Multiple images of a supernova created by gravitational lensing have been captured for the first time by an international team of astronomers using the Hubble Space Telescope (HST). The "Einstein cross" pattern comprises four images of a distant supernova created by the gravitational lensing of its light as it passed a distant galaxy within a cluster of galaxies on its way to Earth. In addition to giving us a closer look at the dynamics of distant supernovae, the team says that its discovery will help to improve our understanding of the distribution of dark matter in the lensing galaxy and galaxy cluster, as well as to test Einstein's general theory of relativity and measure the rate of cosmic expansion in the universe.

A gravitational lens is a large galaxy or group of galaxies that bends or "lenses" light from a distant source as it travels towards an observer. The effect was predicted by Einstein's general theory of relativity and the first such lens was discovered in 1979. Sometimes, the distant light source, lensing galaxy and the observer line up precisely, and we can see an "Einstein ring" – a perfect loop of light from the source encircling the lensing mass. But if there is any misalignment along the way, we observe partial arcs or spots. Depending on the relative positions of the bodies, four such spots can be seen, forming an Einstein cross. The lensing effect serves as a "natural telescope" for astronomers, who can determine the mass of the lensing galaxy and its dark-matter content based on the amount of distortion observed.

Long search

"It's a wonderful discovery," says Alex Filippenko of the University of California, Berkeley, who is part of the team that found the latest quadruple-lensed supernova image, explaining that researchers have been "searching for a strongly lensed supernova for 50 years, and now we've found one". Thanks to the many conditions that need to be fulfilled for a gravitational lens to be seen from Earth, and the relatively short lifetime of a supernova, such a lensed supernova with four images has never been seen before.

Even more interesting, thanks to an understanding of the peculiarities of gravitational lensing, the team already knows that a fifth image will appear in the next decade. This will give astronomers a "replay" of the supernova, because light can take various paths around and through a gravitational lens and therefore arrive at Earth at different times. This is particularly rare and useful, because astronomy is not normally a predictive science. "The longer the pathlength, or the stronger the gravitational field through which the light moves, the greater the time delay," says Filippenko.

The team used a computer model to predict the pathways that the light from the supernova can take around the lensing cluster, which also suggests that we already missed out on seeing earlier images of the exploding star 10 and 50 years ago. The team has dubbed the distant supernova SN Refsdal (after the late pioneering astrophysicist Sjur Refsdal), and it is located about 9.3 billion light-years away (redshift 1.5), near the edge of the observable universe, while the lensing galaxy is about 5 billion light-years (redshift 0.5) from Earth.

Multiple replays

"Basically, we get to see the supernova four times and measure the time delays between its arrival in the different images, hopefully learning something about the supernova and the kind of star it exploded from, as well as about the gravitational lenses," says team member Patrick Kelly, also at Berkeley, who discovered the supernova while looking through infrared images taken by the HST last November.

The galaxy that splits the supernova's light is part of a large cluster – MACS J1149.6+2223 – that was discovered more than 10 years ago. In 2009 astronomers reported that the cluster created the largest known image of a spiral galaxy ever seen through a gravitational lens. The more distant galaxy appears in multiple images around the foreground lensing cluster and it hosts the supernova in one of the galaxy's spiral arms. "We get strong lensing by a red galaxy, but that galaxy is part of a cluster of galaxies, which is magnifying it more. So we have a double lensing system," explains Kelly.

Kelly hopes that measuring the time delays between the phases of the supernova in the four images will let them put better constraints on the mass distribution of the foreground galaxies, as well as the expansion and geometry of the universe. If the researchers identify it as a Type Ia supernova (these have relatively standard brightness) by studying its spectrum, they could place even stronger limits on both the matter distribution and cosmological parameters.

The work is published in Science.