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The Fastest Stars in Our Milky Way Are High-Speed Runaways From Another Galaxy

Don't get in their way.

6 JUL 2017

Astronomers might finally have an explanation for why the fastest-moving stars in our galaxy have picked up speeds that seem to defy the laws of physics – they're actually runaway stars from other galaxies orbiting the Milky Way.

That's the conclusion drawn by scientists who studied the latest data from our most powerful telescopes and ran computer simulations to back up their new hypothesis.


These stars, known as hypervelocity stars, are moving so fast they're capable of escaping the gravitational pull of the Milky Way altogether, and that's a much quicker pace than we'd expect: a velocity of up to 1,200 km/s (746 miles/sec) as opposed to the maximum 100 kilometres per second (62 miles/sec) velocity of most stars.

The team of researchers from the University of Cambridge in the UK thinks that this breakneck pace could be set by stellar explosions from other galaxies, including the Large Magellanic Cloud (LMC) dwarf galaxy.

"These stars have just jumped from an express train – no wonder they're fast," says one of the researchers, Rob Izzard.

"This also explains their position in the sky, because the fastest runaways are ejected along the orbit of the LMC towards the constellations of Leo and Sextans."

The astronomers used data from the Sloan Digital Sky Survey against their own models to show that runaway stars could be created from binary star systems with two stars at their centre rather than one.

If one of these stars explodes as a supernova, then the other can be catapulted across space at the speed it was orbiting, creating a runaway star.


We've known about runaway stars for a while, but those created in our own Milky Way don't reach hypervelocity speeds – that's because binary stars can't pick up that sort of pace without merging together.

Another factor must be at play to kick up the speed before the stars can merge.

And so if the runaway star was thrown out by a galaxy also travelling at high speeds outside the Milky Way, then we could be talking about hypervelocity speeds. The LMC orbits our galaxy at 400 km/s (249 miles/sec).

Astronomers had thought hypervelocity speeds could have been generated by supermassive black holes, until a high-speed binary star was found without any such black hole in sight. Other ideas put forward in the past included disintegrating dwarf galaxies or chaotic star clusters.

None of these hypotheses satisfied the University of Cambridge researchers, and none of them really explained why hypervelocity stars are only found in a certain part of the sky. Around 20 hypervelocity stars have been spotted so far, and they've all been around the Leo and Sextans constellations.

The models put together by the researchers simulated the life and death of stars in the LMC over a 2 billion year period, and predicted that there should be around 10,000 runaway stars somewhere in the night sky, with around half capable of getting up to hypervelocity speeds.


And as our space mapping technology improves, we'll have a chance to put this new hypothesis to the test soon enough.

"We'll know soon enough whether we're right," says lead researcher Douglas Boubert. "The European Space Agency's Gaia satellite will report data on billions of stars next year, and there should be a trail of hypervelocity stars across the sky between the Leo and Sextans constellations in the North and the LMC in the South."

The research has been published in the Monthly Notices of the Royal Astronomical Society.

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