How do you find when a supernova blew? Run the clock backwards.

About 200,000 years ago, a massive star in a nearby companion galaxy to the Milky Way exploded. Blasting octillions of tons of debris outwards at high velocity, the explosion has been expanding into space ever since. Today it looks like a swirl of cirrus clouds, its huge velocity diminished almost to motionlessness by distance.

But the actual expansion is measurable, and by using a clever technique that runs the clock on it backwards, astronomers have determined when the light of the explosion ...