On 2017 August 17, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst,GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively-powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve in-2 dicates that SSS17a produced at least ∼0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the Universe.The discovery of gravitational waves (GWs) from coalescing binary black holes by the Laser Interferometer Gravitational Wave Observatory (LIGO) has transformed the study of compact objects in the Universe (1, 2). Unlike black holes, merging neutron stars are expected to produce electromagnetic radiation. The electromagnetic signature of such an event can provide more information than the GW signal alone: constraining location of the source, reducing the degeneracies in GW parameter estimation (3), probing the expansion rate of the Universe (4,5), and producing a more complete picture of the merger process (6, 7).Short gamma-ray bursts (GRBs) have long been expected to result from neutron star mergers (8, 9), and therefore would be a natural electromagnetic counterpart to GWs (10). Unfortunately, their emission is beamed, so that it may not intersect our line of sight (11). The possibility that only a small fraction of GRBs may be detectable has motivated theoretical and observational searches for more-isotropic electromagnetic signatures, such as an astronomical transient powered by the radioactive decay of neutron-rich ejecta from the merger. (12)(13)(14)(15)(16)(17). Referred to as a macronova or kilonova, the detection of these events would provide information on the origin of many of the heaviest elements in the periodic table (18).It has long been realized that approximately half of the elements heavier than iron are created via r-process nucleosynthesis-the capture of neutrons onto lighter seed nuclei on a timescale more rapid than β-decay pathways (19,20). However, it is less clear where the r-process predominantly occurs, namely whether the primary sources of these elements are core-collapse supernovae or compact binary mergers (black hole-neutron star or neutron starneutron star) (21,22). For supernovae, direct detection of the electromagnetic signatures from r-process nucleosynthesis is obscured by the much larger luminosity originating from hydrogen 3 recombination (for hydrogen-rich supernovae) or nickel-56 and cobalt-56 decay (for hydrogenpoor supernovae). By contrast, it may be possible to measure the r-process nucleosynthesis after a compact ob...

The elements heavier than zinc are synthesized through the (r)apid and (s)low neutroncapture processes 1,2 . The primary astrophysical production site of the r-process elements (such as europium) has been debated for nearly 60 years 2 . Chemical abundance trends of old Galactic halo stars initially suggested continual r-process production from sources like core-collapse supernovae 3,4 , but evidence in the local universe favored r-process production primarily from rare events like neutron star mergers 5,6 . The appearance of a europium abundance plateau in some dwarf spheroidal galaxies was suggested as evidence for rare rprocess enrichment in the early universe 7 , but only under the assumption of no gas accretion into the dwarf galaxies. Invoking cosmologically motivated gas accretion 8 actually favors continual r-process enrichment in those systems. Furthermore, the universal r-process pattern 1,9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultra-faint dwarf galaxies assembled shortly after the formation of the first stars and are ideal systems to study nucleosynthesis processes such as the r-process 10,11 . Reticulum II is a recently discovered ultra-faint dwarf galaxy [12][13][14] . Like other such galaxies, the abundances of non-neutron-capture elements are similar to those of other old stars 15 . Here we report that seven of nine stars in Reticulum II observed with high-resolution spectroscopy show strong enhancements in heavy neutroncapture elements with abundances that exactly follow the universal r-process pattern above barium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitude higher than what is seen in any other ultra-faint dwarf galaxy 11,16,17 . This implies that a single rare event produced the r-process material in Reticulum II, whether or not gas accretion was significant in ultra-faint dwarf galaxies. The r-process yield and event rate is incompatible with ordinary core-collapse supernova 18 but consistent with other possible sites, such as neutron star mergers 19 .Ultra-faint dwarfs (UFDs) are small galaxies that orbit the Milky Way and have been discovered by deep wide-area sky surveys 12,13 . Although physically close to us, they are also relics from the era of the first stars and galaxies and thus an ideal place to investigate the first metal enrichment events in the universe 10 . Observations of UFDs provide evidence that they form all their stars within 1-3 Gyr of the Big Bang 20 , their stars contain very small amounts of elements heavier than helium ("metals") 21 , and they are enriched by the metal output of only a few generations of stars 11,20 . The chemical abundances of light elements (less heavy than iron) suggested that corecollapse supernovae were the primary metal sources in these systems 11,16,17 . This conclusion was supported by unusually low neutron-capture element abundances that are consistent with small amounts of neutron-capture element production associated with massive star evolution...