NASA's Webb and Hubble Telescopes Reveal Terzan 5's Four Stellar Generations

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.