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How close to each other do two stars have to be for one to steal material from the other?

Eddie Hackett, Tampa, Florida
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If stars in a binary system are close enough to each other, tides can become so strong that the denser companion (the “accretor”) gravitationally pulls gas from the surface of the less dense star (the “donor”). This “tidal mass transfer” depends on the size of the donor sun relative to the binary separation and, to a lesser extent, the ratio of donor-to-accretor mass. For two stars of the same mass, if the donor star’s radius exceeds 38 percent of the binary separation, the accretor will steal mass no matter the separation.

Stars grow enormously in radius as they evolve. For example, the Sun’s radius could reach to Earth’s current orbit (1 astronomical unit [AU]) or farther when it becomes a red giant. If it had a denser companion star orbiting in the middle of the asteroid belt, the future Sun would lose material to such a star. On the other hand, binary stellar remnants called neutron stars are so dense that they only would need to orbit at separations just about 20 miles (30 kilometers) from each other (and with periods less than 2 milliseconds) before they began transferring mass.

Binary systems are extremely common, accounting for about half of all the stars in the solar neighborhood, and perhaps as many as half of them have orbital separations small enough to undergo mass transfer at some time during their evolution. Nevertheless, two stars do not need to be gravitationally bound for mass transfer to occur if they pass close enough to each other. That is an improbable event in our galaxy’s disk, where typical distances between stars are hundreds of millions of solar radii (trillions of miles); but in the cores of the densest globular clusters, the odds can favor a star having a near-collision during its lifetime.

More widely separated binary stars also can transfer mass as one sun captures a portion of the wind from its companion — so it’s not so much “stealing” as “receiving a gift.” A well-known example of this kind of interaction occurs with the pulsating red giant Mira (Omicron [ο] Ceti). It has a companion white dwarf about 60 AU from it that flickers because of donated Mira material. — Ronald Webbink, University of Illinois, Urbana-Champaign
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