Chemical compositions of the atmospheres of giant planets are determined primarily by spectroscopy. Computer models of the density are required for an understanding of compositions deep in the planets’ cores. Astronomers are interested in differences in composition because these differences are clues to the formation and history of the planet. These observers use the relative abundance of the elements in the Sun as a standard reference, termed solar abundance. As illustrated in Figure 10.5, hydrogen (H) is the most abundant element in the Sun, followed by helium (He).