Harvard Astronomers on the ‘Detective Story’ of a 1,000-Light-Year-Wide Bubble Surrounding Earth 

Local Bubble

 

“This is really an origin story; for the first time we can explain how all nearby star formation began,” says astronomer and data visualization expert Catherine Zucker a NASA Hubble Fellow at the Harvard Center for Astrophysics and Space Science Institute about the discovery that Earth sits in a 1,000-light-year-wide void surrounded by thousands of young stars.

Creation of a Vast Superbubble

The discovery begs the question: how did those stars form? In a paper appearing in Nature, astronomers at the Harvard & Smithsonian Center for Astrophysics (CfA) and the Space Telescope Science Institute (STScI) reconstruct the evolutionary history of our galactic neighborhood, showing how a chain of events beginning 14 million years ago led to the creation of a vast bubble that’s responsible for the formation of all nearby, young stars.

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“Suppebubbles are the outcome of the explosion of several supernovae. Massive stars form in groups, most of the time, so when they die and explode you do not get one explosion but a group of explosions that will create a superbubble,” explains  co-author João Alves, a professor at the University of Vienna in an email to The Daily Galaxy. “Note that several massive stars near the Sun will still explode as supernovae ‘soon’, like Antares, a red supergiant about 12 times the mass of the sun and more than 75,000 more luminous than the sun at the heart of constellation Scorpius,” Alves added.

The superbubble cavity is much less dense than its surrounding environment and is filled with thin, hot gas that can reach up to 1 million degrees Kelvin, Zucker told The Daily Galaxy.

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The paper’s central figure, a 3D spacetime animation, reveals that all young stars and star-forming regions — within 500 light years of Earth — sit on the surface of a giant bubble known as the Local Bubble. While astronomers have known of its existence for decades, scientists can now see and understand the Local Bubble’s beginnings and its impact on the gas around it.

The Source of Our Stars: The Local Bubble

Using a trove of new data and data science techniques, the spacetime animation shows how a series of supernovae that first went off 14 million years ago, pushed interstellar gas outwards, creating a bubble-like structure with a surface that’s ripe for star formation. Today, seven well-known star-forming regions or molecular clouds — dense regions in space where stars can form — sit on the surface of the bubble.

“The seven star-forming regions are Taurus, Ophiuchus, Pipe, Corona Australis, Lupus, Musca, Chameleon,” Alves told The Daily Galaxy,  You can see them in this short YouTube video:

“We’ve calculated that about 15 supernovae have gone off over millions of years to form the Local Bubble that we see today,” says Zucker who is now a NASA Hubble Fellow at STScI.

The oddly-shaped bubble is not dormant and continues to slowly grow, the astronomers note. “It’s coasting along at about 4 miles per second,” Zucker says. “It has lost most of its oomph though and has pretty much plateaued in terms of speed.”

The expansion speed of the bubble, as well as the past and present trajectories of the young stars forming on its surface, were derived using data obtained by Gaia, a space-based observatory launched by the European Space Agency. 

“This is an incredible detective story, driven by both data and theory,” says Harvard professor and Center for Astrophysics astronomer Alyssa Goodman, a study co-author and founder of Glue, a data visualization software that enabled the discovery. “We can piece together the history of star formation around us using a wide variety of independent clues: supernova models, stellar motions and exquisite new 3D maps of the material surrounding the Local Bubble.”

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Supernovae Driven Bubbles Everywhere?

“When the first supernovae that created the Local Bubble went off, our Sun was far away from the action,” says co-author João Alves, a professor at the University of Vienna. “But about five million years ago, the Sun’s path through the galaxy took it right into the bubble, and now the Sun sits — just by luck — almost right in the bubble’s center.”

Today, as humans peer out into space from near the Sun, they have a front row seat to the process of star formation occurring all around on the bubble’s surface. 

“Think of the supernova explosion snowplowing surrounding gas, accumulating it as the edge of the bubble. At some point, it can accumulate enough gas that can cool and become unstable to collapse, forming new stars. Things are more complicated than this, but this is the general idea,” João Alves wrote in an email to The Daily Galaxy.

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Superbubble Physics

“A single superbubble forms as powerful supernova explosions set off a shock wave that sweeps up the ambient gas around it into an expanding dense shell with a surface that is ripe for star formation,” Zucker told The Daily Galaxy. “However, when multiple supernova-driven superbubbles touch, this snow plow effect is amplified, so we expect even more star formation where bubbles touch. We have an idea that the Local Bubble might be interacting with other bubbles in our Galactic neighborhood, and we hope to explore this line of research in future work.” 

Astronomers first theorized that superbubbles were pervasive in the Milky Way nearly 50 years ago. “Now, we have proof — and what are the chances that we are right smack in the middle of one of these things?” asks Goodman. Statistically, it is very unlikely that the Sun would be centered in a giant bubble if such bubbles were rare in our Milky Way Galaxy, she explains.

Goodman likens the discovery to a Milky Way that resembles very hole-y swiss cheese, where holes in the cheese are blasted out by supernovae, and new stars can form in the cheese around the holes created by dying stars.

The Last Word –“Triggered Star Formation”

“Molecular clouds are the birthplaces of stars,” wrote Michael Foley, co-author and an NSF Fellow at the Harvard-Smithsonian Center for Astrophysics in an email to The Daily Galaxy. “Since many of the local molecular clouds are found on the surface of the Local Bubble, it suggests that supernovae played a large role in compressing gas enough to form nearby stars,” Foley explains. “This mechanism.” he continues, “is known as “triggered” star formation – supernovae may push gas together to “trigger” the gravitational collapse of gas into stars, instead of gas collapsing without external influence. The presence of both the Local Bubble and the PerTau shell within only a few hundred parsecs of our Sun suggest that this “triggered” approach may be a major, if not the dominant, mechanism for forming new stars.

“By charting out new bubbles in our galaxy,” Foley notes, “we can study exactly how common it is for dying stars to “trigger” the birth of new ones. These bubbles will also help us understand the evolution of larger structures in our galaxy, such as its spiral arms and ‘galactic chimneys’ – cavities in the interstellar medium produced by multiple supernova explosions that can channel gas outside of the galactic plane and into the halo. We believe the Local Bubble may be one example of a galactic chimney.

Turbulence is the Catalyst

“Additionally,” Foley explains in his email, “the gas within molecular clouds is highly turbulent. Turbulence is very important in star formation for two reasons: 1) it is capable of generating individual dense subregions in gas that may begin to collapse and form stars and 2) it keeps gas moving fast enough to prevent gravitational collapse of the entire molecular cloud at once. The debate is ongoing regarding how clouds originally obtain this turbulence though. One possibility is that the turbulence can be produced by interacting shocks from the interstellar medium that compress and stir the gas within clouds. These shocks could be produced by events like supernovae, so studying the 3D structure of bubbles and their relationship to molecular clouds may give us clues about the ways in which supernovae contribute to the generation of turbulence.”

Next, the team plans to map out more interstellar bubbles to get a full 3D view of their locations, shapes and sizes. Charting out bubbles, and their relationship to each other, will ultimately allow astronomers to understand the role played by dying stars in giving birth to new ones, and in the structure and evolution of galaxies like the Milky Way..

Image credit: top of page, The Local Bubble,  Leah Hustak (STScI)

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via João Alves Michael FoleyCatherine Zucker and Harvard CfA

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