Sun

The Sun, sometimes called Sol, is the star in our solar system. The planet Earth and all of her sister planets, both the other terrestrial planets and the gas giants, orbit the Sun. Other bodies that orbit the Sun include asteroids, meteoroids, comets, Trans-Neptunian objects, and, of course, dust.

Physical and other characteristics

The Sun is a main sequence star, with a spectral class of G2, meaning that it is somewhat bigger and hotter than the average star but far smaller than a red giant star. A G2 star has a main sequence lifetime of about 10 billion years, and the Sun is probably about 5 billion years old, as determined by nucleocosmochronology.

At the center of the Sun, where its density is 1.5 × 10⁵ kg m^-3, thermonuclear reactions (nuclear fusion) convert hydrogen into helium. 3.9 × 10⁴⁵ atoms undergo nuclear reactions there every second. This releases energy which escapes from the surface of the Sun as light. Physicists are able to replicate thermonuclear reactions with hydrogen bombs. Sustained nuclear fusion on earth for electricity generation may be possible in the future, with nuclear fusion reactors.

All matter in the Sun is in the form of plasma due to its extreme temperature. This makes it possible for the sun to rotate faster at its equator than it does at higher latitudes, since the Sun is not a solid body. The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences[?].

The corona has 10¹¹ atoms/m³, and the photosphere has 10²³ atoms/m³.

For some time it was thought that the number of neutrinos produced by the nuclear reaction in the Sun was only one third of the number predicted by theory, a result that was termed the solar neutrino problem. When it was recently found that neutrinos had mass, and could therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth, measurement and theory were reconciled.

Observation of the Sun can reveal such phenomena as:

• Sunspots
• Faculae[?]
• Solar flares
• Solar prominences[?]
  • quiescent prominences[?]
  • eruptive prominences[?]

Caution: looking directly at the sun can damage the retina and one's eyesight[?].

See also:

• Solar radiation
• Solar wind
  • Aurora borealis
  • Aurora australis
• Photosphere
• Chromosphere
• Corona
• Airglow[?]
• Eclipse
• Timeline of solar astronomy
• Sun mythology

Solar system:
Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets

Several newspapers are called The Sun.

Sun is a commonly used name for the computer company Sun Microsystems.