Our Milky Way Galaxy formed by expanding out from the center, suggests analysis of first data from the Gaia-ESO survey – the ground-based extension to the Gaia space mission, launched by the European Space Agency at the end of 2013.

The astronomers involved with the Gaia-ESO project took detailed observations of stars with a wide range of ages and locations in the Galactic disc to accurately determine their ‘metallicity’: the amount of chemical elements in a star other than hydrogen and helium, the two elements most stars are made from.

Immediately after the Big Bang, the Universe consisted almost entirely of hydrogen and helium, with levels of “contaminant metals” growing over time. Consequently, older stars have fewer elements in their make-up – so have lower metallicity.

“The different chemical elements of which stars are made are created at different rates – some in massive stars which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes,” said Prof Gerry Gilmore from the University of Cambridge, who is a co-author of the paper submitted to the journal Astronomy and Astrophysics (arXiv.org version).

Massive stars, which have short lives and die as ‘core-collapse supernovae’, produce huge amounts of magnesium during their explosive death throes. This catastrophic event can form a neutron star or a black hole, and even trigger the formation of new stars.

The astronomers have shown that older, ‘metal-poor’ stars inside the Solar Circle – the orbit of our Sun around the center of the Milky Way, which takes roughly 250 million years to complete – are far more likely to have high levels of magnesium. The higher level of the element inside the Solar Circle suggests this area contained more stars that lived fast and die young in the past.

The stars that lie in the outer regions of the Galactic disc – outside the Solar Circle – are predominantly younger, both ‘metal-rich’ and ‘metal-poor’, and have surprisingly low magnesium levels compared to their metallicity.

This discovery signifies important differences in stellar evolution across the Milky Way disc, with very efficient and short star formation times occurring inside the Solar Circle; whereas, outside the Sun’s orbit, star formation took much longer.

“We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form,” said Maria Bergemann from Cambridge’s Institute of Astronomy, who led the study.

“This supports theoretical models for the formation of disc galaxies in the context of Cold Dark Matter cosmology, which predict that galaxy discs grow inside-out.”

The study also sheds further light on another much debated double structure in the Milky Way’s disc – the so-called ‘thin’ and ‘thick’ discs.

“The thin disc hosts spiral arms, young stars, giant molecular clouds – all objects which are young, at least in the context of the Galaxy,” said co-author Dr Aldo Serenelli from the Institute of Space Sciences.

“But astronomers have long suspected there is another disc, which is thicker, shorter and older. This thick disc hosts many old stars that have low metallicity.”

This study provides exciting new evidence that the inner parts of the Milky Way’s thick disc formed much more rapidly than did the thin disc stars, which dominate near our Solar neighborhood.”

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Bergemann M et al. 2014. The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk. A&A, submitted for publication; arXiv: 1401.4437