Star at the centre of our Solar System. It is about 5 billion years old, with a predicted lifetime of 10 billion years; its diameter is 1.4 million km/865,000 mi; its temperature at the surface (the photosphere ) is about 5,800 K/5,530°C/9,986°F, and at the centre 15 million K/about 15 million°C/about 27 million°F. It is composed of about 70% hydrogen and 30% helium, with other elements making up less than 1%. The Sun's energy is generated by nuclear fusion reactions that turn hydrogen into helium, producing large amounts of light and heat that sustain life on Earth. Space probes to the Sun have included NASA's series of Orbiting Solar Observatory satellites, launched between 1963 and 1975, the Ulysses space probe, launched in 1990, and Genesis, launched in 2001. Since 1995 the Sun has been continuously observed by SOHO, a joint European-US satellite permanently stationed between the Earth and the Sun. At the end of its life, it will expand to become a red giant the size of Mars's…