How Calcium Is Born in Stellar Explosions express
Deep inside a dying star, just before it goes supernova, nuclear reactions determine which chemical elements will be scattered across the cosmos. One such reaction is the collision of an oxygen nucleus with a proton, producing a nitrogen nucleus and an alpha particle (a helium nucleus). The rate of this reaction controls how much calcium and sulfur are produced — elements astronomers use to infer the composition of the progenitor star.
For a long time, data from different experiments contradicted each other: some measurements gave a reaction rate seven times higher than the standard value, others lower. To sort it out, physicists used the MUSIC detector, which catches particles right inside a gas chamber, like a microphone picking up sound. The new measurement at energies typical of an explosion showed the reaction rate is 1.5 times higher than the REACLIB reference, but not seven times. The scatter in the old data has been greatly reduced.
But even with the new reaction rate, we still can't explain why supernova remnants of different ages have different calcium-to-sulfur ratios. That means the answer lies in other oxygen-burning reactions — for instance, the fusion of two oxygen nuclei, or oxygen with carbon. Improving the precision of those measurements is what future experiments need to do to understand how stars forge elements.
Original abstract
as written by the authors arXiv:2603.06307
The $$^{16}$$O$$(p,\alpha)^{13}$$N reaction plays a key role in shaping the $$\alpha$$-particle abundance during explosive oxygen burning in Type Ia supernovae. By enhancing $$\alpha$$-production, this reaction directly affects the calcium-to-sulphur (Ca/S) and argon-to-sulphur (Ar/S) ratios, which serves as a tracer of progenitor metallicity. However, recent work suggests that the rate must be enhanced by a factor of up to seven over the standard value to explain observed Ca/S ratios across a range of progenitor metallicities. To explore this impact, available experimental cross-section data for the $$^{16}$$O$$(p,\alpha)^{13}$$N reaction have been compiled and critically evaluated. Significant discrepancies are identified in the low-energy region ($$E_{\mathrm{cm}}$$ = 5.7--7.0 MeV), primarily due to limitations of the activation method. To resolve this, the first direct measurement at astrophysical energies has been performed using the MUSIC active-target detector. The new $$^{16}$$O$$(p,\alpha)^{13}$$N thermonuclear reaction rate is found to be approximately 1.5 times higher than the REACLIB rate in the temperature range T = 3--4 GK, with more constrained uncertainties that resolve the previously large spread among existing data. The suggested factor of seven enhancement is excluded and these results indicate that this reaction alone cannot fully explain the variation in the Ca/S and Ar/S ratios observed across different progenitor metallicities. Therefore, future work should focus on reducing the uncertainties in other key oxygen-burning reactions, particularly $$^{16}$$O+$$^{16}$$O and $$^{12}$$C+$$^{16}$$O. Further reducing the constraints on the $$^{16}$$O$$(p,\alpha)^{13}$$N rate is also needed to fully determine to whether a nuclear physics solution to this discrepancy is possible.
The MUSIC detector works like a gas chamber where particles leave tracks, letting physicists literally 'hear' each nuclear collision.
In Stanisław Lem's novel 'Solaris', the planet's ocean changes its orbit by manipulating gravity — much like nuclear reactions in supernovae govern the chemical makeup of entire galaxies.