Elements heavier than zinc are synthesized through the (r)apid and (s)lowneutron-capture processes. The main site of production of the r-processelements (such as europium) has been debated for nearly 60 years. Initialstudies of chemical abundance trends in old Milky Way halo stars suggestedcontinual r-process production, in sites like core-collapse supernovae. Butevidence from the local Universe favors r-process production mainly during rareevents, such as neutron star mergers. The appearance of a europium abundanceplateau in some dwarf spheroidal galaxies has been suggested as evidence forrare r-process enrichment in the early Universe, but only under the assumptionof no gas accretion into the dwarf galaxies. Cosmologically motivated gasaccretion favors continual r-process enrichment in these systems. Furthermore,the universal r-process pattern has not been cleanly identified in dwarfspheroidals. The smaller, chemically simpler, and more ancient ultra-faintdwarf galaxies assembled shortly after the first stars formed, and are idealsystems with which to study nucleosynthesis events such as the r-process.Reticulum II is one such galaxy. The abundances of non-neutron-capture elementsin this galaxy (and others like it) are similar to those of other old stars.Here, we report that seven of nine stars in Reticulum II observed withhigh-resolution spectroscopy show strong enhancements in heavy neutron-captureelements, with abundances that follow the universal r-process pattern abovebarium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitudehigher than that detected in any other ultra-faint dwarf galaxy. This impliesthat a single rare event produced the r-process material in Reticulum II. Ther-process yield and event rate are incompatible with ordinary core-collapsesupernovae, but consistent with other possible sites, such as neutron starmergers.
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