There are man many ways to measure the mass of the galaxy. One way is too first measures the gravitational mass from the speed of rotation of a galaxy as a function of distance from the center, taking advantage of Newton's Laws to determine the mass around which the stars orbit. Another way also to measure is measures the total light produced by a galaxy; since we know how much light individual stars produce, we can determine how many stars are in the galaxy, and, hence, its luminous mass. The third way is to determine masses is to use our knowledge of physics. All mass is expected to interact gravitationally whether or not we can see it. We should be able to measure the mass with our eyes by their effects. These are some ways that astronomers use to measure the mass of the galaxy.