A supernova will appear in the sky in the first few months of 2016, according to astronomers working on the Hubble Space Telescope. The prediction is possible because they first saw the star explode in 2014 in a gravitationally lensed galaxy, which will make it visible again next year.
Gravitational lenses happen when a massive object (or objects such as a cluster of galaxies) magnifies and distorts the light of background galaxies. In this case, the galaxy cluster is so massive that it deforms space and time so that it acts like a gigantic magnifying glass.
Sometimes, these distortions produce multiple images of the same object. Although they belong to the same galaxy, the images we see were not emitted at the same time. Because light travels at a finite speed, photons will take a different amount of time to travel around the massive object depending on the path they follow – with some routes taking longer than others.
The supernova explosion that we will see in 2016 is a re-run of the 2014 one, known as the Refsdal Supernova. It was generated in a galaxy nine billion light-years away, and the lens is created by a massive galaxy cluster, called MACS J1149+2223, five billion light-years from us.
When the object was first discovered, astronomers saw the supernova four times, as one of the images was perfectly aligned with an elliptical galaxy in the cluster, producing an extra lensing boost. By studying the matter distribution in the cluster, the team was able to predict that some of the photons emitted by the supernova are still traveling and are yet to arrive at Earth. For this reason, Hubble will now periodically gaze at the lensed galaxy in the hope of spotting the star going off.
The supernova was named Refsdal in honor of the Norwegian astrophysicist Sjur Refsdal who, in 1964, first suggested using a gravitationally lensed supernova to study the expansion of the universe. “Astronomers have been looking to find one ever since,” said Tommaso Treu, lead author of the paper which described Refsdal Supernova, in a statement earlier this year. “And now the long wait is over!”