NASA's Webb and Hubble Telescopes Reveal Mysterious History of a Galactic Formation Relic

Researchers using humanity's two most powerful space observatories — NASA's James Webb Space Telescope (JWST) and the Hubble Space Telescope — have definitively confirmed that Terzan 5 is not the globular cluster it was long believed to be, offering a new window into how galaxies like the Milky Way form and evolve.

Ordinary globular clusters contain a single population of ancient stars. However, the latest data not only confirmed the existence of two distinct stellar populations within Terzan 5, but also revealed two additional, more recent rounds of star formation. Despite being located within the Milky Way's crowded bulge — the dense, spherical region of older stars at the galaxy's center — Terzan 5 was massive enough to maintain its identity over billions of years, even as lighter systems dispersed and merged into the bulge. It is akin to a lump in cake batter that never fully dissolves, no matter how long you stir.

"Webb's new near-infrared observations, combined with Hubble's historical data, have given us a much clearer picture of Terzan 5's history," said lead researcher Giorgia Zullo, a doctoral student at the University of Bologna, Italy.

The findings were presented at a press conference during the 248th meeting of the American Astronomical Society (AAS), held in Pasadena, California, and published in the journal Astronomy & Astrophysics.

Four Generations of Stars

Terzan 5 was discovered by astronomer Alop Terzan in 1968 and had long appeared similar to a globular cluster. In 2009, however, scientists found that the system contained two distinctly different stellar populations. In 2016, Hubble first estimated the ages of these two populations: one formed approximately 12.5 billion years ago — when the Milky Way itself was still assembling — and another roughly 4.7 billion years ago, just before Earth began to form. This pointed to a far more complex evolutionary history than that of a typical globular cluster.

Because Terzan 5 lies in the heavily star-crowded, dust-obscured region toward the galactic center, research has been extremely challenging — and this is where Webb proved critical. Its infrared capabilities allowed the team to peer through the dust and observe and catalog more stars, and fainter ones, than ever before. By measuring stellar color and brightness, astronomers can sort stars by age and chemical composition.

Webb was able to measure these key properties for every star in its field of view — whether a member of Terzan 5 or a foreground star. To isolate true Terzan 5 members, the team relied on Hubble's unmatched long-baseline observing record. Spanning 12 years of data, Hubble enabled measurements of the tiny positional shifts of individual stars — known as proper motions — making it possible to determine which stars belong to Terzan 5 and which belong to the Milky Way bulge.

Combining Webb and Hubble data, researchers found compelling evidence for two additional stellar populations: one formed approximately 3.8 billion years ago, and another as recently as 2.5 billion years ago. They also re-confirmed the ages of the two previously known populations — 12.5 billion and 4.7 billion years ago — with unprecedented precision.

With only two known stellar generations, astronomers could not rule out the possibility that Terzan 5 had acquired fresh gas through interactions with other objects — such as globular clusters or giant molecular clouds — triggering a second round of star formation. The discovery of four generations eliminates those explanations.

Chemical composition measurements of Terzan 5's stellar populations, conducted using the W. M. Keck Observatory and the European Southern Observatory's Very Large Telescope (VLT), also point to distinctly different populations. "Beyond the ages of these populations, the cluster preserves a fossil record of progressive heavy-element enrichment by supernovae," said co-author R. Michael Rich, a research astronomer at the University of California, Los Angeles (UCLA).

Terzan 5's ability to generate multiple generations of stars stems from its capacity to retain the necessary raw materials. Evidence suggests that powerful supernova explosions within Terzan 5 forged heavier elements that were subsequently incorporated into later generations of stars. In lower-mass systems, the force of those explosions alone is sufficient to expel those elements along with residual gas and dust. Terzan 5's progenitor, however, was massive enough to hold onto the ejecta from those stellar explosions, allowing new generations of stars to continue forming over billions of years.

A 'Bulge Fossil Fragment'

The findings suggest that Terzan 5 is most likely the remnant of a far more massive stellar system that formed 12.5 billion years ago. What makes Terzan 5 remarkable is that it survived — never fully merging or "blending" into the Milky Way's bulge. "For some reason, this particular stellar clump formed independently and was not destroyed during the very process of bulge formation itself," said Francesco R. Ferraro, a professor at the University of Bologna and principal investigator of the Webb observing program. "Terzan 5 is what we now call a 'bulge fossil fragment,' because it closely resembles the primordial gaseous clumps that contributed to building the bulge."

Only one other known object resembles Terzan 5: Liller 1, the second object to be reclassified from globular cluster to bulge fossil fragment, which also contains multiple generations of stars. More such objects may yet be found. Ferraro's team plans to examine another 40 to 50 globular clusters orbiting within the bulge to determine whether their stellar populations are all the same age — as in a typical globular cluster — or span multiple generations, as in a bulge fossil fragment.

Implications for Galaxy Formation Near and Far

This research may ultimately deepen our understanding of how galactic bulges assemble over hundreds of millions of years. "Based on observations and detailed simulations, we believe that early galaxies had massive gaseous disks that fragmented into clumps and formed stars. These clumps migrated toward the galactic center, where many merged to form the bulge," said co-author Barbara Lanzoni, an associate professor at the University of Bologna. Webb has already observed multiple 'clumpy' galaxies actively forming stars when the universe was only a few hundred million years old — including the clumps within the so-called Firefly Sparkle galaxy. "Terzan 5 may provide direct evidence to help explain how galactic bulges formed throughout the universe," Lanzoni said.

The James Webb Space Telescope is the world's premier space science observatory. Webb is unraveling the mysteries of our solar system, peering into distant worlds around other stars, and probing the structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners ESA (European Space Agency) and CSA (Canadian Space Agency).

The Hubble Space Telescope has been operating for more than three decades, continuing to make groundbreaking discoveries that have profoundly shaped our fundamental understanding of the cosmos. Hubble is a joint international NASA/ESA project managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland. Science operations are conducted by the Space Telescope Science Institute (STScI) in Baltimore, which carries out Hubble's science mission for NASA.

---

原文來源： 查看原文