The discovery of four distinct generations of stars in the ancient stellar system Terzan 5 challenges our understanding of globular clusters. This finding, made possible by the James Webb Space Telescope and the Hubble Space Telescope, reveals a complex history that defies the conventional wisdom of these celestial objects. Terzan 5, located in the inner bulge of the Milky Way, was initially thought to be a typical globular cluster with a single ancient star population. However, the revelation of two distinct populations of stars, one formed 12 billion years ago and the other 5 billion years ago, hinted at a more intricate past. The new data now confirms not only these two populations but also two more recent rounds of star formation, dating back 3.8 and 2.5 billion years. This discovery raises profound questions about the formation and evolution of globular clusters, suggesting that Terzan 5 may have interacted with another object, such as a globular cluster or a giant molecular cloud, to enrich itself with new gas and dust, triggering subsequent star formation. The study of Terzan 5 is further complicated by its location in a crowded region of the galaxy, obscured by dust, which the James Webb Space Telescope's infrared capabilities helped to overcome. The measurements of star colors and brightnesses, along with the identification of very small movements of individual stars, allowed astronomers to classify the stars into populations of different ages and chemistries. The findings also highlight the cluster's ability to retain the necessary raw materials for star formation, even in the face of powerful supernova explosions that forged heavier elements. This resilience is attributed to the cluster's initial mass, which enabled it to survive and form multiple generations of stars without merging or fully integrating with the Milky Way's bulge. Terzan 5, therefore, stands as a testament to the complexity and diversity of stellar systems, offering a unique glimpse into the formation and evolution of our galaxy.
