- noun sunlight or the rays of the Sun
- noun (written as Sun)a very hot star round which the earth and other planets orbit and which gives energy in the form of light and heat
- (written as Sun)The star which dominates the solar system and contains most of its matter. The Sun is by far the most familiar star to astronomers, and is known to be a fairly ordinary G2 star (see star classification) with a surface temperature of some 6000K. It is about 329,000 times as massive as the Earth and is notable among other stars for being solitary rather than part of a multiple system. During the 20th century, the energy source which powers the Sun was identified as nuclear fusion, ending millennia of speculation, and models of the Sun’s formation were produced as part of wider studies of the origin of the solar system. The Sun is known to be a variable star with regular cycles of sunspot activity over 22 years, although its the total variation in energy output is far less than that of other variables observed by astronomers. This variation is closely linked to the Sun’s intense magnetism, which is also intimately involved in the other surface and atmosphere effects studied by the many astronomers who study the Sun. In the last few decades, their work has been expanded by the use of telescopes to observe the Sun in a wide variety of wavelengths. The Sun was one of the first objects observed by radio astronomy and has since been viewed with telescopes operating across almost the whole of the electromagnetic spectrum, as well as more arcane instruments like neutrino telescopes. As well as being fascinating in its own right – and as the energy source for life on Earth – the Sun fascinates astronomers because it is close and well-known enough to provide the baseline to which observations and theories of other stars are referred.
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