Milky Way’s fastest stars are ‘runaways’ from another galaxy

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The fastest-moving stars in our galaxy are in fact runaways from a much smaller galaxy in orbit around our own, according to a recent study. The researchers, from the University of Cambridge, used data from the Sloan Digital Sky Survey and computer simulations to demonstrate that these stellar sprinters originated in the Large Magellanic Cloud (LMC), a dwarf galaxy in orbit around the Milky Way.

These fast-moving stars, known as hypervelocity stars, were able to escape their original home when the explosion of one star in a binary system caused the other to fly off with such speed that it was able to escape the gravity of the LMC and get absorbed into the Milky Way.

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Astronomers first thought that the hypervelocity stars, which are large blue stars, may have been expelled from the centre of the Milky Way by a supermassive black hole. Other scenarios involving disintegrating dwarf galaxies or chaotic star clusters can also account for the speeds of these stars but all three mechanisms fail to explain why they are only found in a certain part of the sky.

To date, roughly 20 hypervelocity stars have been observed, mostly in the northern hemisphere, although it’s possible that there are many more that can only be observed in the southern hemisphere.

“Earlier explanations for the origin of hypervelocity stars did not satisfy me,” said lead author Douglas Boubert. “The hypervelocity stars are mostly found in the Leo and Sextans constellations – we wondered why that is the case.”

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An alternative explanation to the origin of hypervelocity stars is that they are runaways from a binary system. In binary star systems, the closer the two stars are, the faster they orbit one another. If one star explodes as a supernova, it can break up the binary and the remaining star flies off at the speed it was orbiting. The escaping star is known as a runaway. Runaway stars originating in the Milky Way are not fast enough to be hypervelocity because blue stars can’t orbit close enough without the two stars merging. But a fast-moving galaxy could give rise to these speedy stars.

The LMC is the largest and fastest of the dozens of dwarf galaxies in orbit around the Milky Way. It only has 10% of the mass of the Milky Way, and so the fastest runaways born in this dwarf galaxy can easily escape its gravity. The LMC flies around the Milky Way at 400 kilometres per second and, like a bullet fired from a moving train, the speed of these runaway stars is the velocity they were ejected at plus the velocity of the LMC. This is fast enough for them to be the hypervelocity stars.

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“These stars have just jumped from an express train – no wonder they’re fast,” said co-author 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 results are published in the Monthly Notices of the Royal Astronomical Society.