600 light years away is a cloud full of star stuff dubbed the Perseus molecular cloud. A new radio survey of this cloud could have big implications for the way our solar system looked billions of years ago. A team of astronomers can only explain what the survey shows if all sunlike stars formed as binaries at first.

Multi-star systems are common. How common is still up for debate, but our closest neighbor Alpha Centauri is a triple star system. Astronomers have grappled with multi-star systems for a long time. Are binary and other types of multi-star systems born this way? Does one star capture another? Can a binary system eventually become a single star system?

“We ran a series of statistical models to see if we could account for the relative populations of young single stars and binaries of all separations in the Perseus molecular cloud, and the only model that could produce the data was one in which all stars form initially as wide binaries. These systems then either shrink or break apart within a million years,” says co-author Steven Stahler, a UC Berkeley research astronomer.

binary star system

Young binary star system in Perseus molecular cloud. Credit: Sarah Sadavoy

The team of astronomers believes our sun had a buddy a long time ago nicknamed Nemesis. It’s named after a theory that proposes this companion star nudged the asteroid into Earth’s orbit that killed the dinosaurs. But let’s be clear. This companion star has never been found.

Many companion stars orbit fairly close together. Take the Alpha Centauri example I mentioned above. Alpha Centauri A and B sit between 11 AU and 35 AU apart (1 AU = distance between Earth and Sun).

Stahler and the other astronomers on the team propose these binaries start as “wide binaries.” This means the two stars would have been more than 500 AU apart. Nemesis would have been 17 times farther away from the sun than Neptune.

Where is Nemesis today? The model suggests 60 percent of the binaries split up over time. The other 40 percent shrink into tight binaries. Since Earth isn’t Tatooine, we can assume Nemesis (if it existed) spilt from the sun a long time ago. It’s out there mingling with other stars in the region of the Milky Way Galaxy we call home.

Sarah Sadavoy, the study’s first author, says the idea of most stars forming with a companion isn’t a new one. But their study aimed to figure out how common it could be. Based on the model of the Perseus molecular cloud, it appears nearly all have a companion star at first. “But our model needs to be checked in other clouds,” Sadavoy adds.

Like many studies of the cosmos, the data and findings are still fluid. More observations are needed across other parts of the sky. Future observations with the world’s strongest radio telescopes like Chile’s ALMA will continue filling in our picture of how stars and galaxies take shape.

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