Runaway stars flung into the Milky Way by extragalactic supernovas

Hypervelocity stars may all come from the Large Magellanic Cloud, Andrew Masterson reports.

An artist’s impression of a hypervelocity star.
An artist’s impression of a hypervelocity star.
Amanda Smith

Hypervelocity stars – rare super-suns that blast through the Milky Way at up to 500 kilometres a second – are the subjects of increasing research, and the latest findings throw up a surprising twist on their origins.

So far there have been only 20 high-speed stars identified, and until recently most astronomers thought they had been ejected from their original orbits by a supermassive black hole at the heart of the galaxy.

However, researchers from the University of Cambridge, using data from the Sloan Digital Sky Survey, suggest the explanation is way wide of the mark.

Far from coming from deep within the Milky Way, they say, hypervelocity stars actually began their breakneck journeys in another galaxy altogether – specifically the Large Magellanic Cloud (LMC).

“These stars have just jumped from an express train – no wonder they're fast,” says Rob Izzard, one of the researchers and co-author of the paper published in the journal Monthly Notices of the Royal Astronomical Society.

Izzard and his colleagues began their research in a quest to explain why high-speed stars have mainly been recorded in the skies above the northern hemisphere, notably in the constellations of Leo and Sextans.

Dissatisfied with the black hole theory, the astronomers combed the data looking for an alternative explanation. They realised that observations of hypervelocity star trajectories were consistent with an origin point outside the Milky Way.

Specifically, they concluded, a fast-moving star could be the result of one half of a binary star system – two stars orbiting each other very fast – if the other star went supernova and exploded.

The force of the supernova would be enough to knock the remaining star out of its trajectory and send it towards the edge of its host galaxy. With enough momentum to escape that galaxy’s gravitational pull, the rogue star would effectively slingshot out of its original home and zoom off into deep space.

Izzard and his team deduced that the Large Magellanic Cloud was a perfect candidate for the birthplace of the hypervelocity stars. Contrary to its name, the cloud is actually a rather small galaxy orbiting the Milky Way. It has only about 10% of the mass of our galaxy – a fact that, combined with its speed, means that a dislodged star could easily escape its gravity and power away with enough velocity to also resist the gravitational pull of its enormous neighbour.

Along with other research teams, the Cambridge crew is confident the 20 runaway stars so far identified are just the start of the haul.

“We are the first to simulate the ejection of runaway stars from the LMC – we predict that there are 10,000 runaways spread across the sky,” says co-author Douglas Boubert.

“We’ll know soon enough whether we’re right. 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.”

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Andrew Masterson is news editor of Cosmos.
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