Astronomers are still discovering strange things in space, and the latest is something they’ve named the Radcliffe Wave. This wave-shaped chain of star-forming clouds is the largest coherent structure ever seen in our galaxy — 9,000 light-years from end to end, stretching across the night sky from Canis Major to Cygnus, with Orion in between.
Now it turns out the Radcliffe Wave is actually waving. So claims a paper published Tuesday in the journal Nature.
The star-forming clouds are rising far above the plane of the galaxy and then back down again. This kind of oscillation is known as a traveling wave, which is akin to sports fans “doing the wave” by popping up from their seats in a synchronized round-the-stadium pattern.
“This issue of the wave — you can find papers that hint at it in the past — but it’s nailed down now. This is a brick in the wall and it’s not coming out,” said Bob Benjamin, an astronomer at the University of Wisconsin at Whitewater who was not part of this new research. “This newest paper is a really neat step in understanding the origin of this structure.”
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This structure is within our galaxy and virtually right next door. It’s within spitting distance — if you could spit 500 light-years.
The story has another twist: It appears that our solar system passed through the Radcliffe Wave about 13 million years ago. And that might have been an interesting time for life on Earth. These star-forming regions have more than their fair share of exploding stars.
“Thirteen million years ago, we think we could have passed through a festival of supernovae going off,” said study co-author Catherine Zucker, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.
Motion in the Milky Way
Until just a few years ago, no one recognized that the many star-forming clouds relatively near the sun were part of a coherent structure. That’s because astronomers can see distant galaxies better than the one that surrounds us, the Milky Way. There is no telescope out there in intergalactic space, a couple million light-years away, obtaining beautiful images of the entirety of our galaxy. (If there is, it’s not one of ours.)
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“It’s really hard to see what the structure of your hand is if you put it very close to your face,” explains Alyssa Goodman, an astronomy professor at Harvard and co-author of the new report. “We don’t get to fly outside the galaxy.”
Astronomers have known for a century that the Milky Way is just one of many galaxies. They have also known that ours is a large spiral galaxy that’s much like the neighboring Andromeda Galaxy.
The cloudlike ribbon of milky light that you can see on a clear night — and which, as Galileo discovered four centuries ago with a telescope, is filled with individual stars — is an edge-on view of the plane of our home galaxy. The galaxy is a pancake-like disk, made from a relatively thick batter, if you will. We’re right there in the mix, and we can see stars in all directions that are part of the pancake.
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But only in recent years has it been possible to create a precise three-dimensional map of stars and gas in our sector of the galaxy. This is in part thanks to the European Space Agency’s Gaia spacecraft, which is designed to measure with unprecedented precision the distances to millions of stars in our galaxy and their motion relative to each other.
The “fixed stars,” as astronomers and sailors call them, are not actually just sitting there in deep space. Everything’s moving. Our solar system makes one orbit of the center of the galaxy over the course of about 226 million Earth-years.
Using Gaia data, Joao Alves, Zucker, Goodman and six colleagues described the Radcliffe Wave in a 2020 paper in Nature. They named it in honor of early 20th-century female astronomers associated with Radcliffe College, including Radcliffe graduate Henrietta Leavitt, who discovered that the periodic brightening of certain stars encoded information about their distance from Earth.
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That breakthrough was critical to the discovery that the intriguing “spiral nebulae” seen through telescopes are actually structures outside the Milky Way — distinct galaxies in a universe even more vast than previously imagined.
The Radcliffe Wave appears to be the backbone (or “gas reservoir,” as a 2022 paper put it) of the spiral arm of our galaxy closest to our sun, known as the Orion Arm, or Local Arm. Additional updates from Gaia allowed scientists to create theoretical models to track the motion of star clusters within the wave, revealing its undulations.
The big question now: Why is the Radcliffe Wave waving?
“Who ordered that?” Goodman asked.
Something clearly happened to disturb our galactic neighborhood and impose disorder on the heavens. One possibility is that something — perhaps a dwarf galaxy — came crashing into the Milky Way and caused a big splash, and the wave is a ripple effect.
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Another possibility is that a sequence of supernovae — explosions of stars emitting powerful bursts of radiation — shook things up. Or it could be a combination of factors.
“It might be that stars exploded as supernovae, and pushed the gas and the dust out of the galaxy plane,” said Ralf Konietzka, a PhD candidate at Harvard and lead author of the new paper. This wavelike pattern will disappear in a few tens of millions of years, he said.
Earth’s ride through the wave
There’s more digging to be done here, Zucker and her colleagues say — and more scientific papers are in the offing. There could be signs in the geological record of Earth being affected by supernova explosions in that long-ago transit through the Radcliffe Wave.
Earth has a magnetic field that helps protect it from potentially harmful radiation coming from the sun. And the sun’s solar wind creates a great protective bubble around the entire solar system that helps protect us from dangerous particles racing through space from other points in the galaxy.
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But here’s where “interstellar weather” complicates the picture. A nearby supernova could have compressed that bubble, called the heliosphere, to the point that our planet was fully exposed to the interstellar medium.
The next step is to look into the geological record for signs that Earth was pelted with an isotope of iron consistent with exposure to a supernova about 13 million years ago. And then cross-tab that with anything interesting in the biological record.
“Galaxies may be even more dynamic than we previously thought,” Konietzka said.
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