The 1,000-light-year-wide cosmic bubble around Earth

We are in the middle of a bubble of stars being made by the Milky Way.

As this artist’s concept shows, the Local bubble is driving the formation of stars in the 500 light-years around Earth.

When you hear the word “bubbles,” you might think of soap or gum.

But not Catherine Zucker, who is a Hubble Fellow at the Space Telescope Science Institute and used to work as a researcher at the Harvard-Smithsonian Center for Astrophysics. Zucker is very interested in bubbles. She and her team have also learned new things about a bubble in which our solar system is located.

Astronomers have long known about the 1,000-light-year-wide Local Bubble. In a new paper published Jan. 12 in Nature, Zucker and her co-authors describe it as “a cavity of low-density, high-temperature plasma surrounded by a shell of cold, neutral gas and dust.” But for years, astronomers were in the dark beyond that. The History of the Local Bubble, even its size, remained unknown.

Now, no longer. Zucker and her team became historians by accident when, while working on a different project, they found a kind of creation story for our local stellar neighborhood and strong evidence that supernovae, which are the explosions of dying stars, lead to the birth of new stars. This happens when the things that were blown away come back together somewhere else because of gravity.

What Zucker’s team found, according to their paper, was “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.” She calls it a “eureka moment.” 

In other words, almost all of the young stars in our galaxy’s neighborhood are caused by the shock waves of a series of supernovae. This process of blown-out matter coming back together to make new suns and solar systems is called nucleation. The bubble, which looks more like a pipe cutting through the plane of the Milky Way, seems to have formed 14 million years ago from about 15 supernovae, which set off star formation that is still going on today.

Zucker’s research shows that this kind of supernova happened about 2 million years ago. This fits well with what was already known about the deposition of cosmic iron in the Earth’s crust.

Zucker gave a virtual presentation of her team’s work this week at a much smaller meeting of the American Astronomical Society, which was supposed to take place in Salt Lake City. The COVID-19 pandemic, which is still going on, ruined those plans.

Zucker told Astronomy that while there are “tens of millions of ‘old’ stars [those that are more than 14 million years old] inside the Local Bubble,” there are “on the order of thousands of ‘young’ stars … on its surface that have been birthed by the supernovae.” 

The Sun and our solar system just happen to be inside this bubble right now. The team thinks that the Sun rolled into the Local Bubble about 5 million years ago, but it is likely that it has been in other bubbles before.

“This work is most useful for showing how stars and planets fit into the ‘big picture,’” says Zucker. “One thing that might have been missed is that this study is just the tip of the iceberg. The Local Bubble is the first bubble whose history we’ve figured out. It’s the easiest one to figure out first, since it’s where our Sun is right now. But there are signs that the formation of young stars near our Sun is caused not just by single superbubbles but also by the interactions of many superbubbles.

Zucker compares the process to plowing snow. If one or more superbubbles is “piling up gas in the same region of space … we should get even more enhanced star formation at those intersecting surfaces.” And, in fact, one such bubble, called Perseus-Taurus, is interacting with our Local Bubble “at the site of the Taurus molecular cloud” — home to known protoplanetary disks.

The European Space Agency’s Gaia star-mapping mission was very important for getting the exact data needed to figure out how stars formed in the Local Bubble, which Zucker calls a “origin story.”

Luckily, you don’t need access to high-end data to connect to this work. “The two clusters of stars that hosted the supernovae are still around and they are about 15 to 16 million years old,” says Zucker. “They currently lie near the edge of the Local Bubble’s shell.” (At the time, the supernovae were getting underway, these clusters were in the thick of the action.) 

You can point your telescope at these nearby places where stars are being made. One is in the constellation Taurus, and the other is in Ophiuchus, where the Ophiuchus Nebula is. When you look into these places, you can see how stars were born in the past and how they are still being born in our Local Bubble.