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.
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.