Tidal circularization

Tidal circularization is an effect of the tidal forces between a body in orbit around a central celestial object, whereby the eccentricity of the orbit is reduced over time so that it becomes less and less elliptical.

[1] When the lag angle is non zero as in figure 1, the forces F1 and F2 combine to produce clockwise torque on body 1, because F1 is stronger.

At the same time they torque the orbital motion counter clockwise: if you ignore the portion of F1 and F2 that lie along the line connecting the two bodies the remaining combined force on the entirety of body 1 is F3.

Figure 2 shows the path of one of the stars where G is the center of gravity of the system.

When Body 1 is in the green region of Figure 2, the torque slows down the orbit.

Circularization takes place as a result of lowering apoapsis or raising periapsis.

At a larger scale, it can occur in clusters of stars orbiting an imaginary point in space at the center of gravity.

Figure 1: Torque applied to orbit caused by tidal bulge and lag angle
Figure 2: Varying speeds of elliptical orbits