Element abundance patterns in stars indicate fission of nuclei heavier than uranium

The heaviest chemical elements are naturally produced by the rapid neutron-capture process ( -process) during neutron star mergers or supernovae. The -process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments so must be extrapolated by using nucleosynthesis models. We examined element abundances in a sample of stars that are enhanced in -process elements. The abundances of elements ruthenium, rhodium, palladium, and silver (atomic numbers = 44 to 47; mass numbers = 99 to 110) correlate with those of heavier elements (63 ≤ ≤ 78, > 150). There is no correlation for neighboring elements (34 ≤ ≤ 42 and 48 ≤ ≤ 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in -process events.