Three stars, each of mass $2.0 \times 10^{30}\ \text{kg}$, sit at the corners of an equilateral triangle of side $1.0 \times 10^{11}\ \text{m}$. They move together in one circle round the centre of the triangle, under their own gravity alone, so that the triangle keeps its shape, as shown in the figure. Take $G = 6.67 \times 10^{-11}\ \text{N m}^2/\text{kg}^2$. What is the period of their motion?

Fig. 1: Three stars, each of mass m, sit at the corners of an equilateral triangle of side a. A dashed circle through the stars is centred on the centre of the triangle, marked with a cross, and each star has a velocity arrow tangent to the circle, all counterclockwise.