A team of astronomers has released new information about Terzan 5, a crucial celestial body at the center of the Milky Way. The findings result from the joint analysis of data captured by the James Webb and Hubble space telescopes.
The study reveals that Terzan 5, once thought to be an ordinary globular cluster, now hosts up to four distinct stellar generations. This particularity makes it a rare cosmic object, essential for understanding the formation process of the Milky Way itself. The conclusions were presented at the 248th meeting of the American Astronomical Society and detailed in the publication “Astronomy & Astrophysics”.
The reclassification of Terzan 5: from globular cluster to bulge fragment
The galactic bulge, the central area of the Milky Way, concentrates stars in great density and is surrounded by dense clouds of gas and dust. Terzan 5 is located in this region, about 22,000 light-years away from our planet, in the direction of the constellation Sagittarius.
Conventional globular clusters usually consist of a single set of old stars, formed at a similar period. However, Terzan 5 generated discussions among experts due to previous observations that indicated stars with significant variations in iron. Now, researchers classify it as the prototype of a new class of object: a “Bulge Fossil Fragment,” or BFF.
At the beginning of the universe, gas-rich disks are believed to have fragmented, creating countless massive star clusters. Such clusters migrated to the center of galaxies and came together, culminating in the formation of the bulges that we observe today.
The “bulge formation remnants” are defined as primordial star clusters that survived the merger process, avoiding complete assimilation. These are giant, self-sufficient clusters abundant in heavy elements. Their intense gravity allowed them to retain supernova residues — gases containing elements denser than hydrogen and helium —, enabling the continuous formation of new generations of stars. They are, in fact, cosmic “fossils”, preserving the appearance of billions of years ago, from the time when the bulge of the Milky Way was forming.
According to information from NASA, only two celestial objects are currently recognized as remnants of this bulge formation: Terzan 5 and Liller 1, highlighting the rarity and importance of the discovery.
Located in the bulge of our Milky Way galaxy, Terzan 5 resembles a globular cluster 🪩 a dense ball of ancient stars. However, observations from @Hubble_Space and Webb have revealed that it contains four distinct populations of stars.
This makes it the prototype of a new class… pic.twitter.com/N2JoWqteIK— ESA Webb Telescope (@ESA_Webb) June 16, 2026
Advances in analysis reveal four moments of star formation
The researchers had access to a vast collection of data, totaling more than two decades of observations. They combined new images from the Webb Space Telescope’s NIRCam (Near-Infrared Camera), known for its ability to pierce through dust clouds and produce clear images, with twelve years of historical data from the Hubble Space Telescope.
Observing the galactic bulge region presents significant challenges, as the high concentration of stars and gas can include objects not aligned with the line of sight. Furthermore, the presence of cosmic dust causes the absorption and reddening of light, a phenomenon known as interstellar extinction.
To overcome these difficulties, the team investigated the proper motion of the stars, identifying and selecting only the celestial bodies belonging to Terzan 5. With high-resolution corrections for variations in spatial dimming, it was possible to create an unprecedented color-magnitude diagram, which illustrates the relationship between the brightness and color of the stars.
The analysis detected several “tipping points”, which indicate the stage at which stars evolve from main sequence to giants. Using theoretical models, researchers determined the ages of two main star clusters, formed about 12.5 billion and 4.7 billion years ago. Additionally, evidence of a third cluster dating back 3.8 billion years was found, and evidence of a younger cluster, with stellar activity dating back approximately 2.5 billion years.
Deepening understanding of the formation of galactic centers
Initially, one explanation for the different stellar ages in Terzan 5 would be a past collision with another globular cluster or molecular cloud, which would have triggered a new phase of star formation. However, the recent discovery of four intense star formation events in Terzan 5 disputes scenarios dependent on external factors. Instead, it reinforces the theory that Terzan 5, with a mass about 2 million times that of the Sun, has continually produced stars using only its own material.
Professor Francesco R. Ferraro, from the University of Bologna and co-author of the study, reported that similar research will be carried out in another 40 to 50 globular clusters located in the galactic bulge. The expectation is that the Webb Space Telescope, fundamental to this new methodology, will continue to bring crucial insights into understanding the formation of the central bulge of the Milky Way.