Primordial Open Cluster Groups: The Role of Supernovae in Star Formation

Stars in the Milky Way don’t always form in isolation. Instead, they often emerge in clusters—groups of stars that form from the same giant molecular cloud (GMC). Some clusters remain close together, forming open cluster (OC) groups, but how exactly these groups form and evolve has remained an open question. Thanks to precise astrometric data from the Gaia space mission, Liu et al. (2025) explore how some of these OC groups might have been shaped by supernovae (SNe)—the violent explosions of massive stars.

Identifying Open Cluster Groups

To understand the origins of OC groups, Liu and collaborators used data from Gaia to look for clusters that share similar positions, velocities, and ages. This allowed them to identify four distinct OC groups, which they labeled G1, G2, G3, and G4. Each group consists of multiple OCs that are gravitationally bound—or at least were in the past. These groups span several hundred parsecs in size, which is typical for primordial OC groups.

The authors ran N-body simulations, which track the motion of the clusters over time, to see how these groups have evolved. The results showed that, over millions of years, these OC groups gradually disperse as individual clusters drift apart, eventually becoming independent OCs scattered throughout the galaxy.

Supernovae as Triggers for Star Formation

One of the key discoveries in this study is the potential role of supernova explosions in triggering the formation of OC groups. When a massive star explodes, the shockwave from the explosion can compress nearby gas, triggering new rounds of star formation. The researchers found that in G1 and G2, the clusters’ ages follow an interesting pattern: the older clusters are closer to a predicted supernova explosion site, while the younger ones are farther away. This supports a sequential star formation scenario, where a supernova explosion set off a chain reaction of star formation in a nearby gas cloud.

However, not all OC groups show this pattern—G3 and G4 lack clear evidence of supernova-triggered star formation. This suggests that different mechanisms may be responsible for forming OC groups.

Pulsars as Clues to Past Supernovae

To strengthen their hypothesis, Liu et al. searched for pulsars—the rapidly spinning remnants of dead stars left behind after a supernova explosion. Pulsars act like cosmic clocks, and their motion through space can help trace back to where the original explosion occurred.

By studying pulsar motion, the researchers found that several pulsars likely originated from the same regions where they predicted supernovae had occurred. Specifically, they identified three pulsars near G1 and 26 near G2 whose birthplaces matched the predicted explosion sites. This provides further evidence that supernovae played a role in forming these OC groups.

Conclusions and Implications

This study supports the idea that supernova-triggered star formation is an important process in shaping OC groups. The findings also reinforce the hierarchical star formation model, where clusters form in a sequential, interconnected way within GMCs rather than as isolated events.

While the study provides strong evidence for this scenario, the authors acknowledge that their models rely on assumptions about the initial conditions of star-forming regions. Further studies—perhaps using even more precise data from future space missions—could help refine our understanding of how OC groups form and evolve over time.

Understanding OC groups is not just about star clusters—it also helps us piece together the history of the Milky Way and how massive stars influence their cosmic neighborhoods. The next step will be to investigate more OC groups and determine whether supernova-triggered formation is a widespread phenomenon across the galaxy.

Source: Liu