Thanks to a rare cosmic phenomenon, astronomers were able to observe the same ancient, distant star explode as a supernova four times at different locations in the galaxy.
The effect, known as Einstein’s Cross, is similar to gravitational lensing (whereby a massive galaxy, or a cluster of galaxies, bends the light that comes through it, creating a sort of celestial magnifying glass), but rather than just making the object in front of it look larger, it also multiplies its image.
First predicted by Einstein about a century ago (as a result of his general theory of relativity), this unique phenomenon has been observed with bright, active galaxies and black holes, but never before with a supernova.
“They are definitely very unusual,” said University of California, Berkeley post-doc Patrick Kelly, who spotted the arrangement in images from Hubble, while looking for small supernovas in front of gravitational lenses. “Probably because no one had found a similar object before, I had not even seriously entertained the possibility of finding multiple images of the same supernova.”
Located 9 billion light years away, the exploding star was magnified by two succeeding galaxies, enlarging the image by 20 times.
Since the light from the explosion took slightly different paths on its way to Earth, the images are playing at different speeds and appearing in different places. Scientists predict this explosion will play out again in a different part of the cluster sometime this decade, although it may happen as soon as next year.
By studying the lag time between the images, astronomers hope to learn more about the strength of gravity and the amount of dark matter and dark energy in the universe.
“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,” explained Kelly.
The images, taken by Hubble, are part of a program designed to look for lensed objects that may help us better understand how the expansion rate of the universe has changed over time.
“We can try to reverse engineer the properties of the gravitational lens,” noted Robert Kirshner from the Harvard-Smithsonian Centre for Astrophysics. “The lens itself is not really the bright stars we can see. The thing that is doing the lensing is the dark matter.”
Kelly and his colleagues will continue observing the supernova, named after the Norwegian astrophysicist Sjur Refsdal who predicted the phenomenon in 1964, and try to catch the rerun from the very beginning.
The study was published in the journal Science as part of the celebrations to commemorate the 100th birthday of Einstein’s General Theory of Relativity.