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Why does our galaxy spiral?

Spread your arms out wide, Milky Way

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Video A group of astrophysicists from the Harvard-Smithsonian Center for Astrophysics and the University of Wisconsin-Madison say they've resolved a long-standing question: how long do spiral arms in galaxies like our own last?

The boffins aren't just thinking about our very own Milky Way: the paper, published in The Astrophysical Journal and also available at Arxiv, notes that 70 percent of galaxies in our little bit of the universe have spiral arms.

That makes them both photogenic and contentious, with some theories proposing that the spiral arms come and go and others treating them as permanent features.

Based on simulations of the motions of a hundred million “stellar particles” (that is, representations of stars in the simulator), the team says the formation of spiral arms happens in response to giant molecular clouds – the star-forming regions in galaxies. These clouds, the simulation suggests, are perturbers that both initiate the formation of spiral arms, and sustain the arms “indefinitely”.

“Past theory held the arms would go away with the perturbations removed, but we see that [once formed] the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there,” explained lead author Elena D'Onghia of Wisconsin-Madison.

The simulation ran a Monte Carlo N-body simulation on Harvard's Odyssey supercomputer, first installed in 2008 and originally using 512 Dell PowerEdge M600 modules with a total of 4,096 cores.

Their visualisation is shown in the video below. ®

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