Editing Triple-alpha process (section)

==Discovery==
The triple-alpha process is highly dependent on [[carbon-12]] and [[beryllium-8]] having resonances with slightly more energy than [[helium-4]]. Based on known resonances, by 1952 it seemed impossible for ordinary stars to produce carbon as well as any heavier element.<ref name="Kragh">Kragh, Helge (2010) When is a prediction anthropic? Fred Hoyle and the 7.65 MeV carbon resonance. http://philsci-archive.pitt.edu/5332/</ref> Nuclear physicist [[William Alfred Fowler]] had noted the beryllium-8 resonance, and [[Edwin Salpeter]] had calculated the reaction rate for <sup>8</sup>Be, <sup>12</sup>C, and <sup>16</sup>O nucleosynthesis taking this resonance into account.<ref name="Salpeter">{{Cite journal | last=Salpeter | first=E. E. | title= Nuclear Reactions in Stars Without Hydrogen | journal=The Astrophysical Journal | date=1952| volume=115 | pages= 326–328 | doi=10.1086/145546 | bibcode=1952ApJ...115..326S}}</ref><ref>{{Cite journal | last=Salpeter | first=E. E. | journal=Annu. Rev. Astron. Astrophys. | date=2002| volume=40 | pages= 1–25 | doi=10.1146/annurev.astro.40.060401.093901 | title=A Generalist Looks Back | bibcode=2002ARA&A..40....1S}}</ref> However, Salpeter calculated that red giants burned helium at temperatures of 2·10<sup>8</sup> K or higher, whereas other recent work hypothesized temperatures as low as 1.1·10<sup>8</sup>&nbsp;K for the core of a red giant.

Salpeter's paper mentioned in passing the effects that unknown resonances in carbon-12 would have on his calculations, but the author never followed up on them. It was instead astrophysicist [[Fred Hoyle]] who, in 1953, used the abundance of carbon-12 in the universe as evidence for the existence of a carbon-12 resonance. The only way Hoyle could find that would produce an abundance of both carbon and oxygen was through a triple-alpha process with a carbon-12 resonance near 7.68&nbsp;MeV, which would also eliminate the discrepancy in Salpeter's calculations.<ref name=Kragh/>

Hoyle went to Fowler's lab at [[Caltech]] and said that there had to be a resonance of 7.68&nbsp;MeV in the carbon-12 nucleus. (There had been reports of an excited state at about 7.5&nbsp;MeV.<ref name=Kragh/>) Fred Hoyle's audacity in doing this is remarkable, and initially, the nuclear physicists in the lab were skeptical. Finally, a junior physicist, Ward Whaling, fresh from [[Rice University]], who was looking for a project decided to look for the resonance. Fowler permitted Whaling to use an old [[Van de Graaff generator]] that was not being used. Hoyle was back in Cambridge when Fowler's lab discovered a carbon-12 resonance near 7.65&nbsp;MeV a few months later, validating his prediction. The nuclear physicists put Hoyle as first author on a paper delivered by Whaling at the summer meeting of the [[American Physical Society]]. A long and fruitful collaboration between Hoyle and Fowler soon followed, with Fowler even coming to Cambridge.<ref>''Fred Hoyle, A Life in Science'', Simon Mitton, Cambridge University Press, 2011, pages 205–209.</ref>

The final reaction product lies in a 0+ state (spin 0 and positive parity). Since the [[Hoyle state]] was predicted to be either a 0+ or a 2+ state, electron–positron pairs or [[gamma ray]]s were expected to be seen. However, when experiments were carried out, the [[gamma emission]] reaction channel was not observed, and this meant the state must be a 0+ state. This state completely suppresses single gamma emission, since single gamma emission must carry away at least 1 [[angular momentum quantization|unit of angular momentum]]. [[Pair production]] from an excited 0+ state is possible because their combined spins (0) can couple to a reaction that has a change in angular momentum of 0.<ref>{{cite journal
 |last1=Cook |first1=CW
 |date=1957
 |title=12B, 12C, and the Red Giants
 |journal=[[Physical Review]]
 |volume=107 |issue=2 |pages=508–515
 |doi=10.1103/PhysRev.107.508
 |last2=Fowler
 |first2=W.
 |last3=Lauritsen
 |first3=C.
 |last4=Lauritsen
 |first4=T.
|bibcode = 1957PhRv..107..508C }}</ref>