Clocking the Cosmos: Measuring the Ages of Milky Way’s Ancient Star Clusters
Globular clusters (GCs) are some of the most ancient structures in our galaxy—densely packed groups of stars formed billions of years ago. In a recent study led by Jiaqi (Martin) Ying and colleagues, researchers set out to determine the absolute ages of eight Milky Way globular clusters using a combination of theoretical models and data from the Hubble Space Telescope (HST). These measurements help scientists better understand the history of our galaxy and even provide a check on the age of the universe itself.
Understanding the Data: Stars, Distances, and Brightness
The team used photometric data—essentially, precise measurements of starlight—from the HST Advanced Camera for Surveys to build color-magnitude diagrams (CMDs) for each cluster. A CMD plots the brightness of stars against their color and helps astronomers track the evolution of stars in a cluster. However, before comparing real data with models, the researchers had to account for uncertainties in the distances to the clusters and how much dust reddens the starlight (called “reddening”). To refine their models, they also included calibration stars (single stars with well-known properties) and detached eclipsing binaries (special star pairs whose properties can be precisely measured).
Building Models: Simulating Star Clusters with Monte Carlo Methods
To estimate cluster ages, the researchers used the Dartmouth Stellar Evolution Program (DSEP) to simulate how stars evolve over time. They created over 800,000 synthetic CMDs, each based on different assumptions about stellar physics, such as how efficiently stars mix materials in their interiors or how certain nuclear reactions proceed. These models were then compared to the actual CMDs using sophisticated statistical methods. This Monte Carlo approach allowed them to incorporate uncertainties in both the observations and the physics behind the models.
Fitting the Data: Two Methods, One Goal
The team applied two full-CMD-fitting techniques to determine the best matches between observed and synthetic CMDs. The first, the Voronoi Binning Method, breaks the CMD into regions and compares expected vs. observed star counts using a χ² (chi-squared) statistic. The second, the 2D Kolmogorov–Smirnov (KS) Method, looks at the overall distribution of stars and compares the cumulative patterns. While both methods are statistically rigorous, the KS method is more robust when there are hidden sources of observational uncertainty, like slight errors in measuring star brightness.
Results: Ancient, But Not Ageless
Ying and collaborators found that the clusters studied are between 11.6 and 13.2 billion years old, consistent with previous studies and slightly younger than the age of the universe itself (about 13.8 billion years). They also observed a trend: clusters with lower metallicity (fewer heavy elements) tended to be older. This makes sense, as early stars formed when the universe had not yet been enriched with heavy elements from previous generations of stars.
What’s Behind the Uncertainty?
The authors carefully broke down what contributes most to the uncertainty in determining a cluster's age. They found that the biggest sources of error are the distance to the cluster and the amount of reddening. These factors affect the placement of stars on the CMD more than any single detail of stellar physics. Other significant contributors included the mixing length (a measure of how energy moves within a star) and how elements like helium are distributed inside stars over time.
Complications: Are All Cluster Stars the Same?
Many globular clusters are now known to contain multiple populations of stars—groups that differ slightly in chemical composition or age. While this complicates the picture, the filters used in this study (F606W and F814W) are less sensitive to these subtle differences. The researchers tested for multiple populations in 47 Tucanae, one of the clusters with the clearest evidence for them, and found that any impact on the final age estimate was minimal.
Conclusion: A More Precise Cosmic Clock
By accounting for both observational and theoretical uncertainties, this study provides some of the most precise absolute age estimates for Milky Way globular clusters to date. It also demonstrates that detailed, model-based methods—especially those that use full CMDs—can significantly reduce age uncertainties. These results strengthen our understanding of how the galaxy formed and evolved, and they add another tick to the cosmic clock that tells us just how old the universe really is.
Source: Ying
