I.

Introduction

Sun, the star that, by the gravitational effects of its mass, dominates the solar system—the planetary system that includes the Earth. By the radiation of its electromagnetic energy, the Sun furnishes directly or indirectly all of the energy supporting life on Earth except for that supported by deep-ocean hydrothermal vents, because all foods and fuels except for these are derived ultimately from plants using the energy of sunlight. See Photosynthesis; Solar Energy.

Because of its proximity to the Earth (average distance 149,597,870 km (92,960,116 mi), known as an astronomical unit, (AU), and because it is such a typical star, the Sun is a unique resource for the study of stellar phenomena. No other star can be studied in such detail. Lying at very great distances from Earth, the stars in the night sky appear as unresolved point sources. Spectroscopic studies of distant stars of solar type allow astronomers to infer that these show similar patterns of behaviour to the Sun, including magnetic activity cycles and flares. It is believed that other stars have spots similar to sunspots.