Since the 1980s, astronomers have been searching for the Sun's "evil" twin, dubbed Nemesis due to its habit of slinging deadly asteroids our way every 26 million years or so. Lately, the Nemesis hypothesis has fallen out of favor after decades of sky surveys have turned up no trace of the star, but a new mathematical model from UC Berkeley suggests that almost every star is born with a buddy – including our Sun.

The team probed the Perseus cloud, a stellar nursery some 600 light years away, to take stock of the number of single and binary stars. Combining several data sets from different surveys, the researchers identified 19 binary-star systems and 45 single-star systems.


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Intriguingly, in wide binary systems in which the two stars are further than 500 Astronomical Units (AU) apart, all of the stars were very young – under 500,000 years old. The slightly older stars – between 500,000 and 1 million years – were all closer together, about 200 AU.

"This has not been seen before or tested, and is super interesting," says Sarah Sadavoy, first author of the study. "We don't yet know quite what it means, but it isn't random and must say something about the way wide binaries form."

The Perseus cloud appears in the sky as a black spot, since it's made up of dense gas and dust that blocks light from stars inside and behind it (Credit: FORS Team, 8.2-meter VLT Antu, ESO)

To try to find those answers, the team ran computer simulations to model several scenarios. There was only one way to make all the pieces fit with observations: all stars with masses about that of the Sun must start life as part of a wide binary system. Over time, an estimated 60 percent of them split up to form two single-star systems, while the rest drift closer together into tight binaries.

That means that even though the hypothetical Nemesis has never been detected, the Sun probably does have a long-lost twin, which has since migrated out into the Milky Way – it probably isn't evil, though.

"We are saying, yes, there probably was a Nemesis, a long time ago," says Steven Stahler, co-author of the study. "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 reproduce 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."

To test its mettle, the model needs to be applied to other star-birthing clouds.

The research has been published online, and will appear in a future issue of the Monthly Notices of the Royal Astronomical Society.

Source: UC Berkeley

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