The sun shoots unnoticed through a gigantic empty bubble, a kind of cavity in space. All the thousands of nearby young stars formed on the edge of that bubble. This is a conclusion from a 3D simulation in which the cosmic history of the Sun’s environment was washed back in time 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 of 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 around the center of the Milky Way, a spiral galaxy about 100,000 light-years across. 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 form. Some places contain more gas and dust than others. Large regions several light-years wide with little dust and gas are called super bubbles.
Those super bubbles have been hollowed out by multiple exploding stars. When a massive star, at least eight times as massive as the sun, dies, the outer layers of the star are rapidly flung out into space. That’s a supernova. When multiple supernovae occur relatively close together, the explosions one by one sweep out almost all the nearby gas and dust like brooms. This creates an empty cavity with only some thin, hot gas left from the explosion. The sweeping 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 travels through such a super bubble came at the end of the last century when astronomers with telescopes saw a lot of emptiness around them. Astronomers called it the local bell. It is estimated to be about a thousand light-years wide and the sun has been in it for about five million years, now at the center. The bell does not affect the sun. It is pure coincidence that the orbit of the sun in the Milky Way crossed 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 many young stars there, McKee and Ostriker came up with a theory fifty years ago: that the rims have the ideal conditions for star formation. One of the researchers of the new study compared that theory in a press release with holes cheese. When the supernova explosions push cheese out and create a cavity, there is extra cheese on the edge of the cavity. New stars can be formed from that extra cheese, if it is locally pushed closer together. That happens, according to the theory, when another supernova explosion goes off. This creates a shock wave that compresses the extra cheese in some places on the edge.
To confirm that theory, simulated the American astronomers the evolution of the young stars in the vicinity of the sun over the past seventeen million years in 3D. “We looked at the current positions and velocities of about 1,000 young stars and star-forming regions within 500 light-years,” said Catherine Zucker, one of the astronomers involved. They obtained 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 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 their birth. Fourteen million years ago, 15 supernovae swept away dust and gas, creating the local bubble, the researchers calculated. That gas and dust moved out at a certain speed. Stars that arose from that gas and dust were given the same speed at birth and move outwards together with the edge of the bubble.
Now the bubble is still growing at about 6.5 kilometers per second. That’s slow by cosmic terms. The sun orbits the Milky Way faster than the bubble grows and will come out of the bubble again in the distant future.
“Interestingly, with the simulation, the astronomers may well show a typical star cycle for the rest of the cosmos,” says Ignas Snellen, an astronomer at Leiden University who was not involved in the study. “Some stars that have formed at the edge of the local bubble over the past millions of years will eventually also end up in a supernova. This is how they make new super bubbles. More super bells have been found besides the local bell. It would be very coincidental if the cycle only applies to the local bubble that contains the sun.”
#sun #middle #bubble #stars #edge