Webb confirms – The James Webb Space Telescope has helped verify that Terzan 5—once misclassified as a globular star cluster—acts as a “bulge fossil fragment,” preserving multiple ancient and younger star-formation phases that may mirror how the Milky Way’s bulge formed.
The James Webb Space Telescope didn’t just look deeper into the Milky Way’s crowded heart. It helped settle a long-running argument about what one specific star region actually is—and why it matters.
In a new study. astronomers using Webb observations. paired with archival data from the Hubble Space Telescope. confirmed that Terzan 5—an area in the center of the galaxy often called “the bulge”—is not a globular star cluster. The region had been previously classified that way. but the new findings point to a more complicated past. one with multiple bursts of star formation instead of a single ancient population.
Terzan 5 sits in a challenging environment for observation. The center of the galaxy is packed with stars and tangled with dust. making it hard to disentangle what’s happening there. Webb’s view. combined with Hubble’s archival observations. gave the team a way to see that Terzan 5 holds at least four distinct phases of star formation.
According to the researchers’ survey, Terzan 5 contains two older star populations formed 12.5 billion years ago and 4.7 billion years ago. It also includes two more contemporary populations formed 3.8 billion years ago and 2.5 billion years ago.
That staggered timeline is the core reason the team now calls Terzan 5 a “bulge fossil fragment.” University of Bologna professor Francesco R. Ferraro, the principal investigator of the Webb observations, said the region’s story explains the term:
“For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” Ferraro said. “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.”
The working idea behind the interpretation is rooted in how galaxies formed in the early Universe. Barbara Lanzoni. co-author and an associate professor at the University of Bologna. described a scenario supported by observations and in-depth simulations: early galaxies had huge discs of gas that broke into clumps. which then formed stars. Over time, these clumps migrated toward the center of their galaxies, and many of them merged to build the bulge.
Put together. the finding is simple in its impact even if the subject is not: Terzan 5 appears to preserve something that the bulge should have erased. The timing of its different star populations. combined with Webb’s confirmation that it is not a globular cluster. gives astronomers a window into the early building blocks of the Milky Way’s structure.
The researchers published the findings in the journal Astronomy & Astrophysics.
James Webb Space Telescope Webb telescope Terzan 5 bulge fossil fragment Milky Way formation Hubble Space Telescope globular star cluster Astronomy & Astrophysics star formation history University of Bologna