حوراء الحصن
14-08-2009, 11:58 PM
The Sun is the star (http://en.wikipedia.org/wiki/Star) at the center of the Solar System (http://en.wikipedia.org/wiki/Solar_System). The Earth (http://en.wikipedia.org/wiki/Earth) and other matter (including other planets (http://en.wikipedia.org/wiki/Planet), asteroids (http://en.wikipedia.org/wiki/Asteroid), meteoroids (http://en.wikipedia.org/wiki/Meteoroid), comets (http://en.wikipedia.org/wiki/Comet), and dust (http://en.wikipedia.org/wiki/Cosmic_dust)) orbit (http://en.wikipedia.org/wiki/Orbit) the Sun, which by itself accounts for about 99.86% of the Solar System's mass (http://en.wikipedia.org/wiki/Mass).[ (http://en.wikipedia.org/wiki/Sun#cite_note-8)The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU (http://en.wikipedia.org/wiki/Astronomical_unit)), and its light (http://en.wikipedia.org/wiki/Light) travels this distance in 8 minutes and 19 seconds. This distance varies throughout the year (http://en.wikipedia.org/wiki/Year) from a minimum of 147.1 million kilometers (0.9833 AU (http://en.wikipedia.org/wiki/Astronomical_unit)) on the perihelion (http://en.wikipedia.org/wiki/Perihelion) (around 3 January (http://en.wikipedia.org/wiki/January_3)), to a maximum of 152.1 million kilometers (1.017 AU (http://en.wikipedia.org/wiki/Astronomical_unit)) on the aphelion (http://en.wikipedia.org/wiki/Aphelion) (around 4 July (http://en.wikipedia.org/wiki/July_4)).[] (http://en.wikipedia.org/wiki/Sun#cite_note-USNO-9) Energy (http://en.wikipedia.org/wiki/Energy) from the Sun, in the form of sunlight (http://en.wikipedia.org/wiki/Sunlight), supports almost all life on Earth via photosynthesis (http://en.wikipedia.org/wiki/Photosynthesis),[] (http://en.wikipedia.org/wiki/Sun#cite_note-Simon2001-10) and drives the Earth's climate (http://en.wikipedia.org/wiki/Climate) and weather (http://en.wikipedia.org/wiki/Weather). The Sun consists of hydrogen (http://en.wikipedia.org/wiki/Hydrogen) (about 74% of its mass, or 92% of its volume), helium (http://en.wikipedia.org/wiki/Helium) (about 24% of mass, 7% of volume), and trace quantities of other elements, including iron (http://en.wikipedia.org/wiki/Iron), nickel (http://en.wikipedia.org/wiki/Nickel), oxygen (http://en.wikipedia.org/wiki/Oxygen), silicon (http://en.wikipedia.org/wiki/Silicon), sulfur (http://en.wikipedia.org/wiki/Sulfur), magnesium (http://en.wikipedia.org/wiki/Magnesium), carbon (http://en.wikipedia.org/wiki/Carbon), neon (http://en.wikipedia.org/wiki/Neon), calcium (http://en.wikipedia.org/wiki/Calcium), and chromium (http://en.wikipedia.org/wiki/Chromium).[] (http://en.wikipedia.org/wiki/Sun#cite_note-basu2008-11)
The Sun has a spectral class (http://en.wikipedia.org/wiki/Stellar_classification) of G2V. G2 means that it has a surface temperature of approximately 5,780 K (5,510 °C) giving it a white color, which often appears as yellow when seen from the surface of the Earth because of atmospheric scattering (http://en.wikipedia.org/wiki/Scattering). It is this scattering of light at the blue end of the spectrum (http://en.wikipedia.org/wiki/Spectrum) that gives the surrounding sky its color.[] (http://en.wikipedia.org/wiki/Sun#cite_note-12) The Sun's spectrum contains lines (http://en.wikipedia.org/wiki/Spectral_line) of ionized and neutral ****ls as well as very weak hydrogen lines. The V (Roman five (http://en.wikipedia.org/wiki/Roman_numerals)) in the spectral class indicates that the Sun, like most stars, is a main sequence (http://en.wikipedia.org/wiki/Main_sequence) star. This means that it generates its energy by nuclear fusion (http://en.wikipedia.org/wiki/Nuclear_fusion) of hydrogen nuclei (http://en.wikipedia.org/wiki/Atomic_nuclei) into helium. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now believed to be brighter than 85% of the stars in the galaxy (http://en.wikipedia.org/wiki/Milky_Way), most of which are red dwarfs (http://en.wikipedia.org/wiki/Red_dwarf).] (http://en.wikipedia.org/wiki/Sun#cite_note-14)
The Sun's hot corona (http://en.wikipedia.org/wiki/Corona) continuously expands in space creating the solar wind (http://en.wikipedia.org/wiki/Solar_wind), a hypersonic stream of charged particles that extends to the heliopause (http://en.wikipedia.org/wiki/Heliopause) at roughly 100 AU. The bubble in the interstellar medium (http://en.wikipedia.org/wiki/Interstellar_medium) formed by the solar wind, the heliosphere (http://en.wikipedia.org/wiki/Heliosphere), is the largest continuous structure in the Solar System.[ (http://en.wikipedia.org/wiki/Sun#cite_note-15)] (http://en.wikipedia.org/wiki/Sun#cite_note-16)
The Sun is currently traveling through the Local Interstellar Cloud (http://en.wikipedia.org/wiki/Local_Interstellar_Cloud) in the low-density Local Bubble (http://en.wikipedia.org/wiki/Local_Bubble) zone of diffuse high-temperature gas, in the inner rim of the Orion Arm (http://en.wikipedia.org/wiki/Orion_Arm) of the Milky Way Galaxy, between the larger Perseus (http://en.wikipedia.org/wiki/Perseus_arm) and Sagittarius arms (http://en.wikipedia.org/wiki/Sagittarius_arm) of the galaxy. Of the 50 nearest stellar systems (http://en.wikipedia.org/wiki/Nearest_stars) within 17 light-years (1.6×1014 km) from the Earth, the Sun ranks 4th[] (http://en.wikipedia.org/wiki/Sun#cite_note-17) in mass as a fourth magnitude star (M = +4.83).[ (http://en.wikipedia.org/wiki/Sun#cite_note-nssdc-0)[ (http://en.wikipedia.org/wiki/Sun#cite_note-18)although slightly different values for the magnitude (http://en.wikipedia.org/wiki/Absolute_magnitude) have been published, for example 4.85[ (http://en.wikipedia.org/wiki/Sun#cite_note-19) and 4.81 The Sun orbits the center of the Milky Way (http://en.wikipedia.org/wiki/Milky_Way)galaxy (http://en.wikipedia.org/wiki/Galaxy) at a distance of approximately 24,000–26,000 light years (http://en.wikipedia.org/wiki/Light-year) from the galactic center (http://en.wikipedia.org/wiki/Galactic_center), moving generally in the direction of Cygnus (http://en.wikipedia.org/wiki/Cygnus_(constellation)) and completing one revolution in about 225–250 million years (one Galactic year (http://en.wikipedia.org/wiki/Galactic_year)). Its orbital speed (http://en.wikipedia.org/wiki/Orbital_speed) was thought to be 220 ± 20, km/s (http://en.wikipedia.org/wiki/Km/s) but a new estimate gives 251 km/s.[22] (http://en.wikipedia.org/wiki/Sun#cite_note-space.newscientist.com-21)[23] (http://en.wikipedia.org/wiki/Sun#cite_note-Kerr-22) Since our galaxy is moving with respect to the cosmic microwave background radiation (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation) (CMB) in the direction of Hydra (http://en.wikipedia.org/wiki/Hydra_(constellation)) with a speed of 550 km/s, the sun's resultant velocity with respect to the CMB is about 0 km/s in the direction of Crater (http://en.wikipedia.org/wiki/Crater_(constellation)) or Leo (http://en.wikipedia.org/wiki/Leo_(constellation))
The Sun has a spectral class (http://en.wikipedia.org/wiki/Stellar_classification) of G2V. G2 means that it has a surface temperature of approximately 5,780 K (5,510 °C) giving it a white color, which often appears as yellow when seen from the surface of the Earth because of atmospheric scattering (http://en.wikipedia.org/wiki/Scattering). It is this scattering of light at the blue end of the spectrum (http://en.wikipedia.org/wiki/Spectrum) that gives the surrounding sky its color.[] (http://en.wikipedia.org/wiki/Sun#cite_note-12) The Sun's spectrum contains lines (http://en.wikipedia.org/wiki/Spectral_line) of ionized and neutral ****ls as well as very weak hydrogen lines. The V (Roman five (http://en.wikipedia.org/wiki/Roman_numerals)) in the spectral class indicates that the Sun, like most stars, is a main sequence (http://en.wikipedia.org/wiki/Main_sequence) star. This means that it generates its energy by nuclear fusion (http://en.wikipedia.org/wiki/Nuclear_fusion) of hydrogen nuclei (http://en.wikipedia.org/wiki/Atomic_nuclei) into helium. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now believed to be brighter than 85% of the stars in the galaxy (http://en.wikipedia.org/wiki/Milky_Way), most of which are red dwarfs (http://en.wikipedia.org/wiki/Red_dwarf).] (http://en.wikipedia.org/wiki/Sun#cite_note-14)
The Sun's hot corona (http://en.wikipedia.org/wiki/Corona) continuously expands in space creating the solar wind (http://en.wikipedia.org/wiki/Solar_wind), a hypersonic stream of charged particles that extends to the heliopause (http://en.wikipedia.org/wiki/Heliopause) at roughly 100 AU. The bubble in the interstellar medium (http://en.wikipedia.org/wiki/Interstellar_medium) formed by the solar wind, the heliosphere (http://en.wikipedia.org/wiki/Heliosphere), is the largest continuous structure in the Solar System.[ (http://en.wikipedia.org/wiki/Sun#cite_note-15)] (http://en.wikipedia.org/wiki/Sun#cite_note-16)
The Sun is currently traveling through the Local Interstellar Cloud (http://en.wikipedia.org/wiki/Local_Interstellar_Cloud) in the low-density Local Bubble (http://en.wikipedia.org/wiki/Local_Bubble) zone of diffuse high-temperature gas, in the inner rim of the Orion Arm (http://en.wikipedia.org/wiki/Orion_Arm) of the Milky Way Galaxy, between the larger Perseus (http://en.wikipedia.org/wiki/Perseus_arm) and Sagittarius arms (http://en.wikipedia.org/wiki/Sagittarius_arm) of the galaxy. Of the 50 nearest stellar systems (http://en.wikipedia.org/wiki/Nearest_stars) within 17 light-years (1.6×1014 km) from the Earth, the Sun ranks 4th[] (http://en.wikipedia.org/wiki/Sun#cite_note-17) in mass as a fourth magnitude star (M = +4.83).[ (http://en.wikipedia.org/wiki/Sun#cite_note-nssdc-0)[ (http://en.wikipedia.org/wiki/Sun#cite_note-18)although slightly different values for the magnitude (http://en.wikipedia.org/wiki/Absolute_magnitude) have been published, for example 4.85[ (http://en.wikipedia.org/wiki/Sun#cite_note-19) and 4.81 The Sun orbits the center of the Milky Way (http://en.wikipedia.org/wiki/Milky_Way)galaxy (http://en.wikipedia.org/wiki/Galaxy) at a distance of approximately 24,000–26,000 light years (http://en.wikipedia.org/wiki/Light-year) from the galactic center (http://en.wikipedia.org/wiki/Galactic_center), moving generally in the direction of Cygnus (http://en.wikipedia.org/wiki/Cygnus_(constellation)) and completing one revolution in about 225–250 million years (one Galactic year (http://en.wikipedia.org/wiki/Galactic_year)). Its orbital speed (http://en.wikipedia.org/wiki/Orbital_speed) was thought to be 220 ± 20, km/s (http://en.wikipedia.org/wiki/Km/s) but a new estimate gives 251 km/s.[22] (http://en.wikipedia.org/wiki/Sun#cite_note-space.newscientist.com-21)[23] (http://en.wikipedia.org/wiki/Sun#cite_note-Kerr-22) Since our galaxy is moving with respect to the cosmic microwave background radiation (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation) (CMB) in the direction of Hydra (http://en.wikipedia.org/wiki/Hydra_(constellation)) with a speed of 550 km/s, the sun's resultant velocity with respect to the CMB is about 0 km/s in the direction of Crater (http://en.wikipedia.org/wiki/Crater_(constellation)) or Leo (http://en.wikipedia.org/wiki/Leo_(constellation))