NASA's James Webb and Hubble Telescopes have revealed a fascinating insight into the formation and evolution of galaxies like our own Milky Way. The discovery of Terzan 5, once thought to be a globular star cluster, has now been proven to be a 'bulge fossil fragment' with multiple generations of stars. This finding challenges our understanding of how galaxies form and evolve, and offers a unique perspective on the history of our own Milky Way.
Terzan 5, discovered in 1968, initially appeared to be a typical globular star cluster with a single ancient star population. However, in 2009, researchers found evidence of two distinct populations of stars, with ages spanning 12 billion to 5 billion years. This discovery pointed to a more complex history than previously thought. The Hubble Space Telescope provided the first estimates of their ages, showing that one population formed as the Milky Way was assembling, and the other just before Earth started forming. This raised a deeper question: how did Terzan 5 maintain its separate identity while lighter weight systems spread out and mixed to form the bulge billions of years ago?
This is where the James Webb Space Telescope stepped in. Its infrared view allowed researchers to peer through the dust and catalog many more stars, including fainter stars, than previous work. By measuring star colors and brightnesses, astronomers could classify them into populations of different ages and chemistries. Webb was able to measure these key properties for every star within the field of view in the sky, both stars within Terzan 5 and unrelated foreground stars. The 12-year separation between Webb and Hubble observations allowed the team to measure very small movements of individual stars, known as proper motions, to determine which stars belong to Terzan 5 and which are part of the Milky Way bulge.
The results were striking. By combining data from both Webb and Hubble, researchers found strong evidence for two more stellar populations, one that formed 3.8 billion years ago and another only 2.5 billion years ago. They were also able to determine the ages of the previously known stellar populations with unprecedented precision, finding that they formed 12.5 billion and 4.7 billion years ago. With the previously known two generations of stars, astronomers could not rule out the possibility that Terzan 5 interacted with another object, like a globular cluster or a giant molecular cloud, becoming enriched with new gas and dust that set off a second round of star formation. With four stellar generations, those explanations are ruled out.
Measurements of the stellar composition of Terzan 5 populations made at the W. M. Keck Observatory and European Southern Observatory’s Very Large Telescope also point toward very distinct populations. Terzan 5 formed multiple generations of stars because it was able to retain the necessary raw materials. There is evidence of powerful supernova explosions in Terzan 5 that forged heavier elements that were swept up by subsequent generations of stars. In lighter weight systems, the force of the explosions themselves could have ejected the resulting elements as well as sweeping out leftover gas and dust. The progenitor of Terzan 5 had enough mass to retain those stars’ ejections, allowing new generations of stars to form over billions of years.
The results show that Terzan 5 is most likely the remnant of a much more massive stellar system that initially formed 12.5 billion years ago. Terzan 5 is extraordinary because it survived — and never merged or fully 'mixed in' with the Milky Way’s bulge. 'For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,' said Francesco R. Ferraro, a professor at the University of Bologna and principal investigator of the Webb observations. 'Terzan 5 is what we now call a bulge fossil fragment because it resembles the primordial clumps that contributed to the formation of the bulge.'
To date, there’s one other known cosmic object like Terzan 5. Liller 1 was the second to be reclassified from a globular star cluster to a bulge fossil fragment. It also contains multiple generations of stars. There may be more objects like it. Between 40 to 50 additional globular clusters that orbit within the bulge will be examined by Ferraro’s team to determine if their stellar populations are all the same, like globular clusters, or have several generations, like bulge fossil fragments.
Ultimately, this research may improve what we know about how the central bulges of galaxies form over hundreds of millions of years. 'Based on observations and in-depth simulations, we think that galaxies in the early universe had huge disks of gas that fragmented into clumps and formed stars. These clumps migrated to the center of the galaxies, and many merged to form their bulges,' said Barbara Lanzoni, a co-author and associate professor at the University of Bologna. For example, Webb has turned up several examples of 'clumpy' galaxies that were actively forming when the universe was only a few hundred million years old, like the clumps in the Firefly Sparkle galaxy. 'Terzan 5 may provide direct evidence that can help explain how bulges formed in galaxies throughout the universe,' Lanzoni said.
This discovery is a testament to the power of human curiosity and the importance of pushing the boundaries of our knowledge. It reminds us that there is always more to learn and explore, and that even the most well-understood concepts can be challenged and transformed by new observations and insights. In my opinion, this finding is a fascinating insight into the complex history of our universe, and a reminder of the importance of continued exploration and discovery.
