One of my favourite astrophysical phenomena was the appearance of the same supernova at various different times and places in the sky. The light from “Supernova Refsdal” was bent by a cluster of galaxies, whose gravity acted like a magnifying glass. This “gravitational lensing” meant that multiple images appeared.
Imagine rays of light heading out in all directions from the exploding star. Normally we would expect only one of those rays or directions to intercept a given telescope, hence the star would appear at a single location. However according to general relativity, heavy masses curve spacetime. As the rays pass through the complicated gravitational field of the galaxy cluster (which incidentally is much closer to us than the supernova), their paths are deflected.
The supernova was first observed in late 2014, as four separate images surrounding a single galaxy. They formed a very rare “Einstein cross” pattern. The galaxy had deflected the passing light. Amazingly, astrophysicists predicted another image of the same supernova would appear about a year later, in a precise location. This is because they had already mapped out the mass distribution in the cluster of galaxies, and already observed multiple copies (up to 7) of various objects. This gave the possible paths in space and time. Indeed, in late 2015 the Hubble telescope detected the supernova as predicted. Researchers also predict (rather “postdict”) the supernova would have appeared decades earlier in a different spot.
Note the video embedded above uses a little artistic license for the supernova. Another video tries to illustrate the various paths that reached us. The supernova was also featured in NASA’s Astronomy Picture of the Day , which has an excellent description.