The Milky Way is a giant ‘smoothie’ of blended stars

A new Australian study has analysed the light from 600,000 stars in the Milky Way to identify which ones originated within it, and which formed in other galaxies and were blended into ours in the distant past.

“Although the Milky Way is our home galaxy, we still do not understand how it formed and evolved,” says first author Dr Sven Buder, from the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the Australian National University (ANU).

“The Milky Way ate up lots of smaller galaxies but, until recently, we did not have enough evidence of that to say for sure. That’s because simple images of stars in our Milky Way look the same – whether they were born inside the galaxy or outside and then blended into the galaxy.”

Published in the Monthly Notices of the Royal Astronomical Society, the researchers analysed the light from stars to help us understand what went in to creating the Milky Way we see today.

A graph of the abundance of sodium and iron, and magnesium and manganese in stars originating from inside or outside the milky way.
Just by looking at how abundant are sodium, iron, magnesium, and manganese in a star, we can tell apart stars born in the Milky Way (green) or outside (yellow). Credit: from paper

The team, from the Galactic Archaeology with HERMES (GALAH) Survey, used Australia’s largest optical telescope – the Anglo-Australian Telescope (ATT) to collect the light from more than 600,000 stars and split it with the HERMES (High Efficiency and Resolution Multi-Element Spectrograph) instrument.

Akin to the way water droplets in the atmosphere split white light into its components to form rainbows, the researchers split the light into its different wavelengths to get 600,000 stellar spectra signatures. Each contains specific bands of light which correspond to the emission spectrum of different chemical elements, acting like tiny barcodes that vary depending on a star’s chemical composition.

“By scanning these ‘stellar barcodes’, we measured how abundant 30 elements – such as sodium, iron, magnesium, and manganese – were, and how they appeared in different concentrations depending on where the star was born,” explains Buder.

This method, known as chemical tagging, looks at the percentage composition of sodium, iron, magnesium and manganese to detect whether a star formed within the galaxy or was absorbed from a satellite galaxy.

This early step in reconstructing the early Milky Way offers an insight into the size of the galaxies it consumed in its early stages. It could also answer questions about several of its special features, for example our galaxy’s  two distinct groups of stars in the disc that we see as the milky band in the night sky.

“The Milky Way spread out across the night sky is a familiar sight, and when we look at it, we are actually gazing into the centre of our galaxy with its billions of stars,” says Buder. “But we are looking at two populations of stars, one much older than the other.

“The old stars have moved, so they look like they bulge out of the main plane of the Milky Way – while the younger stars form a much thinner band in the plane,” he adds. “But we don’t know why this has happened and our latest findings of the remnants of gigantic, galactic collisions may help us understand.”

Young disk old disk with telescope high res
The Milky Way has two distinct populations of stars, one older than the other. The older stars have moved so they look like they bulge out of the main plane of the Milky Way, while the younger stars form a much thinner band in the plane. Credit: author provided

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