Mapping the Many Lives of Omega Centauri: Untangling 14 Stellar Families in the Milky Way’s Most Complex Cluster

Omega Centauri, the largest and most enigmatic globular cluster in the Milky Way, has long puzzled astronomers. In this paper, Callie Clontz and collaborators present a detailed “subpopulation census” using the oMEGACat dataset, which combines space-based imaging from the Hubble Space Telescope and ground-based spectroscopy from the MUSE instrument. Their goal is to identify and characterize the multiple generations of stars that make up the cluster, connecting how they differ in age, chemistry, and formation history.

Introduction: The Puzzle of Omega Centauri

Unlike most globular clusters, Omega Centauri (ω Cen) shows a wide spread in metallicity, the amount of heavy elements in its stars, suggesting it contains multiple stellar populations. Previous studies had already identified metal-poor, intermediate, and metal-rich groups, but the exact number and origin of these populations remained uncertain. Many researchers have proposed that ω Cen is not a typical cluster at all but the leftover core of a small galaxy that merged with the Milky Way.

Data and Methods: Building a Galactic Family Tree

Clontz and colleagues used over 300,000 stars from oMEGACat, each with precise brightness and metallicity measurements. To identify distinct subpopulations, they created three-dimensional “maps” using metallicity and color data derived from specialized Hubble filters. These maps, called chromosome diagrams, allow astronomers to separate stars by subtle differences in chemical makeup.

Their analysis followed four main phases: preparing the sample, identifying clusters of similar stars, propagating these groupings to fainter stars, and refining the results using stellar models. The team used machine-learning methods (including K-means clustering) to isolate 14 subpopulations spanning the red giant branch down to the main sequence turnoff, effectively tracing each “family” of stars through their life stages.

Results: Fourteen Families, Three Main Lineages

Each of the 14 subpopulations falls into one of three main groups, labeled P1 (primordial), Im (intermediate), and P2 (processed). These labels reflect differences in chemical composition: P1 stars are helium- and nitrogen-poor, P2 stars are enhanced in these elements, and the Im group falls between the two. The P2 stars are also found to be, on average, about 1 billion years younger than P1 stars, while the Im populations have intermediate ages.

This age structure is revealed through the age–metallicity relation, which plots a star’s age against its metal content. Interestingly, the team discovered that the familiar “two-stream” pattern in this diagram does not correspond directly to the three branches seen in the chromosome diagram, indicating multiple overlapping formation pathways.

Discussion: A Complicated History of Star Formation

The researchers interpret these findings as evidence that ω Cen underwent several distinct waves of star formation, each producing stars with different chemical fingerprints. The P1 populations likely formed first within the cluster itself (an “in-situ” origin), while the younger, metal-rich P2 populations may have formed from material accreted from outside sources (“ex-situ”). The intermediate populations may bridge these events, suggesting gradual enrichment rather than discrete bursts.

Comparisons with earlier studies, especially the work of Bellini et al. (2017) and Dondoglio et al. (2025), show broad agreement in the number of subpopulations (around 14–15), though the exact details of their origins differ. Clontz et al. emphasize that no single model yet explains all observed features, but their results set strong constraints on future theories of how complex clusters like ω Cen assemble.

Conclusions: A Blueprint for Galactic Archaeology

This study represents the most detailed mapping yet of ω Cen’s internal structure. By connecting the chemical and age properties of 14 distinct stellar groups, Clontz and colleagues show that the cluster’s history cannot be explained by a single formation event. Instead, it likely formed through a mixture of internal evolution and external accretion, consistent with the idea that it was once the nucleus of a small galaxy captured by the Milky Way.

Source: Clontz