Astronomers have found a wave of creation in our corner of the Milky Way. It's a huge river of gas that has the mass of 3 million suns, snaking for 9,000 light-years over and under the galaxy. The structure is about 400 light-years wide but, in places, so tightly packed that the gas condenses into stars.
The cloud is the largest single structure of gas and dust discovered in the galaxy. And it's in our neighborhood, astronomically speaking: The structure's nearest point is only 500 light-years from the sun. (A recent estimate suggests the entire Milky Way is, by contrast, 170,000 light-years across.)
"No astronomer expected that we live next to a giant, wavelike collection of gas — or that it forms the Local Arm of the Milky Way," said Alyssa Goodman, an applied astronomy professor at Harvard University, in a statement. Goodman and her colleagues published a report on the structure on Tuesday in the journal Nature.
They used data from Gaia, the European Space Agency spacecraft, which since 2014 has been surveying the location and motion of a billion stars in the Milky Way.
Early astronomers noticed there was a nearby band of stars, that, to them, looked like the arc of a ring; for 150 years it was known as Gould's Belt, after the 19th century American astronomer Benjamin Gould who first described it.
But the belt isn't a ring, after all. Most of the stars in our galaxy swirl in the same flattish plane, circling the galactic center like water around a drain. This structure rises and falls, for 500 light-years in either direction, outside the Milky Way's disk.
The line of newborn stars and gas "appears to be undulating," the study authors wrote. The scientists described the shape as sinusoidal, meaning it has humps and troughs like a sine function. (Imagine the up-and-down dips of a roller coaster.) The study authors named it the Radcliffe Wave, for the Radcliffe Institute for Advanced Study at Harvard.
It's not clear how the Radcliffe Wave formed or why it undulates. "We don't know what causes this shape," said University of Vienna astrophysicist and study author João Alves. "But it could be like a ripple in a pond, as if something extraordinarily massive landed in our galaxy."
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