In close binary systems, observation finds that stellar rotation synchronizes with the orbital motion, or equivalently become tidally locked as is the moon to the earth, (notice the same side of the moon is always visible). In order to synchronize, the stars do not necessarily need to be in contact though their proximity to one another must be such that friction from the interacting tidal forces can hinder the orbit. In addition, the degree of tidal interaction is dependent upon the ratio of the stellar radius to the separation distance between the stars (Hut 1981).
The presence of a companion introduces a tidal or differential force that acts to elongate the star along the line between the center of mass, which produces a tidal bulge. If the rotational period of the star is shorter than the orbital period, then frictional forces on the surface of the star will drag the bulge axis ahead of the line of centers. As the stars orbit each other, the resulting tides of the stars will tend to either lag or precede the line of centers, which produces a torque on the system. This torque transfers angular momentum between the stellar spin and the orbit, ultimately dissipating orbital energy. As energy dissipates through the tides (typically in the form of heat), the total energy of the binary system diminishes, thus changing the orbital parameters of the binary. The stars may approach an equilibrium state or spiral inwards until merger. The equilibrium state is characterized by co-rotation and a circular orbit, again like the earth and moon, corresponding to a minimum energy for a given total angular momentum and alignment of the spin orbit axes.
Other angular momentum loss mechanisms specific to binary systems such as gravitational radiation and magnetic breaking are also addressed in the BSE. To see in detail how the BSE models dissipation mechanisms see Hurley et al. (2002).
An Example of the Tides - The Earth Moon System
