A resident of a bustling city center one day packs a suitcase and moves to a quiet suburb. The same happens to galactic clusters, except instead of a suitcase, they have stars, and instead of a move, they have billions of years of drifting across the disk.
Berkeley 36 is an open cluster, that is, a group of stars born together from a single cloud. Astronomers checked its ID using three instruments: the Gaia satellite, which precisely measures stellar positions and motions; the Gaia-ESO spectrograph, which reads chemical compositions; and the TESS telescope, which monitors brightness. It turns out the cluster is 6.8 ± 0.5 billion years old. For comparison, the Sun is 4.6 billion years old. Berkeley 36 has aged but not fallen apart.
The most interesting part is where the cluster was born and where it ended up. The chemical composition of stars acts like a registered address: the closer to the galactic center a star is born, the more heavy elements it contains. From the metal content, astronomers calculated that Berkeley 36 was born 6.7 kiloparsecs from the center. It now sits at 11.4 kiloparsecs. The difference is 5 kiloparsecs, or 16,000 light-years. The cluster has moved outward, like a family that left the city center and settled in the suburbs.
Yet its orbit remains nearly circular. Usually, when an object migrates outward, its orbit stretches like a rubber band. That did not happen here. The secret lies in the galaxy's spiral arms: they act like giant stirrers, exchanging angular momentum with stars and clusters. Berkeley 36 was not torn apart but gently moved, preserving its orbital shape.
Before this work, it was unclear how far and in what manner old clusters could migrate. Berkeley 36 shows that even with a nearly circular orbit, a shift of 5 kiloparsecs is possible. It is like learning that your neighbor down the hall once lived in the city center but moved not because they were kicked out, but because they wanted some quiet. Now that neighbor is a valuable witness to how the galaxy stirs its stars.
Berkeley 36 is older than the Sun: it is 6.8 billion years old, while our star is only 4.6 billion. During its lifetime, it has completed about 20 full orbits around the Galactic center, assuming an orbital period of 345 million years.
The idea of star clusters migrating through the galactic disk echoes the plot of Isaac Asimov's novel "Foundation," where the planet Trantor, capital of the Galactic Empire, lies at the very center of the Galaxy, and its inhabitants have forgotten the outer worlds. Berkeley 36, by contrast, was born in the densely populated inner disk and drifted toward the outskirts, like a wandering star leaving the bustling center for the quiet outer frontiers.