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.
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.