US Scientists Crack the Mystery of How Stars Make Strontium

For decades astronomers have been puzzling over a curious detail in the chemical fingerprints of the oldest stars: they contain way more strontium than our textbooks said they should. Strontium, the reddish‑hued metal that lights up fireworks and helps date archaeological finds, is forged deep inside stars, but the exact recipe remained stubbornly fuzzy.

The classic playbook for building heavy elements involves three well‑known processes. The rapid‑neutron‑capture or “r‑process” smashes nuclei together in a flash, the slow‑neutron‑capture or “s‑process” adds neutrons more leisurely, and the rarer “p‑process” creates proton‑rich isotopes. These three mechanisms explain most elements beyond iron, at least they seemed to until the 1990s when high‑precision spectra from ancient halo stars started to shout a different story.

Enter the so‑called i‑process – an intermediate‑speed neutron‑capture scenario that sits between the r‑ and s‑processes. It was a tempting fix because it can produce a broad suite of heavy nuclei. Yet when modelers ran the numbers, the i‑process still fell short on strontium, churning out only a fraction of what the telescopes saw. The missing piece? A stubborn uncertainty in the rate at which krypton‑88 snags an extra neutron.

Measuring that reaction directly is a nightmare. In a lab you can’t easily recreate the searing densities and temperatures of a star’s interior, and the krypton‑88 nucleus is fleeting enough that catching it in the act is nearly impossible. The solution, championed by scientists at the Facility for Rare Isotope Beams (FRIB) and the Argonne Tandem‑Linac Accelerator System (ATLAS), was to take a detour. They produced a heavier sibling, krypton‑89, let it decay, and then watched the cascade of gamma rays it emitted with the high‑resolution Summing‑Nal (SuN) detector. From that glow they could back‑calculate the elusive krypton‑88 neutron‑capture rate.

What they found was a surprise: the real capture rate is consistently lower than the values that theoretical models had been using for years. Plugging the new, slower rate into i‑process simulations caused the predicted strontium yields to jump up, aligning much more closely with the abundances measured in those ancient stars.

“Now that we know this reaction rate, the next step is again on us as modelers,” said Falk Herwig, a professor of physics and astronomy at the University of Victoria. “With the main nuclear uncertainty removed, we can turn to the astrophysics – the neutron densities, the timing of the burning – and work to close the remaining gap with what we see in the oldest stars.”

The results, published in the journal Communications Physics, not only solve a lingering discrepancy in stellar nucleosynthesis but also showcase how clever indirect experiments can untangle the most stubborn nuclear puzzles. As researchers refine the remaining astrophysical variables, we’re edging ever closer to a complete, chemistry‑by‑chemistry map of how the universe builds its elements.