Cosmic Lens Helps Hubble Capture Image of Most Distant Star Ever Seen

Gravitational lensing — the bending of light by massive galaxy clusters in the line of sight — has occurred during a rare cosmic alignment, magnifying a star more than 2,000 times. The phenomenon allowed astronomers to capture an image of a star 9 billion light years from Earth.

The discovery of the star, often referred to as Icarus rather than by its formal name, MACS J1149 Lensed Star 1, kicks off a new technique for astronomers to study individual stars in galaxies formed during the earliest days of the universe.

Icarus, whose official name is MACS J1149+2223 Lensed Star 1, is the farthest individual star ever seen. It is only visible because it is being magnified by the gravity of a massive galaxy cluster, located about 5 billion light-years from Earth. Called MACS J1149+2223, this cluster (left) sits between the Earth and the galaxy that contains the distant star. The astronomy team had been using the Hubble Space Telescope to monitor a supernova in the far-off spiral galaxy when, in 2016, they spotted a point of light near the supernova that began to brighten. Even though the object subsequently became three times brighter in one month, the colors of the light coming from the object did not change. Analysis of these colors showed it was a blue supergiant star in the background galaxy whose magnification grew for several weeks because of an intervening object, probably a star, in the galaxy cluster. The panels at the right show the view in 2011, without Icarus visible, compared with the star's brightening in 2016. Courtesy of NASA, ESA and P. Kelly, University of Minnesota.

“For the first time ever we’re seeing an individual normal star — not a supernova, not a gamma ray burst, but a single stable star — at a distance of 9 billion light years. These lenses are amazing cosmic telescopes,” said Alex Filippenko, professor at University of California (UC), Berkeley.

Patrick Kelly, former postdoc at UC Berkeley and now at the University of Minnesota, noticed the star while monitoring a supernova, SN Refsdal. Suspecting that Icarus might be more highly magnified than SN Refsdal, Kelly and his team analyzed the colors of the light coming from Icarus and discovered it was a single star, a blue supergiant. This B-type star is much larger, more massive, hotter and possibly hundreds of thousands of times intrinsically brighter than our sun.

By modeling the lens, the team concluded that the tremendous apparent brightening of Icarus was probably caused by a unique effect of gravitational lensing. An extended lens, like a galaxy cluster, can only magnify a background object up to 50×, but smaller objects can magnify much more. A single star in a foreground lens, if precisely aligned with a background star, can magnify the background star thousands of times. In this case, a star about the size of our sun briefly passed directly through the line of sight between the distant star Icarus and the Hubble Space Telescope, boosting its brightness more than 2,000×.

That single star within the cluster turned the light from the distant star into an “Einstein ring” — a halo of light created when light from the distant star bends around all sides of the lensing star. The ring is too small to discern from this distance, but the effect made the star easily visible by magnifying its apparent brightness.

“There are alignments like this all over the place as background stars or stars in lensing galaxies move around, offering the possibility of studying very distant stars dating from the early universe, just as we have been using gravitational lensing to study distant galaxies,” Filippenko said. “For this type of research, nature has provided us with a larger telescope than we can possibly build.”

The research was published in Nature Astronomy (doi:10.1038/s41550-018-0430-3).