Astronomers have reclassified Terzan 5, long catalogued as a globular cluster, as a surviving remnant from the Milky Way's formation - a 'bulge fossil fragment' with four distinct stellar generations spanning billions of years.
A dense, crowded object nestled in the heart of the Milky Way has turned out to be something fundamentally different from what astronomers classified it as for decades. Research published June 16, 2026, using NASA’s James Webb Space Telescope and the Hubble Space Telescope, shows that Terzan 5 - long listed as a globular cluster - is instead a “bulge fossil fragment”: a 12.5-billion-year-old survivor from the primordial clumps that built our galaxy’s central bulge. It is, according to the research team, a direct window into the Milky Way’s earliest formation history.
Globular clusters are some of the oldest structures in the universe - tightly bound, roughly spherical groups of hundreds of thousands to millions of stars. They typically contain a single ancient stellar population, all born at roughly the same time, with the same chemical composition. What made Terzan 5 anomalous was already clear from earlier ground-based studies: it contained two distinct populations of stars of noticeably different ages, which contradicted the globular cluster model entirely.
The new observations resolved the picture more sharply. According to findings released through NASA’s Space Telescope Science Institute, the research team - led by Giorgia Zullo, a PhD student at the University of Bologna, and principal investigator Professor Francesco Ferraro - identified not two but four separate stellar populations within Terzan 5:
Spanning ten billion years of episodic star formation, this timeline is incompatible with any standard globular cluster. Globular clusters lack the gravitational mass needed to retain gas ejected by supernova explosions. Terzan 5, by contrast, appears to have held onto that material and recycled it into successive generations of new stars. That capacity points to it once being a far more massive system than it appears today - most likely a proto-galactic clump that formed early in the universe’s history.
The team proposes the classification “bulge fossil fragment” to describe Terzan 5 and objects like it: survivors of the building-block clumps that, over billions of years, gradually merged and homogenized into the Milky Way’s central bulge. Most of those clumps were absorbed. Terzan 5 was not.
“Terzan 5 may provide direct evidence that can help explain how bulges formed in galaxies throughout the universe,” the team stated in materials issued by the Space Telescope Science Institute.
The co-authors include R. Michael Rich, research astronomer at the University of California, Los Angeles, and Barbara Lanzoni, associate professor at the University of Bologna. In addition to JWST and Hubble, the study drew on observations from the W. M. Keck Observatory and the European Southern Observatory’s Very Large Telescope.
Webb’s near-infrared instruments were critical. The galactic center is shrouded by dense interstellar dust that blocks visible wavelengths almost completely. Near-infrared light passes through this dust more cleanly, allowing Webb to resolve individual stars in Terzan 5’s crowded core with clarity that was not achievable before. Cross-referencing those observations with Hubble’s archival data gave the team the broad temporal baseline needed to reconstruct the object’s full star-formation history.
The finding does not resolve every question about how galactic bulges form - there are competing theoretical models and much of the galactic center remains poorly characterized. But Terzan 5 now provides an observational anchor: a preserved, identifiable remnant from the Milky Way’s earliest assembly that can be studied in detail.
How many similar objects remain undiscovered in the galactic bulge is unknown. The central region of our galaxy is densely packed and heavily obscured. The research team notes that Webb’s capacity to cut through that dust could reveal additional bulge fossil fragments in future systematic surveys - objects that have gone unrecognized precisely because no telescope until now had the resolution and infrared sensitivity to see them clearly.
The results were published in Astronomy & Astrophysics. They do not change our understanding of UAP or anomalous phenomena, but they do illustrate the pace at which space telescope observations are revising our picture of the galaxy we inhabit - sometimes by revealing that a familiar object is something it never was.
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