When it comes to viewing far-flung stars, astronomers are usually limited to studying them within groups – or galaxies – or as supernovae. But thanks to a rare cosmic alignment, astronomers have been able to view the most distant individual normal star ever observed. Located some 9 billion light years from Earth, the star was captured by the Hubble Space Telescope thanks to gravitational lensing amplifying the star's feeble glow.

Gravitational lensing is a quirk of nature whereby light from a distant object is bent by the immense gravity of a massive cluster of galaxies located between the object and the observer, magnifying the distant object. It was galaxy cluster MACS J1 149+2223, which sits 5 billion light years away from us, that researchers were originally observing when a flickering light was noticed in the background of images captured by Hubble.

Upon closer inspection, they found the flickering light was coming from a single blue supergiant that has been dubbed Icarus, but whose formal name is MACS J1149 Lensed Star 1. While gravitational lensing typically magnifies galaxies by up to 50 times, the researchers say that the gravity of the galaxy cluster in the foreground magnified the brightness of Icarus more than 2,000 times – and its brightness could potentially be magnified by as much as 10,000 times over the next decade as the cluster stars move around.

"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 nine billion light years," says Alex Filippenko, a professor of astronomy at UC Berkeley and one of many co-authors of the study. "These lenses are amazing cosmic telescopes."

The researchers say Icarus is much larger, more massive, hotter and possibly hundreds of thousands of times brighter than our Sun. Patrick Kelly, who first noticed the star in 2014 while using Hubble to monitor a supernova, also saw a second star in the Hubble image. This could be a second star being gravitationally lensed, or a mirror image of Icarus.

"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," says Filippenko. "For this type of research, nature has provided us with a larger telescope than we can possibly build!"

In addition to providing a way to study individual stars in far-flung galaxies and giving new insights into how stars evolve, the technique also has implications for researchers studying dark matter. Already, the light from Icarus has allowed scientists to discount one possibility for the explanation of dark matter – that it is mostly made up of a large number of primordial black holes with masses tens of times larger than the Sun lurking within galaxy clusters.

Researchers anticipate that many more stars will be discovered using gravitational lensing once the James Webb Space Telescope becomes operational.

The international team's paper about the discovery appears online this week in an advance publication of Nature Astronomy.

Sources: UC Berkeley, Kavli IPMU, HubbleSite

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