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Milky Way may have formed ‘inside-out’

Jenny Winder, News Writer
Jan 23, 2014, 8:00 UTC

Sen—Research on the first data release from the Gaia-ESO project suggests the Milky Way formed by expanding out from the centre, and reveals new insights into the way our Galaxy was assembled, backing up theoretically-predicted divisions in the chemical composition of the stars that make up the Milky Way's disc.

The Gaia-ESO project is a public spectroscopy survey targeting around 105 stars, systematically covering all major components of the MilkyWay.

By tracking fast-produced elements, specifically magnesium, astronomers can determine how rapidly different parts of the Milky Way were formed. The research suggests the inner regions of the Galaxy assembled faster than the outer regions, which took a much longer time to form, supporting ideas that our Galaxy grew from the inside-out.

Using data from the 8-m Very Large Telescope (VLT) in Chile, astronomers 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 - and we - 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 Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.

Radial metallicity gradients and age-metallicity of stars in the Milky Way disc. Image credit: University of Cambridge.

The team found that older, "metal-poor" stars inside the Solar Circle, the orbit of our Sun around the centre of the Milky Way, are more likely to have high levels of magnesium, suggesting this area contained more stars that “lived fast and die young” in the past. Stars 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.

The research also sheds 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," explains Aldo Serenelli from the Institute of Space Sciences in Barcelona. "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."

The team found that stars in the young, "thin" disc aged between 0 and 8 billion years all have a similar degree of metallicity, regardless of age in that range, with many of them considered "metal-rich". There is a "steep decline" in metallicity for stars aged over 9 billion years, typical of the "thick" disc, with no detectable "metal-rich" stars found at all over this age. Stars of different ages and metallicity can be found in both discs.

In theory, say astronomers, the thick disc - first proposed by Gilmore 30 years ago - could have emerged in a variety of ways, from massive gravitational instabilities to consuming satellite galaxies in its formative years.