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Milky Way’s fastest stars came from another galaxy

IANS | London |

Astronomers have shown that the fastest-moving stars in our galaxy, known as hypervelocity stars, are in fact runaways from a much smaller galaxy in orbit around our own.

These stellar sprinters originated in the Large Magellanic Cloud (LMC), a dwarf galaxy in orbit around the Milky Way, the findings showed. 

These fast-moving 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, the study said. 

For the study, published in the journal Monthly Notices of the Royal Astronomical Society, the researchers used data from the Sloan Digital Sky Survey and computer simulations. 

"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," said the paper's lead author Douglas Boubert from the Institute of Astronomy, University of Cambridge. 

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 is 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," Boubert said. 

"The hypervelocity stars are mostly found in the Leo and Sextans constellations – we wondered why that is the case," Boubert added.

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. 

"These stars have just jumped from an express train – no wonder they are fast," co-author Rob Izzard from the Institute of Astronomy said. 

"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," Izzard added.