Stars from other galaxies spotted flying through the Milky Way

Astronomers believe they may have glimpsed stars speeding through our galaxy from intergalactic space after examining data from the Gaia mission.

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Jackson Ryan was CNET's science editor, and a multiple award-winning one at that. Earlier, he'd been a scientist, but he realized he wasn't very happy sitting at a lab bench all day. Science writing, he realized, was the best job in the world -- it let him tell stories about space, the planet, climate change and the people working at the frontiers of human knowledge. He also owns a lot of ugly Christmas sweaters.
Jackson Ryan
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The European Space Agency's Gaia mission is an ambitious project that aims to provide a complete map of the dizzying number of stars in our home galaxy.

New research suggests it might have also picked up a few extragalactic foreign invaders.

Since launching in 2013, the mission has provided some incredible data on 1.7 billion stars. With so much data to work through, astronomers have their work cut out for them -- and on Tuesday, the ESA announced that astronomers found stars not being flung away from the Milky Way, but further into it.

Stars speed around the Milky Way at hundreds of kilometres per second, but the fastest of them can travel upward of 1,000km/s and are known as hyper-velocity stars. The consensus has been that these fast moving stars start their lives in the middle of the Milky Way, before being flung further and further out. Because they travel at such great speed, it is believed that they can escape the gravitational pull of the galaxy.

In research published in Monthly Notices of the Royal Astronomical Society, astronomers at the Leiden Observatory in the Netherlands discovered 20 such hyper-velocity stars but only seven of them appeared to be moving away from the Milky Way. Unusually, 13 of the stars appeared to be extragalactic.

"These could be stars from another galaxy, zooming right through the Milky Way," Tommaso Marchetti told the Royal Astronomical Society.

Of the extragalactic stars the paper proposes, it's the delightfully named Gaia DR2 1396963577886583296, with a velocity of over 700 kilometres per second, that has the highest probability of visiting from another galaxy.

The team suggests that these stars may have come from a galactic neighbour known as the Large Magellanic Cloud, a 7,000 light-year wide cluster of stars around 163,000 light years from home.

However, the paper does note that these 13 stars have a probability of originating from the Milky Way's stellar disk at less than 50 percent, rather than stating this is definitive evidence that deep space stars are speeding through our neighborhood. A super-fast star might not be passing through at all, but rather a lingering effect of interactions the stars may have had with dwarf galaxies during the formation of the Milky Way.

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