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Einstein's Cross: Distant supernova split into four by gravitational lensing

An exploding star directly behind a cluster of enormous galaxies has been split into four images by the light-bending properties of the cluster's gravity.


Thanks to the amount of mass in the universe, light travelling great distances never moves in a straight line. The mass of colossal objects such as galaxies and galaxy clusters creates enough gravity to pull light in a curve around them. This, from Earth, can magnify up to 100 times and otherwise distort the images of objects behind these gravitational effects -- creating an effect known as gravitational lensing.

This effect was predicted by Albert Einstein's General Theory of Relativity, which proposed that dense concentrations of mass in the universe would bend light like a lens. This effect was not observed until 1979, but today astronomers use it to help find out more about the early universe.

The visual effects can be pretty interesting, too. Take the phenomenon known as the Einstein cross. This is a quasar is positioned behind the lensing object in such a way that the light travelling from the object towards the observer splits into four distinct, separate images.

Now, for the first time, astronomers have observed the Einstein cross effect -- on a supernova 9.3 billion light-years from Earth, near the limits of the observable 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," said UC Berkeley postdoctoral scholar Patrick Kelly, who found the photograph among infrared images taken by the Hubble Space Telescope on November 10, 2014.

The supernova is behind a massive galaxy about 5 billion light-years from Earth in a cluster of galaxies, both of which have a lensing effect. Thanks to the unique configuration, the researchers till have the opportunity to see the star explode again in the next 10 years; because of the way gravity bends light, the light can take many different path through the gravitational lens, arriving at Earth at different times.

Computer modelling of the cluster indicates that opportunities to view the explosion were missed 50 years ago and again 10 years ago; but it can also map predictions for when researchers can observe the explosion in the future.

"The longer the path length, or the stronger the gravitational field through which the light moves, the greater the time delay," said Alex Filippenko, UC Berkeley professor of astronomy and a member of Kelly's team.

"It's a wonderful discovery. We've been searching for a strongly lensed supernova for 50 years, and now we've found one. Besides being really cool, it should provide a lot of astrophysically important information."

The cluster, known as MACS J1149.6+2223 and the location of a very strong gravitational lensing effect, has been the site of discoveries in the past. In 2012, researchers found what may be the oldest galaxy ever seen, formed some 500 million years after the beginning of the universe. And earlier, in 2009, it located what, up until that point, was the largest known image of a spiral galaxy seen through a gravitational lens.

This galaxy is the site of the supernova, which is located in one of the galaxy's spiral arms.

Infrared images of the supernova. Science/Kelly et al.

The team notes that, although the first image in which they found the supernova was from November 10, the first image that shows the supernova, very faintly, was dated November 3. So far, the Hubble telescope has taken several dozen images of the supernova, with many more to be expected over the next six months. And, of course, the researchers can continue their observations when the supernova replays in the next 10 years.

"It's perfectly set up, you couldn't have designed a better experiment," said study co-author Dr Bradley Tucker of the Australian National University Research School of Astronomy and Astrophysics.

"You can test some of the biggest questions about Einstein's theory of relativity all at once -- it kills three birds with one stone."

The full study, "Multiple Images of a Highly Magnified Supernova Formed by an Early-Type Cluster Galaxy Lens", can be found online, published in the journal Science.