GW170817 was the first confirmed gravitational-wave event from the merger of two neutron stars.
The Laser Interferometer Gravitational-Wave Observatory and Virgo detected the signal on 17 August 2017. Unlike earlier gravitational-wave detections from black-hole mergers, GW170817 was followed by electromagnetic observations across the spectrum, including gamma rays, visible light, infrared radiation, and radio waves.
The event occurred in the galaxy NGC 4993, about 130 million light-years away. Its observations provided strong evidence that neutron-star mergers produce short gamma-ray bursts and create heavy elements such as gold and platinum through rapid neutron capture.
GW170817 also gave scientists an independent way to estimate the expansion rate of the universe. Because the event supplied both a gravitational-wave distance and a host-galaxy redshift, it became a landmark example of multi-messenger astronomy.