The sun is considered to be a satellite of our galaxy, the Milky-way. The sun revolves around the centre of the galaxy with a radius of $2.2 \times 10^{20}$ m. The period of one revolution $2.5 \times 10^{8}$ years. What is the mass of the galaxy? [Gravitational constant $G=6.67 \times 10^{-11} \mathrm{~N} \cdot \mathrm{m}^{2} / \mathrm{kg}$ ]