Transcript: Stars near the Sun have radial velocities measured by the Doppler shift in the range of ten to twenty kilometers per second. Because us and the stars around us are all moving together at similar speeds around the center of our galaxy the differential speeds are small. Further from the Sun the orbital speeds do vary. The Milky Way does not rotate like a solid object with a constant angular velocity at every radius from the center. Nor does it follow Kepler’s third law because unlike the situation of the planets orbiting the Sun, most of the mass in the disk is distributed and not concentrated at the center of the galaxy. The situation in the Milky Way where the rotation speeds vary with distance is called differential rotation. There is also some component of random motion superimposed on the circular rotation velocities.