NASA's Webb and Hubble Telescopes Reveal the History of Terzan 5, a Relic of the Milky Way's Formation

Using two of humanity's most powerful astronomical observatories — NASA's James Webb Space Telescope (JWST) and the Hubble Space Telescope — researchers have conclusively confirmed that Terzan 5, long classified as a globular cluster, is in fact something far more extraordinary. The discovery offers a new perspective on how galaxies like the Milky Way form and evolve. While globular clusters typically contain only a single, ancient population of stars, new data not only confirms the existence of two distinct stellar populations within Terzan 5 but also provides evidence of two additional, more recent episodes of star formation.

Although Terzan 5 lies within the densely packed star field of the Milky Way's central bulge, its substantial mass allowed it to maintain its independence billions of years ago, even as lighter systems dispersed and merged into the bulge. Researchers describe it as a lump in an otherwise smooth batter.

"Webb's new near-infrared observations, cross-referenced with Hubble's historical data, have given us a much clearer picture of Terzan 5's history," said Giorgia Zullo, a doctoral student at the University of Bologna and lead researcher on the study.

The findings were presented at a press conference during the 248th meeting of the American Astronomical Society and published in the journal Astronomy & Astrophysics.

Four Generations of Stars

Terzan 5 was discovered by astronomer Alain Terzan in 1968 and appeared visually similar to a globular cluster. However, in 2009, scientists identified two distinct stellar populations within the system. In 2016, Hubble provided the first age estimates for these two populations: one formed approximately 12 billion years ago — during the period when the Milky Way was assembling itself — and another formed around 5 billion years ago, shortly before Earth began to take shape. This indicated that Terzan 5 has a far more complex history than a typical globular cluster.

Studying Terzan 5 is challenging due to its location — buried deep within a densely starred, heavily dust-obscured region of the galaxy. This is where Webb proves invaluable. Its infrared observing capabilities allow researchers to pierce through the dust, observing and cataloguing far more stars — and far fainter ones — than ever before. By measuring the color and brightness of individual stars, astronomers can classify them into populations of different ages and chemical compositions.

Webb can measure these key properties for every star in its field of view, including both Terzan 5 members and unrelated foreground stars. To isolate the stars that truly belong to Terzan 5, the team drew on Hubble's 12-year observational baseline, using measurements of the tiny movements of individual stars — known as proper motions — to determine which stars belong to Terzan 5 and which belong to the galactic bulge.

Combining Webb and Hubble data, the researchers found compelling evidence for two additional stellar populations: one formed 3.8 billion years ago, and another formed as recently as 2.5 billion years ago. They also confirmed the ages of the two previously known populations with unprecedented precision — 12.5 billion years and 4.7 billion years ago, respectively.

With only two stellar generations, astronomers could not rule out the possibility that Terzan 5 had interacted with another object — such as a globular cluster or a giant molecular cloud — replenishing gas and dust to trigger a second round of star formation. With four generations, all such alternative explanations can be ruled out.

Measurements of the chemical compositions of stars across Terzan 5's populations, carried out at the W. M. Keck Observatory and the European Southern Observatory's Very Large Telescope (ESO VLT), further point to distinctly different population characteristics. "Beyond the ages of these populations, the cluster preserves a fossil record of the progressive enrichment of heavy elements by supernovae," said R. Michael Rich, research astronomer at UCLA and co-author of the study.

Terzan 5's ability to form multiple generations of stars stems from its capacity to retain the necessary raw materials. Evidence indicates that powerful supernova explosions occurred within Terzan 5, synthesizing heavier elements that were subsequently incorporated into later generations of stars. In lower-mass systems, the shockwaves from such explosions can expel these newly formed elements along with remaining gas and dust. The progenitor of Terzan 5 had sufficient mass to retain this ejected material, enabling new stellar generations to continue forming over billions of years.

A 'Bulge Fossil Fragment'

The findings suggest that Terzan 5 is most likely the remnant of a far more massive stellar system, whose progenitor first formed 12.5 billion years ago. What makes Terzan 5 exceptional is that it survived without fully merging or dissolving into the galactic bulge. "For some reason, this particular stellar clump formed independently of the bulge and was not disrupted during the bulge's own formation," said Francesco R. Ferraro, professor at the University of Bologna and principal investigator of the Webb observing program. "Terzan 5 is what we now call a 'bulge fossil fragment,' because it resembles the primordial clumps that originally contributed to the bulge's formation."

To date, only one other known cosmic object falls into this same category. Liller 1 is the second object to be reclassified from a globular cluster to a bulge fossil fragment, and it too contains multiple stellar generations. More such objects may yet be discovered. Ferraro's team plans to examine another 40 to 50 globular clusters orbiting within the bulge to determine whether their stellar populations are uniform — as in typical globular clusters — or multi-generational, as in bulge fossil fragments.

Implications for Galaxy Formation Research

Ultimately, this research may deepen our understanding of how the central bulges of galaxies form over hundreds of millions of years. "Based on observations and deep simulations, we believe that early-universe galaxies had massive gas disks that fragmented into clumps and formed stars. These clumps migrated toward the galactic center, and many merged to form the bulge," said Barbara Lanzoni, associate professor at the University of Bologna and co-author of the study. Webb has already identified examples of 'clumpy' galaxies actively forming stars when the universe was only a few hundred million years old — such as the clumps observed in the Firefly Sparkle galaxy. "Terzan 5 may provide direct evidence to help explain how galactic bulges across the universe formed," Lanzoni said.

The James Webb Space Telescope is the world's premier space science observatory, led by NASA in partnership with the European Space Agency (ESA) and the Canadian Space Agency (CSA). The Hubble Space Telescope, now more than thirty years into its mission, continues to deliver groundbreaking discoveries that deepen our fundamental understanding of the cosmos and is a joint NASA–ESA international collaboration.

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