Estimate the mass of our galaxy (the Milky Way) if the Sun orbits the center of the galaxy with a period of 250 million years at a mean distance of 30,000 c · y. Express the mass in terms of multiples of the solar mass Ms. (Neglect the mass farther from the center than the Sun, and assume that the mass closer to the center than the Sun exerts the same force on the Sun, as would a point particle of the same mass located at the center of the galaxy.)