Webb and Hubble confirm new class of Milky Way relic

NASA announced Tuesday that Webb and Hubble data confirm Terzan 5 is not a globular cluster but a new type of object called a "bulge fossil fragment."science.nasa+1
The stellar system contains four distinct star populations spanning 10 billion years, making it a self-enriching relic of the Milky Way's early formation.esawebb+1
Findings presented at the American Astronomical Society meeting offer new insight into how galaxies assemble and evolve over cosmic time.facebook+1

Researchers using NASA's James Webb Space Telescope and Hubble Space Telescope have confirmed the existence of a new class of objects within the Milky Way called "bulge fossil fragments," with the stellar system Terzan 5 serving as the prototype. The findings were presented Tuesday at the 248th meeting of the American Astronomical Society in Pasadena, California.science.nasa+1

A Star System Unlike Any Cluster

Long classified as a globular star cluster, Terzan 5 has now been definitively reclassified. A study combining recent Webb observations with 12 years of Hubble data shows that the system experienced up to four distinct episodes of star formation — a trait incompatible with true globular clusters, which typically contain only one ancient star population.stsci+1

The research team determined the ages of the four stellar populations with unprecedented precision: 12.5 billion, 4.7 billion, 3.8 billion, and 2.5 billion years ago. This 10-billion-year span of star formation activity demonstrates that Terzan 5 is a self-contained, self-enriching stellar system capable of retaining and recycling material across cosmic timescales.science.nasa+1

A Massive Relic

Located 22,000 light-years away in the constellation Sagittarius, Terzan 5 contains roughly 2 million solar masses packed into a region only a few tens of light-years across, making it one of the most massive and densely populated globular-cluster-like systems in the Milky Way. The results indicate it is the remnant of a much more massive stellar system that formed 12.5 billion years ago and survived the assembly of the galaxy's central bulge without merging into the surrounding structure.esawebb+3

Rewriting Galactic History

Rather than a simple cluster, Terzan 5 now belongs to a new category — a bulge fossil fragment, defined as a self-enriching stellar system with multiple star populations of different ages and iron abundances. The discovery offers a window into how galaxies like the Milky Way formed and evolved, preserving a record of conditions from the universe's earliest epochs within a single surviving object.esa+1

"Terzan 5 is not a globular star cluster, as originally classified," NASA stated in its release announcing the findings. The lead researchers, Giorgia Zullo and Francesco Ferraro of the University of Bologna, produced the combined Webb-Hubble imagery that underpins the new classification.miragenews+1

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