Sun
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
- See Sun (disambiguation) for other meanings of the word "Sun".
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 blue 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 in the form of electromagnetic radiation and neutrinos (and to a smaller extent as the kinetic and thermal energy of solar wind plasma and as the energy in the Sun's magnetic field). 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 solar activity cycle includes old magnetic fields being stripped off the Sun's surface starting from one pole
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.
To obtain an uninterrupted view of the Sun, the European Space Agency and NASA cooperatively launched the Solar and Heliospheric Observatory (SOHO) on December 2, 1995.
Observation of the Sun can reveal such phenomena as:
The astronomical symbol for the sun is a circle with a point at its centre.
Physical and other characteristics
Caution: looking directly at the sun can damage the retina and one's eyesight.See also
The Solar System
Sun | Mercury | Venus | Earth | Moon | Mars | Asteroids | Jupiter | Saturn | Uranus | Neptune | Pluto
(For other objects and regions, see: List of solar system objects, Astronomical objects)
Large solar flare recorded by 
.
.
Courtesy SOHO(ESA&NASA)