For decades, Terzan 5 sat quietly in astronomical catalogs, labeled as just another globular cluster orbiting within the dense central bulge of our galaxy. It was discovered in 1968 by astronomer Azop Terzan and, like hundreds of similar objects, largely treated as a dense ball of ancient stars—interesting, but unremarkable in the grand scheme of galactic structure.
That classification is now dead. Research presented on June 16 at the 248th meeting of the American Astronomical Society in Pasadena, and published in the journal Astronomy & Astrophysics, has used combined observations from NASA's James Webb Space Telescope and Hubble Space Telescope to prove that Terzan 5 is something fundamentally different: a "bulge fossil fragment," a self-contained stellar system that preserves a layered record of star formation stretching back 12.5 billion years.
It is, in effect, a time capsule from the epoch when the Milky Way was still assembling itself from primordial clumps of gas and stars—and one of only two such objects known to exist.
Four Generations, One System
The key finding is that Terzan 5 contains not one but four distinct populations of stars, each born in a separate episode of star formation. By combining near-infrared data from Webb's NIRCam instrument with visible-light observations from Hubble's Advanced Camera for Surveys, the research team was able to disentangle these populations with unprecedented precision.
The oldest stars formed 12.5 billion years ago, when the universe itself was barely a billion years old. A second generation appeared 4.7 billion years ago—roughly the same era when our own Sun was coalescing from a molecular cloud in a quieter arm of the galaxy. A third wave followed at 3.8 billion years, and the youngest population dates to just 2.5 billion years ago, well into the period when complex multicellular life was already established on Earth.
That 10-billion-year span of star formation is extraordinary. Typical globular clusters formed their stars in a single burst during the early universe and have been passively aging ever since. Terzan 5 kept making new stars across multiple epochs, behaving less like a cluster and more like a miniature galaxy embedded within our own.
How You Separate the Signal from the Noise
Studying anything in the Milky Way's bulge is an exercise in frustration. The region is a dense, chaotic soup of stars, and Terzan 5 sits right in the thick of it. Distinguishing stars that actually belong to Terzan 5 from unrelated bulge stars that happen to lie along the same line of sight requires more than a sharp image—it requires motion data.
This is where Hubble's long operational history became critical. The team leveraged a 12-year baseline of Hubble observations to measure the "proper motions" of individual stars—their tiny lateral movements across the sky over time. Stars physically bound to Terzan 5 share a common motion that differs from the general drift of background bulge stars, allowing the researchers to cleanly separate the two populations before analyzing ages and compositions.
Webb's contribution was different but equally essential. Its NIRCam instrument operates in the near-infrared, which penetrates the thick dust lanes that obscure the bulge at visible wavelengths. Where Hubble provided the motion baseline, Webb provided the depth, cataloging stars that were simply invisible to optical telescopes and revealing the full extent of Terzan 5's stellar populations.
The team also drew on supporting data from the W.M. Keck Observatory and the European Southern Observatory's Very Large Telescope, building a multi-facility picture that left little room for ambiguity.
A Self-Enriching Fossil
The existence of multiple stellar generations tells a story about chemistry as much as chronology. When massive stars die in supernova explosions, they seed their surroundings with heavy elements—metals, in astronomers' parlance—that get incorporated into the next generation of stars. For this recycling to work, the system has to be massive enough to gravitationally retain the material blasted outward by those explosions.
Terzan 5 cleared that bar. Its sufficient mass meant that supernova ejecta stayed put rather than being flung into the surrounding bulge, enriching the local gas reservoir and fueling successive rounds of star formation over billions of years. The team describes it as "a self-contained, self-enriching stellar system"—a phrase that underscores just how different it is from a conventional globular cluster, which lacks the gravitational heft to hold onto supernova debris.
Lighter primordial clumps were not so fortunate. Billions of years ago, during the Milky Way's formative era, countless similar structures existed throughout what would become the galactic bulge. Most of them were too small to survive intact. They spread out, lost their individual identities, and merged together to build the bulge we observe today. Terzan 5 was massive enough to resist that process, maintaining its structural integrity while the galaxy assembled around it.
The Research Team
The study was led by Giorgia Zullo, a PhD student at the University of Bologna in Italy, under the supervision of principal investigator Francesco Ferraro, a professor at the same institution. Co-authors include R. Michael Rich, a research astronomer at UCLA, and Barbara Lanzoni, an associate professor at the University of Bologna. The group has been studying Terzan 5 and objects like it for years, and this paper represents the culmination of a multi-telescope, multi-year observational campaign.
Ferraro's team is not stopping here. They have outlined plans to examine 40 to 50 additional globular clusters within the Milky Way's bulge, searching for other systems that might share Terzan 5's unusual properties. If even a handful turn out to be bulge fossil fragments rather than true globular clusters, it would reshape our understanding of how common these relics are and how much of the bulge's original building material has survived to the present day.
Not Alone, But Nearly
For now, only one other object shares Terzan 5's classification: Liller 1, another dense stellar system in the galactic bulge that has shown evidence of multiple stellar populations. Together, these two objects define an entirely new category—one that sits somewhere between a globular cluster and a dwarf galaxy on the spectrum of stellar systems.
The rarity itself is informative. If most primordial clumps were destroyed during the bulge's assembly, the survivors should be few and massive. Finding exactly that pattern—two known examples, both unusually dense—is consistent with theoretical models of hierarchical galaxy formation, where large structures are built from the merger of many smaller ones.
Why It Matters
Galactic archaeology is one of the few ways we can reconstruct the early history of the Milky Way. We cannot observe our own galaxy's formation from the outside, and we cannot rewind time. What we can do is find objects that have preserved conditions from those early epochs and read the record they carry.
Terzan 5 is that kind of object—a fossil embedded in the living body of the galaxy, carrying chemical and chronological information from 12.5 billion years ago through to 2.5 billion years ago. Its four stellar populations are like sedimentary layers in rock, each one recording the conditions and available materials at the time it formed.
The Webb-Hubble combination that made this work possible also points toward a broader methodological shift. Webb's infrared vision and Hubble's decades-long baseline of precise positional data are complementary in ways that neither telescope achieves alone. As Ferraro's team turns this dual-telescope approach on dozens more bulge clusters, the result could be a systematic census of the Milky Way's surviving primordial fragments—a fossil record of our galaxy's infancy, extracted from the crowded stellar metropolis at its center.
The Milky Way, it turns out, still carries pieces of the mess it was born from. We just needed the right tools to find them.
