The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally referred to as a yellow dwarf. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today.
Some Facts About Sun
1. Minimum distance from the earth (Perihelion):147 million km (January 3)
2. Maximum distance from the earth (Aphelion): 152.1 million km (July 4)
3. Average distance from the earth: 149.8 million km
4. Diameter: 13,92,000 km
5. Volume: 1.3 million times that of the earth
6. Mass: 3,32,000 times that of the earth.
7. Gravity at the surface: 28 times that of the earth
8. Density at the centre: 100 gm/cm3
9. Temperature of the photosphere: 6000°C
10. Temperature at the centre: 15 million °C
11. Temperature of the Sun-spots: 1500°C
12. Energy dissipation: 1026 joules/second
13. Pressure at the centre: 109 atm.
14. Rotational time period: 25.38 days (relative to the equator); 33 days (relative to the poles).
15. Composition: Hydrogen (71%), Helium (26.5%) and others (2.5%)
16. Age: 5 billion years (approx)
17. Estimated life of normal stars: 10 billion years (approx)
18. Time taken by the light to reach the earth: 8 min 18 sec
