At the center of the Milky Way lies a dense star cluster called Terzan 5. Discovered in 1968, it was long considered an ordinary globular cluster — an ancient "ball" of stars born together.
But it turns out the stars in Terzan 5 were not born together. There are four generations here: the oldest is 12.5 billion years old, the youngest just 2.5 billion. Star formation continued in waves over 10 billion years. For a globular cluster, that's impossible — they typically contain a single generation.
Seeing this required both of our most powerful telescopes. Webb sees in infrared — it was able to pierce the dust that obscures the Galaxy's center and resolve even faint stars. Hubble observed this region over 12 consecutive years — enough time to measure tiny star movements and distinguish cluster members from nearby Milky Way stars.
Why could Terzan 5 keep forming stars again and again? It was massive enough. When stars exploded as supernovae, lighter clusters lost their gas forever. Terzan 5 held onto the material — and it became the building blocks for new stars.
Billions of years ago, similar clumps merged to form the Galaxy's central region. Terzan 5 is a rare survivor that kept its separate identity to this day. Astronomers now call such objects "bulge fossil fragments" — essentially, pieces of the primordial Galaxy that reached us nearly unchanged.
Only one other analog is known — Liller 1. But the team plans to examine 40–50 more clusters.