Astronomers have revealed an unusual cocktail of stars in the stellar grouping known as Terzan 5, which could represent a relic building block of our Milky Way's central bulge.

Never observed anywhere in the bulge before, Terzan 5 could be the remains of a dwarf galaxy that merged with the Milky Way during its very early days. “The history of the Milky Way is encoded in its oldest fragments, globular clusters and other systems of stars that have witnessed the entire evolution of our galaxy,” says Francesco Ferraro, lead author of a paper appearing in this week’s issue of the journal Nature. “Our new study opens a new window on yet another piece of our galactic past.”

Terzan 5 is an unusual cluster because, unlike most 'normal' globular clusters, it contains two distinct populations of star –one that formed some 12 billion years ago and another six billion years ago. “Only one globular cluster with such a complex history of star formation has been observed in the halo of the Milky Way: Omega Centauri,” says team member Emanuele Dalessandro. “This is the first time we see this in the bulge.”

The dusty nature of the galactic bulge does not make observations of this region easy: only infrared light can penetrate the thick veil to reveal the myriad stars hidden there.

“It is only thanks to the outstanding instruments mounted on ESO’s Very Large Telescope (VLT) that we have finally been able to ‘disperse the fog’ and gain a new perspective on the origin of the galactic bulge itself,” says co-author Barbara Lanzoni.

Using the sharp eye of the VLT, the astronomers also discovered that Terzan 5 is more massive than previously thought. Combined with its complex star formation history, its size suggests it could be a surviving remnant of a disrupted dwarf galaxy, which merged with the Milky Way during its very early stages, providing the building blocks of the galactic bulge.

“This could be the first of a series of further discoveries shedding light on the origin of bulges in galaxies, which is still hotly debated,” concludes Ferraro. “Several similar systems could be hidden behind the bulge’s dust: it is in these objects that the formation history of our Milky Way is written.”