Sun

Sun Star at the centre of our Solar System, around which all other Solar System bodies revolve in their orbits. The Sun is a typical, average star. It consists of c.70% hydrogen (by weight) and 28% helium, with the remainder mostly oxygen and carbon. Its temperature, pressure, and density increase towards the centre. Like all stars, the Sun's energy generates by nuclear fusion reactions taking place under the extreme conditions in the core. This core is c.400,000km (250,000mi) across. Energy released from the core passes up through the radiative zone, which is c.300,000km (200,000mi) thick, then passes through the 200,000km (125,000mi) thick convective zone to the surface, the photosphere, from where it radiates into space. Most of the Sun's visible activity takes place in this 500km (300mi) -thick photosphere. Above the photosphere lies the chromosphere, which consists of hot gases and extends for thousands of kilometres. Extending outwards from the chromosphere for millions of kilometres is the corona, which emits the solar wind. The solar wind and the Sun's magnetic field dominate a region of space called the heliosphere, which extends to the boundaries of the Solar System.

http://lpl.arizona.edu/nineplanets/nineplanets/sol.html