A quirk of nature that helps magnify extremely distant cosmic objects allowed a space telescope to see a star that existed when the universe was only 900 million years old.
NASA's Hubble observatory made the discovery in 2022, making it the farthest and earliest star ever seen. Astronomers named it Earendel, meaning "morning star" in Old English.
A year later, scientists followed up with the James Webb Space Telescope because it has a larger mirror and collects light at longer infrared wavelengths. At that point, scientists thought they may have spotted a companion star, something they were surprised was technically possible, even with Webb's unprecedented power.
Now, a new study suggests that the astonishingly distant starlight of Earendel may be coming from more than just a single star or close pair. The paper, published in The Astrophysical Journal Letters, analyzes the previous Webb telescope data with computer models and finds a compelling case that Earendel may, in fact, be a star cluster, made up of a collection of stars.
"My secret hope is that it is an individual star," Massimo Pascale, who led the research at UC Berkeley, told Mashable.
Scientists are eager to find and research stars as old as Earendel because such relics may solve mysteries of the ancient universe, thought to be 13.8 billion years old.
Hubble detected Earendel through a phenomenon known as gravitational lensing — when a galaxy cluster in the foreground of a telescope's view magnifies and bends light beyond it. NASA often uses the analogy of a bowling ball placed on a trampoline to illustrate this point, with the ball representing a massive celestial object and the trampoline being the fabric of spacetime. Light that would otherwise travel straight curves gets distorted as it passes through that warped spacetime. It's like adding a more powerful lens to a telescope.
But gravitational lensing also has the potential to replicate or stretch objects, the way a funhouse mirror can create multiple irregular copies of images.
Because galaxy cluster WHL0137-08 is serving as a colossal magnifying glass in the sky, the light of Earendel's incredibly distant galaxy, the Sunrise Arc, popped into Hubble's view. Scientists could see it as it was 12.9 billion years ago. But today, because the universe has also stretched out through cosmic expansion, the Sunrise Arc is estimated to be 28 billion light-years away from Earth.
Astronomers have a lot of experience identifying the effects of gravitational lensing, but that wasn't always the case. In 1987, an enormous blue arc thought to be hundreds of trillions of miles long was at first considered one of the largest things ever detected in the cosmos. Later that year, scientists figured out they were looking at an optical illusion, a distortion caused by a galaxy cluster. The New York Times published a story about the "bizarre" implication of Einstein's General Theory of Relativity, titled "Vast Cosmic Object Downgraded to a Mirage."
For Earendel, some astronomers have continued to wonder whether it is indeed a star. Pascale, now an Einstein Fellow at UCLA, and collaborators decided to reassess its size, which may have been low-balled since estimates didn't account for mini halos of dark matter, a mysterious, yet abundant substance that doesn't shine or interact with light. Such clumps of dark matter could be influencing the magnification. With these effects considered, Earendel's size could be consistent with a star cluster.
The researchers compared Earendel to a widely acknowledged star cluster in the same galaxy, known as 1b. What they found was that Earendel and the 1b cluster have similar features: They're both already between 30 and 150 million years old in Webb's snapshot, they lack heavy elements made by "newer" stars, and they resemble nearby ancient star clusters.
While 1b fit extremely well in star cluster models, so did Earendel, according to the study.
"In order for Earendel to be an individual star, or maybe a binary of two stars very close to each other, the chance alignment of Earendel with the foreground galaxy cluster that's causing the gravitational-lensing effect would have to be an incredible stroke of luck," Pascale told Mashable. "If it is a star cluster, that chance alignment — it doesn't have to be quite as perfect."
Though the new paper doesn't speculate on how many stars could be among such a cluster, Pascale says its mass might be equal to that of hundreds of thousands of suns — or more.
One possible way to answer the question of whether Earendel is a lone star is to watch for flickers. Through multiple observations, scientists might be able to catch the light source suddenly and briefly getting brighter. A star cluster wouldn't show such fluctuations because all of the other stars' light would wash it out.
Though securing the telescope time to conduct the research would be expensive, Pascale said it might be a worthwhile endeavor for the scientific community to explore.
So far, colleagues have seemed open to considering a star cluster as an explanation for Earendel, but Pascale emphasizes that the paper doesn't definitively prove that it is: The previous teams that made the discovery make a compelling case, too. He hopes the new study merely adds to the discourse.
"Maybe everybody keeps what their secret thoughts are about it a little bit more private," he said, "but most people are pretty happy to say, 'Yeah, a star cluster seems like an option.'"
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