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Sky rivers

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Science  05 Oct 2018:
Vol. 362, Issue 6410, pp. 16-21
DOI: 10.1126/science.362.6410.16

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Summary

Stellar streams are like necklaces of stars. They stretch across the Milky Way's halo, a sparse and roughly spherical region of stars that envelops the galaxy's whirling disk like a snow globe. They are the filamentous remains of neighboring small galaxies and star clusters that have been disemboweled as they fall into the galaxy's gravitational grasp. And their study is now entering a golden age, with astronomical surveys like the European Space Agency's Gaia pushing into the far reaches of the Milky Way's halo. Even before Gaia, astronomers were finding streams in data from ground-based observing campaigns such as the Sloan Digital Sky Survey and the Dark Energy Survey. Now, their number is exploding. In just the past 2 years, the count has more than doubled, passing 60. It is more than mere stamp collecting. The streams are particularly useful for what astronomers call galactic archaeology—rewinding the cosmic clock to reconstruct the assembly of the Milky Way. They also are being used as exquisitely sensitive scales to measure the galaxy's mass. The third possible application for stellar streams is perhaps the most intriguing. Patrolling the outskirts of the galaxy's halo, the streams are well-positioned to reveal the presence of dark matter, the unseen stuff thought to dominate ordinary matter by a ratio of nearly six to one. Because the streams are so fragile, theorists say, collisions with marauding clumps of dark matter could leave telltale scars, potential clues to its nature.