A supernova that exploded in 2010 and that appeared to burn much brighter than a normal star explosion is explained by the presence of a giant cosmic magnifying lens, scientists announced Thursday.

The four-year-old star explosion, known as supernova PS1-10afx, had confounded scientists because it was 30 times brighter than normal.

On Thursday, a team of scientists announced that a nearby galaxy's gravity had made the supernova appear superbright.

Writing in the the journal Science, the team said that when cosmic objects align with one right in front of the other, the foreground object's gravity can act as a lens, warping and magnifying the background object's light.

"This has been a huge mystery," says astronomer Andy Howell of Las Cumbres Observatory Global Telescope Network. "But this new work makes a compelling case that this is a type 1a supernova that has been gravitationally lensed."

PS1-10afx has perplexed scientists ever since its discovery by the Hawaii-based Pan-STARRS1 telescope. Like all type 1a supernovae, the explosion resulted from a thermonuclear detonation that ripped apart a white dwarf star. Normally, these blasts shine with a predictable brightness, which is why scientists use them as cosmic distance markers.

But PS1-10afx, which in all other respects looked a lot like a type 1a, was way too bright—30 times brighter than it should have been.

Looking for Explanations

Teams trying to explain the peculiar supernova offered two different explanations. One team suggested that PS1-10afx might belong to a grab bag of blinding stellar oddities known as superluminous supernovas that are ten to a hundred times brighter than normal. A few weeks later, another team suggested it was a magnified, normal type 1a.

"We have good reason to believe that [type 1a supernovae] can't get as bright as PS1-10afx—or that if they did, they would look completely different," says study author Robert Quimby, an astronomer at the Kavli Institute for the Physics and Mathematics of the Universe, near Tokyo. "I thought it had to be gravitationally lensed."

The trouble was, there was no lens in sight.

Late last year, after the supernova had dimmed, Quimby and his colleagues set out to find the missing lens. They aimed the Hawaii-based Keck 1 telescope at the explosion site for 6.5 hours, searching for emission lines that would tell them whether they were seeing one galaxy or two. When the spectra came back, the team had their answer: In the foreground, about 8.2 billion light-years away, is a little galaxy. Much smaller than the Milky Way, it has an observed mass equivalent to about ten billion suns.

In the background? The supernova's host galaxy, sitting about nine billion light-years from Earth.

"I think it's time to throw in the towel on the idea that it was a different type of supernova and admit that there's evidence for a lens," says Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

"Nature's Microscope"

Still, some astronomers would like to see stronger evidence for the foreground galaxy—chiefly, more emission lines or a clear Hubble Space Telescope image of the region, which Quimby is working on.

"They have a reasonably strong case that there are two galaxies there," says Saurabh Jha, an astrophysicist at Rutgers University in Piscataway, New Jersey. Jha and others are also curious about what the foreground galaxy's total mass is—that is, the mass of both visible matter and dark matter, the mysterious material that cannot be seen yet still exerts gravitational force.

"One of the great things about gravitational lensing is that it makes things visible that we wouldn't have been able to see," Jha says. "You wouldn't have been able to see this supernova from the ground except that it got so much brighter with lensing."

In extreme cases, gravitational lensing can produce multiple images of the background object. Depending on the path they take through the cosmos, these images can be seen on Earth at different times. That difference in arrival time could help astronomers measure the Hubble constant, or the rate at which the universe is expanding. Lensed supernovae could also help astronomers probe the distant, dark universe by making it possible to calculate the amount of dark matter in a cosmic lens.

"You have nature's microscope out there, in the form of these gravitational lenses," Kirshner says. "It opens the window to seeing things at much greater distances than we could do otherwise."