Finding the 'DNA' of the galaxy

An international team of astronomers has used the chemical composition of stars to shed light on the formation of our Milky Way.

Artists conception of the Milky Way galaxy (R.Hurt, Clatech, NASA)

The formation and evolution of galaxies, and in particular of our Milky Way, is one of the major puzzles of astrophysics - indeed, a detailed physical explanation is still missing. The team, led by European Southern Observatory astronomer Gayandhi De Silva, used the Very Large Telescope (VLT) to find new ways to address this galactic puzzle, and found that chemical composition of star clusters was key.

“Galactic star clusters are witnesses of the formation history of the Galactic disc,” says Kenneth Freeman, a member of the team. “The analysis of their composition is like studying ancient fossils. We are chasing pieces of galactic DNA!”

The astronomers used the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT to observe a dozen red giants in the open star cluster Collinder 261, located about 25,000 light years from the galactic centre. Red giants are more luminous, hence they are well suited for high-precision measurements.

“We have analysed in great detail the chemical composition of stars in three star-clusters and shown that each cluster presents a high level of homogeneity and a very distinctive chemical signature,” said De Silva. “This paves the way to chemically tagging stars in our Galaxy to common formation sites and thus unravelling the history of the Milky Way,” she added.

Open star clusters are among the most important tools for the study of stellar and galactic evolution, and are composed of a few tens up to a few thousands of stars that are gravitationally bound. The team reason that if clusters have similar composition, then they are likely to have formed in common areas.

“This high level of homogeneity indicates that the chemical information survived through several billion years,” explains De Silva. “Thus all the stars in the cluster can be associated to the same prehistoric cloud. This corroborates what we had found for two other groups of stars.”

The team will now aim to measure the chemical abundances in a larger sample of star clusters. Once the ‘DNA’ of each star cluster is inferred, they hope it will be possible to trace the genealogic tree of the Milky Way.