JWST Tracks 104 Magnified Stellar Events in the Dragon Galaxy

Why astronomers call it the Dragon

The Dragon is a distant galaxy whose light is stretched into a long arc by the gravity of the massive galaxy cluster Abell 370. The cluster sits between the galaxy and Earth, bending the light along multiple paths. This effect is called gravitational lensing.

The arc is not a physical stream of stars stretched across space. It is a distorted image of the background galaxy. Different parts of the cluster magnify the background light by different amounts, and some narrow regions can produce especially strong magnification.

Those high-magnification boundaries are known as caustics. When an individual star in the distant galaxy moves across one of these regions, its apparent brightness can rise dramatically for a limited time. Astronomers call this a caustic-crossing event.

How Webb’s observations revealed 104 events

The research team analysed JWST/NIRCam images collected between 2022 and 2024 in four infrared filters: F150W, F182M, F200W and F210M. Rather than relying on a single picture, they compared observations from different epochs and looked for point-like changes in brightness along the arc.

Image alignment and careful modelling were essential. The team corrected for small differences between observations, constructed point-spread functions, and compared each epoch with a reference made from the other available observations. They also masked bright regions and used a nearby part of the same galaxy—the Dragon’s “head”—to estimate ordinary brightness fluctuations that could otherwise be mistaken for microlensing.

The analysis produced 224 detections before repeated appearances were consolidated. After matching detections that likely represented the same physical event across different filters or epochs, the researchers retained 104 unique candidates. That is the largest sample of highly magnified stellar candidates identified in a single galaxy so far.

What the events reveal about massive stars

Only particularly luminous stars are likely to become detectable at these enormous distances. The study therefore focuses on the brightest members of the stellar population, including blue and red supergiants.

Blue supergiants are very hot and emit strongly at shorter wavelengths. Red supergiants are cooler and radiate more of their energy at longer wavelengths. Comparing the observed colours can help astronomers investigate the ages and properties of the stars that make up the distant galaxy.

The team found a roughly equal blue-to-red ratio among the analysed events. That balance is consistent with a galaxy containing both types of luminous stars and still forming stars. It is not a complete census of every star in the Dragon: the method is biased toward stars that become bright enough to detect through lensing.

Could the pattern help test dark matter?

The cluster’s gravity creates the large-scale lens, but stars and other compact objects inside the cluster can add smaller distortions. These tiny lensing structures—microcaustics—affect where and how often individual stars brighten.

The researchers examined how the events were distributed around the critical curve, a boundary in the lens model associated with very high magnification. They reported a parity asymmetry that remains in their preferred model and noted that it could be compatible with a fuzzy-dark-matter scenario.

That interpretation is still tentative. The predicted position of the critical curve differs among lens models, and an inaccurate curve location could also influence the apparent asymmetry. The study therefore does not establish the nature of dark matter. It shows how a much larger sample of microlensing events can become a new tool for testing models.

What astronomers may do next

The Dragon demonstrates that repeated deep observations can turn rare stellar brightenings into a useful statistical sample. Applying similar methods to other strongly lensed galaxies could help researchers compare stellar populations across cosmic time and improve maps of the matter inside galaxy clusters.

More frequent observations, additional filters and better-constrained lens models will be important. With those improvements, microlensing events may help astronomers study stars that would otherwise remain unresolved even for the most powerful telescopes.