An Interstellar Ribbon of Clouds in the Sun’s Backyard
A 9,000-light-year-long ribbon of matter undulates through our sun’s interstellar neighborhood, made of hundreds of different clouds of dust and gas—the largest such structure of interacting nebulae yet described. Its discovery, announced today by a team of Harvard astronomers in the journal Nature, re-draws the map of our corner of the Milky Way and raises new questions about how stars and nebulae form and move through our galaxy, and those beyond.
The structure, dubbed the “Radcliffe Wave,” after Harvard’s Radcliffe Institute for Advanced Study, crests some 500 light years “above” the central disk of our spiral galaxy, before plunging just as far below. It holds about three million times the mass of the sun, mostly in clouds of dust and gas so diffuse they would register as a vacuum by any earthly standard. Never before have scientists seen interstellar clouds organized in a wave-like pattern like this one, but the team thinks that this wave is the backbone of the Orion Arm, the spiral arm of the Milky Way to which our sun belongs.
“We don’t know what causes this shape,” said João Alves, lead author of the Nature paper and a 2018-19 Radcliffe Fellow, in a statement. The professor of stellar astrophysics at the University of Vienna added, “[I]t could be like a ripple in a pond, as if something extraordinarily massive landed in our galaxy.”
“We have no precedent for this sort of structure in the galaxy,” said coauthor Catherine Zucker, a fifth-year doctoral student in astronomy at Harvard, in a separate interview, nor have wave-like structures such as this “been seen yet in simulations of galaxies like our Milky Way.”
The Harvard team found the wave after building the most accurate map to date of the interstellar clouds within about 7,000 light years of the sun. The map made it possible for the first time to visualize more than two billion cubic light years in three dimensions—and brought the massive wave into clear view. “I’m sure you’re wondering why, if this thing is right up in our face, we didn’t find it sooner,” coauthor Alyssa Goodman, Wilson professor of applied astronomy, said in a press conference. “It’s not apparent in 2-D” images of the sky. (Readers can explore an interactive view of the wave for themselves through 3-D visualizations that the authors have posted online.)
The authors chose the name ‘Radcliffe’ for the wave, they write, “in honour of both the early 20th century female astronomers from Radcliffe College and the interdisciplinary spirit of the current Radcliffe Institute that contributed to this discovery.” Alves is particularly indebted, he said, to Henrietta Leavitt, an 1892 Radcliffe graduate who worked as a “computer” at the Harvard College Observatory (see “Eye on the Cosmos,” November-December 2017, page 104). In 1908, Leavitt painstakingly compared the blinking of thousands of pulsing stars known as “Cepheid Variables,” and discovered a precise mathematical relationship between their brightness and the frequency of their pulses. Because knowing the true brightness of a star allows astronomers to calculate its distance based on its apparent brightness, astronomers would later use this discovery to produce the first precise measurements of intergalactic distances and the expansion of the universe.
Alves said he was inspired at the beginning of his Radcliffe fellowship by an exhibit showcasing the work of artist Anna von Mertens, including quilts sewn to show the stars at the moments of Leavitt’s birth and death (see “Looking at the Cosmos Through a Feminine Lens”). Of Leavitt herself, he declared: “Her incredibly detailed work opened the Universe for us.”
Not a Ring, but a Wave
For more than a century, astronomers have grouped the interstellar clouds near the sun into a ring-like structure called Gould’s Belt, about 3,000 light years across.
The structure is named for Benjamin Gould, A.B. 1844, who in 1879 described a band of stars and nebulae forming an incomplete belt around Earth’s night sky. The belt stretches from the South Pole through constellations including Orion, near the equator, and Perseus, in the northern sky, and back south through Scorpio. Astronomers have long struggled to explain the gaps in the belt, and why it seemed to tilt relative to the disk of the Milky Way.
“One of the big issues with the Gould’s Belt model was that there was no consensus for how the structure actually formed,” said Zucker. Explanations ranged from a clump of dark matter plowing through the region to an ancient supernova in the center triggering star formation around the edges.
Alves, Zucker, and the rest of the team hoped a better map of Gould’s Belt and its surroundings would help answer these questions. “I was expecting Gould’s Belt in high definition,” said Alves. “I was not expecting any wave.” But when the team plotted out the data, it was clear: the most cohesive structure in our neighborhood is a line of clouds, remarkably straight from above but darting in and out of the Milky Way’s disk when viewed from the side.
The Radcliffe Wave provides an explanation for both the gaps and the tilt in Gould’s Belt: rather than a coherent structure, the “belt” is actually a straight, sloping portion of the wave (stretching through the northern sky, roughly from Orion to Perseus) and a clump of matter, unassociated with the wave, in the southern sky, between the constellations Scorpio and Centaurus.
“The Sun lies only 500 light years from the Wave,” said Alves in the statement. “It’s been right in front of our eyes all the time, but we couldn’t see it until now.”
The team deduced that the sun passed through the wave about 13 million years ago, and will do so again in another 13 million years. “There is no obvious record to my knowledge, no obvious mass extinction,” resulting from that event, said Alves. “So the impact might have been subtle,” perhaps merely a mist of iron ions and other supernovae residue settling on Earth, “but I suspect the event left a mark.”
Whether the sun’s passage through the wave left physical evidence or not, the sky at the time would have looked “fabulous,” Goodman said at the press conference. “All these beautiful nebulae and all this star formation would be all around us.”
Interstellar Cloud Atlas
The map the team used to find the wave was based on data from the European Space Agency’s Gaia satellite, which has spent the last five years mapping the stars from its vantage point just beyond Earth’s orbit. Anyone can measure the direction to a star, but Gaia is measuring their distances as precisely as possible by triangulating them from different sites in its orbit.
Unlike stars, however, interstellar clouds like those in the Radcliffe Wave are too diffuse and dim for astronomers to measure their distance from Earth through triangulation. The research group of professor of astronomy and of physics Douglas Finkbeiner, of which Zucker is a part, got around this by using known star distances to measure cloud distances.
“There are stars that are in front of the cloud, and there are stars that are behind the cloud,” explained Zucker during the interview, “and we know the distances to both.” Because the clouds absorb more blue light than red light as starlight passes through them, stars behind the clouds appear dimmer and redder than they otherwise would. To determine the clouds’ distance, she said, “we can essentially bracket clouds between these background and foreground star formations.”
Using this method, Zucker led a pair of studies, published last summer and this month, that combined the Gaia distances with data about the stars’ colors to calculate the most precise distances to date of the clouds in the sun’s neighborhood. Then the team turned to finding structures in the map—and that was when the Radcliffe Wave surprised them.
The discovery also raises new questions. “The Radcliffe Wave shouldn’t be the only wave in the galaxy,” Zucker noted. So how common are waves, in our galaxy and others? How do they form? And do these waves, well, wave?
Ripples in a Pond
The astronomers have yet to confirm whether the wave is, in fact, undulating, instead of sitting static in space. “The major work to be done now,” said Alves, “is to find the young stars that were formed in the wave and use them as proxies for the motion of the wave.”
So far, the team has checked one clump of stars, formed from the nebulae in one part of the wave. “It is doing what would be expected,” said Alves: the stars furthest from the wave’s centerline are moving more slowly relative to the wave’s centerline than those closer in. But it’s not proof.
If the wave is indeed rippling through the galaxy’s disk, Zucker said, there are a few things that could have caused that disturbance, including the dwarf galaxies that circle the Milky Way and fast-moving clouds of gas that stream through it. “We think it’s possible that a high-velocity cloud or one of these dwarf galaxies hit the disk of the Milky Way, and that is what is causing this structure to undulate.”
To learn more, she continued, the team may try to run simulations “where we actually try to hit the Milky Way with objects of different mass from different angles and at different speeds and see if we can reproduce this structure.”
Understanding the wave, Zucker pointed out, will yield a better understanding of how stars form from gas clouds, like those within it. “People have been studying these clouds for hundreds of years, back before Gould. To find that these clouds are connected at a galactic scale transforms our understanding” of stars’ creation.