For almost sixty years, Terzan 5 was catalogued as a globular cluster, a spherical, highly compact group of stars that were born virtually all at once in a single, brief initial burst.
Today, however, we know it is something entirely different. This has been clarified by a new analysis led by a research group from the University of Bologna and the National Institute for Astrophysics (INAF).
The study – published in Astronomy & Astrophysics – shows that Terzan 5 is made up of four distinct generations of stars born in periods extremely far apart from one another, a feature incompatible with the definition of a globular cluster.
"Multiple epochs of star formation separated by billions of years tell us that Terzan 5 is certainly not a globular cluster", confirms Francesco R. Ferraro, Professor at the Department of Physics and Astronomy "Augusto Righi" of the University of Bologna and INAF Associate, Principal Investigator for the observations with the James Webb Space Telescope. “The characteristics of Terzan 5 suggest that it may be a kind of living archaeological relic of the birth of our galaxy”
Terzan 5 is located in the “Galactic bulge”, the central and densest region of the Milky Way, a position obscured by a blanket of interstellar gas and dust that makes it extraordinarily difficult to observe. To overcome these obstacles, the researchers used the James Webb Space Telescope’s near-infrared camera NIRCam and were thus able to acquire a series of extremely detailed images.
"The observations carried out with the James Webb Space Telescope have allowed us to obtain the sharpest view ever achieved of this stellar system", says Giorgia Zullo, a PhD student at the University of Bologna and INAF Associate, first author of the paper. "From the analysis of the data we obtained, we were able to clearly identify two populations of stars: one with an age of about 12.5 billion years, virtually the same age as the oldest stars in the galaxy, and a much younger, metal-rich population with an age of about 4.7 billion years.”
Cristina Pallanca, a Professor at the University of Bologna and INAF Associate, said: “In addition to these two distinct stellar populations, we identified traces of a further generation of stars that are even younger (about 3.8 billion years old), and the presence of a group of particularly bright, blue stars, whose existence suggests that star formation in Terzan 5 may have continued until about 2.5 billion years ago.”
Terzan 5 therefore appears to be a system that continued to form stars repeatedly over almost the entire age of the Universe. This case is however not unique. The researchers point out that very similar characteristics have been found in another stellar system called Liller 1, which, just like Terzan 5, is located near the centre of the Milky Way and was long mistaken for a globular cluster.
“These stellar systems could be fossil remains of the processes that led to the birth of the Milky Way as we know it today”, Ferraro says. “This is the first result of the GENESIS project, funded by the Italian Ministry of University and Research as part of the FIS3 call, which aims to gain a thorough understanding of the nature of these objects, with the potential to revolutionise our view of how galaxies are born and evolve.”
The study was published in Astronomy & Astrophysics with the title “The multi-age stellar populations of Terzan 5 as revealed by JWST”. For the University of Bologna (Department of Physics and Astronomy “Augusto Righi”) and INAF – OAS (Osservatorio di Astrofisica e Scienza dello Spazio di Bologna), Giorgia Zullo, Cristina Pallanca, Francesco R. Ferraro, Barbara Lanzoni, Livia Origlia, Davide Massari, Emanuele Dalessandro, Cristiano Fanelli and Mario Cadelano took part in the project.
