Taking advantage of a cosmic ‘double lens,’ astronomers identified over 40 individual stars in a galaxy so distant that its light has traveled to us from when the universe was only half its present age.
Typically, attempting to view individual stars halfway across the observable universe is unrealistic in astronomy, akin to aiming binoculars at the moon in hopes of seeing dust particles in its craters. However, an international team led by astronomers from the University of Arizona’s Steward Observatory managed to achieve just that, thanks to a remarkable cosmic phenomenon.
Using NASA’s James Webb Space Telescope (JWST), this research group observed a galaxy located about 6.5 billion light-years from Earth, at a time when the universe was half its current age. Thanks to gravitational lensing and JWST’s exceptional light collection ability, the team was able to spot numerous individual stars within that remote galaxy.
This groundbreaking discovery, published in the journal Nature Astronomy, sets a record as the largest number of individual stars detected in the far-off universe. It also provides new avenues for exploring one of the universe’s biggest enigmas: dark matter.
While most galaxies, including our Milky Way, host tens of billions of stars, astronomers can easily observe individual stars in nearby galaxies like Andromeda. However, in galaxies billions of light-years away, stars appear blurred together since their light has traveled vast distances to reach us, presenting a significant challenge for scientists studying galaxy formation and evolution.
“To us, distant galaxies often look like a blurry haze,” explained lead study author Yoshinobu Fudamoto, an assistant professor at Chiba University in Japan and a visiting scholar at Steward Observatory. “Yet, these hazy regions are composed of countless individual stars that we can’t differentiate with our telescopes.”
Recent advancements in astronomy have expanded opportunities by utilizing gravitational lensing—a natural magnification effect caused by the intense gravitational fields of large objects. According to Albert Einstein’s predictions, these gravitational lenses can amplify the light from distant stars by hundreds or even thousands of times, making them detectable with sensitive tools like the JWST.
“Traditionally, these findings have been limited to just one or two stars in a galaxy,” noted Fudamoto. “To analyze stellar populations meaningfully, we require many more individual star observations.”
Fengwu Sun, a former graduate student at the U of A now working as a postdoctoral scholar at the Center for Astrophysics | Harvard & Smithsonian, discovered a wealth of these stars while examining JWST images of a galaxy known as the Dragon Arc. This galaxy is positioned behind a massive cluster of galaxies called Abell 370, which creates a gravitational lensing effect, altering the Dragon Arc’s shape into an elongated form, much like a cosmic funhouse mirror.
In December 2022 and 2023, JWST captured two images of the Dragon Arc. Within these images, astronomers counted 44 individual stars whose brightness shifted over time due to changes in the gravitational lensing conditions.
“This pioneering discovery proves for the first time that it is indeed feasible to study numerous individual stars in a distant galaxy,” said Sun, highlighting nature’s supportive role in this achievement.
Nonetheless, even exceedingly strong gravitational magnification from a galaxy cluster can’t magnify stars in even more distant galaxies. This finding resulted from a fortunate alignment of “lucky stars.”
“Inside the galaxy cluster, there are numerous stars not bound to any galaxies,” explained co-author Eiichi Egami, a research professor at Steward Observatory. “When one of them crosses in front of a background star in the distant galaxy from Earth’s perspective, it acts as a microlens, complementing the macrolensing effect of the galaxy cluster overall.”
The combined effects of macrolensing and microlensing significantly enhance the magnification, enabling JWST to detect individual stars that would otherwise be too distant and dim to observe.
As the stars within the magnifying cluster move in relation to the target stars in the distant galaxy and Earth, the arrangement of microlenses in this natural “telescope” shifts slightly over short intervals—from a few days to a week. When the alignments are just right, the brightness and magnification of the distant stars increase sharply, only to diminish shortly afterward.
“By repeatedly observing the same galaxy, we can notice stars in far-off galaxies appearing to flicker in and out of view,” Fudamoto commented. “This phenomenon occurs due to the fluctuating magnifications from macro- and microlensing as the microlensing stars shift in and out of alignment.”
The research team meticulously examined the colors of each star within the Dragon Arc and discovered that many are red supergiants, akin to Betelgeuse in the Orion constellation, which is nearing the end of its life. This finding contrasts with previous discoveries that predominantly identified blue supergiants like Rigel and Deneb—the brightest stars visible in the night sky. The researchers noted that this variation in star types underscores JWST’s extraordinary capabilities at infrared wavelengths that enable the detection of cooler stars.
Future observations with JWST are anticipated to reveal more magnified stars within the Dragon Arc galaxy. These endeavors could facilitate detailed studies of hundreds of stars in distant galaxies. Furthermore, examining individual stars could provide crucial insights into the structure of gravitational lenses and even help unravel the mysterious essence of dark matter.
This project received support from various sources, including NASA and NSF.
