A study in Astronomy & Astrophysics combines Webb’s infrared vision with more than a decade of Hubble observations to reveal a layered history inside Terzan 5. The crowded stellar system near the Milky Way’s center appears to contain up to four generations of stars, giving astronomers a rare surviving record from the galaxy’s early formation.
By pairing the strengths of two space telescopes, researchers traced stars through dust, sorted true members from foreground and background objects and sharpened the age map of this unusual system. The result turns Terzan 5 into a cosmic archive, one that may preserve clues from the time when the Milky Way’s central bulge was still being assembled.
The work draws on the NASA/ESA/CSA James Webb Space Telescope and the NASA/ESA Hubble Space Telescope. Together, they show that Terzan 5 formed stars in several distinct waves. That pattern points to a self-contained system that held onto enriched material and used it to build new stars again and again.
A relic near the galactic center
Terzan 5 sits in the Milky Way’s bulge, the dense central zone where old stars, thick dust and overlapping stellar traffic make observations difficult. Astronomers have studied the object for decades because it looks compact and crowded, like the familiar spherical star swarms known as globular clusters.
Its history now looks far richer. A typical globular cluster usually forms most of its stars in a short early burst. Terzan 5 carries signs of repeated star formation across billions of years. That makes it valuable for researchers who want to reconstruct how the Milky Way built its center.
The system was discovered in 1968 by astronomer Agop Terzan. Later observations hinted at something unusual in its chemistry and stellar ages. The new Webb and Hubble analysis gives that story a clearer timeline, with several populations that can be measured more precisely than before.
Near the galactic center, dust blocks much of the visible light that astronomers use to study distant stars. Terzan 5 is buried in that difficult region. Its survival and location make it a useful probe of the Milky Way bulge, where the oldest phases of our galaxy’s growth are hard to read directly.
Four generations of stars
The central result is striking. Terzan 5 appears to hold up to four distinct stellar populations, with estimated ages of about 12.5 billion years, 4.7 billion years, 3.8 billion years and 2.5 billion years. Those ages turn the system into a layered timeline rather than a single burst of ancient star birth.
Earlier work had already found two major groups of stars. In 2009, researchers identified two populations with different properties. Hubble observations later helped estimate their ages, placing one group near the early history of the Milky Way and another far later in cosmic time.
Webb expanded that picture by detecting fainter and more deeply embedded stars. Its near-infrared data helped reveal two additional populations, one roughly 3.8 billion years old and another about 2.5 billion years old. The study also refined the older estimates to about 12.5 billion and 4.7 billion years.
Those separate age groups are central to the interpretation. A system that forms stars in several episodes needs a way to retain or gather material over time. In Terzan 5, researchers argue that the evidence favors a long-lived, self-enriching system with its own internal chemical history.
Why Webb changed the picture
Webb’s contribution begins with infrared light. Dust that blocks visible wavelengths can be more transparent at infrared wavelengths, allowing astronomers to see deeper into crowded regions near the galactic center. For Terzan 5, that advantage was essential.
Hubble supplied a second kind of power. Because its observations span about 12 years, researchers could measure tiny shifts in stellar positions. Those proper motions helped separate stars that truly belong to Terzan 5 from unrelated stars in the Milky Way’s bulge.
“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 Giorgia Zullo of the University of Bologna, who led the research.
The combination matters because crowded star fields can fool astronomers. A star can appear to sit inside Terzan 5 while actually lying in front of it or behind it. Hubble’s motion measurements helped clean the sample, while Webb’s sensitivity helped fill in the fainter parts of the population.
Once the team had a clearer census, the color and brightness of the stars could be used to estimate their ages. In stellar astronomy, those patterns work like a clock. Stars of different masses and compositions occupy different positions in a color-magnitude diagram as they age.
How Terzan 5 enriched itself
The multiple populations suggest that Terzan 5 kept enough material to make new generations of stars. That process would require gas, dust and heavy elements produced by earlier stars. Over time, later stars could form from material enriched by stellar deaths.
Supernova explosions are key to that cycle. Massive stars live fast and die violently, forging and dispersing heavy elements into their surroundings. If a stellar system is massive enough, or if its environment helps confine material, some of that enriched gas can remain available for future star formation.
In Terzan 5, the four age groups strengthen the case for internal evolution. The object appears to have experienced repeated episodes of star birth, separated by long intervals. That pattern supports the idea of a self-enriching stellar system with a complex chemical memory.
The word “fossil” fits because the system preserves traces of processes that shaped the young galaxy. Its stars are living records. Their ages and compositions hold information from several eras, starting when the Milky Way was still forming its central structure.
A rare bulge fossil fragment
Astronomers now place Terzan 5 in a rare category called bulge fossil fragment. The phrase describes a surviving stellar clump that resembles the building blocks thought to have helped form the Milky Way’s central bulge.
“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,” said Francesco R. Ferraro of the University of Bologna, principal investigator of the Webb observations.
Ferraro also highlighted the mystery of its survival. “For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” he said.
That survival makes Terzan 5 scientifically important. Many early clumps likely merged into the growing galaxy and became impossible to distinguish. Terzan 5 seems to have kept enough of its identity for astronomers to study it as a remnant of that early construction phase.
Only one other known object, Liller 1, has been reclassified in a similar way. The comparison suggests that Terzan 5 may be the clearest example of a small class of ancient survivors hiding in the Milky Way’s central regions.
What this means for galaxy formation
The story reaches beyond one crowded object in Sagittarius. Galactic bulges are common features in large galaxies, but their formation histories can be difficult to untangle. The Milky Way offers a close laboratory, yet its center is obscured and crowded.
Researchers think early galaxies may have contained massive gas-rich disks that broke into clumps. Those clumps formed stars, interacted and moved inward. Over time, many of them may have merged into the dense central bulges seen in mature galaxies.
Terzan 5 gives astronomers a possible surviving example of such a clump. Its ancient population dates back about 12.5 billion years. Its younger populations show that the system continued forming stars long after that first episode.
“Terzan 5 may provide direct evidence that can help explain how bulges formed in galaxies throughout the universe,” said Barbara Lanzoni, a co-author of the work and associate professor at the University of Bologna.
This is why a small stellar system can matter to a much larger question. If Terzan 5 preserves the signature of a primordial bulge-building clump, its stars can help test models of how galaxies assembled their central regions across cosmic time.
The search for more Milky Way fossils
The Webb and Hubble result also sets up the next phase of the investigation. Ferraro’s team plans to examine dozens of other compact stellar systems orbiting inside the Milky Way bulge. The goal is to learn whether Terzan 5 and Liller 1 are rare exceptions or members of a hidden population.
That search will require the same careful approach. Bulge objects sit behind dust and among many unrelated stars. Infrared observations can pierce the haze, while long-term motion measurements can identify which stars truly belong to each system.
Finding more examples would give astronomers a larger sample of ancient stellar populations linked to bulge formation. Each object could carry a different record of star formation, chemical enrichment and survival. Together, they could reveal how the Milky Way’s center grew from smaller pieces.
For now, Terzan 5 stands as one of the clearest relics of that process. Webb and Hubble have turned its crowded light into a timeline that stretches from the early galaxy to relatively recent cosmic history. The Milky Way’s birth record may still be written in stars near its heart.
