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Astronews: Hubble Space Telescope Found the Most Distant Star Ever Seen

Most stars in the night sky visible to the naked eye are within a few hundred light years from us. By using an astronomical telescope, it is not difficult for us to observe star clusters tens of thousands light years away or galaxies million light years away. Star clusters and galaxies are celestial objects consisting of a large number of stars and their luminosity is high enough to be seen. In a recent analysis of the observational data from the Hubble Space Telescope, astronomers found that Hubble had observed a star 9 billion light years away from Earth in 2016. Apart from supernovas, this star is the farthest single star that has ever been observed by humans.

two photos taken by Hubble Space Telescope in 2011 and 2016
Credits: NASA, ESA, and P. Kelly (University of Minnesota)
The above image compares two photos taken by Hubble Space Telescope in 2011 and 2016 respectively. A star can be seen clearly in the photo (bottom right) taken in 2016.

This star is also called Icarus (its official name being MACS J1149+2223 Lensed Star 1). Astronomers discovered the star by a means called gravitational lensing. Between Icarus and the Earth is a galaxy cluster which is about 5 billion light years away from the Earth. The gravity of the galaxy cluster bends the light emitted from Icarus, creating a lensing effect that amplifies its brightness. Astronomers estimated that under normal circumstances, the galaxy cluster could only magnify the light from Icarus by 600 times, but possibly a celestial object inside the galaxy cluster had moved between the Earth and Icarus coincidentally, creating an extra gravitational lensing effect that amplified its brightness by 2000 times. Thus, the Hubble Space Telescope was able to observe it.

Schematic diagram of the gravitational lensing effect
Schematic diagram of the gravitational lensing effect. Light is bent after passing a gravitational field, just like a convex lens refracting light.

Astronomers analysed the spectrum of Icarus and found that it was a blue supergiant star. This type of star is much larger and more massive, hotter, and brighter than our Sun. Given at such a huge distance, even with Hubble Space Telescope, it would still be impossible to observe it without the amplification of gravitational lensing.

Comparison of a Model of a Blue Supergiant's Spectrum with Observational Data
Credits: NASA, ESA, and A. Feild (STScI)
Comparison of a Model of a Blue Supergiant's Spectrum with Observational Data.

Scientists found that the spectral data of Icarus matches the model for a blue supergiant. The solid blue line shows the model spectrum of the blue supergiant which is 9 billion light years away. The red dots are the actual data measured for Icarus. Due to a phenomenon called redshift, the observed wavelength of the Balmer discontinuity (originally at about 365 nm) in the stellar spectrum is shifted to nearly 1000nm. This phenomenon helps to reveal the distance of the star, whereas the strength of the Balmer discontinuity reveals the strength of the star's gravity at its surface and its temperature.

Reference:
https://www.nasa.gov/feature/goddard/2018/hubble-uncovers-the-farthest-star-ever-seen