Earendel, which means “morning star” in Old English, is the most distant star ever seen and was discovered with the Hubble Space Telescope in 2022. The light of this star, which existed when the universe was only 9.5 million years old or 7% of its current age, took about 12.9 billion years to reach us.
Reference: Hubblesite.org
Seeing a star this far away is impossible with the current telescopes; however, helped by the physical phenomena known as gravitational lensing, astronomers were able to reach far, into the distant universe. With the help of this phenomenon, astronomers have observed many distant galaxies; however, identifying a star is not common, making this discovery something quite amazing. Brian Welch, the astronomer responsible for this discovery and at that time still a doctoral student at the John Hopkins University in Baltimore, Maryland, explained in the Hubble News Release[1] how amazing this discovery was. The team could not believe it at first. Normally all distant galaxies observed via gravitational lenses are small distorted galaxies that look like smudges due to the light produced by millions of stars blending together. However, Earendel appeared only a single time in the lens and was quite defined as compared to the host galaxy that appeared several times in a distorted long curve that they called Sunrise Arc. The name was chosen because its light comes from what we call the early universe.
![Close-up image of Earendel, the farthest individual star ever observed. Traditional telescopes cannot reach stars at this extreme distance. However, the phenomenon of gravitational lensing, a cosmic occurrence, enabled astronomers to probe the depths of the distant universe. Gravitational lensing has previously been used to observe remote galaxies, but the identification of a single star within this context is exceptionally rare and remarkable. Brian Welch, the astronomer responsible for this groundbreaking discovery, was a doctoral student at John Hopkins University in Baltimore, Maryland, at the time. In the Hubble News Release[1], Welch described the astonishment of the team upon making this find. Unlike typical distant galaxies observed through gravitational lenses, which often appear as smeared, distorted smudges due to the confluence of millions of stars' light, Earendel presented as a distinct, solitary point of light within the lensing effect. In contrast, the host galaxy appeared multiple times, creating a distorted, elongated curve known as the Sunrise Arc, aptly named for its connection to the early universe](https://static.wixstatic.com/media/59234a_7acb412353ef4ce6a1ef67569ff3a45f~mv2.jpg/v1/fill/w_147,h_183,al_c,q_80,usm_0.66_1.00_0.01,blur_2,enc_auto/59234a_7acb412353ef4ce6a1ef67569ff3a45f~mv2.jpg)
Reference: Hubblesite.org
The gravitational lens, and hence the distortion, is produced by a huge galaxy cluster called WHL0137-08 sitting between us and the host galaxy. However, to achieve this magnification, the rest of the stars within the host galaxy also had to align in a particular way to contribute to the final brightening that allowed the team to find Earendel[2]. This star is directly on, or extremely close to, a ripple in the fabric of space, which in optics is defined as a “caustic,” magnifying the light of Earendel thousands of times; result that Welsh confirmed using four independent lensing models [3]. The rippling, and resulting magnification, can be better understood if we think about the ripples on the surface of a swimming pool. On a sunny day, the ripples on the surface act as lenses, focusing the sunlight to maximum brightness on the pool's floor. Because the alignment has to be perfect and the stars in the galaxy move, Earendel will only be visible for a few years.
![Image featuring a clustered arrangement of stars, including Earendel, with a magnification line indicating gravitational lensing. The gravitational lens, responsible for the observed distortion, is generated by the massive galaxy cluster known as WHL0137-08, positioned between us and the host galaxy. Achieving this level of magnification required a precise alignment of stars within the host galaxy, contributing to the final intensity increase that facilitated the discovery of Earendel[2]. This star is situated directly on or in extremely close proximity to a spatial ripple, referred to as a 'caustic' in optics, resulting in the magnification of Earendel's light by thousands of times. This phenomenon was verified through four independent lensing models[3]. To grasp the concept of rippling and subsequent magnification, imagine the ripples on the surface of a sunlit swimming pool acting as lenses, concentrating sunlight to its maximum brightness on the pool's floor. Perfect alignment is crucial, and as stars in the galaxy shift, Earendel's visibility is limited to just a few years](https://static.wixstatic.com/media/59234a_62543524b29d41b79f7b68f1107c7d3b~mv2.jpg/v1/fill/w_113,h_88,al_c,q_80,usm_0.66_1.00_0.01,blur_2,enc_auto/59234a_62543524b29d41b79f7b68f1107c7d3b~mv2.jpg)
Reference: Hubblesite.org
Welsh also explained that by studying Earendel, we can get more familiar with star formation in the early universe, when we didn't have all the raw materials as the stars around us today[4]. The observations that led to this discovery were part of a survey called RELICS or “Reionization Lensing Cluster Survey,” which proposed to observe with the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS) on board of HST, 46 fields exceptionally lensed by 41 galaxy clusters. The observation of Earendel was done with the WFC3 infrared detector. Although the team could not determine if Earendel is one single star or a binary star system, if it is one star the team estimated it would be consistent with a star of mass greater than 50 times the mass of the Sun.
Most recently, the team revealed the results of the observations of Earendel and its host galaxy using the James Webb Space Telescope (JWST)[4]. With the resolving power of JWST and using the Near Infrared Camera (NIRCam), the team was able to determine that Earendel is a massive B-type star, more than twice as hot as the Sun, and about a million times more luminous. Although JWST was also unable to reveal if this star has a companion, the observations of Earendel made with different filters show a hint of a cooler and redder companion star. The images of JWST also reveal that the host galaxy has young star-forming regions and older established star clusters as small as ten light-years across. The dots on either side of Earendel are two images of the same star cluster, estimated to be at least 10 million years old. The elongated regions in the arch are forming star regions estimated to be less than 5 million years old[6].
![Close-up image of Earendel, providing detailed observations of this remarkable star and its host galaxy. Most recently, the team harnessed the power of the James Webb Space Telescope (JWST)[4] to explore Earendel and its surroundings. Employing the Near Infrared Camera (NIRCam) aboard JWST, they ascertained that Earendel is a massive B-type star, boasting a temperature over twice that of the Sun and radiating approximately a million times more luminosity. While JWST couldn't definitively determine if Earendel has a companion, observations using various filters hint at the presence of a cooler, redder companion star. The JWST images also unveil young star-forming regions within the host galaxy, alongside well-established star clusters as compact as ten light-years across. Notably, the dots flanking Earendel represent two images of the same star cluster, estimated to be at least 10 million years old. The elongated regions forming an arch signify nascent star-forming regions with an estimated age of under 5 million years](https://static.wixstatic.com/media/59234a_be106f2db1aa4f34aa51ebec1d0bac28~mv2.jpg/v1/fill/w_147,h_75,al_c,q_80,usm_0.66_1.00_0.01,blur_2,enc_auto/59234a_be106f2db1aa4f34aa51ebec1d0bac28~mv2.jpg)
Reference: webtelescope.org
But this is not the end of the story, the team is now analyzing observations of the Sunrise Arc galaxy and Earende taken with the Near-Infrared Spectrograph (NIRSpec) instrument in JWST. With these observations, the team looks to obtain precise composition and distance measurements for the galaxy. We will update this blog once the results of these observations become available.
References
[1] https://hubblesite.org/contents/news-releases/2022/news-2022-003
[2] https://hubblesite.org/contents/media/images/2022/003/01FWS5FJ468Q4HV8C67PEQPVND
[3] Welch, B., Coe, D., Diego, J.M. et al. A highly magnified star at redshift 6.2. Nature 603, 815–818 (2022). https://doi.org/10.1038/s41586-022-04449-y
[4] “https://www.youtube.com/embed/BS0NOgYtkZU”
[5] https://webbtelescope.org/contents/news-releases/2023/news-2023-132?news=true
[6] Welch et al. 2022, ApJ Letters, 940, L1