The sun passes unnoticed through a gigantic empty bubble, a kind of cavity in space. All the thousands of nearby young stars have formed on the edge of this bubble. This is a conclusion of a 3D simulation in which the cosmic history of the Sun’s environment has been traced back in time by 17 million years. American astronomers from Harvard University, among others published the results this week in the magazine Nature. They confirm a fifty-year-old theory by American astronomers Christopher McKee and Jeremiah Ostriker.
The Earth revolves around the sun, and along with hundreds of billions of other stars, the sun orbits the center of the Milky Way, a spiral galaxy about 100,000 light years in diameter. The sun is in Orion’s arm, a small branch of one of the larger spiral arms. Gas and dust float between the stars, from which new stars are formed. Some places contain more gas and dust than others. Large regions several light years wide with little dust and gas are called super bubbles.
These super bubbles were carved out by several exploding stars. When a massive star, at least eight times the size of the sun, dies, the star’s outer layers are quickly thrown into space. It’s a supernova. When multiple supernovae occur relatively close to each other, explosions sweep away almost all nearby gas and dust like brooms. This creates an empty cavity with only a few thin, hot gases left over from the explosion. The sweep continues, at a decreasing rate, millions of years later. In other words, the super bubbles will continue to grow for a while.
cheese with holes
The first indication that the solar system is traveling through such a super bubble came at the end of the last century when astronomers with telescopes saw a lot of vacuum around them. Astronomers called it the local bell. It is estimated to be about a thousand light years wide, and the sun has been there for about five million years, now in the center. The bell does not affect the sun. It is pure coincidence that the sun’s orbit in the Milky Way passed through the bubble. However, it was striking that almost all nearby stars that are much younger than the Sun (maximum fourteen million years old) are located on the edge of the local bubble.
To explain why there were a lot of young stars out there, McKee and Ostriker proposed a theory fifty years ago: that the rims have the ideal conditions for star formation. One of the researchers of the new study compared this theory in a Press release with cheese with holes. When the supernova explosions push the cheese out and create a cavity, there is extra cheese on the edge of the cavity. New stars can be formed from this extra cheese, if it is brought together locally. This happens, according to the theory, when another supernova explosion goes off. This creates a shock wave that compresses the extra cheese in places on the edge.
To confirm this theory, simulated American astronomers the evolution of young stars in the vicinity of the sun over the past seventeen million years in 3D. “We looked at the current positions and speeds of around 1,000 young stars and regions of star formation within a radius of 500 light years,” said Catherine Zucker, one of the astronomers involved. They got the data from the database of Gaia: the European space satellite that has been mapping billions of stars since 2013. “We then calculated how the stars have moved far in the past.”
The sun will leave the local bell again in the distant future
The simulation shows that all nearby young stars have been on the edge of the bubble since birth. Fourteen million years ago, 15 supernovae swept away dust and gas, creating the local bubble, the researchers calculated. This gas and dust moved at a certain speed. The stars from this gas and dust received the same speed at birth and move outward with the edge of the bubble.
Now the bubble continues to grow at around 6.5 kilometers per second. It’s slow in cosmic terms. The sun is orbiting the Milky Way faster than the bubble is growing and will emerge in the distant future.
“Interestingly, astronomers with the simulation may well show a typical star cycle for the rest of the cosmos,” says Ignas Snellen, an astronomer at the University of Leiden and not involved in the study. “Some stars that have formed on the edge of the local bubble over the past million years will also end up in a supernova. This is how they make new super bubbles. Other great bells have been found in addition to the local bell. It would be pure coincidence if the cycle only applied to the local bubble that contains the sun.
A version of this article also appeared in NRC on the morning of January 13, 2022
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