The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields.[12][13] It has a diameter of about 1,392,684 km (865,374 mi),[5] around 109 times that of Earth, and its mass (1.989×1030 kilograms, approximately 330,000 times the mass of Earth) accounts for about 99.86% of the total mass of the Solar System.[14] Chemically, about three quarters of the Suns mass consists of hydrogen, while the rest is mostly helium. The remainder (1.69%, which nonetheless equals 5,600 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron, among others.[15]The Sun formed about 4.6 billion[a] years ago from the gravitational collapse of a region within a large molecular cloud. Most of the matter gathered in the center, while the rest flattened into an orbiting disk that would become the Solar System. The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its core. It is thought that almost all stars form by this process. The Sun is classified as a G-type main-sequence star (G2V) based on spectral class and it is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum, and although it is actually white in color, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.[16] In the spectral class label, G2 indicates its surface temperature, of approximately 5778 K (5505 °C), and V indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen each second.Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs.[17][18] The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is the brightest object in the sky with an apparent magnitude of −26.74.[19][20] The Suns hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.[21][22]The Sun is currently traveling through the Local Interstellar Cloud (near to the G-cloud) in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way galaxy.[23][24] Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light-years away), the Sun ranks fourth in mass.[25] The Sun orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light-years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years. Since the Milky Way is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km/s, the Suns resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.[26]The mean distance of the Sun from the Earth is approximately 1 astronomical unit (150,000,000 km; 93,000,000 mi), though the distance varies as the Earth moves from perihelion in January to aphelion in July.[27] At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life[b] on Earth by photosynthesis,[28] and drives Earths climate and weather. 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. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Suns physical composition and source of energy. This understanding is still developing; there are a number of present day anomalies in the Suns behavior that remain unexplained.4] A mean Earth solar day is approximately 24 hours, while a mean Martian sol is 24 hours, 39 minutes, and 35.244 seconds.[35] CharacteristicsThe Sun is a G-type main-sequence star comprising about 99.86% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths,[36] which means that its polar diameter differs from its equatorial diameter by only 10 kilometres (6.2 mi).[37] Since the Sun consists of a plasma and is not solid, it rotates faster at its equator than at its poles. This behavior is known as differential rotation and is caused by convection in the Sun and the movement of mass, due to steep temperature gradients from the core outwards. This mass carries a portion of the Sun’s counter-clockwise angular momentum (as viewed from the ecliptic north pole), thus redistributing the angular velocity. The period of this actual rotation is approximately 25.6 days at the equator and 33.5 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days.[38] The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Suns equator. The tidal effect of the planets is even weaker and does not significantly affect the shape of the Sun.[39]The core of the Sun is considered to extend from the center to about 20–25% of the solar radius.[45] It has a density of up to 150 g/cm3[46][47] (about 150 times the density of water) and a temperature of close to 15.7 million kelvin (K).[47] By contrast, the Suns surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone.[45] Through most of the Suns life, energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium.[48] Only 0.8% of the energy generated in the Sun comes from the CNO cycle.[49]The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Suns radius, and by 30% of the radius, fusion has stopped nearly entirely. The rest of the star is heated by energy that is transferred outward by radiation from the core to the convective layers just outside. The energy produced by fusion in the core must then travel through many successive layers to the solar photosphere before it escapes into space as sunlight or the kinetic energy of particles.[50][51] the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second, 384.6 yotta watts (3.846×1026 W),[1] or 9.192×1010 megatons of TNT per second.The power production by fusion in the core varies with distance from the solar center. At the center of the Sun, theoretical models estimate it to be approximately 276.5 watts/m3 Therefore it takes a long time for radiation to reach the Suns surface. Estimates of the photon travel time range between 10,000 and 170,000 years, it takes only 2.3 seconds for the neutrinos, which account for about 2% of the total energy production of the Sun, to reach the surface. Since energy transport in the Sun is a process which involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly to be changed.[57]and the blue image of sun that every body seenImage taken by NASA STEREO probes launched in 2006; utilizing two spacecraft to image the Sun at the extreme UV wavelength (171 Å).The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light.[62] Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H− ions, which absorb visible light easily.[62] Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H− ions.[63][64] The photosphere is tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening.[62] The spectrum of sunlight has approximately the spectrum of a black-body radiating at about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of ~1023 m−3 (about 0.37% of the particle number per volume of the Earths atmosphere at sea level). The photosphere is not fully ionized—the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form.[65]...............thankes poula best in science ,,.......poula medhat ,
Posted on: Thu, 28 Nov 2013 09:00:16 +0000
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