A strange object hiding in the crowded center of our galaxy turns out to be a leftover from the Milky Way's birth. It is not a globular cluster as astronomers long believed. It is something far rarer: a surviving fossil fragment from the galaxy's early assembly.
Four bursts of star birth, not one
Researchers using the James Webb Space Telescope and the Hubble Space Telescope have confirmed that Terzan 5 contains four separate generations of stars. Normal globular clusters hold just one ancient population. This object experienced at least four distinct episodes of star formation, the most recent happening about 5 billion years ago, just before Earth began to form. The oldest stars in Terzan 5 date back roughly 12 billion years, when the Milky Way itself was still taking shape.
A lump that never mixed in
Terzan 5 sits in the Milky Way's bulge, the dense, spherical region of old stars at the galaxy's core. Billions of years ago, many similar primordial clumps spread out and merged to create the bulge we see today. Terzan 5 was massive enough to hold its own identity while lighter systems blended in. Lead researcher Giorgia Zullo, a PhD student at the University of Bologna in Italy, said Webb's near-infrared observations combined with 12 years of Hubble data gave a much clearer picture of the object's history. The findings were presented at the 248th meeting of the American Astronomical Society and published in Astronomy & Astrophysics.
Why it matters that Terzan 5 is not a cluster
Discovered in 1968 by astronomer Azop Terzan, the system was first classified as a globular cluster. In 2009, scientists found it harbored two distinct star populations, which hinted at something unusual. Hubble's 2016 age estimates confirmed the two populations and suggested a complex past. But studying Terzan 5 is difficult because it lies in a region crowded with stars and thick with dust. Webb's infrared vision cut through that dust, allowing the team to catalog many more stars, including fainter ones, than ever before. By measuring star colors and brightnesses, astronomers sorted them into populations and confirmed the object as the prototype of a new class: bulge fossil fragments.
This discovery does not rewrite the story of the Milky Way. It fills in a missing piece. Terzan 5 is a relic that stayed intact while the rest of the galaxy's building blocks dissolved into the bulge. It offers a direct window into the processes that shaped our galaxy more than 10 billion years ago.
