The Sun consists of hydrogen (about 74% of its mass), helium (about 24% of mass), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium. The Sun has a spectral class G2V and, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium. There are more than 100 million G2 class stars in our galaxy.
With an orbital speed of 251 km/s, the Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 to 27,000 light-years from the galactic center, moving generally in the direction of Cygnus and completing one revolution in about 225–250 million years. Because of a high abundance of heavy elements such as gold and uranium in the Solar system, it has been suggested that the formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This makes the Sun a Population I, or heavy element-rich, star.
The Sunlight is the Earth's primary source of energy. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface, closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly deadened by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations.