You know that flowers like the ones in Figure 1 need sunlight. In fact, almost all life on Earth depends on sunlight. Without the sun's radiation, Earth would be a cold, dark, and lifeless planet. But what exactly is the sun? Scientists have learned that the sun is a glowing ball of gas. The sun is extremely hot, about 15 million K at its center and 5800 K at its surface. At such temperatures, most of the atoms in the sun have lost most of their electrons, forming a plasma of ionized gas. About 70 percent of the sun's mass is hydrogen and 28 percent is helium.

Figure 1  The sun gives off tremendous amounts of energy. Plants on Earth use sunlight directly in photosynthesis.