Four Generations in One Object
Four distinct generations of stars. In a single object, Terzan 5, located in the central bulge of the Milky Way galaxy. Globular clusters typically contain one ancient star population, according to research published in the journal Astronomy & Astrophysics.
The count doubled not because the object changed, but because the instrument did. In 2009, observations showed that stars in Terzan 5 could be divided into two age groups, researchers reported. No other globular cluster had shown this. In 2016, Hubble Space Telescope data provided the first age estimations: the first generation was about 12 billion years old, the second about 5 billion years ago, according to the study. A 7-billion-year gap.
Now, combining data from both the Hubble Space Telescope and James Webb Space Telescope, researchers have identified four generations, findings published in Astronomy & Astrophysics and discussed at a conference of the American Astronomical Society on June 16, 2026, in Pasadena, California. The youngest is dated to approximately 2.5 billion years ago.
The Sorting Problem
Terzan 5 sits in the crowded central bulge of the Milky Way. Thousands of stars occupy the same sightline. Separating signal from background required proper motion measurements, the tiny apparent changes in positions of stars due to their movements, according to the research team. Without this sorting, the generations blur into noise.
Researchers used proper motion measurements to differentiate Terzan 5's stars from the surrounding central bulge, the study reported. The technique let them filter which stars belonged to the object and which merely appeared nearby. Only then could they resolve the age structure.
The Mass Threshold
Most globular clusters stop at one generation. They form their stars, those stars explode as supernovae, and the debris escapes. The system lacks the gravity to hold onto the material, according to the researchers. Terzan 5 crossed a different threshold.
The large mass of Terzan 5's progenitor prevented material from being ejected from the stellar system, the study found. Supernova explosions occurred within Terzan 5, forming heavier elements. Young star populations in Terzan 5 pulled in material created by supernova explosions. The object retained the raw materials necessary to birth multiple generations of stars. Repeat for 12 billion years.
No Longer a Cluster
Terzan 5 was previously classified as a globular cluster. That classification no longer holds, according to the research team. Terzan 5 has been reclassified as a "bulge fossil fragment," a surviving piece of the early Milky Way that kept making stars when others stopped.
The reclassification reflects mechanism, not taxonomy. A globular cluster forms once and fossilizes. A bulge fossil fragment retains enough mass to recycle its own debris, the researchers explained. The difference is what stays inside versus what escapes.
Francesco R. Ferraro from the University of Bologna was the principal investigator of the Webb observations, according to the conference presentation. Webb's infrared capability cut through dust that Hubble could not, revealing the third and fourth generations that had been hidden. The count doubled because the wavelength changed.
Zero Escape
One generation is typical. Four required 12 billion years, enough mass to hold onto exploded stars, and two space telescopes, according to the findings. The number that matters: zero, the amount of material Terzan 5 let escape.
Everything stayed in. That retention transformed a cluster into a fragment of galactic history, a system that refused to stop, the researchers concluded.
Research Limitations
The discovery represents a significant advance in understanding stellar evolution, but the research remains purely observational astrophysics with no direct applications to human communities or policy identified in the published findings. The study contributes to fundamental knowledge about how galaxies form and evolve over billions of years.
No experimental interventions or community engagement components were included in the methodology. Future observations may target similar objects in other galaxies to determine whether bulge fossil fragments represent a common evolutionary pathway or a rare outcome of specific formation conditions.
