Globular clusters are dense 'balls' of hundreds of thousands of stars that orbit the center of the Milky Way. Some were born in our Galaxy, others were captured when the Milky Way swallowed dwarf satellite galaxies. They can be distinguished by chemical composition, but astronomers now have a new tool — dynamical age.
Dynamical age shows how much a cluster has already 'shaken up' its structure under the tidal forces of the Galaxy. It's like watching a crowd of dancers: some are already tired and huddled together, while others are still moving energetically. It turns out that among 'immigrant' clusters, 61% are dynamically young, while among 'native' ones, only 38% are. If you remove the mysterious low-energy clusters, the fraction of young among immigrants rises to 70%.
Perhaps their parent galaxy plunged into the Milky Way very early, and our Galaxy's gravity had time to 'age' these clusters. Thus dynamical age helps reconstruct the violent history of mergers that the Milky Way has undergone.
as written by the authors arXiv:2604.26012
The bifurcated age-metallicity relation of globular clusters (GCs) in the Milky Way (MW) shows that GCs are either originated in situ or accreted into the Galaxy from former satellites of the MW. The effects of the Galactic tidal field can leave signatures on the dynamical evolution and structural properties of GCs. We present a homogeneous census of dynamical ages for a sample of 93 GCs in the MW, coupled with the knowledge of their common progenitors from the chemo-dynamical parameters from the Gaia mission, unavailable some years ago. We found that the majority of accreted GCs (61%) is dynamically young. This percentage drops to 38% for in situ GCs. Excluding the enigmatic low-energy (LE) GCs, with ambiguous origin, the fraction of dynamically young ex situ GCs raises to 70%. A two tail Kolmogorov-Smirnov test shows that the distribution of dynamical ages of LE GCs cannot be distinguished from the distribution of in situ bulge and disc GCs. Yet, the LE GCs are firmly located on the satellite branch of the age-metallicity relation. An explanation may be that the progenitor of LE GCs plunged very early into the MW, so that the gravitational field of the MW had time enough to act on the associated GCs. The dynamical ages offer a statistically robust evidence corroborating the scenario of an early accretion proposed by several recent studies on the origin of LE GCs.
The most massive globular cluster in the Milky Way, Omega Centauri, contains about 10 million stars, yet its diameter is only about 150 light-years: stars there are packed a million times denser than in the solar neighborhood.
In Arthur C. Clarke's novel 'The City and the Stars', globular clusters are described as natural refuges for ancient civilizations that survived the death of galaxies.