Taking apart the Milky Way finds which parts were stolen from other galaxies

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  Last updated March 4, 2020 at 5:33 pm


Like taking apart a piece of technology, our galaxy – the Milky Way – has been reverse engineered to find out how it was assembled.

Credit: NASA

Why This Matters: To understand the universe, we need to understand the building blocks.

Astronomers have reverse engineered our galaxy to find out how it was formed.

Using ancient star clusters, Swinburne University’s Duncan Forbes traced back the evolution of our Milky Way galaxy to identify which star clusters formed within the original Milky Way, and which were acquired as the Milky Way swallowed up smaller satellite galaxies.

The Milky Way has been pretty greedy over its history. To get to the size we see today, astronomers think it has cannibalised other galaxies for their stars. Through a series of collisions the galaxies have merged, breaking apart the smaller galaxy. These fragmented galaxies then form compact clusters of stars within the Milky Way, which are still recognisable after billions of years.

This cannibalisation is thought to be still occurring today. Earlier this year the ESA’s Gaia satellite found the Milky Way is already acquiring stars from a collision that is yet to happen.

Also: There’s an epic fight under way on the outskirts of the Milky Way

But in the new work, published by the Monthly Notices of the Royal Astronomical Society, Forbes has found that most of the acquired star clusters in the Milky Way are the result of only five previous collisions with satellite galaxies.

Finding the Koala cluster

The five satellite galaxies formed very early, says Forbes, with their collision and absorption by the Milky Way occurring around 8–11 billion years ago. The Milky Way is around 13.5 billion years old itself, meaning the galaxy had a period of rapid growth early in its life.

One of the satellites contained star clusters on low energy orbits, which Forbes named ‘Koala’. The name for a low energy galaxy is fitting, given its namesake sleeps for more than 18 hours each day. After Koala had been absorbed by the Milky Way it formed 21 clusters of stars.

Probably the best known of the absorbed satellite galaxies was Sagittarius. Nine of the galaxy clusters found came from that collision, with one being the former centre of the galaxy.

Deeper: Things we learnt from Stargazing Live: The Milky Way

From the motions, ages and chemical composition of the star clusters, Forbes inferred that several of the satellite galaxies contained bright nuclei at their centres and contained gas, the material needed for new star formation.

These poached star clusters only represent a few percent of the Milky Way’s current stellar mass, says Forbes. However, between them, the 76 clusters that result from these five collisions make up a significant proportion of the Milky Way’s total number of star clusters.

“Although our Milky Way may have undergone a tumultuous past, as it grew by accreting and disrupting other small galaxies, the star clusters known as globular clusters are extremely robust and they largely survived intact to the present day,” Professor Forbes says.

“It is these globular clusters that can be used to re-trace, or reverse engineer, the assembly history of our own galaxy going back billions of years.

“When you look up at the night sky, some of the individual stars and star clusters that you can see were actually formed outside of our galaxy – alien objects if you like, but now part of the Milky Way galaxy as we know it,” says Professor Forbes.

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About the Author

Ben Lewis
Ben Lewis is the Editor of Australia’s Science Channel, and a contributor to Cosmos Magazine. He has worked with scientists and science storytellers including Jane Goodall, Dr Karl, Brian Cox, Chris Hadfield, Robert Llewellyn, astronauts, elite athletes, Antarctic explorers, chefs and comedians. Ben has also been involved in public events around Australia and was co-writer, producer and director of The Science of Doctor Who, which toured nationally in 2014 in association with BBC Worldwide Australia & New Zealand. Want more Ben? You can hear him on ABC and commercial radio in Adelaide, regional SA, across NSW, and the ACT. He also speaks at universities around Australia on communicating science to the public. Around the office he makes the worst jokes known to mankind.

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