For 58 years, astronomers cataloged Terzan 5 as a globular cluster — one of the ancient, densely packed balls of stars orbiting the Milky Way’s central bulge. A peer-reviewed study published June 16, 2026, in Astronomy & Astrophysics has definitively ended that classification. Using the combined power of the James Webb Space Telescope and the Hubble Space Telescope, a team led by PhD student Giorgia Zullo at the University of Bologna has confirmed that Terzan 5 contains not one but four distinct stellar generations spanning 10 billion years — evidence that rules out every globular-cluster explanation and establishes the system as the prototype of an entirely new class of cosmic object: the bulge fossil fragment.

A bulge fossil fragment is, in precise terms, a surviving shard of one of the primordial gas clumps that fell together to build the Milky Way’s central bulge billions of years ago. Most of those clumps dissolved into the galactic mass. Terzan 5 sits 22,000 light-years away in the constellation Sagittarius — near enough for Webb’s cameras to examine star by star — and it preserved, in its iron chemistry and its star-formation history, a record of how the Milky Way’s core assembled. What the researchers now see in Terzan 5 is a direct, observable specimen of hierarchical galaxy formation: the leading cosmological theory that large structures grow by the progressive merging of smaller ones.

The finding also unlocks something bigger than any single object. Ferraro’s team plans to apply the same Webb-and-Hubble methodology to 40 to 50 additional candidate clusters orbiting within the bulge, turning what was a case-by-case discovery into a systematic archaeological survey of the Milky Way’s formation history — something no previous combination of instruments made possible.

Webb’s Infrared Edge: Seeing Through 58 Years of Obscuring Dust

Terzan 5 sits inside the galactic bulge, one of the most dust-saturated environments in the entire galaxy. The average color excess along its line of sight — a measure of how much interstellar dust distorts and absorbs light — is E(B-V) = 2.38 ± 0.55. At those extinction levels, visible-wavelength instruments see only a fraction of the available stars, and the fraction they do see is smeared by spatially variable reddening that varies across the field of view.

Webb’s NIRCam (Near-Infrared Camera) bypasses this problem physically. Infrared photons — those between roughly 1 and 5 micrometers in wavelength — are not scattered the same way as visible-light photons by interstellar dust particles made of carbon, iron, ice, and silicates. Webb’s NIRCam observations used the F115W and F200W filters (1.15 and 2.0 micrometers), wavelengths where the dust that effectively blocks Hubble’s optical view becomes largely transparent. To collect enough of this faint infrared signal without thermal noise swamping the detector, Webb operates at approximately -233°C — a temperature maintained by a five-layer sunshield. The result: NIRCam can resolve roughly six times more stars per area in galactic-center fields than the best previous surveys, reaching stars far fainter and far more numerous than Hubble’s optical cameras could catalog in this region.

The research team combined those new Webb frames with 12 years of archival Hubble observations using the Advanced Camera for Surveys (ACS). That 12-year baseline gave them a critical tool: proper motions — measurements of how individual stars have moved across the sky over time. Because Terzan 5 members share a common velocity, separating them from the far more numerous foreground and background stars of the Milky Way bulge required tracking those small stellar movements, and only a 12-year baseline from a single stable space telescope could make those measurements precise enough.

Four Generations Where Only Two Were Expected

The combined dataset produced what the researchers describe as the deepest color-magnitude diagram ever obtained for Terzan 5. A color-magnitude diagram is essentially a stellar census organized by each star’s color (a proxy for temperature) and brightness (a proxy for age and mass). In a single-population globular cluster, all stars trace one isochrone — one age line — on the diagram. In Terzan 5, the diagram revealed four distinct main-sequence turnoff points.

The oldest population dates to 12.5 billion years ago, making it nearly as old as the universe itself. A second wave of star formation followed approximately 4.7 billion years ago — just before Earth’s own solar system began forming. Webb’s new data added two further episodes: one at roughly 3.8 billion years ago and another at approximately 2.5 billion years ago.

This is not a minor refinement of earlier results. When Hubble provided the first age estimates for Terzan 5 in 2016, it found two populations and could not rule out the possibility that some external event — a collision with another cluster, or gas accretion from a nearby giant molecular cloud — had triggered a second round of star formation. Two populations, while unusual, fall within a range that single-event explanations could accommodate. Four populations spanning 10 billion years cannot. No interaction event, no matter how complex, can explain self-contained star formation episodes at 12.5, 4.7, 3.8, and 2.5 billion years ago in the same system.

“Webb’s new near-infrared observations, cross-referenced with Hubble’s archival observations, have given us a much clearer picture of the history of Terzan 5,” said Zullo, who led the study.

How Terzan 5 Kept Forming Stars When Others Couldn’t

The physical explanation for four consecutive star-formation episodes comes down to mass and self-enrichment. Each generation of stars in Terzan 5 ended with a population of supernovae — massive stars that exploded and scattered heavy elements (iron, calcium, silicon) into the surrounding gas. In lower-mass systems, those explosion shockwaves are energetic enough to blast all remaining gas out of the cluster entirely, ending star formation permanently. Terzan 5’s progenitor was massive enough — estimates place its initial mass at perhaps 100 million times the Sun’s mass — to hold on to that gas despite the supernova blasts. Each wave of heavy elements enriched the retained gas, which then collapsed into the next generation of stars with a progressively higher iron content.

The iron-abundance fingerprints of all four populations, measured through high-resolution spectroscopy at the W. M. Keck Observatory and the European Southern Observatory’s Very Large Telescope, match the iron content found in stars across the galactic bulge field itself. That chemical alignment is a strong independent confirmation that Terzan 5 formed inside the bulge and remained chemically linked to it — not an interloper captured from elsewhere. As Zullo et al. 2026 Astronomy & Astrophysics records, Dr. R. Michael Rich of UCLA put it: Terzan 5 “preserves a fossil record of progressive enrichment of heavy elements by supernovae.”

Milky Way Formation Written in One Surviving Fragment

Astronomers have long debated exactly how the Milky Way’s central bulge assembled. Two broad scenarios compete: monolithic collapse, in which the bulge formed rapidly and all at once from a single collapsing gas cloud, and hierarchical assembly, in which massive primordial clumps migrated inward and merged over billions of years. Computer simulations strongly favor the hierarchical model, but observational proof — an actual surviving specimen of one of those primordial clumps — has been rare.

Terzan 5 now provides that specimen directly. “Based on observations and in-depth simulations, we think that galaxies in the early universe had huge disks of gas that fragmented into clumps and formed stars,” said Barbara Lanzoni, associate professor at the University of Bologna and a co-author of the study. “These clumps migrated to the centre of the galaxies, and many merged to form their bulges.” Terzan 5, she said, may provide direct evidence explaining how bulges formed in galaxies throughout the universe.

Webb’s broad science program adds a third line of evidence here. The telescope has imaged so-called “clumpy” galaxies in the distant early universe — actively forming structures packed with dense, star-forming knots — including objects like the Firefly Sparkle. Terzan 5 may represent what one of those ancient clumps looks like billions of years later: still coherent, still distinguishable from the merged mass around it, still carrying its chemical autobiography.

“For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” said Francesco R. Ferraro, professor at the University of Bologna and principal investigator of the Webb observations. “Terzan 5 is what we now call a bulge fossil fragment because it resembles the primordial clumps that contributed to the formation of the bulge.” — NASA press release on Terzan 5

What Webb Made Possible: Galactic Archaeology at Scale

The Terzan 5 result is significant not only for what it confirms about one object but for the survey it makes possible. Ferraro’s team will apply the NIRCam-plus-Hubble proper-motion methodology to between 40 and 50 additional globular-cluster-like systems orbiting inside the Milky Way’s bulge, according to the ESA Webb press release on Terzan 5.

Only one other object — Liller 1 — has previously been reclassified as a bulge fossil fragment. Terzan 5 is now the prototype of the class and the first confirmed by a JWST-quality color-magnitude diagram. Whether dozens of other globular clusters in the bulge turn out to be misclassified fossil fragments, or whether Terzan 5 and Liller 1 are genuinely rare survivors, will depend on the survey Ferraro’s team is now equipped to run. The question has direct implications for how much of the Milky Way’s bulge assembled from these primordial clumps rather than forming by other mechanisms.

The results were presented at the 248th meeting of the American Astronomical Society in Pasadena, California, and published in Astronomy & Astrophysics (A&A 709, A212 (2026); DOI: 10.1051/0004-6361/202659349). The study was led by Giorgia Zullo at the University of Bologna, with co-investigators from the University of Bologna, INAF, Indiana University, the University of Edinburgh, UCLA, and the European Southern Observatory.

Frequently Asked QuestionsWhat is a bulge fossil fragment, and why does Terzan 5 qualify?

A bulge fossil fragment is a surviving remnant of one of the massive primordial gas clumps that merged billions of years ago to form the central bulge of a galaxy like the Milky Way. Most of those clumps dissolved into the merged mass. Terzan 5 qualifies because it contains four distinct stellar generations — formed at 12.5, 4.7, 3.8, and 2.5 billion years ago — with iron abundances matching the galactic bulge itself. That combination of multi-episode star formation and chemical alignment with the bulge is only explicable if Terzan 5 is a self-contained primordial system massive enough to retain gas and forge new stars repeatedly. No external interaction event can produce four separate episodes spanning 10 billion years.

How does Webb see through the dust in the Milky Way’s center?

Infrared light between roughly 1 and 5 micrometers in wavelength is not scattered the same way as visible light by interstellar dust particles made of carbon, iron, ice, and silicates. Webb’s NIRCam instrument detects exactly those wavelengths, using two filters centered at 1.15 and 2.0 micrometers, while operating at -233°C to prevent thermal noise from drowning out faint signals. The result is that Webb can resolve roughly six times more individual stars per area in galactic-center fields than any previous survey, reaching stars too faint and too obscured for Hubble’s optical cameras. Hubble’s 12-year proper-motion baseline then allowed the team to separate Terzan 5’s own stars from the surrounding bulge field stars — a combination no previous instrument pair could achieve simultaneously. See the NASA Webb and Hubble Terzan 5 study for full methodology details.

What does Terzan 5 tell us about how our galaxy formed — and how many more fossils might exist?

Terzan 5 provides direct observational support for the hierarchical assembly model of galaxy formation, in which the Milky Way’s central bulge grew by the merging of many smaller primordial clumps rather than collapsing all at once. It is now the prototype of the bulge fossil fragment class — the first such object confirmed by a JWST-quality census. Only one other system, Liller 1, has previously been identified as a fossil fragment. The Webb-plus-Hubble methodology used for Terzan 5 will now be applied to 40 to 50 additional globular-cluster-like systems orbiting inside the bulge, which could substantially change the known count of surviving primordial fragments and sharpen the picture of how much of the galactic bulge assembled from these building blocks.

What separates Terzan 5 from an ordinary globular cluster?

Conventional globular clusters form all their stars in roughly one ancient burst and thereafter evolve quietly — a single stellar population with a uniform age and chemical composition. Their color-magnitude diagrams show one main-sequence turnoff point. Terzan 5 shows four. Each represents a distinct star-formation episode separated by billions of years, each with progressively higher iron content from accumulated supernova enrichment. This extended, episodic star-formation history is only possible in a system massive enough to retain gas after its first generation of stars exploded, which is the defining property of a primordial galactic-bulge clump. A standard globular cluster — whether it formed in the halo or was captured from elsewhere — has no mechanism to produce that pattern. Full documentation of Terzan 5’s multi-age populations is in Zullo et al. 2026 Astronomy & Astrophysics.