Astronomers are like the detectives of the cosmos, searching for clues to mysteries in the universe with an ever-growing toolkit of telescopes, spectroscopes, robotic probes, and other instruments. But sometimes, the universe chips in with a gadget of its own: a galactic magnifying glass.
An international team of astronomers has discovered a distant supernova that was magnified in brightness and split into four separate images as its light passed through a galaxy on its way to Earth. The light from the exploding star, called iPTF16geu, took about 4.3 billion years to reach Earth and show up in telescopes—that’s nearly as old as the Earth itself. As it traveled, the light reached a galaxy located about 2 billion light-years away from Earth. The galaxy’s gravitational field bent the light rays around it as they passed, magnifying and swinging them into different paths, a phenomenon known as gravitational lensing. When they finally reached Earth, they appeared as four different orbs around the galaxy—four different images of the same cosmic explosion:
The astronomers say the galaxy also magnified the light from the supernova by a factor of 52.
“I've been looking for a lensed supernova for about 15 years. I looked in every possible survey, I've tried a variety of techniques to do this and essentially gave up, so this result came as a huge surprise,” said Ariel Goobar, a University of Stockholm professor and lead author of the study published Thursday in the journal Science.
The star of the photo is a Type Ia supernova, a class of supernovae that produce a consistent amount of light. Type Ia are known as “standard candles,” which means they’re reliable measuring sticks of distance in the cosmos. Since astronomers know how bright these supernovae can get, they can analyze their brightness to calculate their distance from Earth. The dimmer the glow, the farther the supernova. Astronomers can also use the light of Type Ia supernovae to peer deeper into space and measure vast interstellar distances. The light from each of the four images of the iPTF16geu took different paths around the galaxy to reach Earth, so each arrived at a different time. Astronomers can measure those arrival times to estimate how fast the universe is expanding. These estimates could also provide information about the amount of dark energy in the universe, which astronomers believe is helping speed up the process.
Astronomers first detected iPTF16geu last September using the Samuel Oschin Telescope, a wide-field camera at the Palomar Observatory in California. Goobar took a closer look at its starlight and confirmed it was a Type Ia supernova, but was surprised when he estimated it to be more than 4 billion light-years away. Given the extreme distance, the supernova shouldn’t have appeared as bright as it did. Goobar and his team, being astronomer-detectives, turned to more precise tools and got observation time with the Hubble Space Telescope, the Keck Observatory in Hawaii, and the Very Large Telescope array in Chile.* The resulting high-resolution data allowed astronomers to make detailed observations of the supernova, the galaxy in which it was exploding, and the galaxy that curved its starlight as it traveled through space.
The Palomar Observatory detects supernovae every day, but astronomers say capturing an image of a gravitationally lensed Type Ia is rare, with about one in every 50,000 supernovae they identify. Astronomers hope to detect similar events when the Zwicky Transient Facility, another wide-field camera, begins operating at Palomar in August. Zwicky will be capable of scanning the entire visible sky each clear night, out-powering the Oschin telescope, which could observe only one-fifteenth. But even with powerful technology, astronomers still need the universe’s help. To get a picture like this one, an object big enough to bend light must be precisely lined up with the far-flung target they hope to observe. The stars must, quite literally, align.
* This article originally suggested Ariel Goobar is a woman. We regret the error.