The stars in a cluster are like one big family. They are born together from a single cloud and stay close throughout their lives. Astronomers found such a cluster, Berkeley 36, and read its history like a book.
First, they determined its age: 6.8 billion years. That makes it older than our Sun, which is 4.6 billion years old. Then they looked at the chemical composition of its stars. The closer a star is born to the galactic center, the more heavy elements it contains, like soil rich in fertilizer. From this composition, they deduced that Berkeley 36 was born 6.7 kiloparsecs from the center. Now it sits at 11.4 kiloparsecs. So the cluster has moved outward by 5 kiloparsecs. It's like a family that relocated from a bustling downtown to a quiet suburb.
Previously, it was unclear whether old clusters could travel this far. Now Berkeley 36 shows that even a calm orbit doesn't prevent a grand journey. This helps astronomers map more precisely 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.