The 1,000-light-year-wide cosmic bubble around Earth
When you think of “bubbles,” you might think of “soap” or “gum.”
Catherine Zucker, a Hubble Fellow at the Space Telescope Science Institute and a former researcher at the Harvard-Smithsonian Center for Astrophysics, is one of the exceptions. Zucker has a cosmic fascination with bubbles. She and her colleagues have also discovered new information about a bubble in which our solar system is contained.
The 1,000-light-year-wide Local Bubble has long been known to astronomers. 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” in a new paper published in Nature on Jan. 12. But beyond that, astronomers had been in the dark for years. The origins of the Local Bubble, as well as its size, were unknown.
Not any longer. When Zucker and her team were working on a different project, they discovered a kind of creation story for our local stellar neighborhood, which provided strong support for the theory that supernovae — the explosions of dying stars — lead to the birth of new stars. When the blown-out materials recombine elsewhere due to gravity, this occurs.
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, the young stars in our galaxy are almost entirely due to the expansive shock waves of a series of supernovae, and the blown-out process is still recombining to create new suns and solar systems. The bubble, which looks more like a pipe cutting through the Milky Way’s plane, appears to have formed 14 million years ago from 15 supernovae and triggered star formation, which is still happening today.
According to Zucker’s research, the last such supernova occurred about 2 million years ago, which corresponds to the previously reported deposition of cosmic iron in the Earth’s crust.
Zucker gave a virtual presentation of her team’s work this week at a drastically reduced meeting of the American Astronomical Society, which was supposed to meet in person in Salt Lake City. The ongoing COVID-19 pandemic has thrown those plans into disarray.
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 are currently contained within this bubble. The Sun rolled into the Local Bubble about 5 million years ago, according to the team, but it likely sat in other bubbles at other times.
“This work is most useful for providing the ‘big picture’ context for star and planet formation,” Zucker says. “One takeaway that might have been missed is that this study is really the tip of the iceberg. The Local Bubble is just the first bubble whose history we have mapped out — it’s the easiest one to understand first, since it’s the bubble in which our Sun currently resides. However, we have clues that not just single superbubbles, but the interactions of many superbubbles, are driving the formation of young stars near our Sun.”
Zucker compares the procedure to snow plowing. We should see even more enhanced star formation at those intersecting surfaces if one or more superbubbles are “piling up gas in the same region of space.” Indeed, one of these bubbles, Perseus-Taurus, is interacting with our Local Bubble “at the site of the Taurus molecular cloud,” which is home to known protoplanetary disks.
The Gaia star-mapping mission of the European Space Agency was critical in providing the precise data needed to discover the star-formation nuances of the Local Bubble, which Zucker has nicknamed a “origin story.”
Fortunately, you don’t need high-end data to participate in this project. “The two star clusters that hosted the supernovae are still around and are around 15 to 16 million years old,” Zucker says. “They’re currently near the Local Bubble’s shell’s edge.” (At the time, the supernovae were just getting started, and these clusters were right in the middle of it.)
You can point your telescope in the direction of those nearby star-forming areas. One is in Taurus, while the other is in Ophiuchus, the constellation that contains the Ophiuchus Nebula. Looking into those areas allows you to witness the history of star birth in our Local Bubble as well as the continuation of it.