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Worlds without end: Using the data from Gaia’s second data release astronomers have identified twenty stars that are moving too fast to be permanent members of the Milky Way galaxy.
More significantly, most appeared to be approaching the galaxy, not flying away from it, suggesting they are visitors from other galaxies.
It is possible that these intergalactic interlopers come from the Large Magellanic Cloud, a relatively small galaxy orbiting the Milky Way, or they may originate from a galaxy even further afield. If that is the case, they carry the imprint of their site of origin, and studying them at much closer distances than their parent galaxy could provide unprecedented information on the nature of stars in another galaxy – similar in a way to studying Martian material brought to our planet by meteorites.
…An alternative explanation is that the newly identified sprinting stars could be native to our Galaxy’s halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way during its build-up history. Additional information about the age and composition of the stars could help the astronomers clarify their origin.
At least two more data releases shall come from Gaia, launched by Europe to precisely track the location and motions of a billion stars. So far, they have complete 3D velocity information for about seven million stars. After these additional data releases they expect to have complete 3D velocity information for 150 million stars, and should identify a lot more intergalactic stars at that time.