WHL0137-LS

WHL0137-LS, also known as Earendel, is a star located in the constellation of Cetus. Discovered in 2022 by the Hubble Space Telescope, it is the earliest and most distant known star, at a comoving distance of 28 e9ly. The previous furthest known star, MACS J1149 Lensed Star 1, also known as Icarus, at a comoving distance of 14.4 e9ly, was discovered by Hubble in 2018. Stars like Earendel can be observed at cosmological distances thanks to the large magnification factors afforded by gravitational lensing, which can exceed 1,000. Other stars have been observed through this technique, such as Godzilla.

Observation
Earendel's discovery by the Hubble Space Telescope was reported on 30March 2022. The star was detectable due to gravitational lensing caused by the presence of the galaxy cluster WHL0137-08 between it and the Earth, concentrating the light from the star. Computer simulations of the lensing effect suggest that Earendel's brightness was magnified between one thousand and forty thousand times. The dates of Hubble's exposure to the star's light were 7June 2016, 17July 2016, 4November 2019, and 27November 2019. The star was nicknamed Earendel by the discoverers, derived from the Old English name for 'morning star' or 'rising light'. Eärendil is also the name of a half-elven character in one of J. R. R. Tolkien's books, The Silmarillion, who travelled through the sky with a radiant jewel that appeared as bright as a star. NASA astronomer Michelle Thaller confirmed that the reference to Tolkien was intentional. The star's host galaxy, WHL0137-zD1, was nicknamed "Sunrise Arc", because gravitational lensing distorted its light into a long crescent.

Further observations by Hubble and the James Webb Space Telescope have been proposed to better define the properties of the star. James Webb's higher sensitivity is expected to allow the analysis of Earendel's stellar spectra and determine whether it is actually a single star. The spectral analysis would reveal the presence of elements heavier than hydrogen and helium, if any.

On 30 July 2022, an image of Earendel was captured by the James Webb Space Telescope during its first imaging campaign of the star.

On 8August 2023, the colors of Earendel were detected, and an image was captured by both the Hubble and Webb telescopes. Based on Webb's NIRCam data, Earendel is a "massive B-type star more than twice as hot as our Sun, and about a million times more luminous". Webb's observations, particularly through the Near-Infrared Camera, revealed intriguing details, including hints of a cooler, redder companion star. This discovery offers insights into the universe's distant past, mirroring the early stages of galactic evolution.

Physical properties
The light detected from Earendel was emitted 900 million years after the Big Bang. The star has been determined to have a $6.2$ redshift, meaning the light from Earendel reached Earth 12.9 billion years later. Due to the expansion of the universe, the star's observed position is now 28 billion light-years away. The previous most distant star, MACS J1149 Lensed Star 1, has a redshift of 1.49, and is now 14.4 billion light-years away.

If it is a single star, Earendel has a temperature of 13,000 – 16,000 K and a luminosity of 631,000 – 3,981,000, depending on the magnification. It is possible that Earendel might not be a single star as the spectral energy distribution of Earendel has a strong Balmer break, which is characteristic of stars with temperatures below 13,000 K and a blue UV slope which is present in stars with temperatures aboves 20,000 K. It is possible that Earendel is a binary with two components, where one is more luminous and much hotter (34,000 K) than the other one (9,000 K). Due to the limited amount of data, the parameters are not well constrained. If two stars are in the system, they could have different magnifications, which makes the parameters even more uncertain.