The central star of the solar system. On the range stars cover, the Sun is of medium size and brightness, though the vast majority of stars in the solar neighbourhood are smaller and less luminous. It is a dwarf star of spectral type G2 with a surface temperature of 5,700 K. Like all stars, it is a globe of hot gas and its energy source is nuclear fusion taking place in the centre, where the temperature is 15 million K. Four million tonnes of solar material are annihilated each second in the process, in which hydrogen is converted into helium.
Overlying the core is the radiation zone where the high-energy photons produced in the fusion reactions collide with electrons and ions to be re-radiated in the form of light and heat. Beyond the radiation zone is a convection zone in which currents of gas flow upwards to release energy at the surface before flowing downwards to be reheated. These circulating currents create the Sun's mottled appearance, or granulation. The surface layers, or photosphere, from which the light we see comes, are some hundreds of kilometres thick. In these layers, manifestations of solar activity occur, such as sunspot and flares. High-speed atomic particles released in flares stream through space, affecting the Earth and its environment. They cause radio interference, geomagnetic storm, and aurorae.
The layer over the photosphere is the chromosphere, visible as a glowing pinkish ring during a total solar eclipse. spicules and prominences erupt through the chromosphere. The thinnest, outermost layers, forming the solar corona, merge into the interplanetary medium.
