PASADENA — New observations from the James Webb Space Telescope and the Hubble Space Telescope revealed that the Terzan 5 star cluster contains four distinct populations of stars. This finding has reclassified Terzan 5 as a "bulge fossil fragment" instead of a traditional globular cluster.

Astronomers previously identified two distinct star populations within Terzan 5 in a 2009 discovery. Hubble data analyzed in 2016 estimated the ages of two star populations at approximately 12 billion years and 5 billion years. Webb's infrared capabilities allowed researchers to observe through interstellar dust and catalog fainter stars.

The research team measured the proper motions of stars using a 12-year baseline of Hubble exposures. These proper motion measurements distinguished stars belonging to Terzan 5 from background and foreground stars in the Milky Way bulge. Combined telescope data identified two additional stellar populations formed 3.8 billion years ago and 2.5 billion years ago.

The team refined the age estimates of the two older populations to 12.5 billion years and 4.7 billion years. The presence of four stellar generations rules out external enrichment from interactions with globular clusters or giant molecular clouds. A typical globular star cluster contains only one ancient star population.

Terzan 5 orbits within the central bulge of the Milky Way galaxy. "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 PhD student at the University of Bologna who led the research team. The team classified Terzan 5 as a prototype of a new category of objects termed "bulge fossil fragments."

"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, a research astronomer at the University of California, Los Angeles, and a study co-author. Terzan 5 retained supernova ejecta and gas required for multiple rounds of star formation, unlike stellar systems with lower mass, which typically eject gas and dust following supernova explosions rather than retaining it.

Francesco R. Ferraro, a professor at the University of Bologna and the principal investigator for the Webb observations, commented on the cluster's unique formation. "For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed," Ferraro said. The research team published the study in Astronomy & Astrophysics. Astronomer Azop Terzan discovered the Terzan 5 system in 1968.