Globular clusters are traditionally known to host a single, ancient population of stars. However, groundbreaking data from the NASA/ESA/CSA James Webb Space Telescope and the NASA/ESA Hubble Space Telescope has confirmed the presence of two distinct star populations within the ancient star system Terzan 5. Once classified merely as a globular cluster, Terzan 5 now also shows evidence of two recent rounds of star formation.

Terzan 5, discovered in 1968 by Armenian-Turkish-French astronomer Agop Terzan, is located approximately 19,000 light-years away in the constellation Sagittarius.

Also known as ESO 520-27 and 2MASX J17480455-2446441, this star system is home to hundreds of thousands of varied stars.

Nesting within the inner bulge of the Milky Way, Terzan 5 exhibits many characteristics reminiscent of globular clusters, yet significant findings emerged in 2009 revealing two distinct star populations.

A 2016 study using Hubble provided crucial age estimates: one population formed around 12 billion years ago, pre-dating the Milky Way, while the other emerged approximately 5 billion years ago, shortly before Earth’s formation. This complex history suggests Terzan 5’s evolution diverges from typical globular clusters.

Dr. Giorgia Zullo, a student at the University of Bologna, remarked, “Webb’s new near-infrared observations, in conjunction with Hubble’s archival data, present a clearer narrative of Terzan 5’s history.”

Studying Terzan 5 presents challenges due to its dense star environment and substantial dust cover within the galaxy.

Webb’s infrared capabilities enable astronomers to penetrate this dust, allowing for a comprehensive cataloging of both faint and distant stars.

By analyzing the colors and brightness of the stars, researchers can categorize them based on different ages and chemical compositions.

Webb successfully measured these essential properties for all visible stars, including those in Terzan 5 and unrelated foreground stars.

To distinguish Terzan 5’s stars, researchers leveraged Hubble’s long-term observations. The varying intervals between Hubble’s 12-year exposures allowed them to track tiny stellar movements, known as proper motion, helping to identify which stars are part of Terzan 5 versus those belonging to the Milky Way’s bulge.

By integrating findings from both Webb and Hubble, researchers found compelling evidence for two additional stellar populations, one dating back 3.8 billion years and another 2.5 billion years old.

They also determined the ages of the known stellar populations with remarkable precision, revealing formation timelines between 12.5 billion and 4.7 billion years ago.

The existence of these four distinct generations of stars suggests that Terzan 5 likely interacted with another celestial object, potentially a globular cluster or giant molecular cloud, enriching it with gas and dust to spark a second round of star formation.

Observations made using the W.M. Keck Observatory and ESO’s Very Large Telescope indicate that Terzan 5 hosts a unique stellar population.

Dr. R. Michael Rich, an astronomer at UCLA, noted, “As these populations age, the clusters preserve a fossil record of progressive heavy element enrichment from supernovae.”

Terzan 5 has managed to retain essential raw materials, allowing for the formation of multiple star generations.

There is substantial evidence that Terzan 5 witnessed a powerful supernova explosion that produced heavier elements, which were subsequently dispersed amongst the following generations of stars.

In less massive systems, the explosive force could have expelled residual gases and dust, thereby releasing the resultant elements.

Terzan 5’s progenitor possessed enough mass to sustain ejection, enabling new star generations to take shape over billions of years.

The results indicate that Terzan 5 likely remains from a significantly larger star system that formed around 12.5 billion years ago.

This cluster is remarkable in its survival without merging or fully blending with the Milky Way’s bulge.

Professor Francesco Ferraro from the University of Bologna explains, “For some reason, this extraordinary cluster formed separately from the bulge and was not obliterated during the bulge’s formation.”

“Terzan 5 is considered a bulge fossil fragment, resembling the primordial mass that contributed to bulge formation.”

For further details, consult this study published in Astronomy and Astrophysics.

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G. Zullo et al. 2026. Terzan 5’s multi-age stellar population revealed by JWST. A&A 709, A212; doi: 10.1051/0004-6361/202659349