Hubble spots farthest star ever seen

Astronomers using NASA’s Hubble Space Telescope have discovered an enormous blue star, nicknamed Icarus, located more than halfway across the universe. Icarus is the farthest individual star ever seen. Normally it would be too faint to observe even with the world’s largest telescopes, but the astronomers were able to take advantage of a phenomenon known as gravitational lensing that greatly magnified the star’s faint glow. Astronomers were also able to use Icarus to test out a theory about dark matter, and to examine the make-up of a foreground galaxy cluster.

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. MACS J1149+2223, this cluster, shown at left, sits between Earth and the galaxy that contains the distant star. The panels at the right show the view in 2011, without Icarus visible, compared with the star’s brightening in 2016.

Icarus, located in a distant spiral galaxy and its light took 9 billion years to reach Earth. It appears to astronomers, when the universe was about 30 percent of its current age. Icarus, which was discovered by the use of gravitational lensing has given astronomers a new tool to study individual stars in distant galaxies.

Former University of California at Berkeley postdoc and study leader Patrick Kelly now of the University of Minnesota, Twin Cities said, this is the first time we’re seeing a magnified, individual star. 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.

Gravitational lensing occurs when the gravity of a massive cluster of galaxies (generally in the foreground) bends and amplifies light, acting as a natural lens in space. The light can be extremely magnified, allowing very faint and distant objects to be seen. For Icarus, the gravitational lensing was caused by a galaxy cluster called MACS J1149+2223. This massive cluster of galaxies is about 5 billion light years from Earth, and is situated between Earth and the distant galaxy containing Icarus.  Astronomers can observe and study Icarus by combining the resolution and sensitivity of Hubble with the power of this gravitational lens.

Alex Filippenko, a professor of astronomy at UC Berkeley and one of many co-authors said, 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. These lenses are amazing cosmic telescopes.

Researchers nicknamed the star Icarus (its official name is the mouthful MACS J1149+2223) after the character in Greek mythology who flew too close to the Sun on wings of feathers and wax that melted. Like its namesake, the distant star had a fleeting moment of glory, glowing more than 2,000 times its normal brightness when temporarily magnified. This tremendous brightening was most likely due to the gravitational amplification of a star about the same mass as our Sun, in the foreground galaxy cluster when the star moved in front of Icarus. The foreground cluster’s mass usually magnifies Icarus’s light approximately 600 times.

By studying the contents of the foreground galaxy cluster, researchers were able to test a theory that dark matter may be composed largely of primordial black holes with masses greater than that of the Sun. The results of their observations did not support this hypothesis, because light fluctuations from the background star, measured by Hubble for 13 years, would have looked different if there were a swarm of black holes in the foreground galaxy cluster.

Scientists found that the Hubble data from MACS J1149+2223 Lensed Star 1 (Icarus) matches the model for a blue supergiant. The agreement shows a remarkably good fit, and indicates that Icarus is approximately twice as hot as the Sun. The solid blue line shows the model spectrum of the blue supergiant, adjusted for the distance to the host galaxy of the highly magnified star. The red diamonds are the actual data measured for Icarus. 

Astronomers hope to find many more stars like Icarus when NASA’s James Webb Space Telescope (JWST) is launched (it is hoped the JWST will fly in 2020).  The extreme sensitivity of the JWST should allow for the collection of more details, including whether-or-not these distant stars are rotating. Magnified distant star like Icarus may even be found to be fairly common.

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