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WWW.GOODNEWS.WS The Sun (5 of 6)

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SUPERGOODNEWS
November 01, 2009

http://goodnews.ws/ The Sun is the star at the center of the Solar System. The Sun, by itself accounts for about 99.86% of the Solar System's mass; the remainder consists of the planets (including...

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The Sun is the star at the center of the Solar System. The Sun, by itself accounts for about 99.86% of the Solar System's mass; the remainder consists of the planets (including Earth), asteroids, meteoroids, comets, and dust in orbit. About three-fourths of the Sun's mass consists of hydrogen, most of the rest is helium. Less than 2% consists of other elements, including iron, oxygen, carbon, neon, and others. The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering. It has a spectral class of G2V, informally designated a "yellow star" because the majority of its radiation is in the yellow-green portion of the visible spectrum. The G2 indicates its surface temperature of approximately 5,780 K (5,510 °C.) The V (Roman five) in the spectral class 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. Once regarded as a small and relatively insignificant star, the Sun is now presumed to be brighter than 85% of the stars in the galaxy, most of which are red dwarfs. (Estimates for its magnitude are around 4.8) The Sun's hot corona continuously expands in space creating the solar wind, a hypersonic stream of charged particles that extends to the heliopause at roughly 100 AU. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System. The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way Galaxy. Of the 50 nearest stellar systems within 17 light-years from the Earth, the Sun ranks 4th [20] in mass. The Sun orbits the center of the Milky Way galaxy at a distance of approximately 24,00026,000 light years from the galactic center, completing one revolution in about 225250 million years. The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though this varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the sun to the earth in about 8 minutes 19 seconds. The energy from this sunlight supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather. The enormous impact of the Sun on the Earth has been recognized since pre-historic times, and at times 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 notion of the sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the sun's behavior that remain unexplained. The 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, which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behavior is known as differential rotation. 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. The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun. The Sun is a Population I, or heavy element-rich, star. The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star. The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops approximately exponentially with increasing distance from its center.Nevertheless, it has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye.

WWW.GOODNEWS.WS The Sun (5 of 6)
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