杏吧原创

The speediest stars in the galaxy, moving at hundreds or even thousands of kilometres per second, are also some of the least well-studied. A study of 15 of these fast-moving objects has found that most of them probably came from dwarf galaxies devoured by the Milky Way in the distant past.

鈥淪ome stars that are travelling fast have been [proposed to have] an extragalactic origin, but this is the first time that a relatively large sample has been analysed and evidence has been shown that they are not from this galaxy,鈥� says at the Carnegie Institution for Science in California. He and his colleagues used telescopes at Apache Point Observatory in New Mexico and Las Campanas Observatory in Chile to examine the light from 15 stars with relatively high speeds and learn their chemical compositions.

These stars aren鈥檛 the very fastest in the galaxy, which are called hypervelocity stars and move with speeds in excess of 1000 kilometres per second, but rather a category called extreme velocity stars, which move at hundreds of kilometres per second and are significantly easier to examine. 鈥淭hey are not travelling as fast as the so-called hypervelocity stars, but their atmospheres are not as hot and they can have their chemistry analysed,鈥� says Reggiani.

The researchers examined the relative abundances of 19 elements in this set of 15 stars. Stars from different galaxies tend to have unique combinations of these elements based on their formation histories, so this allowed the researchers to distinguish what kind of galaxies the stars were from.

Of the 15 extreme-velocity stars, the researchers found eight with chemical compositions that were definitively different from Milky Way stars, suggesting they formed elsewhere, while the rest were inconclusive. The stars that came from beyond our galaxy were probably born in smaller galaxies that were swallowed up by the Milky Way.

Over the course of this messy galactic meal, the gravity of the Milky Way would have ripped apart the smaller galaxies, swinging them around and accelerating their stars to the high speeds we see today. The researchers are now working to examine other fast-moving stars to find out how many of them truly came from beyond our galaxy and identify the ill-fated galaxies where they were born.

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