Terzan 5: NASA's Webb and Hubble Reveal Fossil Relic of Milky Way's Formation

A Fossil from the Dawn of the Milky Way

On June 16, 2026, researchers using two of humanity's most powerful observatories — NASA's James Webb Space Telescope and the Hubble Space Telescope — announced a landmark discovery that rewrites our understanding of how the Milky Way galaxy assembled. The team, led by PhD student Giorgia Zullo at the University of Bologna, Italy, presented definitive evidence that the star system Terzan 5 is not a globular cluster as it was long classified, but rather a "bulge fossil fragment" — a surviving relic from the earliest stages of our galaxy's formation.

The findings, presented at the 248th meeting of the American Astronomical Society in Pasadena, California, and published in the journal Astronomy & Astrophysics, show that Terzan 5 contains up to four distinct generations of stars, spanning an extraordinary 12.5 billion years. This multi-generational stellar population is unprecedented for a globular cluster — which typically contains only one ancient population — and proves that Terzan 5 is a self-contained, self-enriching stellar system that preserved its separate identity while the rest of the Milky Way's bulge formed around it.

"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, the lead researcher. The study combined Webb's infrared gaze — which pierced through the dust-heavy galactic bulge — with Hubble's unique ability to measure stellar motions over a 12-year baseline, enabling the team to separate Terzan 5's stars from those of the surrounding Milky Way bulge with exquisite precision.

What Is Terzan 5?

Discovered in 1968 by the French-Armenian astronomer Agop Terzan, Terzan 5 is a dense concentration of stars located in the constellation Sagittarius, deep within the crowded central bulge of our Milky Way galaxy. For decades, astronomers classified it as a globular star cluster — a spherical, tightly bound collection of ancient stars that all formed at roughly the same time from the same primordial gas cloud.

But Terzan 5 always seemed different. Unlike typical globular clusters, which contain a single, uniform population of stars all roughly 12–13 billion years old, Terzan 5 showed hints of complexity. In 2009, astronomers discovered that it harboured two distinct populations of stars with different ages and chemical compositions. In 2016, Hubble provided the first age estimates: one population formed roughly 12 billion years ago — as the Milky Way itself was assembling — and another formed about 5 billion years ago, just before our own Solar System began to take shape.

These early findings hinted that Terzan 5 might be something more than a globular cluster, but the evidence was not conclusive. The problem was that Terzan 5 lies in one of the most obscured regions of the galaxy — the bulge — where thick clouds of dust block visible light, making detailed observations extremely challenging. This is where Webb's infrared capabilities proved decisive.

Four Generations of Stars: A Cosmic Family Portrait

The breakthrough came when Webb turned its NIRCam (Near-Infrared Camera) toward Terzan 5. By measuring star colours and brightnesses in the near-infrared — wavelengths that penetrate dust that blocks visible light — Webb catalogued many more stars, including fainter members, than any previous study. The research team then used these measurements to classify stars into populations of different ages and chemical compositions.

But Webb faced a unique challenge: because Terzan 5 sits in the crowded galactic bulge, many of the stars Webb saw are not actually members of Terzan 5 — they are foreground or background stars in the Milky Way itself. To isolate Terzan 5's true members, the team relied on Hubble's unique long-term monitoring capability. By comparing Hubble images taken 12 years apart, they measured the tiny proper motions of individual stars — their apparent movement across the sky — to determine which stars belong to Terzan 5 and which are part of the general bulge population.

The results were extraordinary. The combined Webb and Hubble data revealed not two but four distinct stellar populations in Terzan 5:

With the previously known two generations, astronomers could not rule out the possibility that Terzan 5 had interacted with another object — like a passing globular cluster or a giant molecular cloud — triggering a second round of star formation. The discovery of four distinct generations rules out these alternative explanations. Terzan 5 formed multiple generations of stars on its own, because it was massive enough to retain the necessary raw materials — gas and dust enriched by earlier generations of supernovae — and keep forming stars over billions of years.

What Is a 'Bulge Fossil Fragment'?

The term "bulge fossil fragment" describes a stellar system that formed separately from the main body of the Milky Way's bulge and survived intact — never merging or fully "mixing in" — as the bulge itself formed around it. Think of it as a time capsule from the early universe, preserving the conditions and composition of the primordial clumps that eventually coalesced to form the galactic bulge we see today.

"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, a 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."

To date, only one other known object has been reclassified as a bulge fossil fragment: Liller 1, which was reclassified in earlier work by the same team. Liller 1 also contains multiple generations of stars, suggesting that these fossil fragments may be more common than previously thought. Ferraro's team plans to examine 40 to 50 additional globular clusters that orbit within the bulge to determine whether they are true globular clusters (single stellar population) or additional bulge fossil fragments (multiple generations).

How Webb and Hubble Worked Together to Solve the Mystery

The discovery is a masterclass in multi-observatory astronomy. Each telescope contributed capabilities that the other could not provide alone:

• Webb's infrared vision pierced through the thick dust of the galactic bulge, revealing stars that are completely invisible in visible-light observations. Webb's NIRCam measured the colours and brightnesses of thousands of stars in the near-infrared, providing the data needed to classify them into age and metallicity populations.
• Hubble's long-term monitoring — the 12-year separation between archival Hubble observations — allowed the team to measure the tiny proper motions of individual stars, definitively separating Terzan 5 members from unrelated bulge stars. Without Hubble's decades-long baseline, Webb's census would have been contaminated by foreground and background stars.
• Ground-based spectroscopy from the W. M. Keck Observatory and the European Southern Observatory's Very Large Telescope (VLT) provided chemical abundance measurements, confirming that each stellar population has a distinct chemical fingerprint — the fossil record of progressive enrichment by supernovae.

"Along with the ages of these populations, the cluster preserves a fossil record of progressive enrichment of heavy elements by supernovae," said co-author R. Michael Rich, a research astronomer at the University of California, Los Angeles. This chemical stratification is a hallmark of self-enrichment — the system generated its own heavier elements through successive generations of stars, rather than acquiring them from external sources.

Why This Matters: Galaxy Formation Near and Far

The significance of Terzan 5 extends far beyond a single star system in our galaxy. This discovery provides direct observational evidence for a key theory of galaxy formation: that galactic bulges formed from the merger of massive star-forming clumps in the early universe.

"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. These clumps migrated to the centre of the galaxies, and many merged to form their bulges," said Barbara Lanzoni, a co-author and associate professor at the University of Bologna. Webb has already turned up several examples of these "clumpy" galaxies in the early universe — including the Firefly Sparkle galaxy, which was actively forming when the universe was only a few hundred million years old.

"Terzan 5 may provide direct evidence that can help explain how bulges formed in galaxies throughout the universe," Lanzoni said. In other words, what we see in Terzan 5 today — a surviving primordial clump — may be a local analogue of the building blocks that assembled galactic bulges across the cosmos. By studying Terzan 5 in exquisite detail, astronomers can test their models of galaxy formation against a real, preserved example, improving our understanding of how galaxies like our own Milky Way came to be.

Can You See Terzan 5 From Your Backyard?

Terzan 5 is a challenging target for amateur astronomers. Located in the constellation Sagittarius, deep within the Milky Way's central bulge, it sits in one of the most crowded and dust-obscured regions of the sky. At magnitude 12.7, it is beyond the reach of typical beginner telescopes and requires at least a 200mm (8-inch) telescope under dark skies to be detected as a faint, unresolved glow.

The system is located near the galactic centre, approximately 4 degrees north-west of the bright star Kaus Media (Delta Sagittarii) and about 6 degrees north-east of the galactic centre itself. It is best observed during the summer months (June–August in the northern hemisphere) when Sagittarius is high in the night sky. Even then, the view through an amateur telescope will show only a dim, fuzzy patch — the individual stars that Webb and Hubble resolve are far too close together and faint for backyard optics.

However, this does not mean the discovery has no practical relevance for amateur observers. The real value for backyard astronomers is contextual: understanding that when you look toward the constellation Sagittarius and the bright Milky Way bulge, you are looking at a region that contains fossil relics of our galaxy's formation. Pairing this knowledge with binocular or telescope sessions targeting brighter objects in the same region — such as the Lagoon Nebula (M8), the Trifid Nebula (M20), or the bright globular cluster M22 — creates a rich, science-connected observing experience that bridges professional discoveries and personal observation.

For those interested in the technical challenge, a 250mm (10-inch) or larger telescope equipped with a narrowband filter and used at a dark-sky site may reveal Terzan 5 as a very faint, small glow under excellent conditions. But for most observers, the far more rewarding approach is to use the Terzan 5 story as motivation to explore the Sagittarius bulge region with binoculars or a wide-field telescope, enjoying the spectacular star fields that surround this ancient fossil fragment.

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Sources & References

This article is based on original reporting from primary sources. The scientific details, images, and quotes were sourced from NASA's official release and the peer-reviewed journal publication.

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  NASA: Webb, Hubble Reveal History of Relic of Milky Way's Formation

  NASA official news release, June 16, 2026. Primary source for images, quotes, and scientific findings.

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  Published Research Paper — Astronomy & Astrophysics

  The peer-reviewed paper by Giorgia Zullo et al. presenting the full analysis of Terzan 5's four stellar populations.

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  Webb Space Telescope: Terzan 5 — Bulge Fossil Fragment

  Additional context from the Space Telescope Science Institute on the Webb observations of Terzan 5.

All data and imagery used under NASA's media usage guidelines. Telescope Advisor editorial team independently verified factual details against NASA primary sources before publication. Images are locally hosted under /images/nasa/.