Sun, Star around which the components of the solar system revolve. It is about 4.6 billion years old and is the dominant body of the system, with more than 99% of its mass. It converts about 4.5 million tons of matter into energy every second by nuclear fusion reactions in its core, producing neutrinos and solar radiation. The small amount of this energy that penetrates Earth’s atmosphere provides the light and heat that support life. A sphere of luminous gas 1,392,000 km (864,950 mi) in diameter, the Sun has about 330,000 times the mass of Earth. Its core temperature is close to 15,000,000 K (27,000,000 °F ) and its surface temperature about 6,000 K (10,000 °F). The Sun, a spectral type G2 V (yellow) star, has fairly average properties for a main-sequence star (see Hertzsprung-Russell diagram). It rotates at different rates at different latitudes; one rotation takes 36 days at the poles but only 25 days at the equator. The visible surface, or photosphere, is in constant motion, with the number and position of sunspots changing in a regular solar cycle. External phenomena include magnetic activity extending into the chromosphere and corona, solar flares, solar prominences, and the solar wind. Effects on Earth include auroras and disruption of radio communications and power-transmission lines. Despite its activity, the Sun appears to have remained relatively unchanged for billions of years.
