This is done primarily by analyzing the light that comes from the star. You can try this simple experiment at home but first ask your parents to supervise. Turn on a burner on the stove. Notice the nice blue glow of the flame. Now take a pinch of salt - just a little - put it on a spoon and put the NaCl into the blue flame. What happens?? The flame should turn very yellow!! Why? Because when sodium (salt= NaCl) is heated, the electrons in the outer shell jump around to different levels. When they do this they emit light of a certain color. In this case its yellow. But this same technique we can figure out the composition of different stars