Under the cosmic microscope: JWST reveals dozens of distant stars in a single galaxy

A quiet revolution is unfolding in the study of distant galaxies, one that lets astronomers pinpoint individual stars across the vast gulfs of time and space. Using the James Webb Space Telescope (JWST), a team of researchers has identified ¹ over 40 individual stars in a single galaxy located halfway across the observable universe. These stars weren’t visible through any ordinary means—they were seen because the universe itself magnified them for us.

This breakthrough pushes the frontier of how we study stars in the early universe and opens a new path toward understanding both galactic evolution and dark matter.

Gravitational lensing: A natural telescope

Imagine a distant star so faint and far that even our most powerful telescopes cannot detect it. Now, place an enormous cluster of galaxies between that star and our telescopes. The gravitational pull of the cluster bends and magnifies the light coming from the background star, making it visible to us. This is gravitational lensing, a cosmic magnifying glass predicted by Einstein’s theory of general relativity.

In rare and lucky circumstances, the gravitational lens doesn’t just brighten the light from an entire galaxy—it can amplify light from individual stars by a factor of hundreds or even thousands. But these stars tend to flare in and out of view because their magnification is boosted further by microlensing—caused by smaller objects like rogue stars within the foreground cluster.

Meet the Dragon arc

The stars identified in this study belong to a galaxy at a redshift of 0.725, meaning we see it as it was when the universe was roughly half its current age. This galaxy is located behind a massive cluster known as Abell 370, whose gravitational field distorts and stretches the background galaxy into a spectacular arc shape. Astronomers have dubbed it the “Dragon arc.”

Over the course of a year, JWST observed this arc in two separate imaging campaigns. These observations were part of broader surveys, but the team noticed something intriguing: faint, point-like sources appearing in only one of the two epochs. Using image subtraction—comparing the two sets of data pixel by pixel—the astronomers identified 44 of these transient sources.

Almost all of them are located near where theoretical models predict the strongest gravitational lensing effects, suggesting these are not just any stars, but ones dramatically brightened by gravitational and microlensing.

Not supernovae, not artifacts—real stars

A major challenge in this kind of research is ensuring the sources aren’t false positives—like cosmic rays, bad pixels, or supernovae. However, the distribution and brightness of these transient sources made those explanations unlikely. The odds of seeing this many supernovae in a single galaxy, within a limited observation window, are vanishingly small.

Furthermore, the transients were spatially compact and aligned with lensing predictions. The most probable explanation? We are seeing individual stars whose light is momentarily boosted by the gravitational fields of both the galaxy cluster and the stars within it.

A Glimpse into stellar evolution and dark matter

Most of the microlensed stars identified in the Dragon arc appear to be red giants or supergiants—stars that are both extremely luminous and cool (with surface temperatures around 3,000–4,000 K). This fits with expectations: massive stars, especially in their late stages of evolution, emit enough light to be magnified to observable levels under extreme lensing.

These detections are far from just astronomical curiosities. They enable astronomers to study the properties of individual stars in galaxies billions of light-years away—something unthinkable just a few years ago. This could revolutionize how we understand stellar populations in the early universe.

There’s another angle, too. Microlensing events are sensitive to the mass distribution of the lensing galaxy cluster, including not just the visible matter but the invisible dark matter that makes up most of the mass in the universe. By mapping where these events occur and how they behave, astronomers can probe the granularity of dark matter—potentially distinguishing between competing theories about its composition.

The power of time-domain astronomy

Why did it take JWST to make this leap? Two key capabilities came together: JWST’s incredible sensitivity in the infrared, and its ability to return to the same spot in the sky at different times. This approach—called time-domain astronomy—lets scientists catch stars in the act of flaring due to microlensing.

Previous instruments, like the Hubble Space Telescope, have identified a few such stars. But this new study marks the first time more than 40 have been found in a single galaxy. It’s a game-changer.

A peek into the future

This work demonstrates that JWST can transform the field of extragalactic stellar astronomy. As more time-domain observations are carried out, we may one day catalogue hundreds or even thousands of individual stars in distant galaxies. This will allow for statistical studies of stellar evolution and feedback, population demographics, and the mass distributions of lensing clusters.

There are even hints that microlensing could reveal exotic physics. In a few cases, astronomers detected pairs of stars that might be two views of the same object—split and shifted by dark matter subhaloes. Confirming such cases could help map the elusive small-scale structure of dark matter, something that has long eluded even the most sensitive experiments.

The discovery of more than 40 microlensed stars in the Dragon arc shows that our universe is not only a vast expanse but also a surprisingly interconnected optical laboratory. With gravitational lensing acting as nature’s telescope and JWST providing the clarity and precision, we are now able to pick out individual stars that lived billions of years ago, in galaxies so distant they appear only as smudges in most telescopes.

It’s a reminder of how far we’ve come in our ability to peer into the cosmos—and how much more is waiting to be discovered.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.