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Our Milky Way galaxy is INSIDE OUT. Just as we suspected, mutter boffins

Standard model of the Big Bang theory ahoy

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New data from the European Space Agency's Gaia-ESO project has confirmed that the Milky Way galaxy grew from the inside out, backing up theories espoused in the standard model of the Big Bang.

On-edge view of the Milky Way's star formation

Right after the Universe exploded into existence, it was made up almost entirely of hydrogen and helium. Levels of so-called "contaminant metals" grew up in galaxies over time, giving boffins a handy way to tell the age of the stars.

Using the Gaia-ESO project, astronomers tracked fast-produced elements like magnesium to see how quickly different parts of the Milky Way were formed. The study suggests that the inner regions of the galaxy assembled faster than the outer regions, confirming ideas that the Milky Way formed inside-out.

"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 are short-lived and die in core-collapse supernovae, produce huge amounts of magnesium as they burn up and leave behind neutron stars or black holes – or even form new stars.

Gaia's evidence shows that older, metal-poor stars inside the orbit of the Sun at the centre of the galaxy are far more likely to have high levels of magnesium, suggesting that they lived fast and died young in the past. But the stars in the outer region of the galactic disc are mostly younger, with both metal-rich and metal-poor constellations, with surprisingly low magnesium levels.

In terms of stellar evolution across the Milky Way, stars were formed quickly and efficiently inside the solar circle, but took much longer to come together outside the Sun's orbit.

“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 (Standard Model of the Big Bang), which predict that galaxy discs grow inside-out.”

The images snapped in the project also shed new light on the much-debated "double structure" of the Milky Way's disc - the idea that there might be "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 (Barcelona), a co-author of the study. “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 researchers found that stars of different ages and metallicity could be found in both the galaxy's discs.

“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 Bergemann. “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.”

The findings are part of the first wave of observations from the Gaia mission, which is trying to provide the largest and most comprehensive map yet of the Milky Way.

The full study, "The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk", can be found here and has been submitted to the journal Astronomy and Astrophysics. ®

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