The Sun Is the Star at the Center of the Solar System. It Is Almost Perfectly
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The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven withmagnetic 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 Sun's 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 4.567 billion[a][16] 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 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.[17] 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 about 620 million metric tons of hydrogen each second.[18][19] 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, most of which are red dwarfs.[20][21] The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is by far the brightest object in the sky with an apparent magnitude of −26.74.[22][23] This is about 13 billion times brighter than the next brightest star, Sirius, with an apparent magnitude of −1.46. The Sun's hot coronacontinuously 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.[24][25] 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.[26][27] 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.[28] The Sun orbits the center of the Milky Way at a distance of approximately 24000–26000 light-years from the galactic center, completingone 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.[29] The mean distance of the Sun from the Earth is approximately 1 astronomical unit (about 150,000,000 km; 93,000,000 mi), though the distance varies as the Earth moves from perihelion in January to aphelion in July.[30] At this average distance, lighttravels 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,[31] 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. Contents [hide] 1 Name and etymology 2 Characteristics o 2.1 Core o 2.2 Radiative zone o 2.3 Convective zone o 2.4 Photosphere o 2.5 Atmosphere o 2.6 Magnetic field 3 Chemical composition o 3.1 Singly ionized iron group elements o 3.2 Solar and planetary mass fractionation relationship 4 Solar cycles o 4.1 Sunspots and the sunspot cycle o 4.2 Possible long-term cycle 5 Life phases o 5.1 Formation o 5.2 Main sequence o 5.3 After core hydrogen exhaustion o 5.4 Earth's fate 6 Sunlight 7 Motion and location 8 Theoretical problems o 8.1 Coronal heating problem o 8.2 Faint young Sun problem 9 History of observation o 9.1 Early understanding o 9.2 Development of scientific understanding o 9.3 Solar space missions 10 Observation and effects 11 See also 12 Notes 13 References 14 Further reading 15 External links Name and etymology[edit] The English proper noun Sun developed from Old English sunne (in around 725, attested in Beowulf), and may be related tosouth. 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 *sunnōn.[32][33] In relation, the Sun is personified as a goddess in Germanic paganism, Sól/Sunna.[33] Scholars theorize that the Sun, as a Germanic goddess, may represent an extension of an earlier Proto-Indo- European sun deity due to Indo-European linguisticconnections between Old Norse Sól, Sanskrit Surya, Gaulish Sulis, Lithuanian Saulė, and Slavic Solntse.[33] 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).[34] The Latin name for the star, Sol, is widely known but is not common in general English language use; the adjectival form is the related wordsolar.[35][36] The term sol is also used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars.[37] A mean Earth solar day is approximately 24 hours, while a mean Martian 'sol' is 24 hours, 39 minutes, and 35.244 seconds.[38] Characteristics[edit] This video takes Solar Dynamics Observatoryimages and applies additional processing to enhance the structures visible. The events in this video represent 24 hours of activity on September 25, 2011. 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 anoblateness estimated at about 9 millionths,[39] which means that its polar diameter differs from its equatorial diameter by only 10 kilometres (6.2 mi).[40] 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-clockwiseangular 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.[41] 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.[42] The Sun is a Population I, or heavy-element-rich,[c] star.[43] The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae.[44] 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 endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star.[43] The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops exponentially with increasing distance from its center.[45]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. The photosphere is the last visible layer as those above it are too cool or too thin to radiate sufficient light to be visible to the naked eye[46] in the presence of the brilliant light from the photosphere.