Halliday, Resnick & Walker — Gravitation: Problem 65

65 The Sun, which is \(2.2 \times 10^{20} \text{ m}\) from the center of the Milky Way galaxy, revolves around that center once every \(2.5 \times 10^{8} \text{ years}\). Assuming each star in the Galaxy has a mass equal to the Sun’s mass of \(2.0 \times 10^{30} \text{ kg}\), the stars are distributed uniformly in a sphere about the galactic center, and the Sun is at the edge of that sphere, estimate the number of stars in the Galaxy.