Beyond the 'Dragon Arc': Unveiling a treasure trove of hidden stars
Hubble image of Abell 370, a galaxy cluster located nearly 45 billion light-years away from Earth that features several arcs of light, including the "Dragon Arc" (lower left of center). These arcs are caused by gravitational lensing, wherein light from distant galaxies far behind the massive galaxy cluster coming toward Earth is bent around Abell 370 by its massive gravity, resulting in contorted images. Image credit: NASA
NASA's James Webb Space Telescope (JWST) has set a new milestone: capturing images of over 40 individual stars in a galaxy so distant that its light has traveled since the universe was only half its current age.
Observing individual stars halfway across the observable universe is generally regarded as impossible in astronomy, akin to trying to use binoculars to see individual grains of dust in the moon's craters. However, due to a remarkable cosmic phenomenon, an international team of astronomers accomplished this seemingly unattainable goal. This groundbreaking discovery is set to change our understanding of the cosmos.
Using JWST data, the astronomers observed a galaxy nearly 6.5 billion light-years from Earth at a time when the universe was half its current age. In this distant galaxy, the team identified many individual stars, which were made visible thanks to an effect known as gravitational lensing and JWST's high light-collecting power.
The paper describing the discovery has been published in Nature Astronomy, is led by Yoshinobu Fudamoto, an assistant professor at Chiba University in Japan and a visiting scholar at University of Arizona's Steward Observatory, and includes Arizona State University co-authors Seth Cohen, Nicholas Foo and Rogier Windhorst.
"I'm amazed at all the different ways gravitational lensing has become a useful tool to study different astrophysical phenomena," said Cohen, an associate research scientist at ASU’s School of Earth and Space Exploration.
The finding marks a record-breaking achievement — the most significant number of individual stars detected in the distant universe. This rare and important event provides an opportunity to investigate one of the universe's greatest mysteries: dark matter.
"When we predicted in 2018 that stars in galaxies at cosmological distances might be observed with Webb individually as they go across these nearly infinite magnification lines (the so-called 'caustics'), I never dreamed of Webb seeing them in such large numbers,'' said Windhorst, an ASU Regents Professor. "And now here we are observing these stars popping in and out of the images taken only a year apart, like fireflies in the night. Webb continues to amaze us all."
Most galaxies, including the Milky Way, contain tens of billions of stars. Astronomers can observe stars one by one in nearby galaxies such as the Andromeda galaxy. However, in galaxies billions of light-years away, stars appear blended together as their light needs to travel for billions of light-years before it reaches us. This presents a long-standing challenge to scientists studying how galaxies form and evolve.
“It was amazing to see the observations taken over time of the Dragon Arc. Stars would appear and disappear from image to image like a twinkling Christmas tree,” said Foo, a graduate research associate at ASU’s School of Earth and Space Exploration.
"To us, galaxies that are very far away usually look like a diffuse, fuzzy blob," said Fudamoto. "But actually, those blobs consist of many, many individual stars. We just can't resolve them with our telescopes."
Recent advances in astronomy have opened new possibilities by leveraging gravitational lensing — a natural magnification effect caused by the strong gravitational fields of massive objects. As predicted by Albert Einstein, gravitational lenses can amplify the light of distant stars by factors of hundreds or even thousands, making them detectable with sensitive instruments like JWST.
How can we observe stars in distant galaxies?
Extremely strong gravitational lens magnifications combining macro lens, micro lens
Observing individual stars in distant galaxies is impossible without a gravitational lens, where the light from distant galaxies is distorted by the intervening large masses as predicted by the theory of general relativity. However, even the extremely strong gravitational magnification from a galaxy cluster is insufficient to magnify individual stars in galaxies far away. To achieve the necessary magnifications for distant stars, combining two gravitational lens is essential.
In the image below, the massive dark matter in the galaxy cluster acts as a first “macrolens,” magnifying and distorting distant galaxies. Simultaneously, stars within the galaxy cluster serves as secondary “microlens,” further magnifying individual stars in distant galaxies in a limited, short duration. Over a brief time frame — ranging a few days to a week — when these two effects perfectly aligned with distant stars, the magnification and the apparent brightness of the stars increase significantly. By observing the same galaxy multiple times, astronomers can detect stars in distant galaxies as they appear to twinkle due to the varying effective magnifications created by the combined macrolens/microlens effect.
"These findings have typically been limited to just one or two stars per galaxy," Fudamoto said. "To study stellar populations in a statistically meaningful way, we need many more observations of individual stars."
Fengwu Sun, a former U of A graduate student who is now a postdoctoral scholar at the Center for Astrophysics | Harvard & Smithsonian, stumbled on a treasure trove of such stars when he was inspecting JWST images of a galaxy known as the Dragon Arc, located along the line of sight from Earth behind a massive cluster of galaxies called Abell 370. Due to its gravitational lensing effect, Abell 370 stretches the Dragon Arc's signature spiral into an elongated shape — like a hall of mirrors of cosmic proportions.
In December 2022 and 2023, JWST obtained two pictures of the Dragon Arc. Within these images, astronomers counted 44 individual stars whose brightness changed over time due to variations in the gravitational lensing landscape.
"This groundbreaking discovery demonstrates, for the first time, that studying large numbers of individual stars in a distant galaxy is possible," Sun said, as long as nature is there to lend a helping hand.
However, even powerful gravitational magnification from a galaxy cluster is not sufficient to magnify individual stars in galaxies even farther away. In this case, the discovery was made possible by a serendipitous alignment of "lucky stars."
"Inside the galaxy cluster, there are many stars floating around that are not bound by any galaxy," said co-author Eiichi Egami, a research professor at Steward Observatory. "When one of them happens to pass in front of the background star in the distant galaxy along the line of sight with Earth, it acts as a microlens, in addition to the microlensing effect of the galaxy cluster as a whole."
The combined effects of microlensing dramatically increase the magnification factor, allowing JWST to pick up individual stars that would otherwise be too far and faint to be detected.
This press release was written by the University of Arizona with contributions from Kim Baptista of ASU's School of Earth and Space Exploration.
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