhancement is well correlated with an enhancement of other alpha process elements.
SometimesC andN are considered alpha process elements since, likeO, they are synthesized in nuclear alpha-capture reactions, but their status is ambiguous: Each of the three elements is produced (and consumed) by theCNO cycle, which can proceed at temperatures far lower than those where the alpha-ladder processes start producing significant amounts of alpha elements (includingC,N, &O). So just the presence ofC,N, orO in a star does not a clearly indicate that the alpha process is actually underway – hence reluctance of some astronomers to (unconditionally) call these three "alpha elements".
The alpha process generally occurs in large quantities only if the star is sufficiently massive – more massive than about 10solar masses.[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 assilicon burning – and thus most commonly occur insupernovae.[8] Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne,Mg,Si,S,Ar,Ca, andTi) whileType Ia supernovae mainly produce elements of theiron peak (Ti,V,Cr,Mn,Fe,Co, andNi).[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.[6]
Typically, the first stage of the alpha process (or alpha-capture) follows from thehelium-burning stage of the star once helium becomes depleted; at this point, free capture helium to produce.[9] This process continues after the core finishes the helium burning phase as a shell around the core will continue burning helium andconvecting into the core.[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. Silicon burning is then later initiated through the photodisintegration of in a similar fashion; after this point, thepeak discussed previously is reached. Thesupernova 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. Starting at and above, all the product elements are radioactive and will therefore decay into a more stable isotope; for instance, is formed and decays into.[9]
The abundance of total alpha elements in stars is usually expressed in terms oflogarithms, 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. It is for the purpose of calculating the number that which elements are to be considered "alpha elements" becomes contentious. Theoreticalgalactic evolution models predict that early in the universe there were more alpha elements relative to iron.
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