The Sun (in Greek: ''Helios'', in Latin: ''Sol'') is the star at the center of the Solar System and is by far the most important source of energy for life on Earth. It is a nearly perfect spherical ball of hot plasma,〔
〕 with internal convective motion that generates a magnetic field via a dynamo process. Its diameter is about 109 times that of Earth, and it has a mass about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System.〔
About three quarters of the Sun's mass consists of hydrogen; the rest is mostly helium, with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.〔
The Sun is a G-type main-sequence star (G2V) based on spectral class and it is informally referred to as a yellow dwarf. It formed approximately 4.567 billion〔All numbers in this article are short scale. One billion is 109, or 1,000,000,000.〕 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 increasingly hot and dense, eventually initiating nuclear fusion in its core. It is thought that almost all stars form by this process.
The Sun is roughly middle aged and has not changed dramatically for four billion〔 years, and will remain fairly stable for another four billion years. However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes and become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury, Venus, and possibly Earth.
The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. Earth's movement around the Sun is the basis of the solar calendar, which is the predominant calendar in use today.
==Name and etymology==
The English proper noun ''Sun'' developed from Old English ''sunne'' and may be related to ''south''. Cognates to English ''sun'' appear in other Germanic languages, including Old Frisian ''sunne'', ''sonne'', Old Saxon ''sunna'', Middle Dutch ''sonne'', modern Dutch ''zon'', Old High German ''sunna'', modern German ''Sonne'', Old Norse ''sunna'', and Gothic ''sunnō''. All Germanic terms for the Sun stem from Proto-Germanic
The English weekday name ''Sunday'' stems from Old English (''Sunnandæg''; "Sun's day", from before 700) and is ultimately a result of a Germanic interpretation of Latin ''dies solis'', itself a translation of the Greek ἡμέρα ἡλίου (''hēméra hēlíou'').〔
〕 The Latin name for the Sun, ''Sol'', is widely known but is not common in general English language use; the adjectival form is the related word ''solar''.〔
(【引用サイトリンク】url=http://www.merriam-webster.com/dictionary/Sol )〕 The term ''sol'' is also used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars.〔
〕 A mean Earth solar day is approximately 24 hours, whereas a mean Martian 'sol' is 24 hours, 39 minutes, and 35.244 seconds.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』