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Our solar system may have formed in a star-making ‘bubble’

Scientists have proposed a new, comprehensive theory for how our solar system could have formed in the wind-blown bubbles around a giant, long-dead star.

The study addresses a nagging cosmic mystery about the abundance of two elements in our solar system compared to the rest of the galaxy.

The general prevailing theory is that our solar system formed billions of years ago near a supernova. But the new scenario instead begins with a giant type of star called a Wolf-Rayet star, which is more than 40 to 50 times the size of our own sun. They burn the hottest of all stars, producing tons of elements which are flung off the surface in an intense stellar wind. As the Wolf-Rayet star sheds its mass, the stellar wind plows through the material that was around it, forming a bubble structure with a dense shell.

A simulation shows how stellar winds carry mass from a giant star over the course of millions of years, forming bubbles around it—which could have served as the origins of our solar system. Simulation by V. Dwarkadas/D. Rosenberg. via GIPHY

“The shell of such a bubble is a good place to produce stars,” because dust and gas become trapped inside where they can condense into stars, says coauthor Nicolas Dauphas, professor in the geophysical sciences department at the University of Chicago. The authors estimate that 1 percent to 16 percent of all sun-like stars could be formed in such stellar nurseries.

This setup differs from the supernova hypothesis in order to make sense of two isotopes that occur in strange proportions in the early solar system, compared to the rest of the galaxy. Meteorites left over from the early solar system tell us there was a lot of aluminium-26. In addition, studies, including a 2015 one by Dauphas and a former student, increasingly suggest we had less of the isotope iron-60.

This brings scientists up short, because supernovae produce both isotopes. “It begs the question of why one was injected into the solar system and the other was not,” says coauthor Vikram Dwarkadas, a research associate professor in astronomy and astrophysics.

This brought them to Wolf-Rayet stars, which release lots of aluminium-26, but no iron-60.

bubble simulation (solar system origins)
A still shot of the simulation showing how bubbles form over the course of 4.7 million years from the intense stellar winds off a massive star. (Credit: U. Chicago)

“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed—trapping the aluminum inside the shell,” Dwarkadas says. Eventually, part of the shell collapses inward due to gravity, forming our solar system.

As for the fate of the giant Wolf-Rayet star that sheltered us: Its life ended long ago, likely in a supernova explosion or a direct collapse to a black hole. A direct collapse to a black hole would produce little iron-60; if it was a supernova, the iron-60 created in the explosion may not have penetrated the bubble walls, or was distributed unequally.

This star is like ‘time travel’ to our early solar system

The scientists lay out their theory in the Astrophysical Journal.

Other authors of the paper include undergraduate student Peter Boyajian of University of Chicago and Michael Bojazi and Brad Meyer of Clemson University. Funding for the research came from NASA.

Source: University of Chicago

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