An international team of astronomers has detected over 40 individual stars in a galaxy located 6.5 billion light-years away. The discovery of more than 40 stars in a single galaxy is unprecedented, as prior observations typically revealed only one or two such stars per galaxy.

For decades, astronomers have sought to study distant galaxies with the same detail as our cosmic neighborhood. But distance doesn’t really work that way. The vastness of space reduced these far-off clusters to blurry patches of light. But researchers have found a clever workaround, thanks to the combined power of NASA’s James Webb Space Telescope (JWST) and an extraordinary cosmic alignment.

“To us, galaxies that are very far away usually look like a diffuse, fuzzy blob,” said Yoshinobu Fudamoto, an assistant professor at Chiba University in Japan and a visiting scholar at the University of Arizona’s Steward Observatory and study lead of the paper published in Nature Astronomy. “But actually, those blobs consist of many, many individual stars. We just can’t resolve them with our telescopes.”

However, thanks to a phenomenon predicted by Einstein—gravitational lensing—coupled with JWST’s extraordinary detection power, astronomers are now privy to sights once thought unattainable.

“I’m amazed at all the different ways gravitational lensing has become a useful tool to study different astrophysical phenomena,” said Seth Cohen, an associate research scientist at Arizona State University’s School of Earth and Space Exploration and study co-author.

The Dragon Arc

Much of the recent success used a cosmic effect triggered by an intervening galaxy cluster—coined Abell 370. Due to its gravitational attraction, this galaxy bends and magnifies the light from a more distant galaxy, called the Dragon Arc for its gracefully elongated spiral shape.

The Dragon Arc can now be imaged so well that astronomers can even identify many stars individually, measuring their brightness, temperature, and possible evolutionary stage. Because this cluster is so old, these insights shed light on how star formation may have differed when the universe was just half its current age. Researchers also spotted several “rogue” stars, that float around the universe outside galaxies.

“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.”

However, the galaxy cluster’s macrolensing—powerful though it is—still cannot by itself produce such astonishing detail. That is where microlensing comes in.

Zooming in on the universe

Microlensing is an astronomy method for finding faraway objects, like planets and stars, by watching how their gravity changes the light from a background star. When one star moves in front of another from our point of view, the star in the center bends the light from the star in the background, making the background star look brighter.

By comparing JWST snapshots taken in December 2022 and again in December 2023, astronomers found that dozens of stars seemed to blink into visibility and then fade across the Dragon Arc’s expanse.

Data from the study reveals that many of these magnified stars are red giants or supergiants—massive, luminous stars that normally remain invisible to us at cosmological distances. These stars have surface temperatures of roughly 3,000 to 4,000 Kelvin, implying they are mature, cooler stars capable of releasing copious amounts of light in the infrared.

These magnified stars represent more than just an observational breakthrough. They can serve as test beacons for one of the grandest challenges in modern physics: determining the nature and distribution of dark matter.

Exploring the cosmos through dark matter’s lens

The study of microlensed stars in the Dragon Arc reveals new insights into the nature of dark matter, a mysterious type of matter seemingly common throughout the universe. By identifying the magnified stars by the gravitational influence of the Abell 370 galaxy cluster, researchers can probe dark matter on sub-galactic scales. These stars, located near critical curves where gravitational lensing is most intense, allow scientists to map the distribution of dark matter subhaloes—small, dense clumps of dark matter that distort light paths.

Some microlensing events were found at unexpected locations, suggesting the presence of dark matter structures that deviate from current models. This discovery challenges the understanding of dark matter’s behavior and distribution, offering a unique way to test competing theories about its composition, such as whether it consists of compact objects or diffuse particles.

By tracking the variability of these stars, scientists can refine lensing models and gain a clearer picture of dark matter’s role in shaping the universe. The study demonstrates the power of time-domain observations by the JWST, opening new possibilities for exploring the unseen matter that constitutes much of the universe’s mass. This marks a significant step toward unraveling the dark matter’s mysteries.

As astronomers refine their methods and use repeated observations, they may uncover entire populations of distant stars, possibly even catch glimpses of Population III stars—the first generation formed after the Big Bang. While the Dragon Arc discovery is extraordinary in itself, it hints that we are only beginning to exploit JWST’s capabilities for time-domain studies.

“When we predicted in 2018 that stars in galaxies at cosmological distances might be observed with Webb individually…I never dreamed of Webb seeing them in such large numbers,” said ASU regents professor and study co-author Rogier Windhorst said. “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.”