While most stars astronomers observe reside within galaxies, intergalactic stars drift independently, unbound by any galaxy's gravity. These solitary stars vary in distance from Earth—but which holds the record for farthest detected?
The champion is MACS J1149 Lensed Star 1, commonly known as Icarus, a brilliant blue supergiant with a redshift of z = 1.49. This places it approximately 14 billion light-years away, corresponding to a lookback time of 9.34 billion years. Its light reached us 4.4 billion years after the Big Bang.
Icarus outshines the runner-up, SDSS J1229+1122, by hundreds of times in distance, securing its title as the most distant individual star observed to date.
Astronomer Patrick Kelly from the University of Minnesota and his team spotted Icarus in Hubble Space Telescope data on supernova SN Refsdal during April and May 2016. Initially noted as a bright spot in 2013 images, it flared dramatically in 2016. Detailed analysis revealed it as a single star, magnified over 2,000 times by gravitational lensing.
Related topic: What are the most distant objects ever discovered in the observable Universe?
This lensing arose from two effects: the massive galaxy cluster MACS J1149+2223, about 5 billion light-years from Earth, provided the primary amplification. A secondary microlens—likely a 3-solar-mass star or black hole within the cluster—fine-tuned the alignment for Hubble's view.
At such vast distances, only galaxies, supernovae, or quasars are typically visible. Gravitational lensing alone made Icarus detectable. Spectral analysis confirmed its stability (ruling out a supernova) and classified it as a blue supergiant with temperatures between 10,000 and 12,000 °C.
Emitting light when the universe was just 30% of its current age, Icarus has since evolved beyond its blue supergiant phase, given typical stellar lifecycles.
As detailed in their Nature Astronomy paper, the researchers note that combining cluster-scale lensing with microlensing could refine dark matter models with unprecedented precision.
