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 orbit, then shrink to become a white dwarf. The Sun is about 149 million km/93 million mi from Earth (the closest star to Earth), with light and heat taking about seven minutes to reach Earth. The Sun spins on its axis every 25 days near its equator, but more slowly towards its poles. Its rotation can be followed by watching the passage of dark sunspots (cooler regions of about 3,600 K/3,300°C/6,000°F) across its disc. Sometimes bright eruptions called flares occur near sunspots. Above the Sun's photosphere (its visible surface which emits light and heat) lies a layer of thinner gas called the chromosphere, visible only by means of special instruments or at eclipses. Tongues of gas called prominences extend from the chromosphere into the corona, a halo of hot, tenuous gas surrounding the Sun. Gas boiling from the corona streams outwards through the Solar System, forming the solar wind. Activity on the Sun, including sunspots, flares, and prominences, waxes and wanes during the solar cycle, which peaks every 11 years or so, and seems to be connected with the solar magnetic field.
A wall of heated hydrogen atoms at temperatures of 20,000–40,000 K (19,700–39,700°C/35,500–71,500°F) that forms in the path of the Sun as it moves through space was discovered in 1995. The wall lies about 2,240 million km/1,555 million mi from the Sun, and its existence had been predicted by theorists.
US astronomers reported finding water on the Sun in 1995. The water, in the form of superheated steam, was located in two sunspots where the temperature was only 3,300 K/3,000°C/5,400°F (as opposed to 5,800 K/5,530°C/9,986°F elsewhere on the surface).
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