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The Sun rotates about the center of the Milky Way Galaxy (Fig. 6–29) at a distance of about 30,000 light-years from the center (1 ly = 9.5 × 1015 m). If it takes about 200 million years to make one rotation, estimate the mass of our Galaxy. Assume that the mass distribution of our Galaxy is concentrated mostly in a central uniform sphere. If all the stars had about the mass of our Sun (2 × 1030 kg), how many stars would there be in our Galaxy?
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