The sun's evil twin

to find Nemesis after finding striking observations while watching recently formed stars in the constellation Perseus. The researchers designed a mathematical model that finds the observations of stars in Perseus can only be explained if all of the stars were born with a companion.

'We are saying, yes, there probably was a Nemesis, a long time ago,' said one of the study's scientists Steven Stahler, a UC Berkeley research astronomer. '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.'

In this study, 'wide' means that the two stars are separated by more than 500 astronomical units, or AU, where one astronomical unit is the average distance between the sun and Earth (150 million kilometres). A wide binary companion to our sun would have been 17 times farther from the sun than its most distant planet today, Neptune. Based on this model, the sun's sibling most likely escaped and mixed with all the other stars in our region of the Milky Way galaxy, never to be seen again.

'The idea that many stars form with a companion has been suggested before, but the question is: how many?' said lead researcher Sarah Sadavoy, a Nasa Hubble fellow at the Smithsonian Astrophysical Observatory. 'Based on our simple model, we say that nearly all stars form with a companion. The Perseus cloud is generally considered a typical low-mass star-forming region, but our model needs to be checked in other clouds.'

The idea that all stars are born in a litter has implications beyond star formation, including the very origins of galaxies, Stahler said. The research has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.