Gaia: Milky Way may have formed 'inside-out'

This is a figure illustrating latest Gaia-ESO research findings. 

Credit: Amanda Smith/Institute of Astronomy

A breakthrough using data from the Gaia-ESO project has provided evidence backing up theoretically predicted divisions in the chemical composition of the stars that make up the Milky Way's disc – the vast collection of giant gas clouds and billions of stars that give our Galaxy its 'flying saucer' shape.

By tracking the fast-produced elements, specifically magnesium in this study, astronomers can determine how rapidly different parts of the Milky Way were formed.

The research suggests that stars in the inner regions of the Galactic disc were the first to form, supporting ideas that our Galaxy grew from the inside-out.

Using data from the 8-m VLT in Chile, one of the world's largest telescopes, an international team of 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'.

Gerry Gilmore

"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.

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 team have shown that older, 'metal-poor' stars inside the Solar Circle – the orbit of our Sun around the centre 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 metallic properties.

During the latest research, the team found that:
Stars in the young, 'thin' disc aged between 0 – 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.

But stars of different ages and metallicity can be found in both discs.

Maria Bergemann

"From what we now know, the Galaxy is not an 'either-or' system. You can find stars of different ages and metal content everywhere!" said Maria Bergemann from Cambridge Institute of Astronomy, who led the study.

"There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs - that's how we know these two discs probably exist – but they could have very different origins."

Added Gilmore: "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 neighbourhood."