The Buddha once described our world as “a star at dawn, a bubble in a stream.” He was speaking more to the transitory nature of reality than cosmology, but he could well have been describing our own Sun and its temporary place in the galaxy — right in the middle of the cosmic bubble of star formation.
For decades now, astronomers have known our Sun sits in the middle of an amorphous space spanning 1,000 light years, a bubble filled with hot, low-density ionized gas, and surrounded by a shell of cold gas and dust. Astronomers call it the Local Bubble and believe it was inflated by a series of supernovae beginning 14 million years ago, with our Sun entering the bubble in its journey around the galactic center just 5 million years ago.
Until work published Wednesday in the journal Nature, scientists didn’t know how the Local Bubble connects with star formation. All young, nearby stars surrounding Earth have formed in clouds of gas along the shell of the Local Bubble.
And now that astronomers understand how those stars formed, “We can now reconstruct the evolutionary history of our galactic neighborhood,” astronomer Catherine Zucker, one of the paper’s authors, said at a press conference for the 239th American Astronomy Society meeting on Wednesday. The study may lead not only to a better understanding of star formation and our place in the galaxy, but insights into the structure and evolution of the galaxy itself.
What’s new?— The main finding of the paper is that all young stars within about 500 light years of Earth have formed on the surface of the Local Bubble in one of seven gas clouds sitting on its shell.
These young stars include some that are well-known, including parts of major constellations such as Taurus, Lupus, and Ophiuchus. “The local bubble is actually the source of some of the most iconic images and astronomy that we as astronomers and the public know and love,” Zucker said, including the Snake Nebulae and the young star HL Tauri with its protoplanetary disk.
How they did it— When visualizing the Local Bubble from an extra-galactic viewpoint, its role as expanding star incubator seems obvious, Zucker said, “but the reason that we didn’t discover it until now is because it’s not apparent from our perspective on Earth.”
When setting out on the research that led to the discovery, Zucker said, she and her colleagues were playing with new data from the European Space Agency’s Gaia Observatory, which is creating a three-dimensional map of millions of stars in our galaxy, and new data visualization software called Glue.
Zucker said her team wanted “to just essentially plot all of the landmarks in our galactic neighborhood together in the same framework.”
When they did, the evident relationship between the Local Bubble and star formation popped out.
“We would have no idea without essentially turning back the clock and using this very exquisite data from the Gaia space mission that the local bubble was the origin of all nearby stars,” Zucker said.
Of course, the most significant bit of luck in the discovery took place long before there were human astronomers, when the Sun stumbled into the midst of the Local Bubble some 5 million years ago.
“When the superbubble first started forming, the Sun was actually 1000 light years away,” Zucker said. “The Sun just sits by chance in the center of the bubble and we get this front row seats to star formation happening all around us.”
Why it matters— By using data imaging software to rewind the clock and trace the paths the stars must have traced as the Local Bubble expanded, the paper also offers strong support for a 50-year-old theory about star formation, “that the supernova can sweep up gas and dense clouds that ultimately form new stars,” Zucker said, “sort of like a snowplow can sweep up snow.”
And that implies that our Local Bubble is just that — the local bubble. “We think that there are many other bubbles that are potentially interacting with each other,” Zucker said. Bubble structures with stars forming at their periphery could dot the galaxy like the cavities in bread or Swiss cheese, possibly even forming part of the galaxy’s structure itself.
What’s next?— Astronomers will first want to go looking for bubbles in the larger landscape of the galaxy, Zucker said, and a lot of work will be devoted to learning how to chart the other bubbles in the neighborhood. Understanding how these bubbles function could help astronomers understand the galaxy’s spiral structure and even its long-term evolution.
“If there are bubbles — bubbles everywhere — these bubbles should be hitting each other,” she said. “So the question is, how can we leverage the new datasets and new techniques on the horizon to chart out the bubbles?”
ABSTRACT — For decades we have known that the Sun lies within the Local Bubble, a cavity of low-density, high-temperature plasma surrounded by a shell of cold, neutral gas and dust1,2,3. However, the precise shape and extent of this shell4,5, the impetus and timescale for its formation6,7, and its relationship to nearby star formation8 have remained uncertain, largely due to low-resolution models of the local interstellar medium. Here we report an analysis of the three-dimensional positions, shapes and motions of dense gas and young stars within 200 pc of the Sun, using new spatial9,10,11 and dynamical constraints12. We find that nearly all of the star-forming complexes in the solar vicinity lie on the surface of the Local Bubble and that their young stars show outward expansion mainly perpendicular to the bubble’s surface. Tracebacks of these young stars’ motions support a picture in which the origin of the Local Bubble was a burst of stellar birth and then death (supernovae) taking place near the bubble’s centre beginning approximately 14 Myr ago. The expansion of the Local Bubble created by the supernovae swept up the ambient interstellar medium into an extended shell that has now fragmented and collapsed into the most prominent nearby molecular clouds, in turn providing robust observational support for the theory of supernova-driven star formation.