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The Milky Way’s ‘flag’ action

Our galaxy flutters, but astronomers are perplexed as to why

New observations suggest that our Milky Way, shown here, flutters along its axis.

JPL-Caltech/NASA, S. Stolovy

How is the Milky Way like a flag? Both can flutter from top to bottom. At least that’s the conclusion of a new study that will be published soon.

While tracking stars in the suburbs of our solar system, Mary Williams of the Leibniz Institute for Astrophysics in Potsdam, Germany, and more than two dozen team members found something unexpected. The stars seemed sway in and out along the galaxy’s north-south axis. They compared this movement to the fluttering of a flag.

What's causing such an undulation is unclear. It could be due to the movement of our galaxy’s spiral arms, the astronomers say. Or it could be due to a collision between the Milky Way and some unseen smaller galaxy. Either hypothesis, Williams’ group says, could explain the apparent fluttering.

Her team will report this unusual motion soon. A paper describing it has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.

Power Words

astronomy  The area of science that deals with celestial objects, space and the physical universe as a whole. People who work in this field are called astronomers.

axis  The line about which something rotates. On a wheel, the axis would go straight through the center and stick out on either side.

galaxy  A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

Milky Way  The galaxy in which Earth’s solar system resides.

star  The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

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