Sun

From Wikipedia, the free encyclopedia

The Sun (Latin: Sol) is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for about 99.8% of the solar system's mass. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.

The Sun is composed of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 25% of mass, 7% of volume), and trace quantities of other elements. The Sun has a spectral class of G2V. G2 implies that it has a surface temperature of approximately 5,780 K, giving it a white color which, because of atmospheric scattering, appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of blue photons (causing the sky color) removes enough blue light to leave a residual reddishness that is perceived as yellow. (When low enough in the sky, the Sun appears orange or red, due to this scattering.)

Its spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) suffix indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium and is in a state of hydrostatic equilibrium, neither contracting nor expanding over time. There are more than 100 million G2 class stars in our galaxy. Because of logarithmic size distribution, the Sun is actually brighter than 85% of the stars in the galaxy, most of which are red dwarfs.^[1]

The Sun orbits the center of the Milky Way galaxy at a distance of approximately 26,000 light-years from the galactic center, completing one revolution in about 225–250 million years. The orbital speed is 217 km/s, equivalent to one light-year every 1,400 years, and one AU every 8 days.^[2]

It is currently traveling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 light years from the Earth, the sun ranks 4th in absolute magnitude as a fourth magnitude star (M=4.83).