Sun

   

For other uses, see Sun (disambiguation).
Sun
The Sun
Observation data
Mean distance from
Earth 		149.6×106 km
(92.95×106 mi)
Visual brightness (V) −26.8m
Absolute magnitude 4.8m
Orbital characteristics
Mean distance from
Milky Way centre 		~2.5×1017 km
(26,000 light-years)
Galactic period ~2.26×108 a
Velocity ~217 km/s
Physical characteristics
Diameter 1.392×106 km
(109 Earths)
Oblateness ~9×10-6
Surface area 6.09 × 1012 km²
(11,900 Earths)
Volume 1.41 × 1018 km³
(1,300,000 Earths)
Mass 1.9891 × 1030 kg

(332,950 Earths)

Density 1.408 g/cm&sup3
Surface gravity 273.95 m s-2

(27.9 g)

Escape velocity from the surface 617.54 km/s
Surface temperature 5780 K
Temperature of corona 5 MK
Approximated core temperature 13.6×106 K
Luminosity (LS) 3.827×1026 J s-1
Rotation characteristics
Obliquity 7.25º
(to the ecliptic)
67.23º
(to the galactic plane)
Right ascension
of North pole 1 (http://www.hnsky.org/iau-iag.htm) 		286.13º
(19 h 4 min 31,2 s)
Declination
of North pole 		63.87º
Rotation period  
At equator: 27 d 6 h 36 min
At 30° latitude: 28 d 4 h 48 min
At 60° latitude: 30 d 19 h 12 min
At 75° latitude: 31 d 19 h 12 min
Rotation velocity 7008.17 km/h
(at the equator)
Photospheric composition
Hydrogen 73.46 %
Helium 24.85 %
Oxygen 0.77 %
Carbon 0.29 %
Iron 0.16 %
Neon 0.12 %
Nitrogen 0.09 %
Silicon 0.07 %
Magnesium 0.05 %
Sulfur 0.04 %

The Sun (also called Sol) is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally captured by the Sun's gravity, but these exceptions are few and their masses are small. More generally the primary stellar body around which an object orbits is colloquially called its "sun", and stars in a multiple star system are referred to as the "suns" of bodies in that system.

Physical and other characteristics

The Sun is a main sequence star, with a spectral class of G2, meaning that it is somewhat more massive and hotter than the average star but far smaller than a blue giant star. A G2 star is on the main sequence, and has a lifetime of about 10 billion years (10 Ga), and the Sun formed about 5 Ga (5 billion years) ago, as determined by nucleocosmochronology. The Sun orbits the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 Ma (226 million years). The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days.

The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths (mostly due to the gravitation of Jupiter), which means the polar diameter differs from the equatorial by 10 km at most. This is partly because the centrifugal effect of the Sun's rather sedate rotation is 18 million times weaker than its surface gravity (at the equator).

Perhaps counterintuitively, the Sun does not have definite boundaries as rocky planets do. Instead, the density of gases comprising the Sun drops off following an exponential relationship with distance from the centre of the Sun. The Sun's radius is measured from centre to the edges of the photosphere.

Large solar flare recorded by SOHO EIT304 instrument in the ultraviolet. (512x512 version), (Animation (980kB MPEG)).
Large solar flare recorded by SOHO EIT304 instrument in the ultraviolet. (512x512 version), (Animation (980kB MPEG)).

At the centre of the Sun, where its density is 1.5×105 kg/m3, thermonuclear reactions (nuclear fusion) convert hydrogen into helium. About 8.9×1037 protons (hydrogen nuclei) are converted to helions (helium nuclei) every second. This releases energy at the matter-energy conversion rate of 4.26 million tonnes per second (about 9.1×1016 tons of TNT per second) 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. 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 and ending at the other.

The corona has a density of 1011 particles/m3, and the photosphere a density of 1023 particles/m3.

For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only one third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were created including the Sudbury Neutrino Observatory to try and measure the amount of neutrinos given off by the Sun. From these observatories and experiments it was recently found that neutrinos had rest mass, and could therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth; thus 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:

Caution: looking directly at the Sun can damage the retina and one's eyesight.

The astronomical symbol for the Sun is a circle with a point at its centre.

The composition of the Sun is not known with any great precision. A solar wind sample return mission, Genesis, returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on reentry to Earth's atmosphere.

The Death of Sol

Our sun does not have the mass to Nova or Supernova. However in 4-5 billion years it will enter its red giant phase, expanding as the hydrogen fuel in the core is consumed and it starts to burn heavier elements that are present. While it is likely that this expansion will reach the current position of Earth's orbit, recent research suggests that mass loss from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being swallowed. Following the red giant phase, the Sun will become a white dwarf, slowly cooling for a further 5 billion years or so.

See also

External links



The Solar System
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroids
Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass



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