Milky Way
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Stars in our part of the Milky Way seem to be doing "the wave," a new study suggests. The finding comes from an analysis of the motions of more than 70,000 red giant stars that lie within 6500 light-years of Earth - a distance that, in one direction, reaches about one-fourth of the way to the center of the galaxy.

Above the horizontal plane that slices through the center of the galaxy, stars closer to the center of the galaxy than the sun are, in general, moving away from the plane at speeds of 10 kilometers per second or less. Meanwhile, those farther from the galactic center than the sun are moving toward the plane - in some cases, as fast as 17 kilometers per second. All together, the complexity of motions observed by the team is similar to that seen among molecules in a gas with a sound wave passing through it, the researchers report this month in the Monthly Notices of the Royal Astronomical Society.

As of yet, the reasons for these anomalous motions aren't clear, the researchers note. The "wave" may indeed be a ripple caused by a long-ago collision with a small companion galaxy, or it may result from perturbations in pressure triggered as the Milky Way's spiral arms (artist's concept above) push their way through space as the galaxy rotates.