Hubble Finds ‘Fossil’ of Early Milky Way

(CN) — A fossilized remnant of the early Milky Way — harboring stars up to several million times larger than the sun — may provide new insights into the galaxy's ancient past.

Terzan 5, long thought to be a traditional globular cluster — a large, compact spherical star cluster — is unique in that star formation appears to have occurred in two separate bursts, with an age gap of roughly seven billion years.

"This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive. At least 100 million times the mass of the sun," said co-author Davide Massari.

Massari is part of a team of Italian researchers who discovered that the stars formed in two distinct bursts. Their work was published Wednesday and is derived from data scoured from the Hubble telescope and a suite of ground-based telescopes.

Besides forming at different times, the elements contained within the stars also differ. These unique properties make Terzan 5 a living fossil from the very beginning of the Milky Way.

Current theories on galaxy formation suggest that large clumps of gas and stars interacted to form the initial bulge of the Milky Way, merging and dissolving in the process.

"We think that some remnants of these gaseous clumps could remain relatively undisrupted and keep existing embedded within the galaxy," said Francesco Ferraro, a researcher from the University of Bologna, Italy, and lead author of the study. "Such galactic fossils allow astronomers to reconstruct an important piece of the history of our Milky Way."

While the properties of Terzan 5 are unique for a globular cluster, they do resemble the stellar population that can be found in the galactic bulge — the tightly packed central region of the Milky Way. These similarities could mean Terzan 5 is a fossilized relic of galaxy formation, representing one of the earliest building blocks of the Milky Way.

Further evidence of this potential connection is the original mass of Terzan 5, which was necessary to create two stellar populations. Such a mass would be similar to the huge clumps that are thought to have formed the galactic bulge during galaxy assembly around 12 billion years ago.

Terzan 5 has managed to survive disruptions for billions of years, serving as a remnant of Milky Way's distant past.

The team's findings present a potential route for astronomers attempting to unravel the mysteries of galaxy formation.

"Terzan 5 could represent an intriguing link between the local and distant universe, a surviving witness of the galactic bulge assembly process," Ferraro said.

The star system is 19,000 light-years from Earth.

Photo: NASA/ESA/Hubble/F. Ferraro

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