Answer to Question 1
Although there are a few ways to estimate the mass of a galaxy, the most direct measurement uses Kepler's third law of planetary motion. We used this method in an earlier chapter to find the masses of binary stars. If astronomers can measure the radius of a galaxy and the velocity with which it rotates, they can easily find the orbital periods of the stars as they orbit the galaxy. Then they can use Newton's version of Kepler's third law to calculate the mass of the galaxy. The most accurate of these studies involve observing the rotation of the galaxy at different distances from its center and drawing a graph called the rotation curve of the galaxy. Using such a graph to find the mass of the galaxy is called the rotation curve method. It is the most accurate way to find the mass, but it works only for the nearer galaxies, whose rotation curves can be observed. More distant galaxies appear so small astronomers cannot measure the radial velocity at different points across the galaxy.
Answer to Question 2
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