According to a team of astronomers from Harvard University and the University of California, Berkeley, low-mass stars are always born with a companion, but many, like the Sun, split up.

This artists impression shows the dust and gas around a binary star system. Image credit: ESO / L. Calcada.

Many stars have companions, including the triple star system of Alpha Centauri, just 4.3 light-years away.

Scientists have long sought an explanation. Are binary and triple stellar systems born that way? Did one star capture another? Do binary stars sometimes split up and become single stars?

Astronomers have even searched for a companion to our Sun, a star dubbed Nemesis because it was supposed to have kicked an asteroid into Earths orbit that collided with our planet and exterminated the dinosaurs. It has never been found.

Radio image of a very young binary star that formed within a dense core (oval outline) in the Perseus molecular cloud. All stars likely form as binaries within dense cores. Image credit: Sarah Sadavoy / Steven Stahler.

The new assertion is based on a radio survey of the Perseus molecular cloud, a giant stellar nursery about 600 light-years away in the constellation Perseus, and a mathematical model that can explain the Perseus observations only if all Sun-like stars are born with a companion.

We are saying, yes, there probably was a Nemesis, a long time ago, said Dr. Steven Stahler, an astronomer at the University of California, Berkeley.

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.

Combined ALMA and VLA image of the L1448 IRS3B system, a member of the Perseus molecular cloud, with two young stars at the center and a third distant from them; spiral structure in the dusty disk surrounding them indicates instability in the disk. Image credit: Bill Saxton / ALMA / ESO / NAOJ / NRAO / AUI / NSF.

In this study, wide means that the two stars are separated by more than 500 AU (astronomical units).

A wide binary companion to our Sun would have been 17 times farther from the Sun than Neptune.

Based on this model, the Suns 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 Dr. Sarah Sadavoy, a NASA Hubble fellow at the Smithsonian Astrophysical Observatory at Harvard University.

Based on our simple model, we say that nearly all stars form with a companion.

The Perseus molecular 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, Dr. Stahler added.

The findings were published recently in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).

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Sarah I. Sadavoy & Steven W. Stahler. 2017. Embedded binaries and their dense cores. MNRAS 469 (4): 3881-3900; doi: 10.1093/mnras/stx1061

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