ABSTRACT
The Sun is a field star, which is to say that it moves through the galaxy without any stellar companions. However, most stars do have stellar companions, usually just one, to form a binary system. The number of binary systems is approximately equal to the number of field stars, so, roughly, two-thirds of sun-like stars are in binary associations. The members of a binary system move round their centre of mass in orbits of similar shape but at a scale proportional to the mass of the companion star. Figure L.1 shows this effect for a star with a planetary-mass companion. There are a few small-scale stable systems with three or more stars, but they are comparatively rare. For example, a three-star system can occur if one pair forms a close binary, while the third is at a greater distance in a binary association with the close pair, which approximates to a compact gravitational source.
