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Alien star clusters fill our galaxy

Image: Messier 80
A Hubble Space Telescope image of the typical globular cluster Messier 80, an object made up of hundreds of thousands of stars and located in the direction of the constellation of Scorpius. NASA
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Many of our galaxy's star clusters may actually foreigners: collections of stars that were born elsewhere and then migrated to our Milky Way, according to a new study.

The foreign-born star clusters actually make up about one-quarter of the Milky Way globular star cluster system, researchers found.

"It turns out that many of the stars and globular star clusters we see when we look into the night sky are not natives, but aliens from other galaxies," said study co-author Duncan Forbes, an astronomer from Swinburne University of Technology in Australia. "They have made their way into our galaxy over the last few billion years."

Astronomers previously suspected that some globular star clusters, which each contain between 10,000 and several million stars, were foreign to our galaxy, but it was difficult to positively identify which ones.

To find them, Forbes, along with his colleague Terry Bridges of Queens University in Ontario, Canada, used data from the Hubble Space Telescope to examine globular star clusters within the Milky Way galaxy.

They then compiled the largest ever high-quality database to record the age and chemical properties of each of these clusters.

"Using this database we were able to identify key signatures in many of the globular star clusters that gave us tell-tale clues as to their external origin," Forbes said.

The research will be detailed in an upcoming edition of Monthly Notices of the Royal Astronomical Society.

The work also suggests that the Milky Way may have swallowed up more dwarf galaxies, or 'mini' galaxies of up to 100 million stars, than was previously thought.

Previous research has shown that two dwarf galaxies within the Milky Way are foreign-born, but the current work suggests there may be as many as six or seven.

"Although the dwarf galaxies are broken-up and their stars assimilated into the Milky Way, the globular star clusters of the dwarf galaxy remain intact and survive the accretion process," Forbes said.

However, future studies will be needed to confirm this finding, the researchers said.