Three data releases from the recently retired Gaia spacecraft show that far-flung parts of the Milky Way are connected by families of stars born in clusters. Some continue to travel the galaxy together, while others appear wildly dispersed, sometimes as chains of related stars. One cluster is even trying to escape the Milky Way. The Gaia data show that open clusters (in particular) and the star formation regions from which they spring are interconnected across the Galaxy, populating the Milky Way in ways astronomers are just now beginning to understand.
Gaia's decade-long survey of the skies was aimed at understanding our Milky Way Galaxy by mapping its stars and other structures. It did this by pinpointing stellar locations, motions, and brightnesses. This is all the kind of data astronomers needed to get a multi-dimensional view of the Milky Way. In addition, the spacecraft gathered data about how stars shrink or swell, monitored "starquakes", and tracked the birth, growth, and eventual deaths of stars.
Most stars are born in batches and stay within their birth clusters for quite some time before dispersing out to the disk of the Galaxy. That's certainly how our own Sun formed, and Gaia gathered data about its birth cluster as well. Open clusters, which tend to have anywhere from a few tens to a thousand young stars, mostly live in the main disk of the Galaxy. Globulars contain millions of very old stars and seem to orbit the galaxy's core. They are tightly gravitationally bound together and some seem older than the Milky Way itself.
Clusters in the Milky Way
Gaia performed what's called a "cluster census", which means that it mapped cluster locations. Within that study, it took data that defined the characteristics of open cluster members - that is, stellar ages, sizes, distances, composition, motions, and more. Thata information helped astronomers figure out which stars actually belong to specific open clusters. It also helped identify those "rogue" stars that don't seem to belong to clusters.
"Thanks to Gaia, we can find and remove rogue stars that don’t actually belong to a cluster, making all of our science far more accurate,” explained Antonella Vallenari of Padova Astronomical Observatory (INAF), Italy, and Deputy Chair of the Gaia Data Processing and Analysis Consortium (DPAC).
“We can also find an incredible number of new clusters. Gaia can spot and group stars that are born together and moving similarly, even if they’re spread out through space. We’ve used Gaia to find new open clusters ranging from the very small – just a few pairs of co-moving stars! – all the way up those a few thousand strong.”
The Legacy Continues
Gaia's data provides astronomers a full view of how different parts of the Galaxy are interconnected, particularly in the spiral structure. For example, the data shows that clusters appear to be unevenly placed throughout the spiral arms. They also seem to move at different speeds and in slightly different ways depending on their location within the disk and arms. That could provide clues to the longevity of spiral arms in a galaxy's evolution. There are several schools of thought about how spiral arms arise in a galaxy, some related to starburst activity, others related to the action of density waves in a rotating galaxy system.
Large-scale galactic dynamics in a galaxy can influence how young star clusters form in the first place, and how they eventually dissolve. There are certainly plenty of examples of "dissolving" clusters in the Milky Way, and astronomers would like to understand more about how clusters form and dissipate over time. The two best-known "dissolvers" are the Pleiades and Hyades. They appear to still be moving together, but their structures indicate that they'll disperse from each other over time.
Gaia data reveals quite a bit about young clusters, mapping the dark clouds and dusty regions in which they form. It's not a straightforward process, with structures resembling streams, beads, strings, pearls, snakes, rings, or filaments of stars evident in some young cluster formation. In particular, Gaia found "tidal tails" of material created when clumps of material tug at a cluster and pull on its stars. The result is chains of stars stretching across the Galaxy, linking different parts together with similar stars. Their placement helps astronomers track the motions of the spiral arms, as well as cluster evolution.
Tidal tails aren’t just remnants of a cluster's past: they’re powerful dynamical tracers that tell the tale of a cluster's lifetime and place in the galaxy,” said Tereza Jerábková of Masaryk University, the Czech Republic, an expert in star formation and regular user of Gaia data. “Historically we’d only seen these tails around clusters in sparser areas of the Milky Way, as they stand out better against emptier skies. It was far harder to spot them in our galaxy’s densest regions – but Gaia changed all that.”
Gaia's Past and Future
The Gaia spacecraft shut down in March 2025, after a decade of data gathering. Most of its data is still being analyzed and should be ready for release in the next few years. What has been released - taken during its first three years in orbit - is what allowed scientists to make many discoveries about stars, clusters, and the structure of our Galaxy.
According to Johannes Sahlmann, ESA Project Scientist for Gaia, the still-unreleased data promises an even closer peek at the structure of the Milky Way and its stars. “Gaia’s spacecraft operations may have ended, but its contributions to science are in full swing,” he said. “It’s a really exciting time for stellar scientists. As more data are released to the scientific community in coming years, we’ll see another flurry of discoveries, further reshaping what we know about our skies in a truly transformative way.”
For More Information
Gaia Proves Our Skies Are Filled with Chains of Starry Gatherings