While the vast majority of stars observed by scientists are grouped within galaxies, some of them, called intergalactic stars, are gravitationally unrelated to any galaxy. These individual stars may be at a greater or lesser distance from Earth. Who currently holds the distance record?
The record holder responds to the name of MACS J1149 Lensed Star 1, or more simply Icarus. It is a blue supergiant star (hot and luminous) at a redshift z =1.49, or a distance of about 14 billion light-years from Earth, for a cosmic time of 9.34 billion years. The light from the star was therefore emitted 4.4 billion years after the Big Bang.
Hundreds of times farther than the second most distant star — SDSS J1229+1122, Icarus is therefore the most distant individual star ever observed.
It was while studying data relating to the supernova SN Refsdal, collected with the Hubble Space Telescope, that astronomer Patrick Kelly of the University of Minnesota and his colleagues discovered Icarus in April and May 2016.
The supernova had been studied since 2004, but astrophysicists spotted a bright spot in their 2013 images, which became much brighter in 2016. After analysis, it turned out to be a magnified individual star more than 2000 times by a gravitational lensing effect.
Related topic:What are the most distant objects ever discovered in the observable Universe?
This gravitational lensing effect was twofold. The galactic cluster MACS J1149+2223, located about 5 billion light-years from Earth, was the main source of the phenomenon. But a second object of about 3 solar masses, located on the line of sight of the Hubble telescope in the galactic cluster itself, perhaps a star or a black hole, also contributed to the phenomenon for a lesser part (microlens ).
Ordinarily, the only objects that can be observed at this distance are galaxies, supernovae or quasars. It was only the gravitational lensing effect that allowed astronomers to detect Icarus. By analyzing the characteristics of the star, the researchers determined that it was stable — thus ruling out the supernova hypothesis — and that it was a blue supergiant, given its high temperature (between 10 '000 and 12'000 °C).
Since the light from Icarus was emitted when the Universe was only 30% of its current age, the star is currently no longer a blue supergiant given the lifespan and evolutionary pattern of this type of star.
As the researchers point out in their article published in the journal Nature Astronomy , the combined use of a strong gravitational lens (galactic cluster) and a microlens (individual compact object) could help put much more precise constraints on current dark matter models.
