Just a bit too late for New Year’s celebrations, astronomers have discovered that the Milky Way galaxy, our home, is, like champagne, full of bubbles.
As it happens, our solar system is passing through the centre of one of these bubbles. Fourteen million years ago, according to the astronomers, a firecracker chain of supernova explosions drove off all the gas and dust from a region roughly 1,000-light-years wide, leaving it bereft of the material needed to produce new generations of stars.
As a result, all the baby stars in our neighbourhood can be found on the edges of this bubble. There, the staccato force of a previous generation of exploding stars has pushed gas clouds together into forms dense enough to collapse under their own ponderous if diffuse gravity and condense enough to ignite, as baby stars. Our sun, 4.5 billion years old, drifts through the middle of this in a coterie of aged stars.
“This is really an origin story,” Catherine Zucker said in a news release from the Harvard-Smithsonian Center for Astrophysics, US. “...We can explain how all nearby star formation began.”
Zucker, now at the Space Telescope Science Institute in Baltimore, US, led a team that mapped what they call the Local Bubble in remarkable detail. They used data from a number of sources, particularly Gaia, a European spacecraft that has mapped and measured more than a billion stars, to pinpoint the locations of gas and dust clouds.
Last year, a group of scientists led by João Alves, an astrophysicist at the University of Vienna, Austria, announced the discovery of the Radcliffe Wave, an undulating string of dust and gas clouds 9,000-light-years long that might be the spine of our local arm of the galaxy. One section of the wave now appears to be part of our Local Bubble.
The same group of scientists published their latest findings in Nature, along with an elaborate animated map of the Local Bubble and its highlights. The results, the astronomers write, provide “robust observational support” for a long-held theory that supernova explosions are important in triggering star formation, perhaps by jostling gas and dust clouds into collapsing and starting on the long road to thermonuclear luminosity.
Astronomers have long recognised the Local Bubble. What is new, said Alyssa Goodman, a member of the team also from the Harvard-Smithsonian Center for Astrophysics, is the observation that all local star-forming regions lie on the Local Bubble’s surface. Earlier, researchers lacked the tools to map gas and dust clouds in three dimensions. “Thanks to 3D dust-mapping, now we do,” Goodman said.
According to the team’s calculations the Local Bubble began 14 million years ago with a massive supernova, the first of about 15; massive stars died and blew up. Their blast waves cleared out the region. As a result there are now no stars younger than 14 million years in the bubble, Goodman said.
The bubble continues to grow at about 4 miles per second. “Still, more supernovae are expected to take place in the near future, like Antares, a red supergiant star near the edge of the bubble that could go any century now,” Alves said. “So the Local Bubble is not ‘done’.”
The team plans to map more bubbles in our Milky Way flute of champagne. There must be more, Goodman said, because it would be too much of a coincidence for the sun to be smack in the middle of the only one.
The sun’s presence in this one is nonetheless coincidental, Alves said. Our star wandered into the region only 5 million years ago, long after most of the action, and will exit about 5 million years from now.
The motions of the stars are more irregular than commonly portrayed, as they are bumped gravitationally by other stars, clouds and the like, Alves said.
“The sun is moving at a significantly different velocity than the average of the stars and gas in the solar neighbourhood,” he noted. This would enable it to catch up and pass — or be passed by — the bubble.
“It was a revelation,” Goodman said, “how kooky the sun’s path is compared with a simple circle.”
NYTNS
