Astronomers have discovered that Terzan 5 contains four distinct generations of stars, thanks to new observations from the James Webb Space Telescope and data collected over years by the Hubble Space Telescope. The finding reveals a much more complex history than expected for an object once classified as a globular cluster.
The stellar system, located near the center of the Milky Way, has intrigued researchers for years. The latest results suggest it may be the surviving remnant of a much larger structure that formed during the early stages of our galaxy’s evolution.
Terzan 5 was discovered in 1968 by astronomerAgop Terzan and sits about 19,000 light-years away in the constellation Sagittarius. Home to hundreds of thousands of stars, it shares some characteristics with globular clusters, the dense collections of old stars found around many galaxies.
Yet Terzan 5 has never quite fit the mold. In 2009, astronomers found evidence that it hosted two separate stellar populations rather than one. Later observations with Hubble showed that those groups formed at very different times, hinting that the object had experienced a far more eventful history than a typical globular cluster.
Webb Cuts Through The Dust
One reason Terzan 5 has been difficult to study is its location. The system lies in the crowded inner bulge of the Milky Way, where thick clouds of dust block much of the visible light coming from distant stars.
Data gathered by the James Webb Space Telescope allowed researchers to look through that dust and detect many stars that previous observations could not clearly identify. By measuring the brightness and colors of those stars, astronomers could estimate their ages and chemical makeup.
“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, a Ph.D. student at the University of Bologna.
The improved view enabled the team to build a much more complete census of the stars in and around the system, including faint objects that had escaped earlier studies.
Four Distinct Stellar Generations Emerge
The next challenge was figuring out which stars actually belonged to Terzan 5. To do that, the team turned to Hubble observationstaken 12 years apart. Those images allowed astronomers to track tiny stellar motions known as proper motions. The study reports that these measurements made it possible to separate stars belonging to Terzan 5 from unrelated stars in the Milky Way’s bulge.
After combining Webb and Hubble observations, researchers identified four distinct populations of stars. The oldest formed about 12.5 billion years ago. Another appeared around 4.7 billion years ago, followed by populations that formed roughly 3.8 billion and 2.5 billion years ago.
Earlier explanations proposed that an interaction with another object, such as a globular cluster or a giant molecular cloud, could have triggered a later episode ofstar formation. The newly identified populations make that explanation unlikely.
A Relic From The Galaxy’s Early Days
The team also examined the chemical composition of the stars using observations from the W. M. Keck Observatory and ESO’s Very Large Telescope. Those measurements revealed clear differences between the various stellar populations.
“Along with the ages of these populations, the cluster preserves a fossil record of progressive enrichment of heavy elements by supernovae,” said R. Michael Rich of the University of California.
The research indicates that the ancestor of Terzan 5 was massive enough to retain gas and heavy elements produced by stellar explosions. In less massive systems, that material would often be lost to space. In this case, it remained available to fuel new generations of stars over billions of years. As detailed in the paper, Terzan 5 is likely the remnant of a much larger stellar system that formed around 12.5 billion years ago. It appears to have survived while the Milky Way’s bulge was taking shape around it.
“For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” noted Francesco Ferraro of the University of Bologna.
He also described the object as a “bulge fossil fragment,”a rare survivor that preserves clues about the structures that contributed to the formation of our galaxy’s central regions.
