Summary

On 17 August, scientists around the world witnessed something never seen before: One hundred and thirty million light-years away, two neutron stars spiraled into each other in a spectacular explosion that was studied by observatories ranging from gamma ray detectors to radio telescopes. The blast confirmed several key astrophysical models, revealed a birthplace of many heavy elements, and tested general relativity as never before. That first observation of a neutron-star merger, and the scientific bounty it revealed, is Science’s 2017 Breakthrough of the Year. Especially remarkable was the way the event was spotted: by detecting the infinitesimal ripples in space itself, called gravitational waves, that the spiraling neutron stars radiated before they merged. The discovery of gravitational waves themselves was Science’s 2016 Breakthrough of the Year. The merger poses puzzles that whet astrophysicists’ appetites for more such collisions. Through gravitational-wave astronomy, scientists also hope to see new types of events, such as mergers of a neutron star and a black hole, which theory suggests are rare, or supernova explosions of individual stars in our Milky Way galaxy. Above all, astrophysicists hope to see some signal they haven’t even predicted.