Astro Bob: Phantom supernovae blew a 1,000-light-year wide bubble that sparkles with baby stars
Astronomers discover that a supernovae explosion 14 million years ago initiated a chain of events responsible for the formation of all nearby, young stars.
"Living in a bubble" has taken on new meaning in the time of COVID-19. But you may not have known that we've been living in an even bigger bubble for a long, long time. Astronomers have known for decades that the solar system sits inside a 1,000-light-year-wide void called the Local Bubble . Thousands of young stars and star clouds coruscate its periphery like twinkling lights on a Christmas tree.
How all these stellar youths got there has been something of a mystery. In a paper appearing this week in Nature , astronomer Catherine Zucker and a team of astronomers at the Center for Astrophysics (Harvard and Smithsonian) and the Space Telescope Science Institute (STScI) think they may finally have the answer.
The group found that starting 14 million years ago, a series of supernovae (massive stars that deplete their fuel reserves, collapse and then explode) pushed interstellar gas outwards into an expanding bubble. Interstellar gas permeates much of the galaxy. It's composed primarily of hydrogen gas mixed with minute dust particles sown into space by … supernovae! Their observations reveal that all young stars and star-forming regions within 500 light years of Earth sit on the surface of this giant balloon of compressed gas. Picture blowing a bubble. As the bubble expands, the air around its surface momentarily piles up.
When a massive star explodes, the expanding shock wave and material compress the local interstellar gas, which collapses into denser regions ripe for star formation. The spectacular Orion Nebula , located below Orion's Belt, is a perfect example of a cloud of interstellar gas and dust in the process of forming new stars. It's estimated that around 700 suns are in various stages of formation as denser pockets of material within the nebula undergo gravitational collapse, heat up and blaze their way to life as stellar newborns.
Today, seven well-known star-forming regions nest on the surface of the bubble, carved out by some 15 supernovae over the past 14 million years. Unlike the bubbles you blow with a bubble wand, which eventually pop, this one keeps on expanding.
"It's coasting along at about four miles per second," Zucker says. "It has lost most of its oomph though and has pretty much plateaued in terms of speed."
Zucker and her team used data from the Gaia mission , a space-based observatory, to measure the expansion speed of the bubble as well as the past and present bearings of the young stars forming on its surface. Combining this data with supernova models they were able to create detailed new 3D maps of the Local Bubble and its surroundings.
The sun doesn't sit still but orbits around the center of the Milky Way galaxy. When the first supernovae that created the bubble exploded, the sun was nowhere near the action.
"But about five million years ago, the Sun's path through the galaxy took it right into the bubble, and now the Sun sits — just by luck — almost right in the bubble's center," says co-author João Alves , a professor at the University of Vienna.
It's a perfect perch from which to observe star formation going on all around us. Statistically, it's pretty unlikely the sun would just happen to be sitting in the middle of this bit of froth, which is why astronomers think such bubbles are common. If so, we might imagine the Milky Way as a wheel of Swiss cheese, with holes blasted out by supernovae and the cheesy part thronged with stars and nebulae.
All of it is a testament to the beauty of recycling. Exploding stars create the dusty-gassy environment in the first place, then compact it into new generations of stars. A small proportion of those newborns are massive enough to become supernovae and seed the next cycle. And on and on. Left to its own, the universe has knack for getting things done in ways we are only beginning to fathom.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at