Astronomers have discovered the most distant individual star ever spotted, located nearly 13 billion light years away, in a stunning breakthrough that was achieved using NASA’s Hubble Space Telescope, reports a new study.
Because looking across space is also looking back in time, Hubble saw this star as it was when the universe was only about 900 million years old. The star, nicknamed Earendel after the Old English word for “morning star” or “rising light,” is dramatically farther than the previous record-holder, a star named Icarus, also discovered by Hubble, when the universe was already four billion years old.
A team led by Brian Welch, an astronomer at the Johns Hopkins University, was able to glimpse this ancient star, which is about 50 times as massive as the Sun, thanks to the fortuitous position of a huge galaxy cluster positioned between it and Earth. The gravitational fields of such enormous objects can warp and amplify light from objects situated behind them, an effect known as gravitational lensing. In this case, the “star is magnified by a factor of thousands by the foreground galaxy cluster,” according to a study published in Nature on Wednesday.
“At 900 million years after the Big Bang, it’s a time when the universe looked a lot different and we expect the stars there might look a little bit different too,” Welch said in a call. “It's really a new way of studying this early time.”
The new discovery provides an unprecedented look at an individual star within the first billion years of the universe’s 13.8-billion-year lifespan. Galaxies from this period have been observed before, but it is significantly harder to distinguish an individual star that is so far away and so deep in the past.
For this reason, Welch and his colleagues collected years of observations after they first spotted the star in 2016 with Hubble’s Reionization Lensing Cluster Survey (RELICS), to be sure that they were really seeing a distinct star (or a binary star system). Not only did they confirm the detection, they found that “unlike previous lensed stars, the magnification and observed brightness have remained roughly constant over 3.5 years of imaging and follow-up,” according to the study.
“When we first found it, the main reaction was almost a little bit of disbelief,” said Welch. “We found it by basically identifying that it was at incredibly high magnification, which seemed a little bit unusual, and it's a somewhat different way than previous lensed stars have been detected.”
“It kicked off a long process of guessing and checking and figuring out if there was any other explanation,” he continued. “So at that first moment, we were a little unsure, but once we finally got there, it was really exciting to know that this was something that was so much further than we've been able to see in the past.”
The galaxy that hosts Earendel appears stretched out into a long crescent by the lensing effect, earning it the name the Sunrise Arc. While the discovery is exciting as a standalone event, Welch and his colleagues think that it also raises the possibility of spotting more of these stellar elders in the early days of the universe. Hubble’s successor, the recently launched James Webb Space Telescope (JWST), will be especially useful in the search for stars from this bygone cosmic age.
“We do expect, with the Webb space telescope, that we will be able to find more of these sorts of objects,” Welch said. “Within the first couple of years, at least, of Webb observations, we’re really hoping that we're going to be able to find more objects like this and break this record pretty soon, because it would be really exciting to continue looking further back into the universe with these individual stars.”
These follow-up observations might also be able to determine if Earendel is among the first stars ever born in the universe, a group called Population III which has never been directly observed before. Whereas modern stars are enriched with heavy elements that have been forged in the bellies of their forebears for billions of years, Population III stars are thought to be made almost entirely of hydrogen and helium, with a possible trace of other light elements, making them curious relics rich with information about the evolution of stars.
“It is very unlikely to be a Population III star,” Welch noted. “About 900 million years after the Big Bang is almost certainly enough time for at least one other star to have gone off and slightly enriched the area that this star formed in.”
“If we did find that it was a Population III star, that would obviously be huge,” he continued. “If we were able to confirm that with future observations, that would be the first Population III star ever discovered and confirmed, so it would be an incredibly exciting thing to find. There's a small chance that we'll get that lucky, but I would say, probably, don't count on it.”
Update: This article has been updated to include comments from study lead author Brian Welch.