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Giant galactic bubble is driving star formation, new study finds

This visualization shows the Local Bubble with star formation happening on its surface. (Image credit: Leah Hustak (STScI))

Earth is surrounded by a vast bubble about 1,000 light-years wide whose borders drive the formation of all nearby young stars, a new study finds.

For decades, astronomers have known the solar system lies within the so-called "Local Bubble," a giant void surrounded by thousands of young stars. However, much has remained unclear about this bubble — everything from its precise size and shape to its origins and evolution has remained unknown.

In a new study, researchers investigated this bubble and found some revealing new insights into how this bubble supports star formation. 

Unexpectedly, the astronomers found "that all nearby star-forming regions lay exactly on the Local Bubble's surface," study lead author Catherine Zucker, an astronomer at the Space Telescope Science Institute in Baltimore, told Space.com. "We stumbled upon this discovery completely by chance."

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With the study, they sought to create a map of the major landmarks in the solar system's galactic neighborhood. They analyzed the 3D positions, shapes and motions of dense gases and young stars within about 650 light-years of the sun.

"One of the most challenging aspects of the research was the sheer number of dimensions needed to build a real 3-D physical picture of star formation on the bubble's surface," Zucker said.

"The research involved mapping three dimensions of space, three dimensions of motion and a time dimension. Now we can literally 'turn back the clock' and see how these star-forming regions evolved over the past millennia." In contrast, "most of our traditional understanding of stellar birth has been based on static 2-D images of star-forming regions," Zucker added.

Analyzing the motions of these young stars helped the scientists reconstruct the chain of events behind the creation and growth of the Local Bubble. They discovered that these stars were traveling mainly straight away from the bubble's surface, which suggested they were moving because the bubble was expanding over time.

The researchers found that a series of about 15 catastrophic star explosions known as supernovae likely began taking place near the Local Bubble's center about 14 million years ago. 

"The supernova explosions triggered a shock wave, and this expanding shockwave subsequently swept up a shell of dense, cool gas — that is, the surface of the Local Bubble — which has now collapsed to form thousands of new stars," Zucker said. Today, seven well-known molecular clouds — dense regions in space where pockets of gas may collapse to form stars — sit on the bubble's surface.

Astronomers have long-theorized that supernovae could sweep up gas into dense clouds that ultimately form new stars, but the researchers in this study were quite surprised to learn that pretty much every single new star near the sun is forming on the surface of the Local Bubble, the researchers said in an email.

"Basically, we can explain how all nearby star formation began, and in doing so, provide very strong observational support for this long-held theory of supernova-driven star formation, where stellar death can trigger stellar birth," Zucker said.

The new findings suggest that a supernova linked with the bubble has detonated about every million years since the first one exploded about 14 million years ago.

"We think we know which clusters were responsible for the supernovae that powered the expansion of the bubble — these two clusters, called Upper Centaurus Lupus and Lower Centaurus Crux in the famous Sco-Cen stellar association, formed very close to each other 15 million to 16 million years ago, so all the stars in these two clusters have approximately the same age," Zucker said.

The stars in these two clusters were born with a range of masses. The biggest of the stars large enough to detonate as supernovas also had the shortest lifetimes. "Those most massive stars will go supernova first, with the less massive ones exploding later," Zucker said.

The Local Bubble is not dormant — it continues to slowly grow at about 4 miles (6.4 kilometers) per second. However, it has lost most of its oomph and has pretty much plateaued in terms of speed, Zucker said.

When the first supernovae that created the Local Bubble went off, the sun was far away from the explosions, study co-author João Alves, a researcher at the University of Vienna, said in a statement. However, about five million years ago, the sun's path through the galaxy took it into the bubble, and now it sits by luck almost right in the bubble's center, he said.

The fact the sun is currently in the middle of the Local Bubble suggests that such "superbubbles" may be pervasive across the Milky Way. "Otherwise, what are the chances that our sun is right in the middle of one?" Zucker said.

The Milky Way may resemble Swiss cheese, with holes in the cheese blasted out by supernovae and new stars forming in the cheese around the holes created by dying stars, Goodman said. The scientists next plan to map out the locations, sizes and shapes of more bubbles in the Milky Way.

"The Local Bubble may be interacting with other bubbles in our galactic neighborhood, and we hope to map out other bubbles and their interactions with each other in future work," Zucker said. "One of the most challenging aspects will be trying to determine the ages, and progenitor stellar clusters going supernova, of these bubbles as we get farther and farther away from the sun. However, new data from the Gaia mission, Gaia DR3, will definitely help, as it will provide 3-D space motions for 30 million stars, a key ingredient in piecing together this puzzle."

The scientists detailed their findings online Jan. 12 in the journal Nature. They also presented their results at an American Astronomical Society press conference that day.

Originally published on Space.com.

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Charles Q. Choi

Charles Q. Choi is a contributing writer for Space.com and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica. Visit him at http://www.sciwriter.us