(13) (3 + 1 = 4 points.) (a) (3 points.) The discovery of a star orbiting the center of our Galaxy with a 15-year period has led to the nding that the mass of the giant black hole at the center of our Galaxy is about 4 million times that of the Sun (as described in Chapter 15). Using Newton's version of Kepler's third law and ratios, calculate the semimajor axis of the star's orbit. (b) (1 point.) Convert your answer to light years. Chapter 6 (14) (3 points.) If you were standing on a spinning turntable holding barbells with your extended arms, and then you brought your hands close to your chest, how would your spin rate be affected? Why? (15) (3 points.) What carries the continental plates around over Earth's surface