Joined: 28 Feb 15
Long, long ago, about 4 billion years before now and in a faraway galaxy, a pair of black holes collided. Typically such an event would leave no visible trace, just a shuddering of space-time — gravitational waves — and a bigger black hole. (Black holes emit no light.) But these black holes were part of a swirl of star parts, gas and dust surrounding a third, gigantic black hole, a supermassive black hole 100 million times more massive than the sun. As a result, the merging pair generated a shock wave of heat and light that allowed the collision to be seen as well as heard.