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 first reached Earth: 1746 years ago, give or take 175
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A wide field of view Hubble image of the supernova remnant 1E 0102.2-7219 (below center, blue) shows it lies just a few dozen light years from the huge star-forming nebula N76 in the Small Magellanic Cloud, a companion galaxy to the Milky Way. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)