Discuss the occurrence of binary (double) stars in a astronomy.

Explanation

If there are no external forces, the centre of mass of a double star moves like a free particle, as shown in figure (a).

The trajectories of the two stars of equal mass are also shown in the figure (a). They look complicated.
If we go to the centre of mass frame, then we find that there the two stars are moving in a circle, about the centre of mass, which is at rest. The position of the stars have to be diametrically opposite to each other figure (b).
In our frame of reference, the trajectories of the stars are a combination of
(i) Uniform motion in a straight line of the centre of mass and
(ii) Circular orbits of the stars about the centre of mass.