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Sun

 Observation data Orbital characteristics Mean distancefrom Earth 1.496×108 km8 min 19 s at light speed Visual brightness (V) −26.74[1] Absolute magnitude 4.83[1] Spectral classification G2V Metallicity Z = 0.0122[2] Angular size 31.6′ – 32.7′[3] Adjectives Solar Mean distancefrom Milky Way core ~2.5×1017 km26000 light-years Galactic period (2.25–2.50)×108 a Velocity ~220 km/s (orbit around the center of the Galaxy) ~20 km/s (relative to average velocity of other stars in stellar neighborhood) ~370 km/s[4] (relative to the cosmic microwave background) Mean diameter 1.392684×106 km[5] Equatorial radius 6.96342×105 km[5] 109 × Earth[6] Equatorial circumference 4.379×106 km[6]109 × Earth[6] Flattening 9×10−6 Surface area 6.0877×1012 km2[6]11990 × Earth[6] Volume 1.412×1018 km3[6]1300000 × Earth Mass 1.9891×1030 kg[1]333000 × Earth[1] Average density 1.408×103 kg/m3[1][6][7] Density Center (model): 1.622×105 kg/m3[1]Lower photosphere: 2×10−4 kg/m3Lower chromosphere: 5×10−6 kg/m3Corona (avg): 1×10−12 kg/m3[8] Equatorial surface gravity 274.0 m/s2[1]27.94 g27542.29 cgs28 × Earth[6] Escape velocity(from the surface) 617.7 km/s[6]55 × Earth[6] Temperature Center (modeled): ~1.57×107 K[1]Photosphere (effective): 5778 K[1] Corona: ~5×106 K Luminosity (Lsol) 3.846×1026 W[1]~3.75×1028 lm~98 lm/W efficacy Mean intensity (Isol) 2.009×107 W·m−2·sr−1 Age 4.57 billion years[9] Obliquity 7.25°[1](to the ecliptic)67.23°(to the galactic plane) Right ascensionof North pole[10] 286.13°19 h 4 min 30 s Declinationof North pole +63.87°63° 52' North Sidereal rotation period (at equator) 25.05 days[1] (at 16° latitude) 25.38 days[1]25 d 9 h 7 min 12 s[10] (at poles) 34.4 days[1] Rotation velocity(at equator) 7.189×103 km/h[6] Hydrogen 73.46%[11] 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%
For the newspaper, see The Sun (newspaper)
The Sun as it is seen from Earth.

The Sun is the star at the center of the Solar System. It is seen in the sky and gives light to the Earth. When the Sun is in the sky, it is day. When the Sun is not in the sky, it is night. The planets, including Earth, go around it.

The Sun gives off energy as electromagnetic radiation. That includes light, infra-red energy (heat), ultraviolet light and radio waves. It also gives off a stream of particles, which reaches Earth as "solar wind". The source of all this energy is the reaction in the star which turns hydrogen into helium and makes huge amounts of energy.

The Sun is a star like many others in our Milky Way galaxy. It has existed for a little over 4.5 billion years, and is going to continue for at least as long. The Sun is about a hundred times as wide as the Earth. It has a mass of 1.9891×1030 kg, which is 333,000 times the mass of the Earth. The Earth can also fit inside the Sun 1.3 million times!

In Astronomy

The sun is a massive ball of plasma in the middle of space.

Scientists think that the Sun started from a very large cloud of dust and small bits of ice 5 billion years ago. At the center of that huge cloud, some of the material started to build up into a ball called the Sun. Once this Sun got big enough, fusion reactions inside it caused that ball make heat and for it to shine.

The light that was made from fusion in the Sun pushed away all the rest of the cloud from itself, and the planets formed from the rest of this cloud.

How the Sun works

At its very center, hydrogen atoms collide together at great temperature and pressure so that they fuse to form atoms of helium. This process is called nuclear fusion. This fusion changes a very small part of the hydrogen atoms into a large amount of energy. This energy then travels from the core to the surface of the Sun. The Sun's surface is called the photosphere and is where it shines the energy into space. Energy can take thousands of years to reach the Sun's surface because the Sun is so huge and most of the way the energy is passed from atom to atom.

Features of the Sun

Since the Sun is all gas, surface features come and go. If the Sun is viewed through a special solar telescope, dark areas called sunspots can be seen. These areas are caused by the Sun's magnetic field. The sunspots only look dark because the rest of the Sun is very bright.

Some space telescopes, including the ones that orbit the Sun have seen huge arches of the Sun's matter extend suddenly from the Sun. These are called solar prominences. Solar prominences come in many different shapes and sizes. Some of them are so large that the Earth could fit inside of them, and a few are shaped like hands. Solar flares also come and go.

Sunspots, prominences and flares become rare, and then numerous, and then rare again, every 11 years.

Photosphere

This is the surface of the Sun. The light that the Earth receives from the Sun is radiated from this layer. Below this layer, the Sun is opaque, or not transparent to light.

Atmosphere of the Sun

Five layers make up the atmosphere of the Sun. The chromosphere, transition region, and corona are much hotter than the outer photosphere surface of the Sun.[12] It is believed that Alfvén waves may pass through to heat the corona.[13]

The minimum temperature zone, the coolest layer of the Sun, is about 500 km above the photosphere. It has a temperature of about 4100 K.[12] This part of the Sun is cool enough to allow simple molecules such as carbon monoxide and water to form. those molecules can be seen on the Sun with special instruments called spectroscopes.[14]

The chromosphere is the first layer of the Sun which can be seen, especially during a solar eclipse when the moon is covering most of the Sun and blocking the brightest light.

The solar transition region is the part of the Sun's atmosphere, between the chromosphere and outer part called the corona.[15] It can be seen from space using telescopes that can sense ultraviolet light. The transition is between two very different layers. In the bottom part it touches the photosphere and gravity shapes the features. At the top, the transition layer touches the corona.

The corona is the outer atmosphere of the Sun and is much bigger than the rest of the Sun. The corona continuously expands into space forming the solar wind, which fills all the Solar System.[16] The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K. In the hottest regions it is 8,000,000–20,000,000 K.[17] We do not understand why the corona is so hot.[16][17]

The heliosphere is the thin outer atmosphere of the Sun, filled with the solar wind plasma. It extends out past the orbit of Pluto to the heliopause, where it forms a boundary where it collides with the interstellar medium.[18]

Solar eclipses

See Eclipse A solar eclipse appears when the moon is between the Earth and sun.

The fate of the Sun

Astrophysicists say our Sun is a main-sequence star in the middle of its life. In about another 4 to 5 billion years, they think it will get bigger and become a red giant star. The Sun would be up to 250 times its current size, as big as 1.4 AU and will swallow up the earth.

Earth's fate is still a bit of a mystery. Calculations suggest that Earth could escape to a higher orbit. This due to the solar wind, which drops 30% of the Sun's mass, but a newer study shows that Earth would possibly vanish due to the tidal forces.[source?] This would happen while the Sun continues to get bigger. However, the Sun will lose mass.

Anyway, Earth's ocean and air would have long since worn out. This is even though the Sun is still in its main sequence stage. After the Sun reaches a point where it can no longer get bigger, it will figuratively explode with passion, but not like a supernova. Rather, it will expand rapidly and lose its layers, forming a planetary nebula. Eventually, the Sun will shrink into a white dwarf. Then, over several hundred billion or even a trillion years, the Sun would fade into a black dwarf.

• Lang, Kenneth R. (2001). The Cambridge Encyclopedia of the Sun. Cambridge University Press.
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References

1. Williams, D. R. (2004). "Sun Fact Sheet". NASA. Retrieved 2010-09-27.
2. Asplund, M.; N. Grevesse and A. J. Sauval (2006). "The new solar abundances - Part I: the observations". Communications in Asteroseismology 147: 76–79. .
3. "Eclipse 99: Frequently Asked Questions". NASA. Retrieved 2010-10-24.
4. Hinshaw, G.; et al. (2009). "Five-year Wilkinson Microwave Anisotropy Probe observations: data processing, sky maps, and basic results". The Astrophysical Journal Supplement Series 180 (2): 225–245. .
5. Emilio, Marcelo; Kuhn, Jeff R.; Bush, Rock I.; Scholl, Isabelle F. (March 5, 2012), "Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits", arXiv, retrieved March 28, 2012
6. Ko, M. (1999). "Density of the Sun". In Elert, G.. The Physics Factbook.
7. "Principles of Spectroscopy". University of Michigan, Astronomy Department. 30 August 2007.
8. Bonanno, A.; Schlattl, H.; Paternò, L. (2008). "The age of the Sun and the relativistic corrections in the EOS". Astronomy and Astrophysics 390 (3): 1115–1118. .
9. Seidelmann, P. K.; et al. (2000). "Report Of The IAU/IAG Working Group On Cartographic Coordinates And Rotational Elements Of The Planets And Satellites: 2000". Retrieved 2006-03-22.
10. "The Sun's Vital Statistics". Stanford Solar Center. Retrieved 2008-07-29., citing Eddy, J. (1979). A New Sun: The Solar Results From Skylab. NASA. p. 37. NASA SP-402.
11. Abhyankar K.D. (1977). "A survey of the solar atmospheric models". Bull. Astr. Soc. India 5: 40–44.
12. De Pontieu B. et al (2007). "Chromospheric Alfvénic waves strong enough to power the solar wind". Science 318 (5856): 1574–77. . .
13. Solanki S.K; Livingston W. & Ayres T (1994). "New light on the heart of darkness of the solar chromosphere". Science 263 (5143): 64–66. . .
14. "The Transition Region". Solar Physics, NASA Marshall Space Flight Center. NASA.
15. Russell, C.T. (2001). "Solar wind and interplanetary magnetic filed: A tutorial". In Song, Paul; Singer, Howard J. and Siscoe, George L. (PDF). Space weather (Geophysical Monograph). American Geophysical Union. pp. 73–88. .
16. Erdèlyi R. & Ballai I. 2007. Heating of the solar and stellar coronae: a review. Astron. Nachr. 328 (8): 726–733
17. European Space Agency (2005). "The distortion of the Heliosphere: our interstellar magnetic compass". Press release. Retrieved 2006-03-22.