Finding a supernova—the huge explosion that marks the death of a star—in a distant galaxy is lucky enough, but one group of astronomers also got the bonus of an instant replay, thanks to gravity. The team first witnessed the supernova last year, as it exploded behind a massive cluster of galaxies 5 billion light-years from Earth called MACS J1149.5+2223. They noticed four images of the same supernova arranged around a galaxy in what is known as an “Einstein cross.” This lensing effect happens when the gravity of a galaxy bends the light of an object behind it so that, from Earth, we see four images of the same object. The team realized that other galaxies in the cluster might be gravitationally lensing light from the same supernova. But, as the light would follow different paths, it would take more or less time to reach Earth. So they set out to carefully model all the matter, conventional and dark, in the galaxy cluster to predict when and where lensed images of the supernova might appear (pictured). One appearance, they calculated, must have happened in 1998, but no telescopes were watching. Another one, they reckoned, was due to happen just about now. On 11 December, the Hubble Space Telescope struck oil: An image of the same supernova appeared just as predicted, the first time such an event has been successfully forecast. The sighting is also a powerful demonstration of astronomers’ ability to model the effect of gravitational mass on light.
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