Sun

== Sun== 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<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">. <sup class="reference" id="cite_ref-12" style="color:rgb(0,0,0);font-family:sans-serif;line-height:1em;">[12] <sup class="reference" id="cite_ref-13" style="color:rgb(0,0,0);font-family:sans-serif;line-height:1em;">[13] <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">It has a <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  diameter<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">of about 1,392,684 km, <sup class="reference" id="cite_ref-arxiv1203_4898_5-2" style="color:rgb(0,0,0);font-family:sans-serif;line-height:1em;">[5] <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">about 109 times that of <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  Earth<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">, and its mass (about 2 <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;margin-right:0.15em;margin-left:0.25em;">× <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">10 <sup style="color:rgb(0,0,0);font-family:sans-serif;line-height:1em;">30 <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;"> kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. <sup class="reference" id="cite_ref-Woolfson00_14-0" style="color:rgb(0,0,0);font-family:sans-serif;line-height:1em;">[14] <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">Chemically, about three quarters of the Sun's mass consists of <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  hydrogen<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">, while the rest is mostly <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  helium<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">. The remainder (1.69%, which nonetheless equals 5,628 times the mass of Earth) consists of heavier elements, including <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  oxygen<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">, <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  carbon<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">, neon<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">and <span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">  iron<span style="color:rgb(0,0,0);font-family:sans-serif;line-height:19.1875px;">, among others. <sup class="reference" id="cite_ref-basu2008_15-0" style="color:rgb(0,0,0);font-family:sans-serif;line-height:1em;">[15]

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<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;color:rgb(0,0,0);font-family:sans-serif;">The Sun formed about 4.6 billion 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 other stars form by this process. The Sun's stellar classification, based on spectral class, is G2V, and is informally designated as a yellow dwarf, because its visible radiation is most intense in the yellow-green portion of thespectrum and although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light.<sup class="reference" id="cite_ref-16" style="line-height:1em;">[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.

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;color:rgb(0,0,0);font-family:sans-serif;">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.<sup class="reference" id="cite_ref-17" style="line-height:1em;">[17] <sup class="reference" id="cite_ref-18" style="line-height:1em;">[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.<sup class="reference" id="cite_ref-19" style="line-height:1em;">[19] <sup class="reference" id="cite_ref-20" style="line-height:1em;">[20]  The Sun's 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.<sup class="reference" id="cite_ref-21" style="line-height:1em;">[21] <sup class="reference" id="cite_ref-22" style="line-height:1em;">[22]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;color:rgb(0,0,0);font-family:sans-serif;">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.<sup class="reference" id="cite_ref-23" style="line-height:1em;">[23] <sup class="reference" id="cite_ref-24" style="line-height:1em;">[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.<sup class="reference" id="cite_ref-25" style="line-height:1em;">[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 Sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.<sup class="reference" id="cite_ref-26" style="line-height:1em;">[26]

<p style="margin-top:0.4em;margin-bottom:0.5em;line-height:19.1875px;color:rgb(0,0,0);font-family:sans-serif;">The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves fromperihelion in January to aphelion in July.<sup class="reference" id="cite_ref-USNO_27-0" style="line-height:1em;">[27]  At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energyof this sunlight supports almost all life on Earth by photosynthesis,<sup class="reference" id="cite_ref-Simon2001_28-0" style="line-height:1em;">[28]  and drives Earth's 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 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