Sun, the star at the center of the solar system. Like other stars, the sun is a huge, hot, gaseous sphere. It is held together by its own massive gravity and is heated by nuclear reactions in its core. Radiation from the sun provides the energy that drives virtually all the natural processes that occur on the earth and in the earth's atmosphere.

No object in the universe is more important to the earth than the sun. Although only one two-billionth of the sun's radiation reaches the earth, that tiny portion is enough to make life possible. Without the heat of the sun, the earth would be frozen and lifeless. By a process called photosynthesis, most plants and algae and some bacteria use the energy of sunlight to produce their own food. Virtually all other life depends on these organisms for nourishment. Sunlight absorbed by them millions of years ago is available as energy today from fuels such as coal, oil, and natural gas. These fossil fuels were formed from decayed plant and animal matter.

From the sun's Latin name, sol, comes the term solar system for the sun together with the group of planets and other bodies whose motions are governed by the sun's gravitational attraction. The vast region dominated by the sun's attraction extends far beyond the orbits of Neptune and Pluto, the most remote planets. Within this region, the motions of all bodies are influenced by the pull of the sun's enormous mass, which is estimated to contain more than 99 per cent of all the matter in the solar system.

As a star, the sun is of only average size and temperature. Astronomers classify it within the main-sequence group of stars, along with millions of other similar stars in the Milky Way galaxy. Although it appears billions of times brighter than any other star seen from earth, the sun differs in appearance from other stars only because of its nearness to earth. If the sun were as distant as Proxima Centauri, the next nearest star to earth, it would appear as a moderately bright, yellow star.

The distance between the sun and the earth varies during the year because of the earth's elliptical orbit around the sun. The average distance is just under 93,000,000 miles (150,000,000 km). Light leaving the surface of the sun requires about eight minutes to reach the earth and about four hours to reach Neptune.