It might look like a speck amid a bejewelled vista of the universe, but scientists say a pinprick of light in an image captured by the Hubble space telescope is the most distant individual star ever seen that is not a supernova. The team behind the find say the light was emitted from the star – dubbed Icarus but officially named MACS J1149+2223 Lensed Star 1 – when it was more than 9bn light years from Earth. Icarus is now much further away but will have died, forming either a black hole or a neutron star. “We are looking back three-quarters of the way almost to the big bang,” said Dr Patrick Kelly, first author of the research from the University of Minnesota. Stars at such distance are normally too faint to be identified individually unless they explode in a supernova. But it seems a chance alignment of the heavens made Icarus visible. “It’s more than 100 times farther away than the next most distant individual star we can observe,” said Kelly.
Writing in the journal Nature Astronomy, an international team of researchers reveal how their curiosity was piqued in 2016. The team were studying a supernova known as SN Refsdal in a galaxy more than 9bn light years away when they noticed a pinprick of light that appeared four times brighter than in previous images. This seemed to come from an object in the same galaxy as the supernova and appeared in the environs of a well-known galaxy cluster that lay just over 5bn light years from Earth. “As we monitored the cluster due to SN Refsdal, we obtained imaging of the cluster regularly, and we saw the ‘Icarus region’ brightening up,” said Dr Mathilde Jauzac, another author of the study from Durham University. The speck, the team determined, was down to a star whose brightness had been magnified by a phenomenon known as gravitational lensing whereby light emitted by the star is bent by the gravitational effect of objects in front of it – in this case, the galaxy cluster.
The increase in brightness of Icarus, the researchers say, is due to an additional magnification boost from a star within the galaxy cluster. “Usually the stars [are magnified] by about 600 [times]; that is just due to the cluster itself. But sometimes a star floating in the middle of a cluster will also get in the right place, and that will contribute additional magnification,” said Kelly. “That is what was responsible for the brightening of [Icarus] by a factor of four.” The upshot, says Kelly, is that overall Icarus was magnified more than 2,000 times.
Analysis of the light emitted by the star revealed it to be a blue supergiant, a rare type of star that is larger than the Sun and far more luminous – a well-known example is a star known as Rigel in the constellation of Orion. The work also allowed the team to test one theory regarding the possible nature of dark matter, which suggested it could be made of black holes. If that were the case, these objects within the cluster galaxy would also be expected to boost the brightness of Icarus. However, looking back on Hubble images over more than a decade, the team found the brightness of Icarus did not vary in a way that would be expected if the dark matter in the cluster of galaxies was made of black holes. “The light fluctuations that we have observed do not agree with predictions from such theory,” said Jauzac.
Credit: Nicola Davis for The Guardian, 2 April 2018.