Discovery Highlights
The James Webb Space Telescope, working hand‑in‑hand with archival Hubble observations, has uncovered a startlingly complex history inside what astronomers once catalogued as a mundane globular cluster. Within the dense core of the Milky Way, the object known as Terzan 5 hosts four separate generations of stars, each born at a different epoch spanning more than ten billion years. This layered population transforms Terzan 5 from a simple spherical congregation of ancient suns into a rare “bulge fossil fragment,” a surviving core of the primordial gas cloud that forged the Galactic bulge.
The enigmatic Terzan 5
Located roughly 6,500 light‑years from the Galactic centre, Terzan 5 lies behind a veil of interstellar dust that blocks most visible light. Traditional surveys classified it as a typical globular cluster—a compact ball of hundreds of thousands of stars formed in a single burst. Yet, spectroscopic work as early as 2009 hinted at a puzzling spread in iron abundance, a trait unheard of in ordinary clusters. JWST’s infrared instruments penetrated the obscuring veil, delivering crisp colour‑magnitude diagrams, while Hubble’s precise astrometry, comparing images taken twelve years apart, allowed researchers to tease apart genuine members from foreground interlopers.
Four distinct stellar epochs
The new study, led by Giorgia Zullo of the University of Bologna and published in Astronomy & Astrophysics, identifies four discrete stellar cohorts. The oldest, dating back about 12.5 billion years, formed shortly after the Big Bang, bearing a low metallicity signature. A second wave emerged roughly 4.7 billion years ago, coinciding with the era that produced our own Sun, and exhibits a markedly higher concentration of heavy elements. Two younger bursts, dated to 3.8 billion and 2.5 billion years ago, add further layers to the chronology. The presence of these successive populations implies that Terzan 5 was massive enough to retain the ejecta from supernova explosions, recycling enriched material to spark fresh star formation—a capability absent in most lighter clusters.
Why this matters
If Terzan 5 is indeed a fragment of the original bulge‑building clump, it provides a unique laboratory for probing the early assembly of our Galaxy. Rather than offering a single‑snapshot view, it delivers a time‑resolved record of chemical enrichment, star‑formation efficiency, and dynamical evolution within the inner Milky Way. The fact that such a relic survived the chaotic environment near the Galactic centre challenges existing models of cluster disruption and suggests that the bulge may contain more hidden “fossil” systems awaiting discovery with JWST’s infrared prowess.
Another fossil: Liller 1
Terzan 5 is not alone in this category. Liller 1, an even more massive conglomerate situated closer to the Galactic nucleus, shows a strikingly similar multi‑generational makeup. Both objects blur the line between classic globular clusters and dwarf‑galaxy nuclei, hinting at a broader population of ancient bulge fragments that have persisted despite the Milky Way’s tumultuous history. Future JWST surveys targeting the crowded central regions are poised to uncover additional candidates, potentially rewriting the narrative of how the Milky Way’s central bulge was constructed.
Source: https://scientias.nl/jwst-terzan-5-bulge-fossil-fragment/
