Terzan 5 Reclassified as a Bulge Fossil Fragment by JWST, Hubble

Astronomers using NASA’s James Webb Space Telescope (JWST) and archival Hubble data have shown that the stellar system Terzan 5 in the Mil Way’s bulge contains four distinct stellar generations. The results overturn its classification as a globular cluster and identify it as a “bulge fossil fragment,” a relic of the early building blocks that formed the Galactic bulge.

NASAEngadgetFour generations of stars discovered in region near the center of the Milky Way

Terzan 5 was discovered in 1968 by Azop Terzan. In 2009 it was found to host two stellar populations, and Hubble data in 2016 gave ages of 12 billion and 5 billion years. Dust and crowding in the bulge hampered earlier studies, a limitation JWST’s infrared capability now overcomes.

NASA

The study was led by PhD student Giorgia Zullo and professor Francesco R. Ferraro of the University of Bologna, with co-authors R. Michael Rich (UCLA) and Barbara Lanzoni (University of Bologna); observations included NASA, ESA, CSA, Space Telescope Science Institute, W. M. Keck Observatory and ESO’s Very Large Telescope.

NASA

Four stellar populations have ages of 12.5, 4.7, 3.8, and 2.5 billion years. Chemical analyses show enrichment of heavy elements by successive supernovae, indicating the progenitor retained ejecta and enabled repeated star formation. Only one other object, Liller 1, shares this profile; 40–50 additional bulge clusters will be examined.

NASA

Multiple, well-separated generations support models where early galactic disks fragment into massive clumps that migrate inward and coalesce into bulges. Terzan 5 thus provides a tangible fossil record, informing bulge-formation scenarios for distant galaxies.

NASA

NASA described JWST as the world’s premier space-science observatory and highlighted Hubble’s three-decade legacy of discoveries. Results were presented at the 248th American Astronomical Society meeting in Pasadena and published in *Astronomy & Astrophysics*.

NASA

Age estimates for the oldest population differ (12 billion vs. 12.5 billion years). The progenitor’s exact mass and formation history remain uncertain, and the prevalence of similar histories in other bulge clusters is still under investigation.

Engadget

• “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,” — Giorgia Zullo, PhD student, University of Bologna

NASA

• “Along with the ages of these populations, the cluster preserves a fossil record of progressive enrichment of heavy elements by supernovae,” — R. Michael Rich, UCLA

NASA

• “For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” — Francesco R. Ferraro, University of Bologna

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• “ "Based on observations and in-depth simulations, we think that galaxies in the early Universe had huge discs of gas that fragmented into clumps and formed stars.” — Barbara Lanzoni, University of Bologna

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Ferraro’s team will apply the JWST-Hubble method to 40–50 more bulge clusters to map multi-generational frequencies. Spectroscopic follow-up with ground-based observatories will refine enrichment histories and test bulge-formation models across the Mil Way and beyond.

NASA