Globular clusters are some of the oldest and most fascinating structures in our galaxy. They are dense groups of stars bound together by gravity, and they often contain multiple stellar populations (MPs) with distinct chemical and dynamical characteristics. This study, led by E. I. Leitinger and collaborators, investigates the 3D kinematics of stars in 30 globular clusters in the Milky Way, offering insights into their history and evolution.

What Are Stellar Populations?

Globular clusters host two primary types of stellar populations: the “primordial” (P1) stars, which have chemical similarities to field stars outside the cluster, and “enriched” (P2) stars, which are richer in certain elements like nitrogen and sodium. The origins of these populations remain a puzzle. Scientists hypothesize that the stars formed in separate phases, with enriched stars being born from the material processed by the first generation of stars. Observing how these populations move within clusters can reveal how these clusters and their stars formed billions of years ago.

How Were the Clusters Studied?

Leitinger’s team used data from two major sources: the Hubble Space Telescope (HST) and the European Space Agency’s Gaia mission. These telescopes provided detailed measurements of star movements across the sky (proper motions) and their speeds toward or away from us (radial velocities). For a deeper look, the researchers also analyzed data from powerful ground-based instruments like the MUSE and MEGARA spectrographs.

The study extended observations to the outer regions of each cluster, where the stars’ movements are less affected by interactions and more likely to retain clues about their formation. By combining data from different sources, the team reconstructed the 3D motion of thousands of stars, giving a complete picture of how these clusters rotate and how their stars move.

What Did They Find?

Out of 30 clusters, 21 showed significant rotation. Interestingly, the study found few differences in the rotation speeds or axes of P1 and P2 stars, with one notable exception: the cluster NGC 104, where P2 stars rotated faster. This suggests that, in most clusters, dynamical processes over time have erased much of the initial differences between these populations.

The researchers also measured anisotropy, which describes whether star movements are more radial (inward/outward) or tangential (circular). They found that younger clusters, which have experienced less dynamical evolution, often show radial anisotropy in their outskirts. In contrast, more evolved clusters exhibit isotropic (random) or tangential motions, reflecting the effects of billions of years of gravitational interactions.

Why Does This Matter?

Understanding the kinematics of globular clusters is like peeling back layers of history. For example, the rotation and anisotropy of a cluster can reveal whether it formed within the Milky Way or was captured from another galaxy. Additionally, studying the differences (or lack thereof) between stellar populations provides clues about how stars were born and interacted in these ancient environments.

Conclusion

This study highlights the power of combining space- and ground-based observations to study the oldest structures in our galaxy. While many questions remain about the origins of multiple stellar populations, the findings bring us closer to understanding the life and times of globular clusters. With ongoing improvements in telescope technology, the mysteries of these stellar relics may soon be fully unraveled.

Source: Leitinger