An international team of astronomers has identified a rare remnant of the early Milky Way displaying unusual properties, which will help scientists to fill the gap in our understanding between the galaxy's past and present. The structure is a collection of stars of hugely different ages resembling a globular cluster – spherical groupings of stars which are usually found in the galactic halo – but unlike any other known cluster.

Since it was first detected 19,000 light years from Earth around 40 years ago, Terzan 5, as it is known, was classified as a globular cluster. An Italian-led team of astronomers has now shown that Terzan is, in fact, very different from the norm.

Using data from Hubble and other telescopes, the team found that Terzan harboured two very different types of stars, which not only differed in their elemental make-up, but also had an age-gap of around seven billion years.

The ages of the two different types of stars suggest that star formation in Terzan 5 was not continuous but was dominated by two very distinct bursts of star formation. "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 Davide Massari, a co-author of the study.

These special properties, and the fact that Terzan 5, contains stars which are remarkably similar to the most ancient stars in our galaxy, means that it is a kind of "living fossil", a relic from the early days of the Milky Way.

Terzan 5 stellar region
This wide-field image, based on data from Digitized Sky Survey 2, shows the whole region around the stellar grouping Terzan 5 ESO/Digitized Sky Survey 2

Current theories on galaxy formation suggest that vast clumps of gas and stars interacted to form the primordial bulge of the Milky Way – the tightly packed central region of the galaxy – 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 lead author of the study. "Such galactic fossils allow astronomers to reconstruct an important piece of the history of our Milky Way."

Terzan 5
Peering through the thick dust clouds of the galactic bulge an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5 Nasa/ESA/Hubble/F. Ferraro

While the properties of Terzan 5 are uncommon for a globular cluster, they are very similar to the huge clumps which are thought to have formed, during the galaxy's formation around 12 billion years ago.

"Some characteristics of Terzan 5 resemble those detected in the giant clumps we see in star-forming galaxies at high-redshift, suggesting that similar assembling processes occurred in the local and in the distant universe at the epoch of galaxy formation," said Ferraro.

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