Of local bubbles, and star birth and death

Almost all of the young stars within hundreds of light-years of Earth are on the fringe of a giant “local bubble” in the interstellar medium shaped a bit like the crown of a tooth, scientists say.

The bubble, says Catherine Zucker, an astronomer and data visualisation expert who did her work with the Harvard/Smithsonian Center for Astrophysics in Massachusetts, US, was formed by a chain of events that began with a powerful supernova explosion 14 million years ago.

That explosion created a shock wave that swept gas and dust outward, “sort of like how a snowplough can sweep up snow,” Zucker said on 12 January, in an online press conference sponsored by the American Astronomical Society (AAS).

Subsequent supernova explosions—perhaps as many as 14 more of them – reinforced the process, contributing to the bubble’s expansion, until now it is approximately 1,000 light-years wide.

At the time, the Sun was well outside of the bubble. But about 5 million years ago the combination of the expansion of the bubble and the Sun’s motion carried it bubble-wards, giving us what is now a front-row seat.

“When the first supernovae that created the Local Bubble went off, our Sun was far away from the action,” says Zucker’s colleague, João Alves of the University of Vienna. “Now the Sun sits – just by luck – almost right in the bubble’s centre.”

The existence of the bubble, Zucker says, has long been known to astronomers studying the interstellar medium, but it is only now that 3D mapping of star locations and movements provided by ESA’s Gaia Space Telescope revealed that virtually all nearby young stars and star-forming regions are on its periphery.

It was a startling discovery, she adds. “We didn’t have a hypothesis that said we wanted to find out the relationship between the local bubble and star formation.”

Rather, she says, all that her team was doing was using a computer program called Glue to organise the GAIA data into a nice 3D map. “We were just dragging and dropping data, when this popped out,” she says.

It was a discovery, though, that instantly made sense and contributes to our understanding of star formation not only in our local vicinity, but throughout the Milky Way, and probably the universe—including the role dying stars play in giving birth to new ones in a vast, interstellar, cycle of death and birth.

What’s happening, Zucker says, is that the edges of the bubble, with their accumulations of gas and dust, are perfect places for star formation. The gas is difficult to see, but the stars aren’t, and once formed, they retain the momentum of the gas and dust clouds from which they condensed. That means they continue to move with the bubble’s edge as it continues to expand, making them handy tracers of the process that gave them birth.

Meanwhile, the search is on for new bubbles, because if one exists in our neighborhood, it is likely, Zucker says, that there are many others, colliding, interacting, and in general helping to produce the Milky Way galaxy as we know it today.

Zucker’s research was published on 12 January in Nature and is summarised on her team’s website.