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The source of the Milky Way’s spirals, a new way to spot gravitational waves and more in this week’s news

Galactic close encounter
The Milky Way should have ducked: Some 1.75 billion years ago a smaller galaxy swung by our neck of the cosmic woods, smearing out the Milky Way’s stars in a gravitational tug-of-war. Astronomers have simulated this galactic close encounter in the best detail yet, and suggest that it may have formed the Milky Way’s dramatic spiral arms. Small-galaxy interactions could thus play a bigger role in shaping galaxies than previously thought, write Chris Purcell, of the University of Pittsburgh and the University of California, Irvine, and his colleagues in the Sept. 15 Nature. —Alexandra Witze

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