n that day, astronomers bore witness to the titanic collision of two neutron stars, the densest things in the universe besides black holes. In the collision’s wake, astronomers answered multiple major questions that have dominated their field for a generation. They solved the origin of gamma-ray bursts, mysterious jets of hardcore radiation that could potentially roast Earth. They glimpsed the forging of heavy metals, like gold and platinum. They measured the rate at which the expansion of the universe is accelerating. They caught light at the same time as gravitational waves, confirmation that waves move at the speed of light. And there was more, and there is much more yet to come from this discovery. It all happened so quickly and revealed so much that astronomers are already facing a different type of question: Now what?