For hundreds of millions of years, two city-sized stars — each outweighing our sun — circled one another in a fatal dance. They were neutron stars, the collapsed cores left behind after giant stars explode into supernovas. Then, 130 million years ago, the dance ended. Their collision was fast and violent, likely spawning a black hole. And a shudder — a gravitational wave — rippled across the fabric of space-time. Light from the cataclysm followed seconds later. The spa…