The four spots of light around the bright galaxy blob in the middle all come from the same exploding star (Image: Patrick L. Kelly et al (arXiv:1411.6009 [astro-ph.CO]))

Seeing quadruple? For the first time, astronomers have seen an image of a single supernova split into four by a gravitational lens. The splintered stellar explosion may help calibrate distances across the universe.

Gravitational lenses are the result of massive celestial objects, like stars, galaxies or even dark matter, bending light as it passes near them. Sometimes gravitational lenses produce multiple images of a single object behind them. The effect is similar to looking at a candle through the base of a wine glass.

Exactly 50 years ago, Norwegian astrophysicist Sjur Refsdal suggested that if astronomers can find a supernova whose light had been bent in several directions by a gravitational lens, that might tell them how quickly the universe is expanding. Each image takes a different path to telescopes near Earth, depending on how much mass is in the way. The differences in the time it takes each image to reach Earth is proportional to the universe’s expansion rate.


Despite decades of searching, no such stellar explosions had turned up. But now, Patrick Kelly at the University of California, Berkeley and his colleagues say they’ve found one.

The MACS J1149.6+2223 galaxy cluster (Image: NASA, ESA, and M. Postman (STScI), and the CLASH team)

The supernova appeared in images of the galaxy cluster MACS J1149.6+2223 taken on 10 November by the Hubble Space Telescope. Around one of the giant galaxies in the cluster, the researchers saw the smeared arcs of a distorted, smaller spiral galaxy behind it. Embedded within those smears were four bright light sources. The wavelengths of the light in those arcs suggest a single origin, the same distance from Earth, meaning the bright light sources are probably all images of the same star within the spiral galaxy. Because the object did not appear in earlier images of the same galaxy cluster, the researchers think it is the bright, fatal explosion of a supernova.

“This is a fantastic discovery,” says Robert Quimby at the University of Tokyo, Japan. “The authors make a good case that this is a supernova seen through a gravitational lens.”

Universal speed-up

Supernovae come in several varieties, but one kind, called a type Ia, has been an important tool for astronomers. These explosions put out nearly the same amount of light, no matter where in the universe they occur. So when scientists find a type Ia supernova, they can tell how far away it is by how much of this light makes it to Earth, or how bright it appears. These supernovae revealed to astronomers in the late 1990s that the universe’s expansion is speeding up – a discovery that received the 2011 Nobel prize in physics.

Kelly’s team doesn’t yet know what kind of supernova they found, but it could help put the strongest limits yet on cosmological parameters if it is. Because the images all come from the same supernova, comparing their time delays would give a more precise value of the supernova’s actual brightness than any distant type Ia supernova measured yet. That value would help them better gauge distances to all such blasts across the universe, and pin down the universe’s expansion rate.

It can also give an estimate of the mass of the lensing galaxy or cluster, and help map the distribution of matter in the cluster, says Brian Schmidt of the Australian National University, one of the 2011 Nobel laureates. That could give astronomers a bead on the nature of dark matter. “We can see if dark matter and atoms are distributed as expected,” he says. “Seriously cool discovery!”

Journal Reference: ArXiv: arxiv.org/abs/1411.6009, submitted to Science