Astronomers using NASA’s James Webb and Hubble Space Telescopes have reclassified Terzan 5, once considered a globular star cluster, as a “bulge fossil fragment” containing four distinct stellar populations. According to research presented by Giorgia Zullo at the 248th American Astronomical Society meeting and published in Astronomy & Astrophysics, the object’s ability to retain gas and dust from supernova explosions allowed it to form new stars over billions of years, rather than existing as a single-generation cluster.
Why was Terzan 5 reclassified?
Terzan 5 fails to meet the definition of a traditional globular cluster, which typically hosts only one ancient population of stars. Data from the Webb and Hubble telescopes confirm the object contains four distinct generations of stars, with ages ranging from 12.5 billion years to 2.5 billion years. According to researchers at the University of Bologna, this multi-generational structure indicates the object is a self-contained, self-enriching system that survived the chaotic formation of the Milky Way’s central bulge. While lighter clusters were dispersed and mixed into the galactic bulge, Terzan 5’s significant mass allowed it to remain a distinct “fossil” of the galaxy’s early assembly.
Terzan 5 is not alone. Astronomers have identified Liller 1 as another “bulge fossil fragment” that shares similar characteristics, including multiple generations of stars. Researchers are now planning to examine 40 to 50 additional globular clusters in the Milky Way’s bulge to determine if they are actually fossil fragments.
How does Webb’s infrared technology improve stellar observation?
Studying objects in the Milky Way’s bulge is challenging because the region is densely packed with stars and obscured by thick cosmic dust. According to the research team, Webb’s near-infrared capabilities allow astronomers to peer through this dust to catalog fainter, previously invisible stars. By analyzing the colors and brightness of these stars, scientists can determine their chemical composition and age. This precision allowed the team to rule out external interactions—such as collisions with molecular clouds—as the cause for the star formation, confirming that Terzan 5’s evolution was an internal, self-driven process.
What is the significance of these star populations?
The four distinct stellar generations act as a “fossil record” of heavy element enrichment. According to co-author R. Michael Rich of UCLA, the system captured the heavy elements dispersed by powerful supernova explosions within its own borders. In smaller systems, the energy from these explosions would have blown the gas and dust away. Because Terzan 5 held onto these materials, it fueled subsequent rounds of star formation. This process provides a local, observable model for how early galaxies may have assembled their structures.
Pro Tip: Tracking Stellar Evolution
Astronomers determine the age of a star population by measuring its “metallicity,” or the presence of elements heavier than hydrogen and helium. Higher concentrations of these heavy elements typically indicate that a star formed later in the universe’s history, after previous generations of stars had enriched the gas supply through supernovae.
How does this change our understanding of galaxy formation?
Terzan 5 provides a potential solution to the “clumpy galaxy” puzzle. According to Barbara Lanzoni of the University of Bologna, early galaxies likely featured massive gas disks that fragmented into clumps, which eventually migrated to the center to form bulges. By studying Terzan 5, scientists can observe a surviving example of these early building blocks. These findings suggest that the Milky Way’s bulge is a composite of many such fragments that merged billions of years ago.
Frequently Asked Questions
- What is the difference between a globular cluster and a fossil fragment? A globular cluster typically contains one generation of stars, while a fossil fragment contains multiple generations resulting from internal enrichment.
- Why was Terzan 5 hard to study? Its location in the Milky Way’s central bulge means it is hidden behind massive amounts of interstellar dust that block visible light.
- How old are the stars in Terzan 5? The populations formed in four distinct waves: 12.5 billion, 4.7 billion, 3.8 billion, and 2.5 billion years ago.
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