Astronomers have reclassified Terzan 5, a system previously categorized as a globular cluster, as a bulge fossil fragment that serves as a direct record of the Milky Way’s early development. A study published June 16, 2026, in the journal Astronomy & Astrophysics confirms that this object holds four separate generations of stars spanning a 10-billion-year history.
Led by Giorgia Zullo, a PhD student at the University of Bologna, the research team utilized data from the James Webb Space Telescope and the Hubble Space Telescope to analyze the system. Located 22,000 light-years away in the constellation Sagittarius, Terzan 5 resides within the dense, dust-saturated environment of the galactic bulge. The study indicates that the system is not a standard cluster but a surviving remnant of the primordial gas clumps that merged to construct the center of the galaxy.
The James Webb Space Telescope provided the necessary infrared resolution to penetrate the thick interstellar dust that obscured previous optical surveys. By utilizing the NIRCam instrument with F115W and F200W filters, researchers resolved stars with significantly higher precision than was possible with Hubble alone. This infrared capability allowed the team to bypass the reddening effects caused by carbon, iron, and silicate particles in the galactic core.
Integrating 12 years of archival Hubble observations with new Webb data enabled the team to calculate the proper motions of individual stars. This long-term baseline was essential for distinguishing the members of Terzan 5 from the dense foreground and background star fields of the Milky Way. The resulting color-magnitude diagram revealed four distinct main-sequence turnoff points, indicating separate star-formation events.
The oldest population of stars in the system dates back 12.5 billion years, placing its origin near the beginning of the universe. Subsequent waves of star formation occurred approximately 4.7 billion, 3.8 billion, and 2.5 billion years ago. These findings contradict previous models that suggested only two populations existed, as no external collision or capture event could account for such a prolonged, self-contained history of stellar birth.
The system’s ability to retain gas despite repeated supernova explosions is attributed to its massive initial size. Researchers estimate the progenitor mass reached 100 million times that of the Sun, providing enough gravity to hold onto heavy elements ejected by dying stars. This self-enrichment process allowed each successive generation to form with higher iron content, creating a chemical signature that matches the broader galactic bulge.
High-resolution spectroscopy from the W. M. Keck Observatory and the Very Large Telescope confirmed this chemical alignment. Dr. R. Michael Rich of UCLA noted that the system preserves a fossil record of progressive enrichment of heavy elements by supernovae. This chemical link proves that Terzan 5 formed in situ rather than being an interloper captured from the galactic halo.
The discovery provides observational evidence for the hierarchical assembly model of galaxy formation. While computer simulations have long suggested that large galaxies grow through the merging of smaller primordial clumps, physical specimens of these remnants have remained elusive. Terzan 5 acts as a surviving shard of this process, offering a window into the conditions of the early universe.
Barbara Lanzoni, an associate professor at the University of Bologna, explained that galaxies in the early universe contained massive gas disks that fragmented into clumps. These clumps migrated toward the galactic center and merged to form the bulge. Terzan 5 represents a rare instance where one of these primordial fragments avoided total dissolution into the galactic mass.
The methodology established by the team opens a new path for galactic archaeology. Researchers intend to apply these combined telescope techniques to 40 to 50 additional candidate objects within the bulge. This systematic survey aims to map the history of the Milky Way by identifying other potential fossil fragments that have preserved their internal star-formation records over billions of years.
