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Star at the center of the Solar System From Wikipedia, the free encyclopedia
The Sun is the star at the center of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is by far the most important source of energy for life on Earth. The Sun has been an object of veneration in many cultures. It has been a central subject for astronomical research since antiquity.
The Sun, viewed through a clear solar filter | |
| Names | Sun, Sol,[1] Sól, Helios[2] |
|---|---|
| Adjectives | Solar[3] |
| Symbol | .svg){kind=small} |
| Observation data | |
| Mean distance from Earth | 1 AU 149,600,000 km 8 min 19 s, light speed[4] |
| −26.74 (V)[5] | |
| 4.83[5] | |
| G2V[6] | |
| Metallicity | Z = 0.0122[7] |
| Angular size | 0.527–0.545°[8] |
| Orbital characteristics | |
Mean distance from Milky Way core | 24,000 to 28,000 light-years[9] |
| Galactic period | 225–250 million years |
| Velocity |
|
| Obliquity |
|
Right ascension North pole | 286.13° (286° 7′ 48″)[5] |
Declination of North pole | +63.87° (63° 52′ 12"N)[5] |
Sidereal rotation period |
|
Equatorial rotation velocity | 1.997 km/s[11] |
| Physical characteristics | |
Equatorial radius | 6.957 x 108 m[12] 109 × Earth radii[11] |
| Flattening | 0.00005[5] |
| Surface area | 6.09×1012 km2 12,000 × Earth[11] |
| Volume |
|
| Mass | |
| Average density | 1.408 g/cm3 0.255 × Earth[5][11] |
| Age | 4.6 billion years[13][14] |
Equatorial surface gravity | 274 m/s2[5] 27.9 g0[11] |
| ≈0.070[5] | |
Surface escape velocity | 617.7 km/s 55 × Earth[11] |
| Temperature |
|
| Luminosity | |
| Color (B-V) | 0.656[15] |
| Mean radiance | 2.009×107 W·m−2·sr−1 |
Photosphere composition by mass | |
The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years. From Earth, it is 1 astronomical unit (1.496×108 km) or about 8 light-minutes away. Its diameter is about 1,391,400 km (864,600 mi), 109 times that of Earth. Its mass is about 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.
The Sun is a G-type main-sequence star (G2V), informally called a yellow dwarf, though its light is actually white. It formed approximately 4.6 billion[lower-alpha 1] years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. Every second, the Sun's core fuses about 600 billion kilograms (kg) of hydrogen into helium and converts 4 billion kg of matter into energy.
About 4 to 7 billion years from now, when hydrogen fusion in the Sun's core diminishes to the point where the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature which will cause its outer layers to expand, eventually transforming the Sun into a red giant. This process will make the Sun large enough to render Earth uninhabitable approximately five billion years from the present. After the red giant phase, models suggest the Sun will shed its outer layers and become a dense type of cooling star (a white dwarf), and no longer produce energy by fusion, but will still glow and give off heat from its previous fusion for perhaps trillions of years. After that, it is theorized to become a super dense black dwarf, giving off negligible energy.
The English word sun developed from Old English sunne. Cognates appear in other Germanic languages, including West Frisian sinne, Dutch zon, Low German Sünn, Standard German Sonne, Bavarian Sunna, Old Norse sunna, and Gothic sunnō. All these words stem from Proto-Germanic *sunnōn.[17][18] This is ultimately related to the word for sun in other branches of the Indo-European language family, though in most cases a nominative stem with an l is found, rather than the genitive stem in n, as for example in Latin sōl, ancient Greek ἥλιος (hēlios), Welsh haul and Czech slunce, as well as (with *l > r) Sanskrit स्वर् (svár) and Persian خور (xvar). Indeed, the l-stem survived in Proto-Germanic as well, as *sōwelan, which gave rise to Gothic sauil (alongside sunnō) and Old Norse prosaic sól (alongside poetic sunna), and through it the words for sun in the modern Scandinavian languages: Swedish and Danish sol, Icelandic sól, etc.[18]
The principal adjectives for the Sun in English are sunny for sunlight and, in technical contexts, solar (/ˈsoʊlər/),[3] from Latin sol[19]—the latter found in terms such as solar day, solar eclipse and Solar System. From the Greek helios comes the rare adjective heliac (/ˈhiːliæk/).[20] In English, the Greek and Latin words occur in poetry as personifications of the Sun, Helios (/ˈhiːliəs/) and Sol (/ˈsɒl/),[2][1] while in science fiction Sol may be used to distinguish the Sun from other stars. The term sol with a lower-case s is used by planetary astronomers for the duration of a solar day on another planet such as Mars.[21]
The English weekday name Sunday stems from Old English Sunnandæg "sun's day", a Germanic interpretation of the Latin phrase diēs sōlis, itself a translation of the ancient Greek ἡμέρα ἡλίου (hēmera hēliou) 'day of the sun'.[22] The astronomical symbol for the Sun is a circle with a center dot, .svg){kind=small}
.[23] It is used for such units as M☉ (Solar mass), R☉ (Solar radius) and L☉ (Solar luminosity).[24][25]
The Sun is a G-type main-sequence star that makes up about 99.86% of the mass of the Solar System.[26] It has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs.[27][28] It is more massive than 95% of nearby stars within 7 pc. (~23lt yrs)[29] The Sun is a Population I, or heavy-element-rich,[lower-alpha 2] star.[30] Its formation approximately 4.6 billion years ago may have been triggered by shockwaves from one or more nearby supernovae.[31][32] 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. The heavy 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.[30]
The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74.[33][34] This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46.[35]
One astronomical unit (about 150 million kilometres; 93 million miles) is defined as the mean distance between the centres of the Sun and the Earth. The instantaneous distance varies by about ± 2.5 million km or 1.55 million miles as Earth moves from perihelion on ~ January 3rd to aphelion on ~ July 4th.[36] At its average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds,[37] while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports almost all life[lower-alpha 3] on Earth by photosynthesis,[38] and drives Earth's climate and weather.[39]
The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere.[40] For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun.[41] By this measure, the Sun is a near-perfect sphere with an oblateness estimated at 9 millionths,[42][43][44] which means that its polar diameter differs from its equatorial diameter by only 10 kilometers (6.2 mi).[45] The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.[46]
The Sun rotates faster at its equator than at its poles. This differential rotation is caused by convective motion due to heat transport and the Coriolis force due to the Sun's rotation. In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at the poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days.[47] Viewed from a vantage point above its north pole, the Sun rotates counterclockwise around its axis of spin.[lower-alpha 4][48]
A survey of solar analogs suggest the early Sun was rotating up to ten times faster than it does today. This would have made the surface much more active, with greater X-ray and UV emission. Sun spots would have covered 5–30% of the surface.[49] The rotation rate was gradually slowed by magnetic braking, as the Sun's magnetic field interacted with the outflowing solar wind.[50] A vestige of this rapid primordial rotation still survives at the Sun's core, which has been found to be rotating at a rate of once per week; four times the mean surface rotation rate.[51][52]
The Sun consists mainly of the elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere.[53] All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant.[54]
The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.[53] The hydrogen and most of the helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements were produced by previous generations of stars before the Sun was formed, and spread into the interstellar medium during the final stages of stellar life and by events such as supernovae.[55]
Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over the past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the center of the Sun because of gravity. The proportions of heavier elements are unchanged. Heat is transferred outward from the Sun's core by radiation rather than by convection (see Radiative zone below), so the fusion products are not lifted outward by heat; they remain in the core,[56] and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium. In the current photosphere, the helium fraction is reduced, and the metallicity is only 84% of what it was in the protostellar phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the main sequence and become a red giant.[57]
The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.[58] Typically, the solar heavy-element abundances described above are measured both by using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by the settling of heavy elements. The two methods generally agree well.[59]
The core of the Sun extends from the center to about 20–25% of the solar radius.[60] It has a density of up to 150 g/cm3[61][62] (about 150 times the density of water) and a temperature of close to 15.7 million kelvin (K).[62] By contrast, the Sun's surface temperature is about 5800 K. Recent analysis of SOHO mission data favors the idea that the core is rotating faster than the radiative zone outside it.[60] Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through the proton–proton chain; this process converts hydrogen into helium.[63] Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous.[64][65]
The core is the only region of the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).[66][67]
The proton–proton chain occurs around 9.2×1037 times each second in the core, converting about 3.7×1038 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9×1056 free protons in the Sun), or about 6.2×1011 kg/s. However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain.[66] Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,[68] so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen[69]), for 384.6 yottawatts (3.846×1026 W),[5] or 9.192×1010 megatons of TNT per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per cubic metre. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core,[70] which, according to Karl Kruszelnicki, is about the same power density inside a compost pile.[71]
The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.[72][73]
The radiative zone is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer.[74] The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.[62] This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why the transfer of energy through this zone is by radiation instead of thermal convection.[62] Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.[74] The density drops a hundredfold (from 20 000 kg/m3 to 200 kg/m3) between 0.25 solar radii and 0.7 radii, the top of the radiative zone.[74]
The radiative zone and the convective zone are separated by a transition layer, the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear between the two—a condition where successive horizontal layers slide past one another.[75] Presently, it is hypothesized that a magnetic dynamo, or solar dynamo, within this layer generates the Sun's magnetic field.[62]
The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface. In this layer, the solar plasma is not dense or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry most of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped 350-fold to 5,700 K (9,800 °F) and the density to only 0.2 g/m3 (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone).[62]
The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun.[62] The Sun's thermal columns are Bénard cells and take the shape of roughly hexagonal prisms.[76]
_as_of_January_20,_2020.jpg)
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light.[77] Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of H− ions, which absorb visible light easily.[77] Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H− ions.[78][79]
The photosphere is tens to hundreds of kilometers thick, and is 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.[77] The spectrum of sunlight has approximately the spectrum of a black-body radiating at 5,772 K (9,930 °F),[12] 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 Earth's 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.[80]
The Sun's atmosphere is composed of five layers: the photosphere, the chromosphere, the transition region, the corona, and the heliosphere.
The coolest layer of the Sun is a temperature minimum region extending to about 500 km above the photosphere, and has a temperature of about 4,100 K.[77] This part of the Sun is cool enough to allow for the existence of simple molecules such as carbon monoxide and water, which can be detected via their absorption spectra.[81] The chromosphere, transition region, and corona are much hotter than the surface of the Sun.[77] The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona.[82]
Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines.