At the heart of the Milky Way, hidden within a dense stellar cluster, astronomers have identified an object that defies standard classification. Terzan 5, previously regarded as a typical globular cluster, has emerged as the prototype for a new class: a "fossilized bulge fragment."
A team led by Francesco R. Ferraro from the University of Bologna, Italy, utilized data from the James Webb Space Telescope along with 12 years of archival Hubble observations. In June 2026, the findings were presented at the 248th meeting of the American Astronomical Society and published in the journal Astronomy & Astrophysics. The object is located 22,000 light-years away in the constellation Sagittarius, situated within our Galaxy’s central bulge.
Researchers identified four distinct generations of stars in Terzan 5, which formed 12.5 billion, 4.7 billion, 3.8 billion, and 2.5 billion years ago. Unlike typical globular clusters that contain only a single ancient population, this system retained gas and heavy elements from its supernovae, enabling sustained star formation. The object has a mass of approximately two million suns and spans only a few dozen light-years.
This discovery transforms our understanding of bulge formation, suggesting that billions of years ago, similar massive clumps merged and mingled to create the Galaxy's core. Terzan 5 remained intact—much like a lump in well-kneaded dough—preserving its own unique history.
According to NASA/ESA/STScI press releases from June 16–17, 2026, Terzan 5 is a living witness to the early stages of the Milky Way’s assembly, having survived the era of mergers to remain in orbit at the very heart of the Galaxy.
