The explosive death of a star is an incredible event, but not an unusual one for astronomers to observe. But now, for the first time, scientists have spotted a star’s death — known as a supernova — in an unprecedented way: the light from a distant explosion was warped on its way to Earth by a galaxy that got in the way. This warping magnified the supernova’s light and even split the explosion into four different images. It’s a unique find that could tell us more about the structure of our Universe.
What the astronomers observed is known as “gravitational lensing,” and it was predicted by Albert Einstein in his theory of general relativity. The idea is that massive objects curve space and time around them, and these curvatures in space-time can affect the paths on which light travels. Light passing by a particularly massive object, for instance, will not follow a straight path but a warped one. In the case of this supernova, its light encountered a galaxy that acted like an optical lens would on Earth — focusing the light and splitting it in four different ways.
It’s not the first time that a supernova’s light has been warped by gravitational lensing before. In fact, astronomers also saw an exploding star split four different ways in 2014. But that supernova was lensed by a cluster of galaxies. This one, detailed today in the journal Science, was lensed by just one galaxy — something that’s never been seen before. And it’s the first time a supernova of this kind, known as a “standard candle,” has been warped like this.
All these unique traits make the discovery extra exciting for astronomers, since the four supernova images can be used to learn more about our expanding Universe. Experts agree that the Universe is growing, but exactly how fast that’s happening is still up for debate. Studying this warped supernova more closely could provide a more definitive answer of that expansion rate. “This is a new tool that we hadn’t thought we had,” Ariel Goobar, a cosmologist at the University of Stockholm and the lead author of the study, tells The Verge.
Gravitational lensing has been used before to learn more about the cosmos — to better understand dark matter, for instance. This mysterious type of material is thought to make up about 27 percent of our Universe, but it can’t be seen directly. So the only way to study dark matter is to observe how it warps the space-time around it. “We’re not looking at dark matter itself, but the effects of dark matter,” Liliya Williams, a gravitational lensing expert at the University of Minnesota, who was not involved in the study, tells The Verge.
To use gravitational lensing to study supernovae, though, things have to line up just right. Goobar and his team stumbled upon this fortuitous event in September 2016, while searching for star explosions with the intermediate Palomar Transient Factory telescope in California. They found a lot of supernovae in the sky, but one particular star caught Goobar’s attention. He studied the supernova’s light a little more closely and found that this star was much farther away than any of the others they had seen. “That immediately made me think that this was something special,” says Goobar. “We were looking for supernovae, but we were never looking for supernovae that far.”
The team then realized that there was a galaxy next to the distant supernova, and that the galaxy was pretty much halfway between Earth and the exploding star. That’s when it clicked: the galaxy had magnified the light of the supernova, making it 50 times brighter than normal and easier to spot from Earth. When the astronomers studied the event with other instruments like the Hubble Space Telescope, they found that the supernova had been multiplied by the galaxy into four separate images. That happened because the galaxy is shaped in such a way that it creates four different paths in space-time the light can take, Goobar says.
The discovery was made extra special by the fact that this exploding star is a “standard candle” supernova. These supernovae explode in very predictable ways, with the same level of brightness. By knowing how luminous this star would have been without the lensing effect, Goobar and his team were able to measure very precisely just how much the galaxy magnified the supernova’s light. In turn, that gives them a good idea of what the galaxy is made of and how its matter is distributed.
In addition to all that, this particular type of supernova will also allow the astronomers to figure out how fast the Universe is expanding. The astronomers can measure how long it takes for the light from each of the four images to arrive at Earth. And comparing these arrival times can be used to figure out the expansion rate.
As exciting as that is, those measurements may not fully settle the debate over the growth of the Universe. But the good news is there will probably be more opportunities like this in the future. This discovery tells astronomers how to look for similar warped supernovae, Goobar says. And the more they find, the closer we’ll get to understanding just how fast the cosmos is growing. “It’s a breakthrough in the methodology — we know how to look for more,” he says. “And with a bit of luck and patience, we’ll be able to have answers in the next few years.”