The single most distant star detected

Icarus (MACS J1149+2223 Lensed Star 1)
Icarus (MACS J1149+2223 Lensed Star 1)

An article published in the journal “Nature Astronomy” describes the observation of the most distant star and therefore also the oldest observed so far, nicknamed Icarus, about 9 billion light years from Earth. A team of researchers exploited a double gravitational lensing effect that magnified the image of the star, which was called MACS J1149+2223 Lensed Star 1 but for that reason it’s simply called Lensed Star 1 (LS1). That effect made it become bright enough to be detectable by the Hubble Space Telescope.

Normally, only the stars of nearby galaxies are visible individually from the Earth, even at observations made with the most powerful space telescopes. Supernovae are a special case because their light can be equal to that of billions of common stars but at distances greater than 100 million light years the light of individual stars is indistinguishable and mixes with that of the others in the same galaxy.

A rare space alignment made it possible to make an observation that is generally impossible. The story began about two years ago, with the observation of the evolution of a supernova with the Hubble space telescope. A gravitational lensing effect generated by the galaxy cluster MACS J1149+2223 helped those observations and the supernova was nicknamed Refsdal after an astronomer who was a pioneer in the field of gravitational lensing.

The surprise came a few weeks later, when a star was observed near the supernova. Patrick Kelly of the University of Minnesota, the first author of the research, explained that the star was magnified just like the supernova by a gravitational lens. It’s a star about 9 billion light years from Earth, almost a hundred times farther than any star normally detectable individually by the instruments in the solar system.

The image (NASA, ESA, and P. Kelly (University of Minnesota)) shows the galaxy cluster MACS J1149+2223, about 5 billion light years from Earth. In the two panels on the right side there’s the area where the star LS1 Icarus is visible in 2016 while it wasn’t in 2011. The combination of the gravitational lensing effect of the galaxy cluster and the supernova Refsdal magnified that star about 2,000 times, allowing the Hubble Space Telescope to detect it.

The analysis of the star LS1 Icarus’ light suggests that it’s a blue supergiant, much more massive and warm than the Sun. This type of star consumes its hydrogen very quickly so, despite its mass, has a short life in astronomical terms, no more than a few tens of millions of years. This means that LS1 Icarus exploded into a supernova long before the Earth formed.

That space alignment also allowed to test the hypothesis that dark matter is actually composed mostly of primordial black holes that formed immediately after the birth of the universe. According to the researchers, in that case the fluctuations of the light coming from the area where the star LS1 Icarus became visible would be different. It will be possible to conduct more tests if other cases of this kind are discovered, especially in the next few years, after the launch of the James Webb space telescope.

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