Astronomers have known since the 1970s that our sun lies at the center of a vast cavity within the hot gas that fills the gaps between stars in the Milky Way. But the origins of that ever-growing void, known as the Local Bubble, and its relationship to our starry neighbors remained elusive.
Now, using new data from the European Space Agency’s Gaia space telescope, scientists have revealed that the genesis and growth of that bubble were due to a series of 15 supernovas—powerful explosions of collapsing stars—over the past 14 million years, and that every young star and star-forming region within 500 light years of Earth sits on the Local Bubble’s surface.
When stars die, the resulting explosions set off shock waves that travel outward, sweeping up and aggregating interstellar material like gas and cosmic dust. Eventually enough gas accumulates, condenses and cools at the edge of this shock wave to start birthing stars.
The Local Bubble is the result of such shock waves, as are the stellar nurseries that pepper its outer shell.
“For the first time, we can explain how nearby star formation began, and it’s because of this 1,000-light-year-wide bubble,” said
Catherine Zucker,
an astronomer with the Harvard-Smithsonian Center for Astrophysics and lead author of a study about the new findings published Wednesday in the journal Nature.
“Imagine it’s like a snowplow,” she said, “At the edge of the shell there’s a sharp edge of piled-up interstellar material that essentially hosts all of these star-forming regions.”
How the Local Bubble Formed
Powerful supernovas begin to explode, creating a cavity of very thin, hot gas in space called the Local Bubble.
The Sun’s path takes it into the ever-expanding bubble. As the bubble expands, regions of dense, cold gas that can birth stars collect on its surface.
Today, many young stars sit on the bubble’s surface, but have not formed inside of it.
approx. 1,000 light years
Source: Space Telescope Science Institute with the
Center for Astrophysics, Harvard and Smithsonian
How the Local Bubble Formed
Powerful supernovas begin to explode, creating a cavity of very thin, hot gas in space called the Local Bubble.
The Sun’s path takes it into the ever-expanding bubble. As the bubble expands, regions of dense, cold gas that can birth stars collect on its surface.
Today, many young stars sit on the bubble’s surface, but have not formed inside of it.
approx. 1,000 light years
Source: Space Telescope Science Institute with the
Center for Astrophysics, Harvard and Smithsonian
How the Local Bubble Formed
Powerful supernovas begin to explode, creating a cavity of very thin, hot gas in space called the Local Bubble.
The Sun’s path takes it into the ever-
expanding bubble. As the bubble expands, regions of dense, cold gas that can birth stars collect on its surface.
How the Local Bubble Formed
Powerful supernovas begin to explode, creating a cavity of very thin, hot gas in space called the Local Bubble.
The Sun’s path takes it into the ever-
expanding bubble. As the bubble expands, regions of dense, cold gas that can birth stars collect on its surface.
Today, many young stars sit on the bubble’s surface, but have not formed inside of it.
approx. 1,000 light years
Source: Space Telescope Science Institute with
the Center for Astrophysics, Harvard
and Smithsonian
Gaia, which launched in 2013, has helped astronomers map the location and movement of stars in these regions with unprecedented accuracy over the past two years. By combining that data with models of supernova behavior, Dr. Zucker’s team could trace back where these stars were millions of years in the past, and calculate how many supernovas would have been required for the Local Bubble to grow to its current size—with those star-forming regions embedded on its surface.
They created an animation that reconstructs the bubble’s rate of expansion, and when those explosions began.
“We found that there was this massive cluster of stars lying right in the center of where we think the bubble started forming 14 million years ago,” Dr. Zucker said.
“This study is a perfect illustration of what the ESA Gaia space observatory allows,” said
Rosine Lallement,
a researcher who studies the solar environment at the Paris Observatory who wasn’t involved in the study, adding, “it is like going from simple photography to 3-D movies” of the Milky Way.
The study found seven well-known star-forming regions, or molecular clouds, that sit on the Local Bubble’s shell—including Corona Australis, which sits within the constellation of the same name. The young stars therein “go along for the ride” and get pushed farther away from the bubble’s center as the cavity expands, Dr. Zucker said.
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Most stars, including the sun, are at least 1 billion years old, but the stars in these clouds are less than about 10 million years old, she added. The study suggests that these clouds, and the supernovas that helped form them, happened in at least four bursts, each roughly 4 million years apart beginning 14 million years ago.
“It would’ve been a series of correlated supernova explosions, it wouldn’t just be like, ‘Boom, boom, boom’ steadily,” said study co-author
Alyssa Goodman,
an astronomy professor at Harvard University.
That is because stars form in clusters, she said, and those of similar mass in these clusters all live about the same amount of time before exploding.
The Local Bubble is still growing, though the speed of its expansion has decreased to 14,400 miles an hour—at its peak, the void expanded about 15 times faster than that, Dr. Zucker said.
“Most of the oomph that’s been injected into this expansion happened many millennia ago” when the most massive stars in our galactic neighborhood went supernova, she said, adding the bubble will eventually run out of steam.
It is only by chance that the sun and our solar system currently sit at the heart of this void—over the past 5 million years, our star’s path through the galaxy took it to what the study authors think is the bubble’s center.
But the sun won’t stay there forever, Dr. Zucker said, adding, “In like 8 million years we should be out of the bubble.”
“But in the future, we might be in another bubble,” she said.
“‘The whole structure of the interstellar medium may be bubbly, and stars may form at the intersection of these bubbles where stuff is smushed together.’”
In a September 2021 study, a team led by Dr. Zucker and Dr. Goodman identified a similar cavity in the Milky Way. They hypothesize that such bubbles are pervasive throughout our galaxy, as well as other galaxies in the universe.
“We actually do think that when supernovas go off in the interstellar medium, they all create bubbles,” Dr. Goodman said. “The whole structure of the interstellar medium may be bubbly, and stars may form at the intersection of these bubbles where stuff is smushed together.”
Write to Aylin Woodward at Aylin.Woodward@wsj.com
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