Do Most Stars Form in Clusters? A New Look at Our Galaxy’s Star Birthplaces
Astronomers have long debated whether stars are mostly born together in dense groups, or whether they form in looser, scattered arrangements. Two main ideas exist: the clustered model, where most stars emerge in tightly packed clusters before many of them drift apart, and the hierarchical model, where stars form across many scales of density, with only a few forming in bound clusters. In this new paper, Alexis L. Quintana and collaborators use the latest data from ESA’s Gaia satellite to re-examine this question, and their results strongly favor the clustered model.
How the Question Was Approached
Quintana and colleagues set out to measure how much of the Milky Way’s star formation happens in compact open clusters, small, dense groups of stars that may or may not remain bound over time. Thanks to Gaia’s precise measurements, astronomers now have more complete catalogs of star clusters than ever before. In particular, the team used a catalog by Hunt & Reffert (2024), which identified hundreds of young clusters and corrected for missing data. This provided a solid starting point for calculating the surface density star formation rate (essentially, how many stars are being born per unit area per million years in these clusters).
Two Different Methods
The authors applied two approaches to estimate the star formation rate in compact clusters. The first method used a power-law fit to the mass distribution of clusters younger than 10 million years, combined with statistical simulations to account for uncertainties. The second, simpler method directly counted the total stellar mass in these clusters within 1,000 light years of the Sun. Both methods gave consistent results, with star formation rates between about 740 and 875 solar masses per million years per square kiloparsec.
How the Results Compare
These values are much higher than older estimates, which were often based on incomplete catalogs. For example, a 2006 study suggested only 350 solar masses per million years per square kiloparsec formed in clusters, while a 2021 analysis gave an even lower fraction of stars forming this way. By contrast, Quintana’s team finds that at least half, and probably more than 80%, of stars in the local Milky Way are born in compact clusters. This is a big shift from previous work that leaned toward the hierarchical model.
What This Means for Star Formation
The findings suggest that the clustered model, where most stars begin life in dense groups, better describes the Milky Way. However, many of these clusters do not last very long. Some disperse within a few million years due to residual gas expulsion (the clearing out of leftover gas by young, massive stars), while others dissolve over hundreds of millions of years. The result is that many stars that look “isolated” today probably started life in a compact cluster that has since broken apart.
Challenges and Next Steps
There are still uncertainties. For example, it is difficult to know for sure whether a cluster is truly gravitationally bound, since velocity data can be tricky to interpret. Future surveys and upcoming Gaia data releases will help clarify these details. Extending studies to larger regions of the Milky Way will also test whether the same patterns hold outside the solar neighborhood.
Conclusion
Quintana and collaborators make a strong case that most stars in our corner of the Milky Way formed in compact clusters, rather than scattered individually. This not only supports the clustered star formation model but also suggests that studying how clusters form and dissolve is key to understanding the origin of most stars, including our own Sun.
Source: Quintana
