A supernova exploded more than 9 billion years ago, and its light has since been traveling to Earth. In between our planet and this supernova, however, lies a massive galaxy with a cluster of galaxies that causes an interesting effect on the path that the light from the exploded star takes.
The galaxy, which lies within a cluster of galaxies, is so massive that it has the ability to warp space-time around it and thus bend the light that travels to Earth.
The galaxy cluster was large enough that the gravity bends and intensifies the supernova's light through an effect known as gravitational lensing, which was first predicted by Albert Einstein.
Gravitational lensing is similar to the effect brought about by a glass lens that bends light, distorting the image of the object behind it. This results in the galaxy creating four separate images of the supernova, arranged in the shape of a cross, a phenomenon known as "Einstein Cross."
"You just see the four dots and they form this beautiful Einstein Cross, and it's just kind of spooky," said University of Chicago astrophysicist Daniel Holz.
Astronomers spotted the Einstein Cross after looking at several images that were transmitted by NASA's Hubble Space Telescope. The four images of the supernova appeared within several days or weeks of each other partly because of the varying length of the pathways the light takes to reach Earth.
Each of the images takes a different route through the galaxy cluster before heading towards our planet. Astronomers expect that the Cross will be replaced by other images of the supernova within the next five years, as the gravitational lens distorts it into different configurations.
The absence of the cluster would have resulted in the astronomers detecting only the light from the supernova that is headed straight at Earth and see only one image of the supernova.
"The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected," Patrick Kelly from the Department of Astronomy of the University of California, Berkeley and colleagues reported the multiple lensed supernova in the journal Science on March 6. "The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses."
