The Hubble Space Telescope has revealed a single star whose light has traveled for 12.8 billion years to Earth, having come from a universe just 900 million years old. It’s currently the most distant star known, and the team has dubbed it Earendel*.
The discovery is a huge jump, as the previous record-holder for more distant star existed in a universe 4 billion years old.
The iconic observatory had some help from nature’s own optics: The vast mass of a foreground cluster of galaxies, sitting just so between us and the distant star, acts like a lens, its gravity magnifying the star’s light thousands-fold. The discovery is published in the March 31st Nature.
A Star Among Galaxies
Astronomers had already found the star’s distant host galaxy among the nearer galaxies of the foreground cluster in earlier Hubble observations, taken in 2016 as part of the Reionization Lensing Cluster Survey (RELICS). The team followed up with additional Hubble images in 2019.
The cluster’s gravitational lens had distorted the distant galaxy’s light into a long, thin crescent that the team nicknames “Sunrise Arc” in the current study. (Look through the bottom of a wine glass to see a similar arcs on less astronomical scales.)
But not all of the galaxy is magnified and distorted in the same way. Just as light passing through rippled water produces ripples of its own on the bottom of the swimming pool, the galaxy’s light ripples too, with some parts of it more highly magnified than others. The most highly magnified points are those on what astronomers call the critical curve.
“As objects approach the critical curve, their magnification increases,” Welch says. “For very small objects, such as stars, they can occasionally align so precisely with the critical curve that their magnification increases dramatically, up to a factor of thousands.”
Welch and his colleagues fit the Hubble images with multiple gravitational-lens computer models. They keyed the models to other star clusters in the Sunrise Arc galaxy, two of which appear on either side of Earendel, their light split into multiple images. (Only one of these clusters is labeled in the image above.) The star itself isn’t split, which suggests it’s nearer to the critical curve than the star clusters are. While the models disagree on the exact amount of magnification, all of them agree that the star is magnified by at least 1,000 times and maybe up to 40,000 times.
The models also calculate the size of the star: less than about 2 light-years across and possibly far smaller than that. The size rules out the possibility that what Hubble has spotted is a star cluster — it’s more likely to be a single star.
Earendel: An Early Star
Based on Earendel’s brightness, the group estimates that it’s massive, containing the equivalent of tens or even hundreds of solar masses, with few elements other than hydrogen and helium. Given the age of the universe it resides in, Earendel is not likely to be one of the very first stars, known as Population III, but at its great mass, it’s nevertheless an early fuser of new heavy elements.
“Knowing that massive stars were present that early on in the universe is an important confirmation of what many of us have predicted and in fact used as assumptions in the work with old Milky Way stars,” says Anna Frebel (MIT), who was not involved in the study. For Frebel, who studies ancient nearby stars for clues to stellar evolution in the infant universe, the discovery is a fantastic find. “Early massive stars must have produced the first elements and driven chemical evolution, so having observations at hand that support this notion is wonderful.”
With these preliminary findings in hand, the team will now look to the James Webb Space Telescope to learn the star’s true nature and how it fits into the universe’s evolution. Team member Dan Coe (also at Johns Hopkins) is the principal investigator of an approved request for observations using the new telescope once engineers have finished aligning its optics. JWST images and spectra will improve estimates of the star’s mass, temperature, and spectral type.
*Tolkien fans may recognize this name, though spelled slightly differently as Eärendil, the half-elf mariner who carried a heavenly jewel into the sky in The Silmarillion. Tolkien found inspiration for his works in the 10th century poem Crist, one line of which (translated from Old English) reads: “Hail Éarendel, brightest of angels, sent over middle-earth to mankind.” The nature of Éarendel is debated but it is commonly identified as the “morning star.”
“I'm a huge Tolkien nerd, so that was one of the first name ideas that came to mind for a distant star,” explains Welch. “When I looked into it further and found that the old English word Earendel actually refers to a morning star, I was pretty much sold on the name.”