The Sun, the central body of the solar system and the Earth's closest star, is an immense sphere of glowing gas 1.39 million km (860,000 mi) in diameter at an average distance from the Earth of 149,591,000 km (92,960,000 mi). It is composed mainly of hydrogen, with about 5%, by number of atoms, of helium and heavier elements. Its mass of 1.99 X 1033 g is sufficient for the mutual gravitational attraction of the molecules to prevent the hot gases from expanding rapidly into the relative vacuum of space. The Sun generates energy at the rate of 3.9 X 1033 ergs/sec by burning hydrogen to helium through nuclear fusion reactions in its interior. This energy radiates into space, mostly in visible and infrared radiation, and is responsible for the continuation of life on Earth.

Compared with the largest known stars, with diameters a thousand times larger and masses several hundred times greater, the Sun merits its astronomical designation as a dwarf star. Its mass and radius are close to the average mass and size of all stars in the Galaxy, however, because many stars are even smaller and less massive. The Sun's spectrum, surface temperature, and color lead to its further classification as a G2 dwarf in the scheme of spectral types used by astronomers. The spectral intensity of light radiated by its surface gases is a maximum at wavelengths near 5000 A, giving sunlight its characteristic yellow color.

Modern study of the Sun began in 1611 with observations of sunspots by Galileo Galilei and his contemporaries and their discovery of solar rotation from the sunspot motions. The first approximately correct determination of the Sun's size and distance from the Earth was made in 1684, from data obtained by the French Académie des Sciences from triangulation observations of Mars during its close approach to the Earth in 1672. The discovery of the solar absorption-line spectrum by Joseph von Fraunhofer in 1814, and its physical interpretation by Gustav Kirchhoff in 1859, opened the era of solar astrophysics, during which the effective study of the physical state and chemical composition of the solar material became possible. The strong magnetic fields of sunspots were detected by George Ellery Hale in 1908, and the role of nuclear fusion in producing solar energy was elucidated by Hans Bethe in 1939. Modern developments continue to change scientists' perception of the Sun. The solar wind was not detected directly until 1962, and the sources of its high-speed recurrent streams awaited the observations of holes in the corona in 1969.