Science
Jun 16, 20261
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NASA's Webb and Hubble Telescopes Reveal Terzan 5's Four Stellar Generations

NASA's James Webb and Hubble Space Telescopes have revealed that Terzan 5 contains four distinct generations of stars formed over billions of years, overturning its previous classification as a simple globular cluster. The discovery demonstrates that Terzan 5 maintained its identity while surrounding systems merged to form the Milky Way's central bulge, providing new insights into galactic formation and evolution.




Quick Facts
Who
Giorgia Zullo (University of Bologna)
What
Discovery of four distinct stellar populations in Terzan 5
When
Discovery: 1968
Where
Terzan 5
- Discovery of four distinct stellar populations in Terzan 5
- Confirmation that Terzan 5 is not a simple globular cluster
- Combined analysis using Webb infrared observations and Hubble proper motion measurements
- Chemical composition measurements from Keck Observatory and ESO's Very Large Telescope
- Identification of stellar populations with different ages and chemistries
Using NASA's James Webb and Hubble Space Telescopes, researchers have conclusively demonstrated that Terzan 5 is not a simple globular star cluster as once classified, but rather a complex system containing four distinct generations of stars. The study, led by Giorgia Zullo, a PhD student at the University of Bologna, reveals stellar populations that formed at approximately 12.5 billion, 4.7 billion, 3.8 billion, and 2.5 billion years ago—a discovery that fundamentally changes our understanding of this celestial object's history.
Terzan 5, discovered in 1968 by astronomer Azop Terzan, was initially thought to be a conventional globular cluster, which typically contains only one ancient star population. However, 2009 observations revealed the presence of two distinct populations, and subsequent Hubble measurements in 2016 provided initial age estimates. The new combined observations from both telescopes have now provided unprecedented clarity. Webb's infrared capabilities allowed researchers to penetrate the dust-obscured region of the Milky Way's bulge where Terzan 5 resides, enabling them to measure colors and brightnesses of individual stars. Hubble's archival data, separated by 12 years from newer observations, allowed astronomers to measure the minute proper motions of stars to definitively determine which belonged to Terzan 5 and which were part of the surrounding galactic bulge.
The discovery of four stellar generations rules out previous hypotheses that Terzan 5 could have acquired new star-forming material through interaction with other cosmic objects. Instead, it demonstrates that Terzan 5 was massive enough to retain the gas and dust necessary to sustain multiple rounds of star formation over billions of years, while remaining gravitationally bound as a distinct system. This suggests Terzan 5 maintained its identity as lighter-weight stellar systems around it mixed together to form the Milky Way's central bulge—a process comparable to "a lump in otherwise well-mixed cake batter," as researchers described it.
Measurements from the W. M. Keck Observatory and the European Southern Observatory's Very Large Telescope confirmed that the four stellar populations have distinctly different chemical compositions, preserving what researchers call a "fossil record of progressive enrichment of heavy elements by supernovae." According to co-author R. Michael Rich of UCLA, these chemical signatures further support the conclusion that Terzan 5 underwent multiple distinct star-formation episodes. The findings provide crucial insights into how galaxies like the Milky Way formed and evolved, revealing that even systems within galactic bulges can maintain complex, multi-generational histories.
The research was presented at the 248th meeting of the American Astronomical Society in Pasadena and published in the journal Astronomy & Astrophysics.
Why This Matters
This discovery fundamentally reshapes our understanding of galaxy formation and stellar evolution. By demonstrating that Terzan 5 retained its identity through multiple star-formation episodes while surrounding systems merged, researchers gain insight into how complex structures form and persist within galaxies like the Milky Way. For astronomers and space enthusiasts, this reveals that stellar systems can have far richer histories than previously thought, opening new avenues for studying galactic archaeology and the chemical evolution of our universe.
Timeline & Sources
Jan 1, 1968
WireTerzan 5 discovered by astronomer Azop Terzan
Jan 1, 2009
WireTwo distinct stellar populations identified in Terzan 5
Jan 1, 2016
WireHubble provides first age estimates for the two known stellar populations
Jun 16, 2026
WireCombined James Webb and Hubble analysis presented at American Astronomical Society meeting revealing four stellar populations