We've got a new distance record! Hubble has gazed across 9 billion light years and spotted the most distant star ever seen. Nicknamed Icarus, it's a massive blue supergiant but so faint that even the biggest telescopes would never have detected it were it not for a lucky accident. A massive foreground galaxy cluster happens to lie in the same line of sight. The powerful gravity exerted by the cluster acted as a "lens" to magnify its light and lift the star from the shadows.
Icarus appears to us as it did when the universe was about 30 percent of its current age. Since blue supergiants are massive, hot stars with much shorter lifetimes compared to smaller, cooler stars like the sun, the star is long gone, having ended its life billions of years ago as a supernova. We see Icarus in the deep past, 9 billion years ago, when it was just a young thing.
"This is the first time we're seeing a magnified, individual star," explained former University of California at Berkeley postdoc and study leader Patrick Kelly now of the University of Minnesota, Twin Cities. "You can see individual galaxies out there, but this star is at least 100 times farther away than the next individual star we can study, except for supernova explosions."
The cosmic quirk that makes this star visible is a phenomenon called gravitational lensing. Einstein predicted that massive objects bend and warp the fabric of space. The more massive the object, the more severe the warping. You can picture this by imagining a child standing on a trampoline. Her weight makes a small depression in the rubber material, while a 200-lb. adult creates a much deeper dip in the fabric. In a similar way — except in all three dimensions, not just a surface — gravity from a foreground, massive cluster of galaxies curves the fabric of space into a sort of lens, what astronomers call a gravitational lens. This natural lens bends and magnifies the light from objects in the distant background making impossibly faint object bright enough to see.
In the case of Icarus, a natural "magnifying glass" is created by a galaxy cluster called MACS J1149+2223. Located about 5 billion light-years from Earth, this massive cluster of galaxies sits between the Earth and the galaxy that contains the distant star. By combining the strength of this gravitational lens with Hubble's keen resolution and sensitivity, astronomers can see and study Icarus.
The star's official name is the unwieldy MACS J1149+2223 Lensed Star 1, so the team dubbed it "Icarus," after the Greek mythological character who flew too near the Sun on wings of feathers and wax that melted. Much like its namesake, the star's glory was fleeting — it momentarily skyrocketed to 2,000 times its true brightness when magnified before fading away.
Normally, the galaxy cluster would have increased its brightness by just 600 times. Astronomers suspect the extra boost came from an individual star with a mass similar to the sun that moved exactly in front of Icarus at the time. Even a single star can also act as gravitational lenses and jack up a background's star brightness when the alignment is just so.
The discovery was made in 2016, when the team was monitoring a supernova in Icarus' home galaxy. They spotted a new point of light not far from the magnified supernova. From the position of the new source, they calculated that it should have been much more highly magnified than the supernova. After analyzing its light they discovered it was a blue supergiant, a star much larger, massive and probably hundreds of thousands of times intrinsically brighter than the sun. But it was still too dim at this distance to see without the help of gravitational lensing. Rigel, located in the "knee" of Orion diagonally opposite Betelgeuse, is a blue supergiant and one of the brightest stars in the sky.
Kelly and his team knew Icarus wasn't a supernova because the object no heating, explosion or flying debris was detected. When NASA's next generation James Webb Space Telescope is launched, astronomers expect to find many more stars like Icarus. Because the alignment that caused the brightening of Icarus was temporary, one wonders if we'll ever see it again.