Webb Counts 104 Magnified Stars in the Dragon Galaxy, a Record, and Hints at Fuzzy Dark Matter
JWST images of a galaxy 6.5 billion light-years away more than double the previous count of 44 individual stars brightened by gravitational lensing.
The James Webb Space Telescope has tracked 104 individual stars flaring in brightness inside a single distant galaxy, more than double the previous record of 44 in the same object and the first statistical study of its kind.
The galaxy, nicknamed the Dragon arc, sits about 6.5 billion light-years away, at redshift 0.725. Its light is magnified by the massive galaxy cluster Abell 370, which acts as a gravitational lens. Where the lens focuses light most strongly along a line called the critical curve, individual stars can briefly brighten by hundreds or even thousands of times before fading over days to weeks. Astronomers call these caustic-crossing events.
The study, led by José María Palencia of the Institute of Physics of Cantabria, was published Sept. 30 in Astronomy & Astrophysics (DOI 10.1051/0004-6361/202660640). It uses Webb's NIRCam images taken from 2022 to 2024 in three infrared filters covering roughly 1.3 to 4.3 microns, within a 60-by-60 arcsecond cutout around the arc.
The method was a careful subtraction exercise. The team compared images of the same region at different times to find brightness changes, corrected for small differences in viewing angle so that subtraction would not leave false residues, and masked bright unrelated objects and the galaxy's core. They estimated the noise from a region far from the critical curve, then merged repeat detections across filters and epochs to arrive at 104 unique events.
Sorting the events by color let the team estimate what kinds of stars were involved. The ratio of blue to red supergiants came out at about one, meaning roughly equal numbers of each. The authors say both types contribute to the events, which points to a relatively evolved system that is still forming stars.
The more provocative result concerns dark matter. Far from the critical curve, the event counts match on both sides. Near the curve, the count drops on one side but not the other. That pattern fits a model called fuzzy dark matter, in which the dark matter consists of extremely light, wave-like particles. Standard cold dark matter would not be expected to produce it.
The authors caution that the signal could instead come from an incorrectly placed critical curve in the lens model, which was taken from earlier work by Diego and colleagues in 2025. The finding is tentative. What the study does establish is that Webb can turn a single lensed galaxy into a census of individual stars at cosmological distances, and that larger event samples will let astronomers test competing dark matter models directly.
