One image tracks the same cosmic object across space and time. It's all down to a strange quirk of physics

The hottest topic in astronomy is what exactly the little red dots (LRDs) hiding in JWST images really are.

Everyone agrees they’re high-redshift (and hence distant) systems, which are compact yet bright – but beyond that, everything is still up for debate. 

The leading theory is that their luminosity is powered by material falling onto supermassive black holes at the centre of small galaxies, but understanding how such beasts grow is difficult.

Adding to the mystery, the dots appear faint in X-rays and radio, both of which would normally be bright if accretion onto black holes is involved.

Getting to the bottom of the mystery

While theorists play with ever-more exotic ideas, observers are seeking ways to constrain what these objects are really like.

I was delighted, then, to see a paper by a large international team, led by Peking University’s Zijian Zhang, that takes a close look at an unusual dot.

The subject is a galaxy cluster known as RXC J2211-0350. It serves as a foreground source behind which sit two LRDs whose light is bent by gravity as it passes through the cluster, an effect known as gravitational lensing.

This lensing helpfully magnifies the more distant dots by a factor of a few, according to the paper. But for one of them, there’s a more dramatic result.

One dot, four images

Because of its position, light from little red dot RX1 is split into four separate images: an Einstein cross.

And what’s especially exciting is that, because of the geometry of the lens, light travels slightly different distances to produce each image – so we’re really seeing four snapshots of the dot taken at slightly different times.

By modelling the cluster, the team estimate that the difference between the youngest and oldest image is about 130 years.

Slight changes in brightness and colour between the images show how these objects evolve over more than a century. 

So what can these changes tell us about the source of the dot’s brightness?

The team suggest that there’s an envelope of hot gas around the supermassive black hole believed to lie at the centre of RX1 and that, heated by accretion onto the central object, it’s pulsing like a giant variable star.

These pulses would explain the brightness changes we see.

Not all little red dots should, the authors explain, show similar behaviour; as with variable stars, whether a pulsing mode can be sustained depends on the exact temperature and pressure of the gas.

But for RX1, this is a compelling argument, if we believe the brightness variations follow a nice, smooth periodic pattern – the paper suggests that a period between maxima of 32 years would fit the data.

Follow-up observations of the different dot images over the next few years should, if this model is right, reveal steady changes in brightness.

If some other mechanism is at work – if, for example, the brightness is varying because different amounts of material are falling on to the black hole at different times – then we should see more random changes in brightness.

Either way, RX1 has a big part to play in our understanding of these enigmatic and exciting new objects.

Chris Lintott was reading Little Red Dot Variability Over a Century Reveals Black Hole Envelope via a Giant Einstein Cross by Zijian Zhang et al. Read it online at: arxiv.org/abs/2512.05180.

This article appeared in the February 2026 issue of BBC Sky at Night Magazine