Deep inside a massive star, two carbon nuclei collide and merge into a single magnesium nucleus. This process determines how quickly the star burns its fuel and which chemical elements it manages to create. But calculating such a fusion on paper has proven incredibly difficult: nuclei are not billiard balls but quantum systems that can rearrange themselves in many ways.
The new work proposes a model where carbon nuclei can fuse directly or via an intermediate step: first, a small piece (an alpha particle) breaks off from one nucleus, and the remainder combines with the other nucleus. It's as if two dancers could perform a lift in two different ways — by a jump or through a rise. Accounting for both paths simultaneously has, for the first time, allowed scientists to explain both nuclear scattering experiments and fusion data.
The model also predicts that in the magnesium nucleus born from fusion, there are no pure 'molecular' states where two carbon nuclei orbit each other like planets. Instead, everything is mixed: the magnesium nucleus resembles a cauldron of bubbling porridge, where the original ingredients cannot be distinguished.
This is only the first step. In the future, the authors plan to add channels with proton and neutron emission so that the model works at even higher temperatures — in the hottest stellar cores. Then we will be able to know more precisely how much carbon, oxygen, and other elements are born in the universe.
as written by the authors arXiv:2602.22763
I present a fully microscopic description of the 12C+12C fusion reaction at stellar energies. Utilizing the multichannel Resonating Group Method (RGM), my model explicitly includes 12C+12C and alpha+20Ne reaction channels (with excited states). This approach provides a consistent, simultaneous, description of fusion, elastic scattering, and 24Mg spectroscopy. Results for 12C+12C elastic scattering show excellent agreement with experimental data, significantly improving the single-channel approximations. Spectroscopic analysis reveals that 24Mg states and resonances are highly mixed configurations, contradicting the concept of pure "molecular states." The calculated fusion S-factor is consistent with available experimental data and predicts both narrow and broad resonances near the Coulomb barrier. Main resonance widths originate primarily from the alpha+20Ne exit channels. The S-factor exhibits a decrease at low energies, providing a microscopic support for the hypothesis of fusion hindrance. This work is a first step towards a reliable theoretical extrapolation of the 12C+12C reaction to deep stellar burning temperatures. Future works should include the neutron and proton channels to provide a complete description of the 12C+12C fusion.
The carbon-12 nucleus consists of 6 protons and 6 neutrons, and the magnesium-24 nucleus consists of 12 protons and 12 neutrons. That is, when two carbon nuclei fuse, you get exactly twice as many particles, but with different properties.