Webb Spots 104 Flickering Stars in One Distant Galaxy
By comparing Webb telescope images taken years apart, astronomers caught over a hundred individual stars brightening and fading in a galaxy far behind a massive cluster. The pattern of flickers may offer a new way to test what dark matter is.
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Nov 8, 22:00 UTC ·
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Astronomers using the James Webb Space Telescope have found 104 distinct magnified stellar events in a single faraway galaxy, more than twice the previous record of 44 and a possible new probe of dark matter. [1][2]
The galaxy is known as the Dragon, a stretched arc of light lying behind the galaxy cluster Abell 370, at a redshift of about 0.725. [2] The study was led by José María Palencia of the Institute of Physics of Cantabria, with co-authors including Fengwu Sun, José María Diego and Yoshinobu Fudamoto. [1][2] It appeared in Astronomy & Astrophysics on September 30, 2026, and Phys.org reported it on November 8. [1]
The Dragon was the first giant arc ever identified, and it has long been a favourite target for people who want to see single stars at great distance. [2] Earlier JWST work reported dozens of magnified stars in it. [2] The new paper turns that string of discoveries into a sample large enough to do statistics on.
A telescope made of a cluster
A galaxy cluster is so massive that its gravity bends light passing near it. Anything behind the cluster is stretched and brightened, as if seen through a vast, uneven lens. [2] This is gravitational lensing, and it lets Webb see things it could never resolve on its own.
The brightening is strongest along a line called the critical curve. Near it, the lensing also picks up small distortions from individual stars in the cluster, and possibly from dark matter. These create tiny regions called microcaustics. [2]
Sunlight on the floor of a swimming pool shows the same idea. Ripples focus the light into bright, shifting lines. When a star in the background galaxy drifts across one of these lines, its light jumps sharply, then fades. [2]
That is what makes the events findable. The team compared JWST images taken between 2022 and 2024 and looked for points of light that had changed brightness from one visit to the next. [1][2] The flickers are short-lived. The study says the observed timescales are typically days to weeks, though the largest stars can stay visible for months. [2]
The brightest stars in the galaxy
A star that can be caught this way has to be luminous, so the sample is a catalogue of the galaxy's heavyweights. The study reports that the detected stars are among the most luminous in the Dragon and expects the sample to be dominated by supergiants. [1]
Their colours split almost evenly. The team found roughly equal numbers of blue and red supergiant candidates, which fits a galaxy that is still forming stars and has an already evolved population. [1]
The spatial spread of the events also yields a number. The authors estimate the slope at the high end of the stellar luminosity function, the rule for how quickly star counts thin out as brightness rises, at 2.18, with an uncertainty of plus 0.20 and minus 0.30. [2] If the slope is fixed instead, the same data can constrain the microlens surface mass density, a measure of how much mass sits in compact objects along the line of sight. [2]
The events also mark where magnification is highest, so the authors propose using them to map the system's critical curves. [2]
Where dark matter comes in
Dark matter is the unseen material that makes up most of the matter in the universe and reveals itself only through gravity. What it is made of is still unknown.
One proposal is fuzzy dark matter, in which the particles are extremely light and behave like waves rather than like tiny billiard balls. [1][2] Such waves would ripple the mass inside the cluster on small scales, and that would change where microcaustics form.
The authors looked at the number of events on opposite sides of the critical curve. A difference between the two sides has been proposed as a signature of wave-like dark matter. [2] The Dragon sample shows an asymmetry that could fit the fuzzy model. [1]
That is as far as the finding goes. The study treats fuzzy dark matter as one possible reading of the pattern. It is not a detection of a dark matter particle and it does not settle what dark matter is. [1][2] The same asymmetry could appear if the critical curve is misplaced in the lens model, the map of the cluster's mass that astronomers use to work out where the magnification lies. [1][2]
The paper is peer-reviewed. The dark matter reading is a tentative interpretation within it. [1]
Why a hundred matters
With 44 events, the Dragon was a collection of curiosities. With 104, it is a population. That is the real change here. A pattern in the left-versus-right count means little in a handful of cases and much more in a hundred.
It also means the lens model can be tested against the stars themselves. If the asymmetry vanishes when the critical curve is redrawn, the dark matter hint goes with it. If it survives better models, the case for a physical cause gets stronger.
The authors say the method can grow with more strongly lensed galaxies, more frequent repeat observations and improved lens models. [1][2] The Dragon is one galaxy behind one cluster. If the same asymmetry turns up behind others, it would be hard to blame on a single misplaced line.
An open question is how many more events more frequent imaging would turn up, since the shortest flickers fade in days and can slip between visits. No one has yet said whether the asymmetry holds in another cluster, and that comparison is what the authors are asking for.
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