In a universe filled with cosmic wonders, scientists have just unlocked a long-standing mystery about how some of the densest star systems in the cosmos come into being. For the first time ever, astronomers have directly observed a celestial spectacle that has only existed in theory—star clusters in dwarf galaxies merging in real-time.
This remarkable discovery, led by Postdoctoral Researcher Mélina Poulain from the University of Oulu, Finland, was published in the prestigious journal Nature. The observation not only provides a crucial piece to the puzzle of galaxy evolution but also breathes life into a decades-old debate about how nuclear star clusters form in small galaxies.
Small Galaxies, Big Questions
Dwarf galaxies might be small in size, but they play an outsized role in the story of the universe. These galactic underdogs, containing as little as one-hundredth the number of stars found in the Milky Way, are believed to be the foundational building blocks of larger galaxies. Piecing together their formation and evolution is like uncovering the blueprints for how the universe assembled its grand architecture.
Interestingly, many dwarf galaxies host a compact, ultra-dense cluster of stars smack dab in their centers. These central concentrations, called nuclear star clusters, can contain up to hundreds of millions of stars packed into a volume only a few light-years across. That makes them the densest stellar systems known.
But the question that has kept astronomers scratching their heads is: how do these monstrous clusters form in such tiny galaxies?
The Long-Debated Theory
One prominent theory has suggested that nuclear star clusters in dwarf galaxies form from merging globular clusters—smaller, gravitationally bound groups of stars that orbit the outskirts of galaxies. Over time, these globular clusters are believed to migrate toward the galaxy’s core, pulled inward by a process known as dynamical friction. Eventually, they’re thought to collide and coalesce, creating the ultra-dense nuclear clusters observed in many dwarf galaxies.
However, until now, this scenario was purely speculative. No one had ever seen these clusters mid-merger—until Poulain and her colleagues caught the universe in the act.
A Galactic Detective Story
The breakthrough came while examining deep-space images from the Hubble Space Telescope, which had captured nearly 80 dwarf galaxies as part of a survey led by Professor Francine Marleau from the University of Innsbruck, Austria. Within this cosmic catalog, researchers noticed something unusual: a few galaxies displayed oddly shaped features near their centers.
Some of the galaxies had two bright clusters situated unusually close together, while others displayed faint, thread-like streams of light seemingly spilling from their nuclear star clusters. These light trails looked like celestial wisps, barely visible but unmistakably there.
“We were surprised by the streams of light that were visible near the center of the galaxies, as nothing similar has been observed in the past,” said Poulain.
These ghostly features hinted at a dramatic and rare process unfolding within these galaxies—the merging of star clusters in real time.
Confirming a Galactic Collision
But astronomers needed more than visual clues to confirm what they were witnessing. To verify the origin of the mysterious light streams, the team turned to ultra-high-resolution computer simulations.
Led by Dr. Rory Smith at Universidad Técnica Federico Santa María in Santiago, Chile, these simulations recreated the gravitational ballet of merging star clusters under various conditions. By adjusting variables like the clusters’ mass, motion, and numbers, researchers could test whether such interactions could produce the same light trails seen in the Hubble images.
The answer was a resounding yes.
The simulations showed that when two globular clusters—with notably different masses—merge, they create exactly the kind of faint stellar streams observed in the Hubble data. These streams are essentially gravitational stretch marks, left behind as the smaller cluster is torn apart and absorbed by the larger one.
Interestingly, the greater the difference in mass between the clusters, the longer and more pronounced the stream. But these trails don’t linger. The merging process wraps up in under 100 million years—a blink of an eye on cosmic timescales—and the streams themselves fade even faster. That’s why they’re so hard to spot and why this discovery is so groundbreaking.
A Rare Glimpse into Galaxy Evolution
This first-of-its-kind observation provides long-awaited confirmation of the cluster cannibalization theory, showing that nuclear star clusters in dwarf galaxies can indeed form from merging globular clusters. It’s a striking example of how the universe builds complexity from simplicity—assembling monumental structures from tiny, repetitive processes over eons.
But the implications stretch even further.
Dwarf galaxies are relics of the early universe. If nuclear clusters are forming this way today, it means the same process likely occurred billions of years ago, helping shape the larger galaxies we see now—including our own Milky Way. In fact, the Milky Way itself has a nuclear star cluster, nestled tightly around its central supermassive black hole. While larger galaxies may have more complicated formation histories, dwarf galaxies offer a simpler environment in which to isolate the core processes.
Moreover, these findings shed new light on the life cycle of globular clusters and their long-term evolution. Understanding how and when they migrate and merge could help astronomers piece together galactic family trees, tracing the lineage of stars and stellar systems from past to present.
Cosmic Snapshots and What Comes Next
Discovering such a fleeting phase of galaxy evolution is like catching a supernova mid-burst or a planet forming in real time. It’s a cosmic lucky break, and one that underscores the value of persistent, high-resolution observation.
But astronomers aren’t done yet. With next-generation telescopes like the James Webb Space Telescope (JWST) and the upcoming Nancy Grace Roman Space Telescope, researchers hope to capture even more detailed views of these dynamic environments. JWST, in particular, with its unparalleled sensitivity to infrared light, could unveil new merging clusters hidden behind clouds of gas and dust.
Further studies may also explore how common these mergers are, how they vary across different galaxy types, and what role they play in seeding supermassive black holes—another astronomical enigma closely tied to galactic centers.
Conclusion
With this pioneering discovery, astronomers have pulled back the curtain on one of the most elusive processes in galaxy evolution. By capturing dwarf galaxies in the act of merging their central star clusters, scientists have confirmed a key formation mechanism for one of the universe’s most extreme stellar environments.
From stargazers to seasoned astrophysicists, this glimpse of cosmic construction in action is a powerful reminder: in the grand theater of the universe, there’s always a new scene unfolding—if we know where to look.
Reference: Mélina Poulain, Evidence of star cluster migration and merger in dwarf galaxies, Nature (2025). DOI: 10.1038/s41586-025-08783-9. www.nature.com/articles/s41586-025-08783-9