Using Kepler's third law and knowing the velocity of bodies in orbit about a central mass M, we can determine the value of the mass M. In order to get the velocity we measure the Doppler shift of the light from mass m.
 

Suppose we have a mass m, in a circular orbit around a mass M, with m<<M. If the radius of the orbit is a, and the period of the orbit is T, then from Kepler's third law T2/a3 = 4(pi)2/(GM),  where G is Newton's gravitational constant. The velocity in orbit is related to the period T and the orbit's size a by v = 2(pi)a/T .  Substituting this into Kepler's law lets us determine that M = (av2 )/G. G = 6.67x10 -11Nm2/kg2    , 1pc = 3.09x1013 km = 3.09x1016m

The graph below shows the velocity of objects orbiting the galaxy as a function of distance from the galactic center.  From this graph you can determine the mass of the galaxy contained within the radius of the sun's orbit around the galaxy. This is very important information because it gives us direct evidence for the existence of "dark matter".

graph source : http://www.angelfire.com/electronic/isolderadford/galacdm.html