Giant galactic bubble is driving star formation, new study finds

 

The Local Bubble is seen here with star formation taking on on its surface. (Photo courtesy of STScI's Leah Hustak)

"We simply happened upon this discovery," researcher Catherine Zucker remarked.

According to a recent study, Earth is surrounded by a gigantic bubble roughly 1,000 light-years across, the borders of which control the development of all surrounding young stars.

Astronomers have known for decades that the solar system is located within the "Local Bubble," a massive gap surrounded by hundreds of newborn stars. However, there is still a lot of mystery surrounding this bubble, from its specific size and structure to its origins and evolution.

Researchers looked into this bubble in a new study and discovered some surprising new information about how it aids star formation.

Surprisingly, the researchers discovered "that all adjacent star-forming regions sat perfectly on the Local Bubble's surface," according to study lead author Catherine Zucker of the Space Telescope Science Institute in Baltimore. "We came across this discovery purely by accident."

They wanted to produce a map of the important features in the solar system's galactic neighbourhood with their research. Within 650 light-years of the sun, they studied the 3D positions, forms, and motions of thick gases and nascent stars.

"The sheer number of dimensions required to generate a meaningful 3-D physical picture of star formation on the bubble's surface was one of the most challenging elements of the research," Zucker said.

"Three dimensions of space, three dimensions of motion, and a time dimension were all mapped as part of the study. We can now literally'rewind the clock' and witness how these star-forming regions have grown over millennia." "Most of our traditional understanding of stellar birth, on the other hand, has been based on static 2-D views of star-forming areas," Zucker added.

The scientists were able to reconstruct the chain of events that led to the formation and expansion of the Local Bubble by analysing the movements of these young stars. They determined that these stars were mostly heading straight away from the bubble's surface, implying that they were moving due to the bubble's expansion.

The researchers discovered that some 14 million years ago, a series of about 15 catastrophic star explosions known as supernovae began to occur near the Local Bubble's centre.

"The supernova explosions generated a shock wave, and the expanding shockwave swept up a shell of dense, cool gas — the Local Bubble's surface — which has now collapsed to form thousands of new stars," Zucker explained. On the bubble's surface today, seven well-known molecular clouds — dense regions in space where pockets of gas may collapse to produce stars — may be found.

Supernovae had long been thought to sweep up gas into dense clouds that eventually form new stars, but the researchers in this study were astonished to see that nearly every new star near the sun is developing on the surface of the Local Bubble, according to an email from the researchers.

"Basically, we can explain how all local star creation began," Zucker said. "In doing so, we provide very strong observational evidence for this long-held notion of supernova-driven star formation, where stellar death can cause stellar birth."

Since the first one erupted around 14 million years ago, a supernova linked to the bubble has exploded about every million years, according to the latest findings.

"We think we know which clusters were responsible for the supernovae that powered the bubble's expansion," Zucker said. "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 roughly the same age."

The stars in these two clusters had a variety of masses when they were born. The stars that were huge enough to explode as supernovas also had the shortest lives. "The most massive stars will go supernova first, followed by the less massive stars," Zucker explained.

The Local Bubble isn't dormant; it's still growing at a rate of roughly 4 miles per second (6.4 kilometres per second). However, it has lost much of its oomph and has reached a speed plateau, according to Zucker.

According to study co-author Joo Alves, a researcher at the University of Vienna, when the initial supernovae that generated the Local Bubble exploded, the sun was far distant from the explosions. However, the sun's course across the galaxy took it into the bubble some five million years ago, and it now sits almost exactly in the bubble's core by chance, he said.

The fact that the sun is now located in the centre of the Local Bubble suggests that such "superbubbles" may exist throughout the Milky Way. "However, what are the odds that our sun will be exactly in the middle of one?" According to Zucker.

According to Goodman, the Milky Way may resemble Swiss cheese, with holes blasted out by supernovae and new stars developing in the cheese surrounding the holes left by dead stars. The scientists intend to map out the locations, sizes, and shapes of more bubbles in the Milky Way in the coming months.

"We aim to map out other bubbles and their relationships with each other in future study," Zucker said, adding that "the Local Bubble may be interacting with other bubbles in our galactic neighbourhood." "One of the most difficult aspects will be determining the ages of these bubbles as we travel further and further away from the sun, as well as the parent star clusters that will go supernova. However, fresh data from the Gaia mission, Gaia DR3, will be invaluable in piecing together this jigsaw, as it will offer 3-D space movements for 30 million stars, a crucial component."

References:

Zucker, C., Goodman, A.A., Alves, J. et al. Star formation near the Sun is driven by expansion of the Local Bubble. Nature (2022). https://doi.org/10.1038/s41586-021-04286-5


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