In a first for astronomy, a distant supernova has been observed that was previously predicted to appear, has been imaged by a team of astronomers from the University of South Carolina, using the Hubble Space Telescope. The prediction utilized the effect of gravitational lensing, where the gravity of a massive object will bend light from a source behind it around itself, as if it were an optical lens.
The supernova in question, being named "Refsdal" by the astronomers studying it, was initially spotted last month by the Hubble Space Telescope, erupting in a cluster of galaxies called MACS J1149+2223, 9.34 billion light years away. This supernova appeared on the edge of a large spiral galaxy, but due to the gravitational lensing caused by the galaxy’s combined mass, it’s image appeared in four separate spots around the galaxy’s edge. The four-point formation is called an "Einstein Cross", named after Albert Einstein’s prediction that large masses could produce the effect of gravitational lensing.
The effect didn’t end there, though: the astronomers realized that the image of the galaxy doing the lensing was itself being gravitationally-lensed by the galactic cluster that it was a part of, allowing them to calculate when the next image of the nova would appear from that lensing. While their prediction for the nova’s reappearance was for sometime in the first quarter of 2016, the nova reappeared this month instead, allowing the team to observe the nova all over again.
"While studying the supernova, we realized that the galaxy in which it exploded is already known to be a galaxy that is being lensed by the cluster," explains study co-author Steve Rodney. "The supernova’s host galaxy appears to us in at least three distinct images caused by the warping mass of the galaxy cluster."