Sun

The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma,[15][16] with internal convective motion that generates a magnetic field via a dynamo process.[17] It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System.[18] 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.[19]

The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion[a][11][20] 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. It is thought that almost all stars form by this process.

The Sun is roughly middle-aged; it has not changed dramatically for more than four billion[a] years, and will remain fairly stable for more than another five billion years. It currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to eventually become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.

The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.

Sun Sun symbol.svg
Sun white
Sun with sunspots and limb darkening as seen in visible light with solar filter.
The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819
False-color photo of the Sun as seen in ultraviolet light (wavelength of 30.4 nm)
Observation data
Mean distance
from Earth
1 au1.496×108 km[1]
8 min 19 s at light speed
Visual brightness (V)−26.74[2]
Absolute magnitude4.83[2]
Spectral classificationG2V[3]
MetallicityZ = 0.0122[4]
Angular size31.6–32.7 minutes of arc[5]
AdjectivesSolar
Orbital characteristics
Mean distance
from Milky Way core
≈ 2.7×1017 km
27,200 light-years
Galactic period(2.25–2.50)×108 yr
Velocity≈ 220 km/s (orbit around the center of the Milky Way)
≈ 20 km/s (relative to average velocity of other stars in stellar neighborhood)
≈ 370 km/s[6] (relative to the cosmic microwave background)
Physical characteristics
Equatorial radius695,700 km,[7]
696,342 km[8]
109 × Earth[9]
Equatorial circumference4.379×106 km[9]
109 × Earth[9]
Flattening9×10−6
Surface area6.09×1012 km2[9]
12,000 × Earth[9]
Volume1.41×1018 km3[9]
1,300,000 × Earth
Mass1.9885×1030 kg[2]
333,000 × Earth[2]
Average density1.408 g/cm3[2][9][10]
0.255 × Earth[2][9]
Center density (modeled)162.2 g/cm3[2]
12.4 × Earth
Equatorial surface gravity274 m/s2[2]
28 × Earth[9]
Moment of inertia factor0.070[2] (estimate)
Escape velocity
(from the surface)
617.7 km/s[9]
55 × Earth[9]
TemperatureCenter (modeled): 1.57×107 K[2]
Photosphere (effective): 5,772 K[2]
Corona: ≈ 5×106 K
Luminosity (Lsol)3.828×1026 W[2]
≈ 3.75×1028 lm
≈ 98 lm/W efficacy
Color (B-V)0.63
Mean radiance (Isol)2.009×107 W·m−2·sr−1
Age≈ 4.6 billion years[11][12]
Rotation characteristics
Obliquity7.25°[2]
(to the ecliptic)
67.23°
(to the galactic plane)
Right ascension
of North pole[13]
286.13°
19 h 4 min 30 s
Declination
of North pole
+63.87°
63° 52' North
Sidereal rotation period
(at equator)
25.05 d[2]
(at 16° latitude)25.38 d[2]
25 d 9 h 7 min 12 s[13]
(at poles)34.4 d[2]
Rotation velocity
(at equator)
7.189×103 km/h[9]
Photospheric composition (by mass)
Hydrogen73.46%[14]
Helium24.85%
Oxygen0.77%
Carbon0.29%
Iron0.16%
Neon0.12%
Nitrogen0.09%
Silicon0.07%
Magnesium0.05%
Sulfur0.04%

Name and etymology

The English proper name Sun developed from Old English sunne and may be related to south. 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.[21][22]

The Latin name for the Sun, Sol, is not commonly used in everyday English. Sol is also used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars.[23]

The related word solar is the usual adjectival term used for the Sun,[24][25] in terms such as solar day, solar eclipse, and Solar System.

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).[26]

General characteristics

The Sun is a G-type main-sequence star that comprises about 99.86% of the mass of the Solar System. The Sun 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] The Sun is a Population I, or heavy-element-rich,[b] star.[29] The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae.[30] 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.[29]

The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74.[31][32] This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is approximately 1 astronomical unit (about 150,000,000 km; 93,000,000 mi), though the distance varies as Earth moves from perihelion in January to aphelion in July.[33] At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, 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[c] on Earth by photosynthesis,[34] and drives Earth's climate and weather.

The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere.[35] For the purpose of measurement, however, 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.[36] By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths,[37] which means that its polar diameter differs from its equatorial diameter by only 10 kilometres (6.2 mi).[38] The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.[39] 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.[40]

Sunlight

The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m2 (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth).[41] Sunlight on the surface of Earth is attenuated by Earth's atmosphere, so that less power arrives at the surface (closer to 1,000 W/m2) in clear conditions when the Sun is near the zenith.[42] Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.[43] The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.[44] Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere.[45]

The Sun's color is white, with a CIE color-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. When measuring all the photons emitted, the Sun is actually emitting more photons in the green portion of the spectrum than any other.[46][47] When the Sun is low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta. Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate.[48] The Sun is a G2V star, with G2 indicating its surface temperature of approximately 5,778 K (5,505 °C, 9,941 °F), and V that it, like most stars, is a main-sequence star.[49][50] The average luminance of the Sun is about 1.88 giga candela per square metre, but as viewed through Earth's atmosphere, this is lowered to about 1.44 Gcd/m2.[d] However, the luminance is not constant across the disk of the Sun (limb darkening).

Composition

BLUE STEREO 3D Time for Space Wiggle
False-color wiggle animation of the Sun by STEREO

The Sun is composed primarily of the chemical elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively.[51] 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.[52]

The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have contained about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.[51] 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.[53]

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. Within the core, the proportion of helium has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere towards the center of the Sun because of gravity. The proportions of metals (heavier elements) is 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[54] 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.[55]

The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.[56] The solar heavy-element abundances described above are typically measured both 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 settling of heavy elements. The two methods generally agree well.[19]

Singly ionized iron-group elements

In the 1970s, much research focused on the abundances of iron-group elements in the Sun.[57][58] Although significant research was done, until 1978 it was difficult to determine the abundances of some iron-group elements (e.g. cobalt and manganese) via spectrography because of their hyperfine structures.[57]

The first largely complete set of oscillator strengths of singly ionized iron-group elements were made available in the 1960s,[59] and these were subsequently improved.[60] In 1978, the abundances of singly ionized elements of the iron group were derived.[57]

Isotopic composition

Various authors have considered the existence of a gradient in the isotopic compositions of solar and planetary noble gases,[61] e.g. correlations between isotopic compositions of neon and xenon in the Sun and on the planets.[62]

Prior to 1983, it was thought that the whole Sun has the same composition as the solar atmosphere.[63] In 1983, it was claimed that it was fractionation in the Sun itself that caused the isotopic-composition relationship between the planetary and solar-wind-implanted noble gases.[63]

Structure and fusion

Sun poster
The structure of the Sun

The structure of the Sun contains the following layers:

  • Core – the innermost 20-25% of the Sun's radius, where temperature (energies) and pressure are sufficient for nuclear fusion to occur. Hydrogen fuses into helium (which cannot currently be fused at this point in the Sun's life). The fusion process releases energy, and the helium gradually accumulates to form an inner core of helium within the core itself.
  • Radiative zoneConvection cannot occur until much nearer the surface of the Sun. Therefore, between about 20-25% of the radius, and 70% of the radius, there is a "radiative zone" in which energy transfer occurs by means of radiation (photons) rather than by convection.
  • Tachocline – the boundary region between the radiative and convective zones.
  • Convective zone – Between about 70% of the Sun's radius and a point close to the visible surface, the Sun is cool and diffuse enough for convection to occur, and this becomes the primary means of outward heat transfer, similar to weather cells which form in the earth's atmosphere.
  • Photosphere – the deepest part of the Sun which we can directly observe with visible light. Because the Sun is a gaseous object, it does not have a clearly-defined surface; its visible parts are usually divided into a 'photosphere' and 'atmosphere'.
  • Atmosphere – a gaseous 'halo' surrounding the Sun, comprising the chromosphere, solar transition region, corona and heliosphere. These can be seen when the main part of the Sun is hidden, for example, during a solar eclipse.

Core

The core of the Sun extends from the center to about 20–25% of the solar radius.[64] It has a density of up to 150 g/cm3[65][66] (about 150 times the density of water) and a temperature of close to 15.7 million kelvins (K).[66] By contrast, the Sun's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the radiative zone above.[64] Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium.[67] Only 0.8% of the energy generated in the Sun comes from the CNO cycle, though this proportion is expected to increase as the Sun becomes older.[68]

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 Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely. The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space as sunlight or the kinetic energy of particles.[49][69]

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.[49] Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,[70] so the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen [71]), for 384.6 yottawatts (3.846×1026 W),[2] or 9.192×1010 megatons of TNT per second. However, 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 power density, or energy production, of approximately 276.5 watts per cubic metre,[72] which is about the same rate of power production as takes place in reptile metabolism or a compost pile.[73][e]

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.[74][75]

Radiative zone

From the core out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer.[76] The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.[66] 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.[66] Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.[76] The density drops a hundredfold (from 20 g/cm3 to 0.2 g/cm3) from 0.25 solar radii to the 0.7 radii, the top of the radiative zone.[76]

Tachocline

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.[77] Presently, it is hypothesized (see Solar dynamo) that a magnetic dynamo within this layer generates the Sun's magnetic field.[66]

Convective zone

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 enough 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 the majority 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 to 5,700 K and the density to only 0.2 g/m3 (about 1/6,000 the density of air at sea level).[66]

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.[66] The Sun's thermal columns are Bénard cells and take the shape of hexagonal prisms.[78]

Photosphere

EffectiveTemperature 300dpi e
The effective temperature, or black body temperature, of the Sun (5,777 K) is the temperature a black body of the same size must have to yield the same total emissive power.

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light.[79] Above the photosphere visible sunlight is free to propagate into space, and almost all of its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H ions, which absorb visible light easily.[79] Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H ions.[80][81] 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.[79] The spectrum of sunlight has approximately the spectrum of a black-body radiating at 5,777 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 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.[82]

During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were caused by a new element that he dubbed helium, after the Greek Sun god Helios. Twenty-five years later, helium was isolated on Earth.[83]

Atmosphere

2017 Solar Eclipse Weiser Idaho
During a total solar eclipse, the solar corona can be seen with the naked eye, during the brief period of totality.

During a total solar eclipse, when the disk of the Sun is covered by that of the Moon, parts of the Sun's surrounding atmosphere can be seen. It is composed of four distinct parts: 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.[79] This part of the Sun is cool enough to allow the existence of simple molecules such as carbon monoxide and water, which can be detected via their absorption spectra.[84]

The chromosphere, transition region, and corona are much hotter than the surface of the Sun.[79] The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona.[85]

Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines.[79] It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses.[76] The temperature of the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top.[79] In the upper part of the chromosphere helium becomes partially ionized.[86]

171879main LimbFlareJan12 lg
Taken by Hinode's Solar Optical Telescope on 12 January 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.

Above the chromosphere, in a thin (about 200 km) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K.[87] The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma.[86] The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion.[76] The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum.[88]

The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 1015 m−3 to 1016 m−3.[86][f] The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K.[87] Although no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.[87][89] The corona is the extended atmosphere of the Sun, which has a volume much larger than the volume enclosed by the Sun's photosphere. A flow of plasma outward from the Sun into interplanetary space is the solar wind.[89]

The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with the solar wind plasma. This outermost layer of the Sun is defined to begin at the distance where the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves,[90] at approximately 20 solar radii (0.1 AU). Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere,[91][92] forming the solar magnetic field into a spiral shape,[89] until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause.[93] In late 2012 Voyager 1 recorded a marked increase in cosmic ray collisions and a sharp drop in lower energy particles from the solar wind, which suggested that the probe had passed through the heliopause and entered the interstellar medium.[94]

Photons and neutrinos

High-energy gamma-ray photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only a few millimeters. Re-emission happens in a random direction and usually at a slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years.[95] In contrast, 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. Because energy transport in the Sun is a process that 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 changed.[96]

Neutrinos are also released by the fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, but neutrino detectors were missing ​23 of them because the neutrinos had changed flavor by the time they were detected.[97]

Magnetic activity

Magnetic field

172197main NASA Flare Gband lg-withouttext
Visible light photograph of sunspot, 13 December 2006
Sunspot butterfly diagram
Sunspot area variation
Butterfly diagram showing paired sunspot pattern. Graph is of sunspot area.
Sun - August 1, 2010
In this false-color ultraviolet image, the Sun shows a C3-class solar flare (white area on upper left), a solar tsunami (wave-like structure, upper right) and multiple filaments of plasma following a magnetic field, rising from the stellar surface.
Heliospheric-current-sheet
The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium.[98]

The Sun has a magnetic field that varies across the surface of the Sun. Its polar field is 1–2 gauss (0.0001–0.0002 T), whereas the field is typically 3,000 gauss (0.3 T) in features on the Sun called sunspots and 10–100 gauss (0.001–0.01 T) in solar prominences.[2]

The magnetic field also varies in time and location. The quasi-periodic 11-year solar cycle is the most prominent variation in which the number and size of sunspots waxes and wanes.[17][99][100]

Sunspots are visible as dark patches on the Sun's photosphere, and correspond to concentrations of magnetic field where the convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, and, so, they appear dark. At a typical solar minimum, few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As the solar cycle progresses towards its maximum, sunspots tend form closer to the solar equator, a phenomenon known as Spörer's law. The largest sunspots can be tens of thousands of kilometers across.[101]

An 11-year sunspot cycle is half of a 22-year Babcock–Leighton dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle.[102][103]

During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size. At solar-cycle minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation shifts magnetic energy back from the poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field.[104][105]

The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the interplanetary magnetic field.[89] In an approximation known as ideal magnetohydrodynamics, plasma particles only move along the magnetic field lines. As a result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind.[89] At great distances, the rotation of the Sun twists the dipolar magnetic field and corresponding current sheet into an Archimedean spiral structure called the Parker spiral.[89] The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun's dipole magnetic field of 50–400 μT (at the photosphere) reduces with the inverse-cube of the distance to about 0.1 nT at the distance of Earth. However, according to spacecraft observations the interplanetary field at Earth's location is around 5 nT, about a hundred times greater.[106] The difference is due to magnetic fields generated by electrical currents in the plasma surrounding the Sun.

Variation in activity

Solar-cycle-data
Measurements from 2005 of solar cycle variation during the last 30 years

The Sun's magnetic field leads to many effects that are collectively called solar activity. Solar flares and coronal-mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from coronal holes at the photospheric surface. Both coronal-mass ejections and high-speed streams of solar wind carry plasma and interplanetary magnetic field outward into the Solar System.[107] The effects of solar activity on Earth include auroras at moderate to high latitudes and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System.

With solar-cycle modulation of sunspot number comes a corresponding modulation of space weather conditions, including those surrounding Earth where technological systems can be affected.

Long-term change

Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance,[108] which, in turn, might influence Earth's long-term climate.[109] For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during a period known as the Maunder minimum. This coincided in time with the era of the Little Ice Age, when Europe experienced unusually cold temperatures.[110] Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures.[111]

A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles.[112][113]

Life phases

The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for over four billion[a] years, and will remain fairly stable for more than five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo dramatic changes, both internally and externally.

Formation

The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars.[114] This age is estimated using computer models of stellar evolution and through nucleocosmochronology.[11] The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago.[115][116] Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that form only in exploding, short-lived stars. This indicates that one or more supernovae must have occurred near the location where the Sun formed. A shock wave from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity.[117] As one fragment of the cloud collapsed it also began to rotate because of conservation of angular momentum and heat up with the increasing pressure. Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion.

HD 162826 and HD 186302 are hypothesized stellar siblings of the Sun, having formed in the same molecular cloud.

Main sequence

Solar evolution (English)
Evolution of the Sun's luminosity, radius and effective temperature compared to the present Sun. After Ribas (2010)[118]

The Sun is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 times the mass of Earth into energy, about 0.03% of the total mass of the Sun. The Sun will spend a total of approximately 10 billion years as a main-sequence star.[119] The Sun is gradually becoming hotter during its time on the main sequence, because the helium atoms in the core occupy less volume than the hydrogen atoms that were fused. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the centre and experience a stronger gravitational force, according to the inverse-square law. This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up as the core gradually becomes denser. It is estimated that the Sun has become 30% brighter in the last 4.5 billion years.[120] At present, it is increasing in brightness by about 1% every 100 million years.[121]

After core hydrogen exhaustion

Sun red giant
The size of the current Sun (now in the main sequence) compared to its estimated size during its red-giant phase in the future

The Sun does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5 billion years and start to turn into a red giant.[122][123] As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth.[123][124]

Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Earth will receive as much sunlight as Venus receives today. Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the red-giant-branch phase where the Sun will spend around a billion years and lose around a third of its mass.[123]

Evolution of a Sun-like star
Evolution of a Sun-like star. The track of a one solar mass star on the Hertzsprung–Russell diagram is shown from the main sequence to the post-asymptotic-giant-branch stage.

After the red-giant branch the Sun has approximately 120 million years of active life left, but much happens. First, the core, full of degenerate helium ignites violently in the helium flash, where it is estimated that 6% of the core, itself 40% of the Sun's mass, will be converted into carbon within a matter of minutes through the triple-alpha process.[125] The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today. It will then have reached the red clump or horizontal branch, but a star of the Sun's mass does not evolve blueward along the horizontal branch. Instead, it just becomes moderately larger and more luminous over about 100 million years as it continues to burn helium in the core.[123]

When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the asymptotic-giant-branch phase, and the Sun is alternately burning hydrogen in a shell or helium in a deeper shell. After about 20 million years on the early asymptotic giant branch, the Sun becomes increasingly unstable, with rapid mass loss and thermal pulses that increase the size and luminosity for a few hundred years every 100,000 years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level and the radius to over 1 AU.[126] According to a 2008 model, Earth's orbit is shrinking due to tidal forces (and, eventually, drag from the lower chromosphere), so that it will be engulfed by the Sun near the tip of the red giant branch phase, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate. Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the red-giant branch produce smaller, less luminous stars at the tip of the asymptotic giant branch, perhaps only 2,000 times the luminosity and less than 200 times the radius.[123] For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a planetary nebula. By the end of that phase—lasting approximately 500,000 years—the Sun will only have about half of its current mass.

The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionised into a planetary nebula as the exposed core reaches 30,000 K. The final naked core, a white dwarf, will have a temperature of over 100,000 K, and contain an estimated 54.05% of the Sun's present day mass.[123] The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to a hypothetical black dwarf.[127][128]

Orbit and location

Artist's impression of the Milky Way (updated - annotated)
Illustration of the Milky Way, showing the location of the Sun

The Sun lies close to the inner rim of the Milky Way's Orion Arm, in the Local Interstellar Cloud or the Gould Belt, at a distance of 7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic Center.[129][130] [131][132][133][134] The Sun is contained within the Local Bubble, a space of rarefied hot gas, possibly produced by the supernova remnant Geminga,[135] or multiple supernovae in subgroup B1 of the Pleiades moving group.[136] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years.[137] The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone. The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels relative to other nearby stars. This motion is towards a point in the constellation Hercules, near the star Vega.

Within 32.6 ly of the Sun there are 315 known stars in 227 systems, as of 2000, including 163 single stars. It is estimated that a further 130 systems within this range have not yet been identified. Out to 81.5 ly, there may be up to 7,500 stars, of which around 2,600 are known. The number of substellar objects in that volume are expected to be comparable to the number of stars.[138] Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being the red dwarf Proxima Centauri at approximately 4.2 light-years), the Sun ranks fourth in mass.[139]

Orbit in Milky Way

The Sun orbits the center of the Milky Way, and it is presently moving in the direction of the constellation of Cygnus. A simple model of the motion of a star in the galaxy gives the galactic coordinates X, Y, and Z as:

where U, V, and W are the respective velocities with respect to the local standard of rest, A and B are the Oort constants, is the angular velocity of galactic rotation for the local standard of rest, is the "epicyclic frequency", and ν is the vertical oscillation frequency.[140] For the sun, the present values of U, V, and W are estimated as km/s, and estimates for the other constants are A = 15.5 km/s/kpc, B = −12.2 km/s/kpc, κ = 37 km/s/kpc, and ν=74 km/s/kpc. We take X(0) and Y(0) to be zero and Z(0) is estimated to be 17 parsecs.[141] This model implies that the Sun circulates around a point that is itself going around the galaxy. The period of the Sun's circulation around the point is . which, using the equivalence that a parsec equals 1 km/s times 0.978 million years, comes to 166 million years, shorter than the time it takes for the point to go around the galaxy. In the (X, Y) coordinates, the Sun describes an ellipse around the point, whose length in the Y direction is

and whose width in the X direction is

The ratio of length to width of this ellipse, the same for all stars in our neighborhood, is The moving point is presently at

The oscillation in the Z direction takes the Sun

above the galactic plane and the same distance below it, with a period of or 83 million years, approximately 2.7 times per orbit.[142] Although is 222 million years, the value of at the point around which the Sun circulates is

(see Oort constants), corresponding to 235 million years, and this is the time that the point takes to go once around the galaxy. Other stars with the same value of have take the same amount of time to go around the galaxy as the sun and thus remain in the same general vicinity as the Sun.

The Sun's orbit around the Milky Way is perturbed due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events.[143] It takes the Solar System about 225–250 million years to complete one orbit through the Milky Way (a galactic year),[144] so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Milky Way is approximately 251 km/s (156 mi/s).[145] At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU.[146]

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, and the Sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.[147]

Theoretical problems

Map of the full sun
Map of the full Sun by STEREO and SDO spacecraft

Coronal heating problem

The temperature of the photosphere is approximately 6,000 K, whereas the temperature of the corona reaches 1,000,000–2,000,000 K.[87] The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.[89]

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating.[87] The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.[87] These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat.[148] The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.[149]

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.[150] In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.[87]

Faint young Sun problem

Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean eon, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today. One theory among scientists is that the atmosphere of the young Earth contained much larger quantities of greenhouse gases (such as carbon dioxide, methane) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching it.[151]

However, examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface albedo brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output.[152]

Observational history

The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity.

Early understanding

Solvognen DO-6865 2000
The Trundholm sun chariot pulled by a horse is a sculpture believed to be illustrating an important part of Nordic Bronze Age mythology. The sculpture is probably from around 1350 BC. It is displayed at the National Museum of Denmark.

The Sun has been an object of veneration in many cultures throughout human history. Humanity's most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural entity. Worship of the Sun was central to civilizations such as the ancient Egyptians, the Inca of South America and the Aztecs of what is now Mexico. In religions such as Hinduism, the Sun is still considered a god. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt; Mnajdra, Malta and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumnal equinoxes.

The Egyptians portrayed the god Ra as being carried across the sky in a solar barque, accompanied by lesser gods, and to the Greeks, he was Helios, carried by a chariot drawn by fiery horses. From the reign of Elagabalus in the late Roman Empire the Sun's birthday was a holiday celebrated as Sol Invictus (literally "Unconquered Sun") soon after the winter solstice, which may have been an antecedent to Christmas. Regarding the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so Greek astronomers categorized it as one of the seven planets (Greek planetes, "wanderer"); the naming of the days of the weeks after the seven planets dates to the Roman era.[153][154][155]

Development of scientific understanding

In the early first millennium BC, Babylonian astronomers observed that the Sun's motion along the ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an elliptic orbit around the Sun, with Earth moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.[156]

One of the first people to offer a scientific or philosophical explanation for the Sun was the Greek philosopher Anaxagoras. He reasoned that it was not the chariot of Helios, but instead a giant flaming ball of metal even larger than the land of the Peloponnesus and that the Moon reflected the light of the Sun.[157] For teaching this heresy, he was imprisoned by the authorities and sentenced to death, though he was later released through the intervention of Pericles. Eratosthenes estimated the distance between Earth and the Sun in the 3rd century BC as "of stadia myriads 400 and 80000", the translation of which is ambiguous, implying either 4,080,000 stadia (755,000 km) or 804,000,000 stadia (148 to 153 million kilometers or 0.99 to 1.02 AU); the latter value is correct to within a few percent. In the 1st century AD, Ptolemy estimated the distance as 1,210 times the radius of Earth, approximately 7.71 million kilometers (0.0515 AU).[158]

The theory that the Sun is the center around which the planets orbit was first proposed by the ancient Greek Aristarchus of Samos in the 3rd century BC, and later adopted by Seleucus of Seleucia (see Heliocentrism). This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century by Nicolaus Copernicus.

Observations of sunspots were recorded during the Han Dynasty (206 BC–AD 220) by Chinese astronomers, who maintained records of these observations for centuries. Averroes also provided a description of sunspots in the 12th century.[159] The invention of the telescope in the early 17th century permitted detailed observations of sunspots by Thomas Harriot, Galileo Galilei and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.[160]

Arabic astronomical contributions include Albatenius' discovery that the direction of the Sun's apogee (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing.[161] (In modern heliocentric terms, this is caused by a gradual motion of the aphelion of the Earth's orbit). Ibn Yunus observed more than 10,000 entries for the Sun's position for many years using a large astrolabe.[162]

Sun-bonatti
Sol, the Sun, from a 1550 edition of Guido Bonatti's Liber astronomiae.

From an observation of a transit of Venus in 1032, the Persian astronomer and polymath Avicenna concluded that Venus is closer to Earth than the Sun.[163] In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun.

In 1666, Isaac Newton observed the Sun's light using a prism, and showed that it is made up of light of many colors.[164] In 1800, William Herschel discovered infrared radiation beyond the red part of the solar spectrum.[165] The 19th century saw advancement in spectroscopic studies of the Sun; Joseph von Fraunhofer recorded more than 600 absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle. Lord Kelvin suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat.[166] Kelvin and Hermann von Helmholtz then proposed a gravitational contraction mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time.[166][167] In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.[168]

Not until 1904 was a documented solution offered. Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested radioactive decay as the source.[169] However, it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass-energy equivalence relation E = mc2.[170] In 1920, Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass.[171] The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne using the ionization theory developed by Meghnad Saha, an Indian physicist. The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.[172][173] In 1957, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle showed that most of the elements in the universe have been synthesized by nuclear reactions inside stars, some like the Sun.[174]

Solar space missions

Sunspots and Solar Flares
The Sun giving out a large geomagnetic storm on 1:29 pm, EST, 13 March 2012
A lunar transit of the Sun captured during calibration of STEREO B's ultraviolet imaging cameras[175]

The first satellites designed for long term observation of the Sun from interplanetary space were NASA's Pioneers 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long time, transmitting data until May 1983.[176][177]

In the 1970s, two Helios spacecraft and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 and 2 probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion.[178] The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station.[88] Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona.[88] Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.[178]

PIA22197-SunFormsQuestionMark-20171222
Coronal hole on the Sun forms a question mark (22 December 2017)

In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity and solar luminosity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-entering Earth's atmosphere in June 1989.[179]

Launched in 1991, Japan's Yohkoh (Sunbeam) satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.[180]

One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on 2 December 1995.[88] Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009.[181] It has proven so useful that a follow-on mission, the Solar Dynamics Observatory (SDO), was launched in February 2010.[182] Situated at the Lagrangian point between Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch.[88] Besides its direct solar observation, SOHO has enabled the discovery of a large number of comets, mostly tiny sungrazing comets that incinerate as they pass the Sun.[183]

Giant prominence on the sun erupted
A solar prominence erupts in August 2012, as captured by SDO

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first travelled to Jupiter, to "slingshot" into an orbit that would take it far above the plane of the ecliptic. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s, which was slower than expected, and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays.[184]

Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material.[185]

The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.[186][187]

The Indian Space Research Organisation has scheduled the launch of a 100 kg satellite named Aditya for 2017–18. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona.[188]

Observation and effects

Anatomy of a Sunset-2
During certain atmospheric conditions, the Sun becomes clearly visible to the naked eye, and can be observed without stress to the eyes. Click on this photo to see the full cycle of a sunset, as observed from the high plains of the Mojave Desert.
STS-134 EVA4 view to the Russian Orbital Segment
The Sun, as seen from low Earth orbit overlooking the International Space Station. This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum

The brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes.[189][190] Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.[191][192] UV exposure gradually yellows the lens of the eye over a period of years, and is thought to contribute to the formation of cataracts, but this depends on general exposure to solar UV, and not whether one looks directly at the Sun.[193] Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced, sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused;[194][195] conditions are worsened by young eyes or new lens implants (which admit more UV than aging natural eyes), Sun angles near the zenith, and observing locations at high altitude.

Viewing the Sun through light-concentrating optics such as binoculars may result in permanent damage to the retina without an appropriate filter that blocks UV and substantially dims the sunlight. When using an attenuating filter to view the Sun, the viewer is cautioned to use a filter specifically designed for that use. Some improvised filters that pass UV or IR rays, can actually harm the eye at high brightness levels.[196] Herschel wedges, also called Solar Diagonals, are effective and inexpensive for small telescopes. The sunlight that is destined for the eyepiece is reflected from an unsilvered surface of a piece of glass. Only a very small fraction of the incident light is reflected. The rest passes through the glass and leaves the instrument. If the glass breaks because of the heat, no light at all is reflected, making the device fail-safe. Simple filters made of darkened glass allow the full intensity of sunlight to pass through if they break, endangering the observer's eyesight. Unfiltered binoculars can deliver hundreds of times as much energy as using the naked eye, possibly causing immediate damage. It is claimed that even brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.[197]

Partial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer.[198] The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.

Actual Sunrise.jpeg
A sunrise
D85 1116 Sunset in Thailand photographed by Trisorn Triboon
A sunset in Thailand in summer

During sunrise and sunset, sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere,[199] and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.[200]

An optical phenomenon, known as a green flash, can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon being bent (usually through a temperature inversion) towards the observer. Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light is scattered more, leaving light that is perceived as green.[201]

Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other biological effects such as the production of vitamin D and sun tanning. Ultraviolet light is strongly attenuated by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color in different regions of the Earth.[202]

Planetary system

The Sun has eight known planets. This includes four terrestrial planets (Mercury, Venus, Earth, and Mars), two gas giants (Jupiter and Saturn), and two ice giants (Uranus and Neptune). The Solar System also has at least five dwarf planets, an asteroid belt, numerous comets, and a large number of icy bodies which lie beyond the orbit of Neptune.

Religious aspects

Solar deities play a major role in many world religions and mythologies.[203] The ancient Sumerians believed that the sun was Utu,[204][205] the god of justice and twin brother of Inanna, the Queen of Heaven,[204] who was identified as the planet Venus.[205] Later, Utu was identified with the East Semitic god Shamash.[204][205] Utu was regarded as a helper-deity, who aided those in distress,[204] and, in iconography, he is usually portrayed with a long beard and clutching a saw,[204] which represented his role as the dispenser of justice.[204]

From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, whose spherical ball of dung was identified with the Sun. In the form of the sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton.[206][207]

In Proto-Indo-European religion, the Sun was personified as the goddess *Seh2ul.[208][209][22] Derivatives of this goddess in Indo-European languages include the Old Norse Sól, Sanskrit Surya, Gaulish Sulis, Lithuanian Saulė, and Slavic Solntse.[22] In ancient Greek religion, the sun deity was the male god Helios,[208] but traces of an earlier female solar deity are preserved in Helen of Troy.[208] In later times, Helios was syncretized with Apollo.[210]

In the Bible, Malachi 4:2 mentions the "Sun of Righteousness" (sometimes translated as the "Sun of Justice"),[211] which some Christians have interpreted as a reference to the Messiah (Christ).[212] In ancient Roman culture, Sunday was the day of the sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. In paganism, the Sun was a source of life, giving warmth and illumination to mankind. It was the center of a popular cult among Romans, who would stand at dawn to catch the first rays of sunshine as they prayed. The celebration of the winter solstice (which influenced Christmas) was part of the Roman cult of the unconquered Sun (Sol Invictus). Christian churches were built with an orientation so that the congregation faced toward the sunrise in the East.[213]

Tonatiuh, the Aztec god of the sun, was usually depicted holding arrows and a shield[214] and was closely associated with the practice of human sacrifice.[214] The sun goddess Amaterasu is the most important deity in the Shinto religion,[215][216] and she is believed to be the direct ancestor of all Japanese emperors.[215]

See also

Notes

  1. ^ a b c All numbers in this article are short scale. One billion is 109, or 1,000,000,000.
  2. ^ In astronomical sciences, the term heavy elements (or metals) refers to all elements except hydrogen and helium.
  3. ^ Hydrothermal vent communities live so deep under the sea that they have no access to sunlight. Bacteria instead use sulfur compounds as an energy source, via chemosynthesis.
  4. ^ 1.88 Gcd/m2 is calculated from the solar illuminance of 128000 lux (see sunlight) times the square of the distance to the center of the Sun, divided by the cross sectional area of the Sun. 1.44 Gcd/m2 is calculated using 98000 lux.
  5. ^ A 50 kg adult human has a volume of about 0.05 m3, which corresponds to 13.8 watts, at the volumetric power of the solar center. This is 285 kcal/day, about 10% of the actual average caloric intake and output for humans in non-stressful conditions.
  6. ^ Earth's atmosphere near sea level has a particle density of about 2×1025 m−3.

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Further reading

External links

Aquarius (astrology)

Aquarius (♒) is the eleventh astrological sign in the Zodiac, originating from the constellation Aquarius. Under the tropical zodiac, the sun is in Aquarius between about January 21 and about February 20, while under the sidereal Zodiac, the sun is in Aquarius from approximately February 15 to March 14, depending on the leap year.

Aries (astrology)

Aries (♈) (meaning "ram") is the first astrological sign in the zodiac, spanning the first 30 degrees of celestial longitude (0°≤ λ <30°). Under the tropical zodiac, the Sun transits this sign from approximately March 20 to April 21 each year. This time duration is exactly the first month of the Solar Hijri calendar (Hamal/Farvardin/Wray). The symbol of the ram is based on the Chrysomallus, the flying ram that provided the Golden Fleece.According to the tropical system of astrology, the Sun enters the sign of Aries when it reaches the March equinox, which occurs on average on March 21 (by design). Because the Earth takes approximately 365.24 days to go around the Sun, the precise time of the equinox is not the same each year, and generally will occur about six hours later from one year to the next until reset by a leap year. February 29 of a leap year causes that year's vernal equinox to fall about eighteen hours earlier compared with the previous year. From 1800 to 2050 inclusive the vernal equinox date has (or will) range(d) from March 19 at 22:34 UT1 in 2048 to March 21 at 19:15 UT1 in 1903.Under the sidereal zodiac, the sun currently transits Aries from April 15 to 14 May (approximately).

Aries is the first fire sign in the zodiac, the other fire signs being Leo and Sagittarius. Individuals born between these dates, depending on which system of astrology they subscribe to, may be called Arians or Ariens.The equivalent in the Hindu solar calendar is Meṣa.

Astronomical unit

The astronomical unit (symbol: au, ua, or AU) is a unit of length, roughly the distance from Earth to the Sun. However, that distance varies as Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year. Originally conceived as the average of Earth's aphelion and perihelion, since 2012 it has been defined as exactly 149597870700 metres, or about 150 million kilometres (93 million miles). The astronomical unit is used primarily for measuring distances within the Solar System or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the parsec.

Earth

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, Earth's only natural satellite. Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.Earth's axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth. The gravitational interaction between Earth and the Moon causes ocean tides, stabilizes Earth's orientation on its axis, and gradually slows its rotation. Earth is the densest planet in the Solar System and the largest of the four terrestrial planets.Earth's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earth's surface is covered with water, mostly by oceans. The remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earth's polar regions are covered in ice, including the Antarctic ice sheet and the sea ice of the Arctic ice pack. Earth's interior remains active with a solid iron inner core, a liquid outer core that generates the Earth's magnetic field, and a convecting mantle that drives plate tectonics.

Within the first billion years of Earth's history, life appeared in the oceans and began to affect the Earth's atmosphere and surface, leading to the proliferation of aerobic and anaerobic organisms. Some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive. In the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that ever lived on Earth are extinct. Estimates of the number of species on Earth today vary widely; most species have not been described. Over 7.6 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans have developed diverse societies and cultures; politically, the world has about 200 sovereign states.

Equinox

An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun. This occurs twice each year: around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator.

The word is derived from the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality primitive cultures noted the day when the Sun rises due East and sets due West and indeed this happens on the day closest to the astronomically defined event.

In the northern hemisphere, the equinox in March is called the Vernal or Spring Equinox; the September equinox is called the Autumnal or Fall Equinox. The dates are slightly variable, dependent as they are on the leap year cycle.Because the Moon (and to a lesser extent the planets) cause the motion of the Earth to vary from a perfect ellipse, the equinox is now officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the longitude of the Sun is 0° and 180°.

Java (programming language)

Java is a general-purpose programming language that is class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible. It is intended to let application developers "write once, run anywhere" (WORA), meaning that compiled Java code can run on all platforms that support Java without the need for recompilation. Java applications are typically compiled to "bytecode" that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax of Java is similar to C and C++, but it has fewer low-level facilities than either of them. As of 2018, Java was according to GitHub one of the most popular programming languages in use, particularly for client-server web applications, with a reported 9 million developers.Java was originally developed by James Gosling at Sun Microsystems (which has since been acquired by Oracle) and released in 1995 as a core component of Sun Microsystems' Java platform. The original and reference implementation Java compilers, virtual machines, and class libraries were originally released by Sun under proprietary licenses. As of May 2007, in compliance with the specifications of the Java Community Process, Sun had relicensed most of its Java technologies under the GNU General Public License. Meanwhile, others have developed alternative implementations of these Sun technologies, such as the GNU Compiler for Java (bytecode compiler), GNU Classpath (standard libraries), and IcedTea-Web (browser plugin for applets).

The latest version is Java SE 12, released in March 2019. Since Java 9 is no longer supported, Oracle advises its users to "immediately transition" to Java 12. Oracle released the last public update for the legacy Java 8 LTS, which is free for commercial use, in January 2019. Java 8 will be supported with public updates for personal use up to at least December 2020. Oracle and others "highly recommend that you uninstall older versions of Java" because of serious risks due to unresolved security issues. Oracle extended support for Java 6 ended in December 2018.

Louis XIV of France

Louis XIV (Louis Dieudonné; 5 September 1638 – 1 September 1715), known as Louis the Great (Louis le Grand) or the Sun King (Roi Soleil), was a monarch of the House of Bourbon who reigned as King of France from 1643 until his death in 1715. Starting on 14 May 1643 when Louis was 4 years old, his reign of 72 years and 110 days is the longest recorded of any monarch of a sovereign country in European history. In the age of absolutism in Europe, Louis XIV's France was a leader in the growing centralisation of power.Louis began his personal rule of France in 1661, after the death of his chief minister, the Italian Cardinal Mazarin. An adherent of the concept of the divine right of kings, Louis continued his predecessors' work of creating a centralised state governed from the capital. He sought to eliminate the remnants of feudalism persisting in parts of France and, by compelling many members of the nobility to inhabit his lavish Palace of Versailles, succeeded in pacifying the aristocracy, many members of which had participated in the Fronde rebellion during Louis' minority. By these means he became one of the most powerful French monarchs and consolidated a system of absolute monarchical rule in France that endured until the French Revolution.

Louis encouraged and benefited from the work of prominent political, military, and cultural figures such as Mazarin, Colbert, Louvois, the Grand Condé, Turenne, Sébastien Le Prestre de Vauban, André Charles Boulle, Molière, Racine, Boileau, La Fontaine, Lully, Marais, Le Brun, Rigaud, Bossuet, Le Vau, Mansart, Charles, Claude Perrault, and Le Nôtre. Under his rule, the Edict of Nantes, which granted rights to Huguenots, was abolished. The revocation effectively forced Huguenots to emigrate or convert in a wave of dragonnades, which managed to virtually destroy the French Protestant minority.

During Louis' long reign, France was the leading European power, and it fought three major wars: the Franco-Dutch War, the War of the League of Augsburg, and the War of the Spanish Succession. There were also two lesser conflicts: the War of Devolution and the War of the Reunions. Warfare defined the foreign policy of Louis XIV, and his personality shaped his approach. Impelled "by a mix of commerce, revenge, and pique", Louis sensed that warfare was the ideal way to enhance his glory. In peacetime he concentrated on preparing for the next war. He taught his diplomats that their job was to create tactical and strategic advantages for the French military.

Mercury (planet)

Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 87.97 days is the shortest of all the planets in the Solar System. It is named after the Roman deity Mercury, the messenger of the gods.

Like Venus, Mercury orbits the Sun within Earth's orbit as an inferior planet, and never exceeds 28° away from the Sun when viewed from Earth. This proximity to the Sun means the planet can only be seen near the western or eastern horizon during the early evening or early morning. At this time it may appear as a bright star-like object, but is often far more difficult to observe than Venus. The planet telescopically displays the complete range of phases, similar to Venus and the Moon, as it moves in its inner orbit relative to Earth, which reoccurs over the so-called synodic period approximately every 116 days.

Mercury is tidally locked with the Sun in a 3:2 spin-orbit resonance, and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two Mercurian years.

Mercury's axis has the smallest tilt of any of the Solar System's planets (about ​1⁄30 degree). Its orbital eccentricity is the largest of all known planets in the Solar System; at perihelion, Mercury's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Mercury's surface appears heavily cratered and is similar in appearance to the Moon's, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no known natural satellites.

Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years before exhausting its fuel and crashing into the planet's surface on April 30, 2015. The BepiColombo spacecraft is planned to arrive at Mercury in 2025.

Moon

The Moon, also known as Luna, is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits (its primary). The Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known.

The Moon is thought to have formed about 4.51 billion years ago, not long after Earth. The most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.

The Moon is in synchronous rotation with Earth, and thus always shows the same side to Earth, the near side. The near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest regularly visible celestial object in Earth's sky. Its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day.

The Moon's average orbital distance is 384,402 km (238,856 mi), or 1.28 light-seconds. This is about thirty times the diameter of Earth. The Moon's apparent size in the sky is almost the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly precisely during a total solar eclipse. This matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is gradually increasing.

The Moon was first reached in September 1959 by the Soviet Union's Luna 2, an unmanned spacecraft. The United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, and six manned landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, and the Moon's later history. Since the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.

Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems, art, and mythology.

Parsec

The parsec (symbol: pc) is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond, which corresponds to 648000/π astronomical units. One parsec is equal to about 3.26 light-years or 31 trillion kilometres (31×1012 km) or 19 trillion miles (19×1012 mi). The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun.The parsec unit was probably first suggested in 1913 by the British astronomer Herbert Hall Turner. Named as a portmanteau of the parallax of one arcsecond, it was defined to make calculations of astronomical distances from only their raw observational data quick and easy for astronomers. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.

In August 2015, the IAU passed Resolution B2, which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 648000/π astronomical units, or approximately 3.08567758149137×1016 metres (based on the IAU 2012 exact SI definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references.

Ra

Ra (; Ancient Egyptian: rꜥ or rˤ; also transliterated rˤw; cuneiform: 𒊑𒀀 ri-a or 𒊑𒅀ri-ia) or Re (; Coptic: ⲣⲏ, Rē) is the ancient Egyptian deity of the sun. By the Fifth Dynasty in the 25th and 24th centuries BC, he had become one of the most important gods in ancient Egyptian religion, identified primarily with the noon sun. Ra was believed to rule in all parts of the created world: the sky, the Earth, and the underworld.Ra was portrayed as a falcon and shared characteristics with the sky god Horus. At times the two deities were merged as Ra-Horakhty, "Ra, who is Horus of the Two Horizons". In the New Kingdom, when the god Amun rose to prominence he was fused with Ra into Amun-Ra.

The cult of the Mnevis bull, an embodiment of Ra, had its center in Heliopolis and there was a formal burial ground for the sacrificed bulls north of the city.

All forms of life were believed to have been created by Ra. In some accounts humans were created from Ra's tears and sweat, hence the Egyptians call themselves the "Cattle of Ra". In the myth of the Celestial Cow it is recounted how mankind plotted against Ra and how he sent his eye as the goddess Sekhmet to punish them.

Roger Ebert

Roger Joseph Ebert (; June 18, 1942 – April 4, 2013) was an American film critic, historian, journalist, screenwriter, and author. He was a film critic for the Chicago Sun-Times from 1967 until his death in 2013. In 1975, Ebert became the first film critic to win the Pulitzer Prize for Criticism.

Ebert and Chicago Tribune critic Gene Siskel helped popularize nationally televised film reviewing when they co-hosted the PBS show Sneak Previews, followed by several variously named At the Movies programs. The two verbally sparred and traded humorous barbs while discussing films. They created and trademarked the phrase "Two Thumbs Up", used when both hosts gave the same film a positive review. After Siskel died in 1999, Ebert continued hosting the show with various co-hosts and then, starting in 2000, with Richard Roeper.

Neil Steinberg of the Chicago Sun-Times said Ebert "was without question the nation's most prominent and influential film critic", Tom Van Riper of Forbes described him as "the most powerful pundit in America", and Kenneth Turan of the Los Angeles Times called him "the best-known film critic in America".Ebert lived with cancer of the thyroid and salivary glands beginning in 2002. In 2006, he required treatment necessitating the removal of his lower jaw, leaving him severely disfigured and costing him the ability to speak or eat normally. His ability to write remained unimpaired and he continued to publish frequently both online and in print until his death on April 4, 2013.

Solar System

The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.

The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.

Solar eclipse

A solar eclipse occurs when an observer (on Earth) passes through the shadow cast by the Moon which fully or partially blocks ("occults") the Sun. This can only happen when the Sun, Moon and Earth are nearly aligned on a straight line in three dimensions (syzygy) during a new moon when the Moon is close to the ecliptic plane. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured.

If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every new moon. However, since the Moon's orbit is tilted at more than 5 degrees to the Earth's orbit around the Sun, its shadow usually misses Earth. A solar eclipse can only occur when the moon is close enough to the ecliptic plane during a new moon. Special conditions must occur for the two events to coincide because the Moon's orbit crosses the ecliptic at its orbital nodes twice every draconic month (27.212220 days) while a new moon occurs one every synodic month (29.530587981 days). Solar (and lunar) eclipses therefore happen only during eclipse seasons resulting in at least two, and up to five, solar eclipses each year; no more than two of which can be total eclipses.Total eclipses are rare because the timing of the new moon within the eclipse season needs to be more exact for an alignment between the observer (on Earth) and the centers of the Sun and Moon. In addition, the elliptical orbit of the Moon often takes it far enough away from Earth that its apparent size is not large enough to block the Sun entirely. Total solar eclipses are rare at any particular location because totality exists only along a narrow path on the Earth's surface traced by the Moon's full shadow or umbra.

An eclipse is a natural phenomenon. However, in some ancient and modern cultures, solar eclipses were attributed to supernatural causes or regarded as bad omens. A total solar eclipse can be frightening to people who are unaware of its astronomical explanation, as the Sun seems to disappear during the day and the sky darkens in a matter of minutes.

Since looking directly at the Sun can lead to permanent eye damage or blindness, special eye protection or indirect viewing techniques are used when viewing a solar eclipse. It is technically safe to view only the total phase of a total solar eclipse with the unaided eye and without protection; however, this is a dangerous practice, as most people are not trained to recognize the phases of an eclipse, which can span over two hours while the total phase can only last a maximum of 7.5 minutes for any one location. People referred to as eclipse chasers or umbraphiles will travel to remote locations to observe or witness predicted central solar eclipses.

Solar mass

The solar mass (M) is a standard unit of mass in astronomy, equal to approximately 2×1030 kg. It is used to indicate the masses of other stars, as well as clusters, nebulae, and galaxies. It is equal to the mass of the Sun (denoted by the solar symbol ⊙︎). This equates to about two nonillion (two quintillion in the long scale) kilograms:

M = (1.98847±0.00007)×1030 kg

The above mass is about 332946 times the mass of Earth (M), or 1047 times the mass of Jupiter (MJ).

Because Earth follows an elliptical orbit around the Sun, the solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. Based upon the length of the year, the distance from Earth to the Sun (an astronomical unit or AU), and the gravitational constant (G), the mass of the Sun is given by:

The value of G is difficult to measure and is only known with limited accuracy in SI units (see Cavendish experiment). The value of G times the mass of an object, called the standard gravitational parameter, is known for the Sun and several planets to much higher accuracy than G alone. As a result, the solar mass is used as the standard mass in the astronomical system of units.

Sun Microsystems

Sun Microsystems, Inc. was an American company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, the Network File System (NFS), and SPARC. Sun contributed significantly to the evolution of several key computing technologies, among them Unix, RISC processors, thin client computing, and virtualized computing. Sun was founded on February 24, 1982. At its height, the Sun headquarters were in Santa Clara, California (part of Silicon Valley), on the former west campus of the Agnews Developmental Center.

On April 20, 2009, it was announced that Oracle Corporation would acquire Sun for US$7.4 billion. The deal was completed on January 27, 2010.Sun products included computer servers and workstations built on its own RISC-based SPARC processor architecture, as well as on x86-based AMD Opteron and Intel Xeon processors. Sun also developed its own storage systems and a suite of software products, including the Solaris operating system, developer tools, Web infrastructure software, and identity management applications. Other technologies included the Java platform and NFS. In general, Sun was a proponent of open systems, particularly Unix. It was also a major contributor to open-source software, as evidenced by its $1 billion purchase, in 2008, of MySQL, an open-source relational database management system. At various times, Sun had manufacturing facilities in several locations worldwide, including Newark, California; Hillsboro, Oregon; and Linlithgow, Scotland. However, by the time the company was acquired by Oracle, it had outsourced most manufacturing responsibilities.

Sun Yat-sen

Sun Yat-sen (; 12 November 1866 – 12 March 1925) was a Chinese politician, medical doctor and philosopher who served as the provisional first president of the Republic of China; and the first leader of the Kuomintang (Nationalist Party of China). He is referred to as the "Father of the Nation" in the Republic of China due to his role in the overthrow of the Qing dynasty during the Xinhai Revolution. Sun remains a unique figure among 20th-century Chinese leaders for being widely revered in both mainland China and Taiwan.

Although Sun is considered to be one of the greatest leaders of modern China, his political life was one of constant struggle and frequent exile. After the success of the revolution and the Han Chinese regaining power after 268 years of living under Manchurian rule (Qing dynasty), he quickly resigned from his post as President of the newly founded Republic of China to Yuan Shikai, and led successive revolutionary governments as a challenge to the warlords who controlled much of the nation. Sun did not live to see his party consolidate its power over the country during the Northern Expedition. His party, which formed a fragile alliance with the Chinese Communist Party, split into two factions after his death.

Sun's chief legacy resides in his developing of the political philosophy known as the Three Principles of the People: nationalism (Han Chinese nationalism: independence from imperialist domination – taking back power from the Manchurian Qing dynasty), "rights of the people", sometimes translated as "democracy", and the people's livelihood (just society).

The Art of War

The Art of War is an ancient Chinese military treatise dating from the Late Spring and Autumn Period (roughly 5th century BC). The work, which is attributed to the ancient Chinese military strategist Sun Tzu ("Master Sun", also spelled Sunzi), is composed of 13 chapters. Each one is devoted to an aspect of warfare and how it applies to military strategy and tactics. For almost 1,500 years it was the lead text in an anthology that would be formalised as the Seven Military Classics by Emperor Shenzong of Song in 1080. The Art of War remains the most influential strategy text in East Asian warfare and has influenced both Eastern and Western military thinking, business tactics, legal strategy and beyond.

The book contained a detailed explanation and analysis of the Chinese military, from weapons and strategy to rank and discipline. Sun Tzu also stressed the importance of intelligence operatives and espionage to the war effort. Because Sun Tzu has long been considered to be one of history's finest military tacticians and analysts, his teachings and strategies formed the basis of advanced military training for centuries to come.

The book was translated into French and published in 1772 (re-published in 1782) by the French Jesuit Jean Joseph Marie Amiot. A partial translation into English was attempted by British officer Everard Ferguson Calthrop in 1905 under the title The Book of War. The first annotated English translation was completed and published by Lionel Giles in 1910. Military and political leaders such as the Chinese communist revolutionary Mao Zedong, Japanese daimyō Takeda Shingen, Vietnamese general Vo Nguyen Giap, and American military general Norman Schwarzkopf Jr. have drawn inspiration from the book.

The Sun (United Kingdom)

The Sun is a tabloid newspaper published in the United Kingdom and Republic of Ireland. As a broadsheet, it was founded in 1964 as a successor to the Daily Herald; it became a tabloid in 1969 after it was purchased by its current owners. It is published by the News Group Newspapers division of News UK, itself a wholly owned subsidiary of Rupert Murdoch's News Corp. Since The Sun on Sunday was launched in February 2012, the paper has been a seven-day operation. The Sun previously had the largest circulation of any daily newspaper in the United Kingdom, but it was overtaken by rival Metro in March 2018.In 2012, The Sun on Sunday was launched to replace the closed News of the World, employing some of its former journalists. The average circulation for The Sun on Sunday in January 2019 was 1,178,687.In January 2019, it had an average daily circulation of 1.4 million. The Sun has been involved in many controversies in its history, including its coverage of the 1989 Hillsborough football stadium disaster. Regional editions of the newspaper for Scotland, Northern Ireland and the Republic of Ireland are published in Glasgow (The Scottish Sun), Belfast (The Sun) and Dublin (The Irish Sun) respectively. There is currently no separate Welsh edition of The Sun, readers in Wales get the same edition as readers in England.

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