Sun
The Sun is the star at the center of the Solar System. The Sun has a diameter of about 1,392,000 kilometres (865,000 mi) (about 109Earths), and by itself accounts for about 99.86% of the Solar System’s mass; the remainder consists of the planets (including Earth), asteroids, meteoroids, comets, and dust in orbit. About three-fourths of the Sun’s mass consists of hydrogen, most of the rest ishelium. Less than 2% consists of other elements, including iron, oxygen, carbon, neon, and others.
The Sun’s color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering. It has aspectral class of G2V, informally designated a “yellow star” because the majority of its radiation is in the yellow-green portion of the visible spectrum. The G2 indicates its surface temperature of approximately 5,780 K (5,510 °C.) The V (Roman five) in the spectral class indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogennuclei into helium. Once regarded as a small and relatively insignificant star, the Sun is now presumed to be brighter than 85% of the stars in the galaxy, most of which are red dwarfs. (Estimates for its magnitude are around 4.8) The Sun’s hot coronacontinuously expands in space creating the solar wind, a hypersonic stream of charged particles that extends to the heliopause at roughly 100 AU. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.
The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way Galaxy. Of the 50 nearest stellar systems within 17 light-years from the Earth, the Sun ranks 4th in mass. The Sun orbits the center of the Milky Way galaxy at a distance of approximately 24,000–26,000 light years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years.
The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though this varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to the earth in about 8 minutes 19 seconds. The energy from this sunlight supports almost all life on Earth via photosynthesis, and drives the Earth’s climate andweather. The enormous impact of the Sun on the Earth has been recognized since pre-historic times, and the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little notion of the Sun’s physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun’s behavior that remain unexplained.
