The sun is a powerful fusion reactor, where hydrogen atoms fuse to form helium and give off a tremendous amount of energy. The surface of the sun, also known as the photosphere, has a temperature of 6,000 K (10,000°F [5,538°C]). The temperature at the core, the region of nuclear fusion, is 36,000,000°F (20,000,000°C). A ball of coal the size of the sun would burn up completely in 3,000 years, yet the sun has already been burning for three billion years and is expected to burn for another four billion. The power emitted by the sun is 3.9 x 10²⁶ watts.

Only a very small fraction of the sun's radiant energy, or insolation, reaches the earth's atmosphere, and only about half of that reaches the surface of the earth. The other half is either reflected back into space by clouds and ice or is absorbed or scattered by molecules within the...