More Than a Hundred Distant Stars in a Single Galaxy express
Light from the distant Dragon Galaxy has been traveling toward us for more than seven billion years. On its way, it passes by a huge galaxy cluster whose gravity bends the path of light like a lens. As a result, the galaxy appears brighter and larger than it really is, just as a magnifying glass enlarges fine print. Inside this cosmic lens, there are also individual stars. When one of these stars passes directly in front of a star in the Dragon, the latter's light is briefly amplified even more, and we see a flare.
Astronomers imaged this region with the James Webb Space Telescope for three consecutive years, searching for such flares — stars that appeared and disappeared in the images. They found more than a hundred such events. This is a record: previously, only a few dozen could be spotted in a single galaxy. Now it's possible to count how many bright and faint stars the Dragon contains. It turns out there are more bright stars than old models predicted.
The positions of the flares also reveal how the invisible dark matter is distributed in the cluster. Flares occur more often where the gravitational lens is strongest. This helps refine the cluster map and test hypotheses about the nature of dark matter. Astronomers also noticed that the flares are not perfectly symmetric: there are more on one side of the center than the other. This asymmetry could be a trace of the wave-like properties of dark matter — if it consists of ultralight particles that behave like waves.
Such a large-scale census of distant stars is a new tool. Previously, knowledge about stars in distant galaxies came from their total light, but now individual stars can literally be counted. It's like going from viewing a crowd from afar to suddenly seeing faces.
Original abstract
as written by the authors arXiv:2604.22702
Highly magnified stars at cosmological distances ($$z \gtrsim 0.7$$) become detectable thanks to microlensing by intracluster stars near the critical curves of galaxy clusters. Multi-epoch photometric campaigns targeting caustic crossing galaxies magnified by massive galaxy clusters enable the detection of these objects as transient events. Such stars provide unique opportunities to study stellar populations at early cosmic times, probe the nature of dark matter, reveal small-scale structure in the cluster, and improve lens models. To date, only a few dozen high-redshift stars have been reported, with a single lensed galaxy, the Dragon, holding the current record of 44 detections. These numbers, however, remain insufficient to exploit their full potential. In this paper, owing to the inclusion of new observations, we report the identification of more than 100 magnified stellar events in the Dragon, behind the massive galaxy cluster Abell 370. The relatively low redshift of the Dragon ($$z\approx0.725$$) facilitates the detection of its most massive stars. Using imaging data from three different cycles (2022--2024) with the James Webb Space Telescope, we apply a time-domain technique to identify flux variations associated with caustic-crossing events. From the spatial distribution of stellar events we constrain the high-end slope of the stellar luminosity function, finding $$\beta=2.18^{+0.20}_{-0.30}$$. Alternatively, assuming a fixed slope, we constrain the microlens surface mass density. In addition, we examine the parity asymmetry of the detected caustic-crossing events, a proposed probe of wave dark matter, and find that it remains present. We also use the events to trace the regions of highest magnification, offering an alternative way to map the system critical curves.
The Dragon Galaxy got its name from its elongated shape: the gravitational lens stretched its image into an arc resembling the curved body of a dragon.
In the series 'Star Trek', gravitational lenses were used to peer into the distant past of the universe — though there it was technology, while here it's nature itself.