12Calphagamma16O

12Calphagamma16O, written in standard nuclear notation as 12C(α,γ)16O, is a radiative alpha-capture reaction in stellar helium burning. In this process, a carbon-12 nucleus captures an alpha particle (helium-4) to form oxygen-16, emitting a gamma ray. The reaction has a Q-value of about 7.16 MeV, and the gamma can originate from transitions to the ground state or to excited states of 16O.

Astrophysical significance is central to helium-burning in stars. After the triple-alpha process creates carbon, 12C(α,γ)16O governs the further conversion of carbon and helium into oxygen, thereby setting the carbon-to-oxygen ratio that influences later stellar evolution, remnant properties, and nucleosynthetic yields. The reaction rate is highly temperature dependent and is described by the astrophysical S-factor because the cross section at stellar energies is extremely small due to the Coulomb barrier. In helium-burning environments, temperatures around 0.1–0.3 GK probe sub-MeV center-of-mass energies within the Gamow window.

Experimental and theoretical status is characterized by substantial challenges. Direct measurements at stellar energies are extremely difficult; results at higher energies are extrapolated to lower energies using models such as R-matrix analyses. Experimental efforts, including underground facilities to reduce background and indirect measurement methods, have improved constraints on the S-factor. Astrophysical reaction-rate compilations (for example, NACRE and NACRE II) provide adopted rates and uncertainties. Nonetheless, 12C(α,γ)16O remains one of the most important and uncertain reactions in stellar evolution models, with small rate changes producing notable shifts in the predicted C/O balance and subsequent evolution.