The largest gaseous structure ever observed

US astronomers have discovered a wave-shaped gaseous structure they say is the largest ever seen in our galaxy.

Made up of interconnected stellar nurseries, it transforms a 150-year-old vision of nearby stellar nurseries as an expanding ring into one featuring an undulating, star-forming filament that reaches trillions of kilometres above and below the galactic disc.

The “Radcliffe Wave” – named after the Radcliffe Institute for Advanced Study at Harvard University – was revealed at the American Astronomical Society’s annual meeting in Hawaii and also is reported in the journal Nature.

The breakthrough was made possible by combining data from the European Space Agency’s Gaia spacecraft with other measurements to construct a detailed 3D map of interstellar matter in the Milky Way. 

The research team then noticed a long, thin structure they report is about 9000 light years long and 400 light years wide, with a wave-like shape, cresting 500 light years above and below the mid-plane of our galaxy’s disc. 

The wave includes many of the stellar nurseries that were previously thought to form part of Gould’s Belt, a band of star-forming regions believed to be oriented around the Sun in a ring.

“No astronomer expected that we live next to a giant, wave-like collection of gas, or that it forms the local arm of the Milky Way,” says Harvard’s Alyssa Goodman. 

“We were completely shocked when we first realised how long and straight the Radcliffe Wave is, looking down on it from above in 3D, but how sinusoidal it is when viewed from Earth. 

“The wave’s very existence is forcing us to rethink our understanding of the Milky Way’s 3D structure.”

Co-author João Alves, from University of Vienna, Austria, says it’s not clear what causes the structure’s shape, but suggests “it could be like a ripple in a pond, as if something extraordinarily massive landed in our galaxy”. 

“What we do know is that our Sun interacts with this structure. It passed by a festival of supernovae as it crossed Orion 13 million years ago, and in another 13 million years it will cross the structure again, sort of like we are ‘surfing the wave’.”

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