Sun

The Sun is the star at the center of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is by far the most important source of energy for life on Earth. The Sun has been an object of veneration in many cultures. It has been a central subject for astronomical research since antiquity.

Sun

The Sun, viewed through a clear solar filter
Names	Sun, Sol,Sól, Helios
Adjectives	Solar
Symbol
Observation data
Mean distance from Earth	1 AU 149,600,000 km 8 min 19 s, light speed
Visual brightness	−26.74 (V)
Absolute magnitude	4.83
Spectral classification	G2V
Metallicity	Z = 0.0122
Angular size	0.527–0.545°
Orbital characteristics
Mean distance from Milky Way core	24,000 to 28,000 light-years
Galactic period	225–250 million years
Velocity	251 km/s orbit Galactic Center 20 km/s to stellar neighborhood 370 km/s to cosmic microwave background
Obliquity	7.25° (ecliptic) 67.23° (galactic plane)
Right ascension North pole	286.13° (286° 7′ 48″)
Declination of North pole	+63.87° (63° 52′ 12"N)
Sidereal rotation period	25.05 days (equator) 34.4 days (poles)
Equatorial rotation velocity	1.997 km/s
Physical characteristics
Equatorial radius	6.957×10^8 m 109 × Earth radii
Flattening	0.00005
Surface area	6.09×10^12 km^2 12,000 × Earth
Volume	1.412×10^18 km^3 1,300,000 × Earth
Mass	1.9885×10^30 kg 332,950 Earths
Average density	1.408 g/cm^3 0.255 × Earth
Age	4.6 billion years
Equatorial surface gravity	274 m/s^2 27.9 g0
Moment of inertia factor	≈0.070
Surface escape velocity	617.7 km/s 55 × Earth
Temperature	15,700,000 K (center) 5,772 K (photosphere) 5,000,000 K (corona)
Luminosity	3.828×10^26 W 3.75×10^28 lm 98 lm/W efficacy
Color (B-V)	0.656
Mean radiance	2.009×10^7 W·m^−2·sr^−1
Photosphere composition by mass	73.46% hydrogen 24.85% helium 0.77% oxygen 0.29% carbon 0.16% iron 0.12% neon 0.09% nitrogen 0.07% silicon 0.05% magnesium 0.04% sulfur

The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years. From Earth, it is 1 astronomical unit (1.496×10⁸ km) or about 8 light-minutes away. Its diameter is about 1,391,400 km (864,600 mi), 109 times that of Earth. Its mass is about 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

The Sun is a G-type main-sequence star (G2V), informally called a yellow dwarf, though its light is actually white. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core. Every second, the Sun's core fuses about 600 billion kilograms (kg) of hydrogen into helium and converts 4 billion kg of matter into energy.

About 4 to 7 billion years from now, when hydrogen fusion in the Sun's core diminishes to the point where the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature which will cause its outer layers to expand, eventually transforming the Sun into a red giant. This process will make the Sun large enough to render Earth uninhabitable approximately five billion years from the present. After the red giant phase, models suggest the Sun will shed its outer layers and become a dense type of cooling star (a white dwarf), and no longer produce energy by fusion, but will still glow and give off heat from its previous fusion for perhaps trillions of years. After that, it is theorized to become a super dense black dwarf, giving off negligible energy.

Etymology

The English word sun developed from Old English sunne. Cognates appear in other Germanic languages, including West Frisian sinne, Dutch zon, Low German Sünn, Standard German Sonne, Bavarian Sunna, Old Norse sunna, and Gothic sunnō. All these words stem from Proto-Germanic *sunnōn. This is ultimately related to the word for sun in other branches of the Indo-European language family, though in most cases a nominative stem with an l is found, rather than the genitive stem in n, as for example in Latin sōl, ancient Greek ἥλιος (hēlios), Welsh haul and Czech slunce, as well as (with *l > r) Sanskrit स्वर् (svár) and Persian خور (xvar). Indeed, the l-stem survived in Proto-Germanic as well, as *sōwelan, which gave rise to Gothic sauil (alongside sunnō) and Old Norse prosaic sól (alongside poetic sunna), and through it the words for sun in the modern Scandinavian languages: Swedish and Danish sol, Icelandic sól, etc.

The principal adjectives for the Sun in English are sunny for sunlight and, in technical contexts, solar (/ˈsoʊlər/), from Latin sol. From the Greek helios comes the rare adjective heliac (/ˈhiːliæk/). In English, the Greek and Latin words occur in poetry as personifications of the Sun, Helios (/ˈhiːliəs/) and Sol (/ˈsɒl/), while in science fiction Sol may be used to distinguish the Sun from other stars. The term sol with a lowercase s is used by planetary astronomers for the duration of a solar day on another planet such as Mars.

The astronomical symbol for the Sun is a circle with a center dot, . It is used for such units as M_☉ (Solar mass), R_☉ (Solar radius) and L_☉ (Solar luminosity). The scientific study of the Sun is called heliology.

General characteristics

Size comparison of the Sun, all the planets of the Solar System and some larger stars. The Sun is 1.4 million kilometers (4.643 light-seconds) wide, about 109 times wider than Earth, or four times the Lunar distance, and contains 99.86% of all Solar System mass.

The Sun is a G-type main-sequence star that makes up about 99.86% of the mass of the Solar System. It has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. It is more massive than 95% of the stars within 7 pc (23 ly). The Sun is a Population I, or heavy-element-rich, star. Its formation approximately 4.6 billion years ago may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars. 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.

The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46.

One astronomical unit (about 150 million kilometres; 93 million miles) is defined as the mean distance between the centers of the Sun and the Earth. The instantaneous distance varies by about ±2.5 million kilometres (1.6 million miles) as Earth moves from perihelion around 3 January to aphelion around 4 July. At its average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 20 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 on Earth by photosynthesis, 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. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. The roundness of the Sun is relative difference between its radius at its equator, ${\displaystyle R_{\textrm {eq}}}$, and at its pole, ${\displaystyle R_{\textrm {pol}}}$, called the oblateness,$${\displaystyle \Delta _{\odot }=(R_{\textrm {eq}}-R_{\textrm {pol}})/R_{\textrm {pol}}.}$$ The value is difficult to measure. Atmospheric distortion means the measurement must be done on satellites; the value is very small meaning very precise technique is needed.

The oblateness was once proposed to be sufficient to explain the perihelion precession of Mercury but Einstein proposed that general relativity could explain the precession using a spherical Sun. When high precision measurements of the oblateness became available via the Solar Dynamics Observatory and the Picard satellite the measured value was even smaller than expected, 8.2 x 10⁻⁶, or 8 parts per million. This makes the Sun the natural object closest to a perfect sphere. The oblateness value remains constant independent of solar irradiation changes. The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.

Rotation

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. Viewed from a vantage point above its north pole, the Sun rotates counterclockwise around its axis of spin.

A survey of solar analogs suggest the early Sun was rotating up to ten times faster than it does today. This would have made the surface much more active, with greater X-ray and UV emission. Sun spots would have covered 5–30% of the surface. The rotation rate was gradually slowed by magnetic braking, as the Sun's magnetic field interacted with the outflowing solar wind. A vestige of this rapid primordial rotation still survives at the Sun's core, which has been found to be rotating at a rate of once per week; four times the mean surface rotation rate.

Composition

The Sun consists mainly of the elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere. 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.

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

Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over the past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the center of the Sun because of gravity. The proportions of heavier elements are unchanged. Heat is transferred outward from the Sun's core by radiation rather than by convection (see Radiative zone below), so the fusion products are not lifted outward by heat; they remain in the core, 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.

The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. Typically, the solar heavy-element abundances described above are measured both by using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by the settling of heavy elements. The two methods generally agree well.

Structure and fusion

Illustration of the Sun's structure, in false color for contrast

Core

The core of the Sun extends from the center to about 20–25% of the solar radius. It has a density of up to 150 g/cm³ (about 150 times the density of water) and a temperature of close to 15.7 million kelvin (K). By contrast, the Sun's surface temperature is about 5800 K. Recent analysis of SOHO mission data favors the idea that the core is rotating faster than the radiative zone outside it. Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through the proton–proton chain; this process converts hydrogen into helium. Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous.

The core is the only region of the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).

Illustration of a proton-proton reaction chain, from hydrogen forming deuterium, helium-3, and regular helium-4

The proton–proton chain occurs around 9.2×10³⁷ times each second in the core, converting about 3.7×10³⁸ protons into alpha particles (helium nuclei) every second (out of a total of ~8.9×10⁵⁶ free protons in the Sun), or about 6.2×10¹¹ kg/s. However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen), for 384.6 yottawatts (3.846×10²⁶ W), or 9.192×10¹⁰ megatons of TNT per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only a fairly small amount of power being generated per cubic metre. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core, which, according to Karl Kruszelnicki, is about the same power density inside a compost pile.

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.

Radiative zone

Illustration of different stars' internal structure based on mass. The Sun in the middle has an inner radiating zone and an outer convective zone.

The radiative zone is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer. The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core. 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.Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions. The density drops a hundredfold (from 20,000 kg/m³ to 200 kg/m³) between 0.25 solar radii and 0.7 radii, the top of the radiative zone.

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. Presently, it is hypothesized that a magnetic dynamo, or solar dynamo, within this layer generates the Sun's magnetic field.

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 or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at the tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry most of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from the top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped 350-fold to 5,700 K (9,800 °F) and the density to only 0.2 g/m³ (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone).

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. The Sun's thermal columns are Bénard cells and take the shape of roughly hexagonal prisms.

Photosphere

Image of the Sun's cell-like surface structures

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of H⁻ ions, which absorb visible light easily. Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H⁻ ions.

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. The spectrum of sunlight has approximately the spectrum of a black-body radiating at 5,772 K (9,930 °F), 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.

Atmosphere

The Sun's atmosphere is composed of five layers: the photosphere, the chromosphere, the transition region, the corona, and the heliosphere.

The coolest layer of the Sun is a temperature minimum region extending to about 500 km above the photosphere, and has a temperature of about 4,100 K. This part of the Sun is cool enough to allow for the existence of simple molecules such as carbon monoxide and water. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona.

The Sun's transition region taken by Hinode's Solar Optical Telescope

Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. 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. The temperature of the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of the chromosphere helium becomes partially ionized.

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. The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. The transition region does not occur at a well-defined altitude, but forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to extreme ultraviolet.

During a solar eclipse the solar corona can be seen with the naked eye during totality.

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. 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. 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. 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.

The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with solar wind plasma and 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, 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, forming the solar magnetic field into a spiral shape, 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. 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, and indeed did so on August 25, 2012, at approximately 122 astronomical units (18 Tm) from the Sun. The heliosphere has a heliotail which stretches out behind it due to the Sun's peculiar motion through the galaxy.

On April 28, 2021, NASA's Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface, the boundary separating the corona from the solar wind, defined as where the coronal plasma's Alfvén speed and the large-scale solar wind speed are equal. During the flyby, Parker Solar Probe passed into and out of the corona several times. This proved the predictions that the Alfvén critical surface is not shaped like a smooth ball, but has spikes and valleys that wrinkle its surface.

Depiction of the heliosphere

Solar radiation

The Sun seen through a light fog

The Sun emits light across the visible spectrum, so its 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. The Solar radiance per wavelength peaks in the green portion of the spectrum when viewed from space. When the Sun is very low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta, and in rare occasions even green or blue. Some cultures mentally picture the Sun as yellow and some even red; the cultural reasons for this are debated. The Sun is classed as a G2 star, meaning it is a G-type star, with 2 indicating its surface temperature is in the second range of the G class.

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, at or near Earth's orbit). 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. 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. The atmosphere filters out over 70% of solar ultraviolet, especially at the shorter wavelengths. Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere.

Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. This radiation causes sunburn, and has other biological effects such as the production of vitamin D and sun tanning. It is the main cause of skin cancer. 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.

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 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. In contrast, it takes only 2.3 seconds for 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.

Electron neutrinos are released by fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. However, measurements of the number of these neutrinos produced in the Sun are lower than theories predict by a factor of 3. In 2001, the discovery of neutrino oscillation resolved the discrepancy: the Sun emits the number of electron neutrinos predicted by the theory, but neutrino detectors were missing 2⁄3 of them because the neutrinos had changed flavor by the time they were detected.

Magnetic activity

The Sun has a stellar magnetic field that varies across its surface. 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. The magnetic field 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.

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. In an approximation known as ideal magnetohydrodynamics, plasma particles only move along 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. 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.

Sunspot

A large sunspot group observed in white light

Sunspots are visible as dark patches on the Sun's photosphere and correspond to concentrations of magnetic field where convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, 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 toward its maximum, sunspots tend to form closer to the solar equator, a phenomenon known as Spörer's law. The largest sunspots can be tens of thousands of kilometers across.

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 the 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 described by Hale's law.

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.

Solar activity

Measurements from 2005 of solar cycle variation during the previous 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 the interplanetary magnetic field outward into the Solar System. 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.

Change in solar irradiance over the 11 year solar cycle has been correlated with change in sunspot number. The solar cycle influences space weather conditions, including those surrounding Earth. 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. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures.

Coronal heating

Unsolved problem in astronomy:

Why is the Sun's corona so much hotter than the Sun's surface?

(more unsolved problems in astronomy)

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

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. The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat. 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.

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. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.

Life phases

Overview of the evolution of a star like the Sun, from collapsing protostar at left to red giant stage at right

The Sun today is roughly halfway through the main-sequence portion of its life. It has not changed dramatically in over four billion years and will remain fairly stable for about 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. This age is estimated using computer models of stellar evolution and through nucleocosmochronology. The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago. 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. As one fragment of the cloud collapsed it also began to rotate due to 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 accumulated more matter from the surrounding disk, eventually triggering nuclear fusion.

The stars HD 162826 and HD 186302 share similarities with the Sun and are thus hypothesized to be its stellar siblings, formed in the same molecular cloud.

Main sequence

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.

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 billion kilograms 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 to 11 billion years as a main-sequence star before the red giant phase of the Sun. At the 8 billion year mark, the Sun will be at its hottest point according to the ESA's Gaia space observatory mission in 2022.

The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main sequence: since the beginning of its main sequence life, it has expanded in radius by 15% and the surface has increased in temperature from 5,620 K (9,660 °F) to 5,772 K (9,930 °F), resulting in a 48% increase in luminosity from 0.677 solar luminosities to its present-day 1.0 solar luminosity. This occurs because the helium atoms in the core have a higher mean molecular weight than the hydrogen atoms that were fused, resulting in less thermal pressure. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the center, releasing gravitational potential energy. According to the virial theorem, half of this released gravitational energy goes into heating, which leads to a gradual increase in the rate at which fusion occurs and thus an increase in the luminosity. This process speeds up as the core gradually becomes denser. At present, it is increasing in brightness by about 1% every 100 million years. It will take at least 1 billion years from now to deplete liquid water from the Earth from such increase. After that, the Earth will cease to be able to support complex, multicellular life and the last remaining multicellular organisms on the planet will suffer a final, complete mass extinction.

After core hydrogen exhaustion

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, when it runs out of hydrogen in the core in approximately 5 billion years, core hydrogen fusion will stop, and there will be nothing to prevent the core from contracting. The release of gravitational potential energy will cause the luminosity of the Sun to increase, ending the main sequence phase and leading the Sun to expand over the next billion years: first into a subgiant, and then into a red giant. The heating due to gravitational contraction will also lead to expansion of the Sun and hydrogen fusion in a shell just outside the core, where unfused hydrogen remains, contributing to the increased luminosity, which will eventually reach more than 1,000 times its present luminosity. When the Sun enters its red-giant branch (RGB) phase, it will engulf (and very likely destroy) Mercury and Venus. According to a 2008 article, Earth's orbit will have initially expanded to at most 1.5 AU (220 million km; 140 million mi) due to the Sun's loss of mass. However, Earth's orbit will then start shrinking due to tidal forces (and, eventually, drag from the lower chromosphere) so that it is engulfed by the Sun during the tip of the red-giant branch phase 7.59 billion years from now, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate.

By the time the Sun reaches the tip of the red-giant branch, it will be about 256 times larger than it is today, with a radius of 1.19 AU (178 million km; 111 million mi). The Sun will spend around a billion years in the RGB and lose around a third of its mass.

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; 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. 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 metallicity 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 react helium in the core.

When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted. This time, however, it all happens faster, and the Sun becomes larger and more luminous. This is the asymptotic-giant-branch phase, and the Sun is alternately reacting 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. Despite this, the Sun's maximum AGB radius will not be as large as its tip-RGB maximum: 179 R_☉, or about 0.832 AU (124.5 million km; 77.3 million mi).

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. For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a planetary nebula.

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 ionized into a planetary nebula as the exposed core reaches 30,000 K (53,500 °F), as if it is in a sort of blue loop. The final naked core, a white dwarf, will have a temperature of over 100,000 K (180,000 °F) and contain an estimated 54.05% of the Sun's present-day mass. Simulations indicate that the Sun may be among the least massive stars capable of forming a planetary nebula. 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 super-dense black dwarf. As such, it would give off no more energy.

Location

Solar System

Location of the Sun within the Solar System, which extends to the edge of the Oort cloud, where at 125,000 AU to 230,000 AU, equal to several light-years, the Sun's gravitational sphere of influence ends.

The Sun has eight known planets orbiting it. 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 nine bodies generally considered as dwarf planets and some more candidates, an asteroid belt, numerous comets, and a large number of icy bodies which lie beyond the orbit of Neptune. Six of the planets and many smaller bodies also have their own natural satellites: in particular, the satellite systems of Jupiter, Saturn, and Uranus are in some ways like miniature versions of the Sun's system.

The Sun is moved by the gravitational pull of the planets. The center of the Sun moves around the Solar System barycenter, within a range from 0.1 to 2.2 solar radii. The Sun's motion around the barycenter approximately repeats every 179 years, rotated by about 30° due primarily to the synodic period of Jupiter and Saturn.

The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. Most of the mass is orbiting in the region between 3,000 and 100,000 AU. The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun. The Sun's Hill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, was calculated by to be 230,000 AU.

Celestial neighborhood

This section is an excerpt from Solar System § Celestial neighborhood.[edit]

Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.

Within 10 light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble's G-Cloud. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to the Sun, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-years. In 2016, a potentially habitable exoplanet was found to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge. Multiple other interstellar clouds exist in the region within 300 light-years of the Sun, known as the Local Bubble. The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.

The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length. All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye.

Groups of stars form together in star clusters, before dissolving into co-moving associations. A prominent grouping that is visible to the naked eye is the Ursa Major moving group, which is around 80 light-years away within the Local Bubble. The nearest star cluster is Hyades, which lies at the edge of the Local Bubble. The closest star-forming regions are the Corona Australis Molecular Cloud, the Rho Ophiuchi cloud complex and the Taurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.

Stellar flybys that pass within 0.8 light-years of the Sun occur roughly once every 100,000 years. The closest well-measured approach was Scholz's Star, which approached to ~50,000 AU of the Sun some ~70 thousands years ago, likely passing through the outer Oort cloud. There is a 1% chance every billion years that a star will pass within 100 AU of the Sun, potentially disrupting the Solar System.

Motion

The general motion and orientation of the Sun, with Earth and the moon as its Solar System satellites.

The Sun, taking along the whole Solar System, orbits the galaxy's center of mass at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph), taking about 220–250 million Earth years to complete a revolution (a Galactic year), having done so about 20 times since the Sun's formation. The direction of the Sun's motion, the Solar apex, is roughly in the direction of the star Vega.

The Sun's idealized orbit around the Galactic Center in an artist's top-down depiction of the current layout of the Milky Way.

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. Since the Sun is moving with respect to the galactic center in the direction of Cygnus (galactic longitude 90°; latitude 0°) at more than 200 km/sec, the resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo (galactic latitude 264°, latitude 48°).

Observational history

Early understanding

The Trundholm sun chariot pulled by a horse is a sculpture believed to be illustrating an important part of Nordic Bronze Age mythology.

In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural entity. 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, moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.

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 a giant flaming ball of metal even larger than the land of the Peloponnesus and that the Moon reflected the light of the Sun.Eratosthenes estimated the distance between Earth and the Sun in the third 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 first century AD, Ptolemy estimated the distance as 1,210 times the radius of Earth, approximately 7.71 million kilometers (0.0515 AU).

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 third 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.

Development of scientific understanding

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

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. 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.

Arabic astronomical contributions include Al-Battani's 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. 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.

From an observation of a transit of Venus in 1032, the Persian astronomer and polymath Ibn Sina concluded that Venus was closer to Earth than the Sun. In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realize that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun. Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value.

Sun as seen in Hydrogen-alpha light

In 1666, Isaac Newton observed the Sun's light using a prism, and showed that it is made up of light of many colors. In 1800, William Herschel discovered infrared radiation beyond the red part of the solar spectrum. 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. The 20th century brought about several specialized systems for observing the Sun, especially at different narrowband wavelengths, such as those using Calcium H (396.9 nm), K (393.37 nm) and Hydrogen-alpha (656.46 nm) filtering.

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.

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. 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. In 1890, Lockyer proposed a meteoritic hypothesis for the formation and evolution of the Sun.

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. 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. 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. The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne using the ionization theory developed by Meghnad Saha. 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. 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.

Solar space missions

Illustration of Pioneer 6, 7, 8, and 9

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.

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. 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. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. 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.

Drawing of a Solar Maximum Mission probe

In 1980, the Solar Maximum Mission probes were 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.

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.

The Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA, was launched on 2 December 1995. Originally intended to serve a two-year mission, SOHO remains in operation as of 2024. 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. 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.

Ulysses spacecraft testing at the vacuum spin-balancing facility

Artist rendition of the Parker Solar Probe

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 traveled 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.

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.

Observation by eyes

Exposure to the eye

The Sun seen from Earth, with glare from the lenses. The eye also sees glare when looked towards the Sun directly.

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. Looking directly at the Sun (sungazing) 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. Viewing of the direct Sun with the naked eye can cause UV-induced, sunburn-like lesions on the retina beginning after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused.

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. Brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.

During sunrise and sunset, sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere, 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.

Phenomena

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.

Religious aspects

Sun and Immortal Birds Gold Ornament by ancient Shu people. The center is a sun pattern with twelve points around which four birds fly in the same counterclockwise direction. Ancient Kingdom of Shu, coinciding with the Shang dynasty.

Solar deities play a major role in many world religions and mythologies.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, known as Surya. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice (for example in Nabta Playa, Egypt; Mnajdra, Malta; and 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 ancient Sumerians believed that the Sun was Utu, the god of justice and twin brother of Inanna, the Queen of Heaven, who was identified as the planet Venus. Later, Utu was identified with the East Semitic god Shamash. Utu was regarded as a helper-deity, who aided those in distress.

Ra from the tomb of Nefertari, 13th century BC

From at least the Fourth 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. 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. 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.

In Proto-Indo-European religion, the Sun was personified as the goddess *Seh₂ul. Derivatives of this goddess in Indo-European languages include the Old Norse Sól, Sanskrit Surya, Gaulish Sulis, Lithuanian Saulė, and Slavic Solntse. In ancient Greek religion, the sun deity was the male god Helios, who in later times was syncretized with Apollo.

In the Bible, Malachi 4:2 mentions the "Sun of Righteousness" (sometimes translated as the "Sun of Justice"), which some Christians have interpreted as a reference to the Messiah (Christ). In ancient Roman culture, Sunday was the day of the sun god. In paganism, the Sun was a source of life, giving warmth and illumination. 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). It was adopted as the Sabbath day by Christians. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. Christian churches were built so that the congregation faced toward the sunrise.

Tonatiuh, the Aztec god of the sun, was closely associated with the practice of human sacrifice. The sun goddess Amaterasu is the most important deity in the Shinto religion, and she is believed to be the direct ancestor of all Japanese emperors.

See also

• Astronomy portal
• Stars portal
• Solar System portal
• Weather portal
• Physics portal

• Advanced Composition Explorer – NASA satellite of the Explorer program, at SE-L1 from 1997
• Analemma – Diagrammatic representation of Sun's position over a period of time
• Antisolar point – Point on the celestial sphere opposite Sun
• Faint young Sun paradox – Paradox concerning water on early Earth
• List of brightest stars – Stars sorted by apparent magnitude
• List of nearest stars – Stars and brown dwarfs within 20 light years of the Solar System
• Midnight sun – Natural phenomenon when daylight lasts for a whole day
• Planets in astrology § Sun
• Solar telescope – Telescope used to observe the Sun
• Sun path – Arc-like path that the Sun appears to follow across the sky
• Sun-Earth Day – NASA and ESA joint educational program
• The Sun in culture – Depictions of the Sun in culture
• Sun in fiction
• Timeline of the far future – Scientific projections regarding the far future

Notes

1. 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 chemical 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. Counterclockwise is also the direction of revolution around the Sun for objects in the Solar System and is the direction of axial spin for most objects.
5. Earth's atmosphere near sea level has a particle density of about 2×10²⁵ m⁻³.

References

1. "Sol". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
2. "Helios". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 27 March 2020.
3. "solar". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
4. Pitjeva, E. V.; Standish, E. M. (2009). "Proposals for the masses of the three largest asteroids, the Moon–Earth mass ratio and the Astronomical Unit". Celestial Mechanics and Dynamical Astronomy. 103 (4): 365–372. Bibcode:2009CeMDA.103..365P. doi:10.1007/s10569-009-9203-8. ISSN 1572-9478. S2CID 121374703. Archived from the original on 9 July 2019. Retrieved 13 July 2019.
5. Williams, D. R. (1 July 2013). "Sun Fact Sheet". NASA Goddard Space Flight Center. Archived from the original on 15 July 2010. Retrieved 12 August 2013.
6. Zombeck, Martin V. (1990). Handbook of Space Astronomy and Astrophysics 2nd edition. Cambridge University Press. Archived from the original on 3 February 2021. Retrieved 13 January 2016.
7. Asplund, M.; Grevesse, N.; Sauval, A. J. (2006). "The new solar abundances – Part I: the observations". Communications in Asteroseismology. 147: 76–79. Bibcode:2006CoAst.147...76A. doi:10.1553/cia147s76. ISSN 1021-2043. S2CID 123824232.
8. "Eclipse 99: Frequently Asked Questions". NASA. Archived from the original on 27 May 2010. Retrieved 24 October 2010.
9. Francis, Charles; Anderson, Erik (June 2014). "Two estimates of the distance to the Galactic Centre". Monthly Notices of the Royal Astronomical Society. 441 (2): 1105–1114. arXiv:1309.2629. Bibcode:2014MNRAS.441.1105F. doi:10.1093/mnras/stu631. S2CID 119235554.
10. Hinshaw, G.; Weiland, J. L.; Hill, R. S.; Odegard, N.; Larson, D.; et al. (2009). "Five-year Wilkinson Microwave Anisotropy Probe observations: data processing, sky maps, and basic results". The Astrophysical Journal Supplement Series. 180 (2): 225–245. arXiv:0803.0732. Bibcode:2009ApJS..180..225H. doi:10.1088/0067-0049/180/2/225. S2CID 3629998.
11. "Solar System Exploration: Planets: Sun: Facts & Figures". NASA. Archived from the original on 2 January 2008.
12. Prša, Andrej; Harmanec, Petr; Torres, Guillermo; et al. (1 August 2016). "NOMINAL VALUES FOR SELECTED SOLAR AND PLANETARY QUANTITIES: IAU 2015 RESOLUTION B3 * †". The Astronomical Journal. 152 (2): 41. arXiv:1510.07674. doi:10.3847/0004-6256/152/2/41. ISSN 0004-6256.
13. Bonanno, A.; Schlattl, H.; Paternò, L. (2002). "The age of the Sun and the relativistic corrections in the EOS". Astronomy and Astrophysics. 390 (3): 1115–1118. arXiv:astro-ph/0204331. Bibcode:2002A&A...390.1115B. doi:10.1051/0004-6361:20020749. S2CID 119436299.
14. Connelly, J. N.; Bizzarro, M.; Krot, A. N.; Nordlund, Å.; Wielandt, D.; Ivanova, M. A. (2 November 2012). "The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk". Science. 338 (6107): 651–655. Bibcode:2012Sci...338..651C. doi:10.1126/science.1226919. PMID 23118187. S2CID 21965292.(registration required)
15. Gray, David F. (November 1992). "The Inferred Color Index of the Sun". Publications of the Astronomical Society of the Pacific. 104 (681): 1035–1038. Bibcode:1992PASP..104.1035G. doi:10.1086/133086.
16. "The Sun's Vital Statistics". Stanford Solar Center. Archived from the original on 14 October 2012. Retrieved 29 July 2008. Citing Eddy, J. (1979). A New Sun: The Solar Results From Skylab. NASA. p. 37. NASA SP-402. Archived from the original on 30 July 2021. Retrieved 12 July 2017.
17. Barnhart, R. K. (1995). The Barnhart Concise Dictionary of Etymology. HarperCollins. p. 776. ISBN 978-0-06-270084-1.
18. Orel, Vladimir (2003). A Handbook of Germanic Etymology. Leiden: Brill. p. 41. ISBN 978-9-00-412875-0 – via Internet Archive.
19. Little, William; Fowler, H. W.; Coulson, J. (1955). "Sol". Oxford Universal Dictionary on Historical Principles (3rd ed.). ASIN B000QS3QVQ.
20. "heliac". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
21. "Opportunity's View, Sol 959 (Vertical)". NASA. 15 November 2006. Archived from the original on 22 October 2012. Retrieved 1 August 2007.
22. Allen, Clabon W.; Cox, Arthur N. (2000). Cox, Arthur N. (ed.). Allen's Astrophysical Quantities (4th ed.). Springer. p. 2. ISBN 978-0-38-798746-0 – via Google Books.
23. "solar mass". Oxford Reference. Retrieved 26 May 2024.
24. Weissman, Paul; McFadden, Lucy-Ann; Johnson, Torrence (18 September 1998). Encyclopedia of the Solar System. Academic Press. pp. 349, 820. ISBN 978-0-08-057313-7.
25. "heliology". Collins Dictionary. Collins. Retrieved 24 November 2024.
26. Woolfson, M. (2000). "The origin and evolution of the solar system" (PDF). Astronomy & Geophysics. 41 (1): 12. Bibcode:2000A&G....41a..12W. doi:10.1046/j.1468-4004.2000.00012.x. Archived (PDF) from the original on 11 July 2020. Retrieved 12 April 2020.
27. Than, K. (2006). "Astronomers Had it Wrong: Most Stars are Single". Space.com. Archived from the original on 21 December 2010. Retrieved 1 August 2007.
28. Lada, C. J. (2006). "Stellar multiplicity and the initial mass function: Most stars are single". Astrophysical Journal Letters. 640 (1): L63 – L66. arXiv:astro-ph/0601375. Bibcode:2006ApJ...640L..63L. doi:10.1086/503158. S2CID 8400400.
29. Robles, José A.; Lineweaver, Charles H.; Grether, Daniel; Flynn, Chris; Egan, Chas A.; Pracy, Michael B.; Holmberg, Johan; Gardner, Esko (September 2008). "A Comprehensive Comparison of the Sun to Other Stars: Searching for Self-Selection Effects". The Astrophysical Journal. 684 (1): 691–706. arXiv:0805.2962. Bibcode:2008ApJ...684..691R. doi:10.1086/589985. hdl:1885/34434. Retrieved 24 May 2024.
30. Zeilik, M. A.; Gregory, S. A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. p. 322. ISBN 978-0-03-006228-5.
31. Connelly, James N.; Bizzarro, Martin; Krot, Alexander N.; Nordlund, Åke; Wielandt, Daniel; Ivanova, Marina A. (2 November 2012). "The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk". Science. 338 (6107): 651–655. Bibcode:2012Sci...338..651C. doi:10.1126/science.1226919. PMID 23118187. S2CID 21965292.
32. Falk, S. W.; Lattmer, J. M.; Margolis, S. H. (1977). "Are supernovae sources of presolar grains?". Nature. 270 (5639): 700–701. Bibcode:1977Natur.270..700F. doi:10.1038/270700a0. S2CID 4240932.
33. Burton, W. B. (1986). "Stellar parameters". Space Science Reviews. 43 (3–4): 244–250. doi:10.1007/BF00190626. S2CID 189796439.
34. Bessell, M. S.; Castelli, F.; Plez, B. (1998). "Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O–M stars". Astronomy and Astrophysics. 333: 231–250. Bibcode:1998A&A...333..231B.
35. Hoffleit, D.; et al. (1991). "HR 2491". Bright Star Catalogue (5th Revised ed.). CDS. Bibcode:1991bsc..book.....H.
36. "Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020". US Naval Observatory. 31 January 2008. Archived from the original on 13 October 2007. Retrieved 17 July 2009.
37. Cain, Fraser (15 April 2013). "How long does it take sunlight to reach the Earth?". phys.org. Archived from the original on 2 March 2022. Retrieved 2 March 2022.
38. "The Sun's Energy: An Essential Part of the Earth System". Center for Science Education. Retrieved 24 May 2024.
39. "The Sun's Influence on Climate". Princeton University Press. 23 June 2015. Retrieved 24 May 2024.
40. Beer, J.; McCracken, K.; von Steiger, R. (2012). Cosmogenic Radionuclides: Theory and Applications in the Terrestrial and Space Environments. Springer. p. 41. ISBN 978-3-642-14651-0.
41. Phillips, K. J. H. (1995). Guide to the Sun. Cambridge University Press. p. 73. ISBN 978-0-521-39788-9.
42. Meftah, M.; Irbah, A.; Hauchecorne, A.; Corbard, T.; Turck-Chièze, S.; Hochedez, J.-F.; Boumier, P.; Chevalier, A.; Dewitte, S.; Mekaoui, S.; Salabert, D. (March 2015). "On the Determination and Constancy of the Solar Oblateness". Solar Physics. 290 (3): 673–687. Bibcode:2015SoPh..290..673M. doi:10.1007/s11207-015-0655-6. ISSN 0038-0938.
43. Gough, Douglas (28 September 2012). "How Oblate Is the Sun?". Science. 337 (6102): 1611–1612. Bibcode:2012Sci...337.1611G. doi:10.1126/science.1226988. ISSN 0036-8075. PMID 23019636.
44. Kuhn, J. R.; Bush, R.; Emilio, M.; Scholl, I. F. (28 September 2012). "The Precise Solar Shape and Its Variability". Science. 337 (6102): 1638–1640. Bibcode:2012Sci...337.1638K. doi:10.1126/science.1223231. ISSN 0036-8075.
45. Jones, G. (16 August 2012). "Sun is the most perfect sphere ever observed in nature". The Guardian. Archived from the original on 3 March 2014. Retrieved 19 August 2013.
46. Schutz, B. F. (2003). Gravity from the ground up. Cambridge University Press. pp. 98–99. ISBN 978-0-521-45506-0.
47. Phillips, K. J. H. (1995). Guide to the Sun. Cambridge University Press. pp. 78–79. ISBN 978-0-521-39788-9.
48. "The Anticlockwise Solar System". Australian Space Academy. Archived from the original on 7 August 2020. Retrieved 2 July 2020.
49. Guinan, Edward F.; Engle, Scott G. (June 2009). The Sun in time: age, rotation, and magnetic activity of the Sun and solar-type stars and effects on hosted planets. The Ages of Stars, Proceedings of the International Astronomical Union, IAU Symposium. Vol. 258. pp. 395–408. arXiv:0903.4148. Bibcode:2009IAUS..258..395G. doi:10.1017/S1743921309032050.
50. Pantolmos, George; Matt, Sean P. (November 2017). "Magnetic Braking of Sun-like and Low-mass Stars: Dependence on Coronal Temperature". The Astrophysical Journal. 849 (2). id. 83. arXiv:1710.01340. Bibcode:2017ApJ...849...83P. doi:10.3847/1538-4357/aa9061.
51. Fossat, E.; Boumier, P.; Corbard, T.; Provost, J.; Salabert, D.; Schmider, F. X.; Gabriel, A. H.; Grec, G.; Renaud, C.; Robillot, J. M.; Roca-Cortés, T.; Turck-Chièze, S.; Ulrich, R. K.; Lazrek, M. (August 2017). "Asymptotic g modes: Evidence for a rapid rotation of the solar core". Astronomy & Astrophysics. 604. id. A40. arXiv:1708.00259. Bibcode:2017A&A...604A..40F. doi:10.1051/0004-6361/201730460.
52. Darling, Susannah (1 August 2017). "ESA, NASA's SOHO Reveals Rapidly Rotating Solar Core". NASA. Retrieved 31 May 2024.
53. Lodders, Katharina (10 July 2003). "Solar System Abundances and Condensation Temperatures of the Elements" (PDF). The Astrophysical Journal. 591 (2): 1220–1247. Bibcode:2003ApJ...591.1220L. CiteSeerX 10.1.1.666.9351. doi:10.1086/375492. S2CID 42498829. Archived from the original (PDF) on 7 November 2015. Retrieved 1 September 2015.
    Lodders, K. (2003). "Abundances and Condensation Temperatures of the Elements" (PDF). Meteoritics & Planetary Science. 38 (suppl): 5272. Bibcode:2003M&PSA..38.5272L. Archived (PDF) from the original on 13 May 2011. Retrieved 3 August 2008.
54. Hansen, C. J.; Kawaler, S. A.; Trimble, V. (2004). Stellar Interiors: Physical Principles, Structure, and Evolution (2nd ed.). Springer. pp. 19–20. ISBN 978-0-387-20089-7.
55. Hansen, C. J.; Kawaler, S. A.; Trimble, V. (2004). Stellar Interiors: Physical Principles, Structure, and Evolution (2nd ed.). Springer. pp. 77–78. ISBN 978-0-387-20089-7.
56. Hansen, C. J.; Kawaler, S. A.; Trimble, V. (2004). Stellar Interiors: Physical Principles, Structure, and Evolution (2nd ed.). Springer. § 9.2.3. ISBN 978-0-387-20089-7.
57. Iben, Icko Jnr. (November 1965). "Stellar Evolution. II. The Evolution of a 3 M_☉ Star from the Main Sequence Through Core Helium Burning". Astrophysical Journal. 142: 1447. Bibcode:1965ApJ...142.1447I. doi:10.1086/148429.
58. Aller, L. H. (1968). "The chemical composition of the Sun and the solar system". Proceedings of the Astronomical Society of Australia. 1 (4): 133. Bibcode:1968PASA....1..133A. doi:10.1017/S1323358000011048. S2CID 119759834.
59. Basu, S.; Antia, H. M. (2008). "Helioseismology and Solar Abundances". Physics Reports. 457 (5–6): 217–283. arXiv:0711.4590. Bibcode:2008PhR...457..217B. doi:10.1016/j.physrep.2007.12.002. S2CID 119302796.
60. García, R.; et al. (2007). "Tracking solar gravity modes: the dynamics of the solar core". Science. 316 (5831): 1591–1593. Bibcode:2007Sci...316.1591G. doi:10.1126/science.1140598. PMID 17478682. S2CID 35285705.
61. Basu, Sarbani; Chaplin, William J.; Elsworth, Yvonne; New, Roger; Serenelli, Aldo M. (2009). "Fresh insights on the structure of the solar core". The Astrophysical Journal. 699 (2): 1403–1417. arXiv:0905.0651. Bibcode:2009ApJ...699.1403B. doi:10.1088/0004-637X/699/2/1403. S2CID 11044272.
62. "NASA/Marshall Solar Physics". Marshall Space Flight Center. 18 January 2007. Archived from the original on 29 March 2019. Retrieved 11 July 2009.
63. Broggini, C. (2003). Physics in Collision, Proceedings of the XXIII International Conference: Nuclear Processes at Solar Energy. XXIII Physics in Collisions Conference. Zeuthen, Germany. p. 21. arXiv:astro-ph/0308537. Bibcode:2003phco.conf...21B. Archived from the original on 21 April 2017. Retrieved 12 August 2013.
64. Goupil, M. J.; Lebreton, Y.; Marques, J. P.; Samadi, R.; Baudin, F. (2011). "Open issues in probing interiors of solar-like oscillating main sequence stars 1. From the Sun to nearly suns". Journal of Physics: Conference Series. 271 (1): 012031. arXiv:1102.0247. Bibcode:2011JPhCS.271a2031G. doi:10.1088/1742-6596/271/1/012031. S2CID 4776237.
65. The Borexino Collaboration (2020). "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun". Nature. 587 (?): 577–582. arXiv:2006.15115. Bibcode:2020Natur.587..577B. doi:10.1038/s41586-020-2934-0. PMID 33239797. S2CID 227174644. Archived from the original on 27 November 2020. Retrieved 26 November 2020.
66. Phillips, K. J. H. (1995). Guide to the Sun. Cambridge University Press. pp. 47–53. ISBN 978-0-521-39788-9.
67. Zirker, J. B. (2002). Journey from the Center of the Sun. Princeton University Press. pp. 15–34. ISBN 978-0-691-05781-1.
68. Shu, F. H. (1982). The Physical Universe: An Introduction to Astronomy. University Science Books. p. 102. ISBN 978-0-935702-05-7.
69. "Ask Us: Sun". Cosmicopia. NASA. 2012. Archived from the original on 3 September 2018. Retrieved 13 July 2017.
70. Cohen, H. (9 November 1998). "Table of temperatures, power densities, luminosities by radius in the Sun". Contemporary Physics Education Project. Archived from the original on 29 November 2001. Retrieved 30 August 2011.
71. "Lazy Sun is less energetic than compost". Australian Broadcasting Corporation. 17 April 2012. Archived from the original on 6 March 2014. Retrieved 25 February 2014.
72. Haubold, H. J.; Mathai, A. M. (1994). "Solar Nuclear Energy Generation & The Chlorine Solar Neutrino Experiment". AIP Conference Proceedings. 320 (1994): 102–116. arXiv:astro-ph/9405040. Bibcode:1995AIPC..320..102H. CiteSeerX 10.1.1.254.6033. doi:10.1063/1.47009. S2CID 14622069.
73. Myers, S. T. (18 February 1999). "Lecture 11 – Stellar Structure I: Hydrostatic Equilibrium". Introduction to Astrophysics II. Archived from the original on 12 May 2011. Retrieved 15 July 2009.
74. "Sun". World Book at NASA. NASA. Archived from the original on 10 May 2013. Retrieved 10 October 2012.
75. Tobias, S. M. (2005). "The solar tachocline: Formation, stability and its role in the solar dynamo". In Soward, A. M.; et al. (eds.). Fluid Dynamics and Dynamos in Astrophysics and Geophysics. CRC Press. pp. 193–235. ISBN 978-0-8493-3355-2. Archived from the original on 29 October 2020. Retrieved 22 August 2020.
76. Mullan, D. J. (2000). "Solar Physics: From the Deep Interior to the Hot Corona". In Page, D.; Hirsch, J. G. (eds.). From the Sun to the Great Attractor. Springer. p. 22. ISBN 978-3-540-41064-5. Archived from the original on 17 April 2021. Retrieved 22 August 2020.
77. Abhyankar, K. D. (1977). "A Survey of the Solar Atmospheric Models". Bulletin of the Astronomical Society of India. 5: 40–44. Bibcode:1977BASI....5...40A. Archived from the original on 12 May 2020. Retrieved 12 July 2009.
78. Gibson, Edward G. (1973). The Quiet Sun (NASA SP-303). NASA. ASIN B0006C7RS0.
79. Shu, F. H. (1991). The Physics of Astrophysics. Vol. 1. University Science Books. ISBN 978-0-935702-64-4.
80. Rast, M.; Nordlund, Å.; Stein, R.; Toomre, J. (1993). "Ionization Effects in Three-Dimensional Solar Granulation Simulations". The Astrophysical Journal Letters. 408 (1): L53–L56. Bibcode:1993ApJ...408L..53R. doi:10.1086/186829.
81. Solanki, S. K.; Livingston, W.; Ayres, T. (1994). "New Light on the Heart of Darkness of the Solar Chromosphere". Science. 263 (5143): 64–66. Bibcode:1994Sci...263...64S. doi:10.1126/science.263.5143.64. PMID 17748350. S2CID 27696504.
82. De Pontieu, B.; et al. (2007). "Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind". Science. 318 (5856): 1574–1577. Bibcode:2007Sci...318.1574D. doi:10.1126/science.1151747. PMID 18063784. S2CID 33655095.
83. Hansteen, V. H.; Leer, E.; Holzer, T. E. (1997). "The role of helium in the outer solar atmosphere". The Astrophysical Journal. 482 (1): 498–509. Bibcode:1997ApJ...482..498H. doi:10.1086/304111.
84. Erdèlyi, R.; Ballai, I. (2007). "Heating of the solar and stellar coronae: a review". Astron. Nachr. 328 (8): 726–733. Bibcode:2007AN....328..726E. doi:10.1002/asna.200710803.
85. Dwivedi, B. N. (2006). "Our ultraviolet Sun" (PDF). Current Science. 91 (5): 587–595. Archived (PDF) from the original on 25 October 2020. Retrieved 22 March 2015.
86. Russell, C. T. (2001). "Solar wind and interplanetary magnetic filed: A tutorial" (PDF). In Song, Paul; Singer, Howard J.; Siscoe, George L. (eds.). Space Weather (Geophysical Monograph). American Geophysical Union. pp. 73–88. ISBN 978-0-87590-984-4. Archived (PDF) from the original on 1 October 2018. Retrieved 11 July 2009.
87. Emslie, A. G.; Miller, J. A. (2003). "Particle Acceleration". In Dwivedi, B. N. (ed.). Dynamic Sun. Cambridge University Press. p. 275. ISBN 978-0-521-81057-9.
88. "A Star with two North Poles". Science @ NASA. NASA. 22 April 2003. Archived from the original on 18 July 2009.
89. Riley, P.; Linker, J. A.; Mikić, Z. (2002). "Modeling the heliospheric current sheet: Solar cycle variations". Journal of Geophysical Research. 107 (A7): SSH 8–1. Bibcode:2002JGRA..107.1136R. doi:10.1029/2001JA000299. CiteID 1136.
90. "The Distortion of the Heliosphere: Our Interstellar Magnetic Compass" (Press release). European Space Agency. 2005. Archived from the original on 4 June 2012. Retrieved 22 March 2006.
91. Landau, Elizabeth (29 October 2015). "Voyager 1 Helps Solve Interstellar Medium Mystery" (Press release). Jet Propulsion Laboratory. Archived from the original on 3 August 2023.
92. "Interstellar Mission". Jet Propulsion Laboratory. Archived from the original on 14 September 2017. Retrieved 14 May 2021.
93. Dunbar, Brian (2 March 2015). "Components of the Heliosphere". NASA. Archived from the original on 8 August 2021. Retrieved 20 March 2021.
94. Hatfield, Miles (13 December 2021). "NASA Enters the Solar Atmosphere for the First Time". NASA. Archived from the original on 27 December 2021. Retrieved 30 July 2022. This article incorporates text from this source, which is in the public domain.
95. "GMS: Animation: NASA's Parker Solar Probe Enters Solar Atmosphere". svs.gsfc.nasa.gov. 14 December 2021. Archived from the original on 4 October 2022. Retrieved 30 July 2022.
96. "What Color is the Sun?". Universe Today. Archived from the original on 25 May 2016. Retrieved 23 May 2016.
97. "What Color is the Sun?". Stanford Solar Center. Archived from the original on 30 October 2017. Retrieved 23 May 2016.
98. Wilk, S. R. (2009). "The Yellow Sun Paradox". Optics & Photonics News: 12–13. Archived from the original on 18 June 2012.
99. "Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present". pmodwrc. 24 May 2006. Archived from the original on 1 August 2011. Retrieved 5 October 2005.
100. El-Sharkawi, Mohamed A. (2005). Electric energy. CRC Press. pp. 87–88. ISBN 978-0-8493-3078-0.
101. Fu, Qiang (2003). "Radiation (Solar)". In Curry, Judith A.; Pyle, John A. (eds.). Radiation (SOLAR) (PDF). Encyclopedia of Atmospheric Sciences. Elsevier. pp. 1859–1863. doi:10.1016/B0-12-227090-8/00334-1. ISBN 978-0-12-227090-1. Archived from the original (PDF) on 1 November 2012. Retrieved 29 December 2012.
102. "Reference Solar Spectral Irradiance: Air Mass 1.5". NREL. Archived from the original on 12 May 2019. Retrieved 12 November 2009.
103. Phillips, K. J. H. (1995). Guide to the Sun. Cambridge University Press. pp. 14–15, 34–38. ISBN 978-0-521-39788-9.
104. Barsh, G. S. (2003). "What Controls Variation in Human Skin Color?". PLOS Biology. 1 (1): e7. doi:10.1371/journal.pbio.0000027. PMC 212702. PMID 14551921.
105. "Ancient sunlight". Technology Through Time. NASA. 2007. Archived from the original on 15 May 2009. Retrieved 24 June 2009.
106. Stix, M. (2003). "On the time scale of energy transport in the sun". Solar Physics. 212 (1): 3–6. Bibcode:2003SoPh..212....3S. doi:10.1023/A:1022952621810. S2CID 118656812.
107. Schlattl, H. (2001). "Three-flavor oscillation solutions for the solar neutrino problem". Physical Review D. 64 (1): 013009. arXiv:hep-ph/0102063. Bibcode:2001PhRvD..64a3009S. doi:10.1103/PhysRevD.64.013009. S2CID 117848623.
108. Charbonneau, P. (2014). "Solar Dynamo Theory". Annual Review of Astronomy and Astrophysics. 52: 251–290. Bibcode:2014ARA&A..52..251C. doi:10.1146/annurev-astro-081913-040012. S2CID 17829477.
109. Zirker, J. B. (2002). Journey from the Center of the Sun. Princeton University Press. pp. 119–120. ISBN 978-0-691-05781-1.
110. Lang, Kenneth R. (2008). The Sun from Space. Springer-Verlag. p. 75. ISBN 978-3-540-76952-1.
111. "The Largest Sunspot in Ten Years". Goddard Space Flight Center. 30 March 2001. Archived from the original on 23 August 2007. Retrieved 10 July 2009.
112. Hale, G. E.; Ellerman, F.; Nicholson, S. B.; Joy, A. H. (1919). "The Magnetic Polarity of Sun-Spots". The Astrophysical Journal. 49: 153. Bibcode:1919ApJ....49..153H. doi:10.1086/142452.
113. "NASA Satellites Capture Start of New Solar Cycle". PhysOrg. 4 January 2008. Archived from the original on 6 April 2008. Retrieved 10 July 2009.
114. "Sun flips magnetic field". CNN. 16 February 2001. Archived from the original on 21 January 2015. Retrieved 11 July 2009.
115. Phillips, T. (15 February 2001). "The Sun Does a Flip". NASA. Archived from the original on 12 May 2009. Retrieved 11 July 2009.
116. Zirker, J. B. (2002). Journey from the Center of the Sun. Princeton University Press. pp. 120–127. ISBN 978-0-691-05781-1.
117. Nandy, Dibyendu; Martens, Petrus C. H.; Obridko, Vladimir; Dash, Soumyaranjan; Georgieva, Katya (5 July 2021). "Solar evolution and extrema: current state of understanding of long-term solar variability and its planetary impacts". Progress in Earth and Planetary Science. 8 (1): 40. Bibcode:2021PEPS....8...40N. doi:10.1186/s40645-021-00430-x. ISSN 2197-4284.
118. Willson, R. C.; Hudson, H. S. (1991). "The Sun's luminosity over a complete solar cycle". Nature. 351 (6321): 42–44. Bibcode:1991Natur.351...42W. doi:10.1038/351042a0. S2CID 4273483.
119. Eddy, John A. (June 1976). "The Maunder Minimum". Science. 192 (4245): 1189–1202. Bibcode:1976Sci...192.1189E. doi:10.1126/science.192.4245.1189. JSTOR 1742583. PMID 17771739. S2CID 33896851.
120. Lean, J.; Skumanich, A.; White, O. (1992). "Estimating the Sun's radiative output during the Maunder Minimum". Geophysical Research Letters. 19 (15): 1591–1594. Bibcode:1992GeoRL..19.1591L. doi:10.1029/92GL01578. Archived from the original on 11 May 2020. Retrieved 16 December 2019.
121. Mackay, R. M.; Khalil, M. A. K. (2000). "Greenhouse gases and global warming". In Singh, S. N. (ed.). Trace Gas Emissions and Plants. Springer. pp. 1–28. ISBN 978-0-7923-6545-7. Archived from the original on 17 April 2021. Retrieved 3 November 2020.
122. Alfvén, H. (1947). "Magneto-hydrodynamic waves, and the heating of the solar corona". Monthly Notices of the Royal Astronomical Society. 107 (2): 211–219. Bibcode:1947MNRAS.107..211A. doi:10.1093/mnras/107.2.211.
123. Parker, E. N. (1988). "Nanoflares and the solar X-ray corona". Astrophysical Journal. 330 (1): 474. Bibcode:1988ApJ...330..474P. doi:10.1086/166485.
124. Sturrock, P. A.; Uchida, Y. (1981). "Coronal heating by stochastic magnetic pumping". Astrophysical Journal. 246 (1): 331. Bibcode:1981ApJ...246..331S. doi:10.1086/158926. hdl:2060/19800019786.
125. Zirker, Jack B. (2002). Journey from the Center of the Sun. Princeton University Press. pp. 7–8. ISBN 978-0-691-05781-1.
126. Amelin, Y.; Krot, A.; Hutcheon, I.; Ulyanov, A. (2002). "Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions". Science. 297 (5587): 1678–1683. Bibcode:2002Sci...297.1678A. doi:10.1126/science.1073950. PMID 12215641. S2CID 24923770.
127. Baker, J.; Bizzarro, M.; Wittig, N.; Connelly, J.; Haack, H. (2005). "Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites". Nature. 436 (7054): 1127–1131. Bibcode:2005Natur.436.1127B. doi:10.1038/nature03882. PMID 16121173. S2CID 4304613.
128. Williams, J. (2010). "The astrophysical environment of the solar birthplace". Contemporary Physics. 51 (5): 381–396. arXiv:1008.2973. Bibcode:2010ConPh..51..381W. CiteSeerX 10.1.1.740.2876. doi:10.1080/00107511003764725. S2CID 118354201.
129. Glozman, Igor (2022). "Formation of the Solar System". Highline College. Des Moines, WA. Archived from the original on 26 March 2023. Retrieved 16 January 2022.
130. D'Angelo, G.; Lubow, S. H. (2010). "Three-dimensional Disk-Planet Torques in a Locally Isothermal Disk". The Astrophysical Journal. 724 (1): 730–747. arXiv:1009.4148. Bibcode:2010ApJ...724..730D. doi:10.1088/0004-637X/724/1/730. S2CID 119204765.
131. Lubow, S. H.; Ida, S. (2011). "Planet Migration". In Seager, S. (ed.). Exoplanets. Tucson: University of Arizona Press. pp. 347–371. arXiv:1004.4137. Bibcode:2010exop.book..347L.
132. Jones, Andrew Zimmerman (30 May 2019). "How Stars Make All of the Elements". ThoughtCo. Archived from the original on 11 July 2023. Retrieved 16 January 2023.
133. "Astronomers Find Sun's Sibling 'HD 162826'". Nature World News. 9 May 2014. Archived from the original on 3 March 2016. Retrieved 16 January 2022.
134. Williams, Matt (21 November 2018). "Astronomers Find One of the Sun's Sibling Stars. Born From the Same Solar Nebula Billions of Years Ago". Universe Today. Archived from the original on 26 March 2023. Retrieved 7 October 2022.
135. Goldsmith, D.; Owen, T. (2001). The search for life in the universe. University Science Books. p. 96. ISBN 978-1-891389-16-0. Archived from the original on 30 October 2020. Retrieved 22 August 2020.
136. "ESA's Gaia Mission Sheds New Light on Past and Future of Our Sun". Sci.News: Breaking Science News. 12 August 2022. Archived from the original on 4 April 2023. Retrieved 15 August 2022.
137. Carroll, Bradley W.; Ostlie, Dal A (2017). An introduction to modern astrophysics (Second ed.). Cambridge, United Kingdom: Cambridge University Press. pp. 350, 447, 448, 457. ISBN 978-1-108-42216-1.
138. Kollipara, Puneet (22 January 2014). "Earth Won't Die as Soon as Thought". Science. Archived from the original on 12 November 2020. Retrieved 24 May 2015.
139. Snyder-Beattie, Andrew E.; Bonsall, Michael B. (30 March 2022). "Catastrophe risk can accelerate unlikely evolutionary transitions". Proceedings of the Royal Society B. 289 (1971). doi:10.1098/rspb.2021.2711. PMC 8965398. PMID 35350860.
140. Redd, Nola Taylor. "Red Giant Stars: Facts, Definition & the Future of the Sun". space.com. Archived from the original on 9 February 2016. Retrieved 20 February 2016.
141. Schröder, K.-P.; Connon Smith, R. (2008). "Distant future of the Sun and Earth revisited". Monthly Notices of the Royal Astronomical Society. 386 (1): 155–163. arXiv:0801.4031. Bibcode:2008MNRAS.386..155S. doi:10.1111/j.1365-2966.2008.13022.x. S2CID 10073988.
142. Boothroyd, Arnold I.; Sackmann, I.-Juliana (1 January 1999) [19 December 1995]. "The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge-up". The Astrophysical Journal. 510 (1). The American Astronomical Society (AAS), The Institute of Physics (IOP): 232–250. arXiv:astro-ph/9512121. Bibcode:1999ApJ...510..232B. doi:10.1086/306546. S2CID 561413.
143. Taylor, David. "The End Of The Sun". Northwestern University. Archived from the original on 22 May 2019. Retrieved 24 May 2015.
144. Vassiliadis, E.; Wood, P. R. (1993). "Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss". The Astrophysical Journal. 413: 641. Bibcode:1993ApJ...413..641V. doi:10.1086/173033.
145. Sackmann, I.-J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". The Astrophysical Journal. 418: 457–468. Bibcode:1993ApJ...418..457S. doi:10.1086/173407.
146. Gesicki, K.; Zijlstra, A. A.; Miller Bertolami, M. M. (2018). "The mysterious age invariance of the planetary nebula luminosity function bright cut-off". Nature Astronomy. 2 (7): 580–584. arXiv:1805.02643. Bibcode:2018NatAs...2..580G. doi:10.1038/s41550-018-0453-9.
147. Bloecker, T. (1995). "Stellar evolution of low and intermediate-mass stars. I. Mass loss on the AGB and its consequences for stellar evolution". Astronomy and Astrophysics. 297: 727. Bibcode:1995A&A...297..727B.
148. Bloecker, T. (1995). "Stellar evolution of low- and intermediate-mass stars. II. Post-AGB evolution". Astronomy and Astrophysics. 299: 755. Bibcode:1995A&A...299..755B.
149. Christensen-Dalsgaard, Jørgen (2021). "Solar structure and evolution". Living Reviews in Solar Physics. 18 (2): 2. arXiv:2007.06488. Bibcode:2021LRSP...18....2C. doi:10.1007/s41116-020-00028-3.
150. Johnson-Groh, Mara (25 August 2020). "The end of the universe may be marked by 'black dwarf supernova' explosions". Live Science. Archived from the original on 2 June 2023. Retrieved 24 November 2023.
151. Lewis, John, ed. (2004). Physics and Chemistry of the Solar System (2 ed.). Elsevier. p. 265. ISBN 9780080470122.
152. Jose, Paul D. (April 1965). "Sun's Motion and Sunspots" (PDF). The Astronomical Journal. 70 (3): 193–200. Bibcode:1965AJ.....70..193J. doi:10.1086/109714. Archived (PDF) from the original on 22 March 2020. Retrieved 22 March 2020.
153. Encrenaz, T.; Bibring, J. P.; Blanc, M.; Barucci, M. A.; Roques, F.; Zarka, P. H. (2004). The Solar System (3rd ed.). Springer. p. 1.
154. Torres, S.; Cai, M. X.; Brown, A. G. A.; Portegies Zwart, S. (September 2019). "Galactic tide and local stellar perturbations on the Oort cloud: creation of interstellar comets". Astronomy & Astrophysics. 629: 13. arXiv:1906.10617. Bibcode:2019A&A...629A.139T. doi:10.1051/0004-6361/201935330. S2CID 195584070. A139.
155. Norman, Neil (May 2020). "10 great comets of recent times". BBC Sky at Night Magazine. Archived from the original on 25 January 2022. Retrieved 10 April 2022.
156. Chebotarev, G. A. (1 January 1963). "Gravitational Spheres of the Major Planets, Moon and Sun". Astronomicheskii Zhurnal. 40: 812. Bibcode:1964SvA.....7..618C. ISSN 0004-6299. Archived from the original on 7 May 2024. Retrieved 6 May 2024.
157. Swaczyna, Paweł; Schwadron, Nathan A.; Möbius, Eberhard; Bzowski, Maciej; Frisch, Priscilla C.; Linsky, Jeffrey L.; McComas, David J.; Rahmanifard, Fatemeh; Redfield, Seth; Winslow, Réka M.; Wood, Brian E.; Zank, Gary P. (1 October 2022). "Mixing Interstellar Clouds Surrounding the Sun". The Astrophysical Journal Letters. 937 (2): L32:1–2. arXiv:2209.09927. Bibcode:2022ApJ...937L..32S. doi:10.3847/2041-8213/ac9120. ISSN 2041-8205.
158. Linsky, Jeffrey L.; Redfield, Seth; Tilipman, Dennis (November 2019). "The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60Fe Accretion". The Astrophysical Journal. 886 (1): 19. arXiv:1910.01243. Bibcode:2019ApJ...886...41L. doi:10.3847/1538-4357/ab498a. S2CID 203642080. 41.
159. Anglada-Escudé, Guillem; Amado, Pedro J.; Barnes, John; et al. (2016). "A terrestrial planet candidate in a temperate orbit around Proxima Centauri". Nature. 536 (7617): 437–440. arXiv:1609.03449. Bibcode:2016Natur.536..437A. doi:10.1038/nature19106. PMID 27558064. S2CID 4451513.
160. Linsky, Jeffrey L.; Redfield, Seth; Tilipman, Dennis (20 November 2019). "The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60 Fe Accretion*". The Astrophysical Journal. 886 (1): 41. arXiv:1910.01243. Bibcode:2019ApJ...886...41L. doi:10.3847/1538-4357/ab498a. ISSN 0004-637X. S2CID 203642080.
161. Zucker, Catherine; Goodman, Alyssa A.; Alves, João; et al. (January 2022). "Star formation near the Sun is driven by expansion of the Local Bubble". Nature. 601 (7893): 334–337. arXiv:2201.05124. Bibcode:2022Natur.601..334Z. doi:10.1038/s41586-021-04286-5. ISSN 1476-4687. PMID 35022612. S2CID 245906333.
162. Alves, João; Zucker, Catherine; Goodman, Alyssa A.; Speagle, Joshua S.; Meingast, Stefan; Robitaille, Thomas; Finkbeiner, Douglas P.; Schlafly, Edward F.; Green, Gregory M. (23 January 2020). "A Galactic-scale gas wave in the Solar Neighborhood". Nature. 578 (7794): 237–239. arXiv:2001.08748v1. Bibcode:2020Natur.578..237A. doi:10.1038/s41586-019-1874-z. PMID 31910431. S2CID 210086520.
163. McKee, Christopher F.; Parravano, Antonio; Hollenbach, David J. (November 2015). "Stars, Gas, and Dark Matter in the Solar Neighborhood". The Astrophysical Journal. 814 (1): 24. arXiv:1509.05334. Bibcode:2015ApJ...814...13M. doi:10.1088/0004-637X/814/1/13. S2CID 54224451. 13.
164. Alves, João; Zucker, Catherine; Goodman, Alyssa A.; et al. (2020). "A Galactic-scale gas wave in the solar neighborhood". Nature. 578 (7794): 237–239. arXiv:2001.08748. Bibcode:2020Natur.578..237A. doi:10.1038/s41586-019-1874-z. PMID 31910431. S2CID 210086520.
165. Mamajek, Eric E.; Barenfeld, Scott A.; Ivanov, Valentin D.; Kniazev, Alexei Y.; Väisänen, Petri; Beletsky, Yuri; Boffin, Henri M. J. (February 2015). "The Closest Known Flyby of a Star to the Solar System". The Astrophysical Journal Letters. 800 (1): 4. arXiv:1502.04655. Bibcode:2015ApJ...800L..17M. doi:10.1088/2041-8205/800/1/L17. S2CID 40618530. L17.
166. Raymond, Sean N.; et al. (January 2024). "Future trajectories of the Solar System: dynamical simulations of stellar encounters within 100 au". Monthly Notices of the Royal Astronomical Society. 527 (3): 6126–6138. arXiv:2311.12171. Bibcode:2024MNRAS.527.6126R. doi:10.1093/mnras/stad3604.
167. "StarChild Question of the Month – Does the Sun move around the Milky Way?". NASA. February 2000. Archived from the original on 30 October 2023.
168. Siegel, Ethan (30 August 2018). "Our Motion Through Space Isn't A Vortex, But Something Far More Interesting". Forbes. Archived from the original on 25 November 2023. Retrieved 25 November 2023.
169. Currin, Grant (30 August 2020). "How long is a galactic year?". Live Science. Archived from the original on 25 November 2023. Retrieved 25 November 2023.
170. Leong, S. (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook. Archived from the original on 22 August 2011. Retrieved 10 May 2007.
171. Raymo, Chet (1990). Three Hundred and Sixty Five Starry Nights: An Introduction to Astronomy for Every Night of the Year. Touchstone. ISBN 9780671766061.
172. Table 3 of Kogut, A.; et al. (1993). "Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps". Astrophysical Journal. 419 (1993): 1. arXiv:astro-ph/9312056. Bibcode:1993ApJ...419....1K. doi:10.1086/173453.
173. Hawthorn, Hannah (2022). The Magick of Birthdays. New York: Penguin. p. 103. ISBN 978-0-593-53854-8.
174. Singh, Madanjeet (1993). The Sun. New York: ABRAMS. p. 305. ISBN 978-0-8109-3838-0.
175. Leverington, David (2003). Babylon to Voyager and beyond: a history of planetary astronomy. Cambridge University Press. pp. 6–7. ISBN 978-0-521-80840-8.
176. Sider, D. (1973). "Anaxagoras on the Size of the Sun". Classical Philology. 68 (2): 128–129. doi:10.1086/365951. JSTOR 269068. S2CID 161940013.
177. Goldstein, B. R. (1967). "The Arabic Version of Ptolemy's Planetary Hypotheses". Transactions of the American Philosophical Society. 57 (4): 9–12. doi:10.2307/1006040. JSTOR 1006040.
178. Stahl, William Harris (1945). "The Greek Heliocentric Theory and Its Abandonment". Transactions and Proceedings of the American Philological Association. 76: 321–332. doi:10.2307/283344. ISSN 0065-9711. JSTOR 283344.
179. Toomer, G. J. (7 March 2016). "Seleucus (5), of Seleuceia, astronomer". Oxford Research Encyclopedia of Classics. Oxford University Press. doi:10.1093/acrefore/9780199381135.013.5799. ISBN 978-0-19-938113-5. Retrieved 27 May 2024.
180. Fraknoi, Andrew; Morrison, David; Wolff, Sidney (9 March 2022). "2.4 The Birth of Modern Astronomy". Astronomy 2e. OpenStax. Retrieved 27 May 2024.
181. Ead, Hamed A. (1998). Averroes As A Physician. University of Cairo. Retrieved 27 May 2024.
182. "Galileo Galilei (1564–1642)". BBC. Archived from the original on 29 September 2018. Retrieved 22 March 2006.
183. Singer, C. (1959). A short History of scientific ideas to 1900. Oxford University Press. p. 151.
184. Ronan, C. (1983). "The Arabian Science". The Cambridge Illustrated History of the World's Science. Cambridge University Press. pp. 201–244. at pp. 213–214.
185. Goldstein, Bernard R. (March 1972). "Theory and Observation in Medieval Astronomy". Isis. 63 (1): 39–47 [44]. Bibcode:1972Isis...63...39G. doi:10.1086/350839. S2CID 120700705.
186. Chapman, Allan (April 2005). Kurtz, D. W. (ed.). Jeremiah Horrocks, William Crabtree, and the Lancashire observations of the transit of Venus of 1639. Transits of Venus: New Views of the Solar System and Galaxy, Proceedings of IAU Colloquium #196, held 7–11 June 2004 in Preston, U.K. Proceedings of the International Astronomical Union. Vol. 2004. Cambridge: Cambridge University Press. pp. 3–26. Bibcode:2005tvnv.conf....3C. doi:10.1017/S1743921305001225.
187. Teets, Donald (December 2003). "Transits of Venus and the Astronomical Unit" (PDF). Mathematics Magazine. 76 (5): 335–348. doi:10.1080/0025570X.2003.11953207. JSTOR 3654879. S2CID 54867823. Archived (PDF) from the original on 3 February 2022. Retrieved 3 April 2022.
188. "Sir Isaac Newton (1643–1727)". BBC Teach. BBC. Archived from the original on 10 March 2015. Retrieved 22 March 2006.
189. "Herschel Discovers Infrared Light". Cool Cosmos. Archived from the original on 25 February 2012. Retrieved 22 March 2006.
190. Wolfschmidt, Gudrun (1998). "Instruments for observing the Corona". In Warner, Deborah Jean; Bud, Robert (eds.). Instruments of Science, An Historical Encyclopedia. Science Museum, London, and National Museum of American History, Smithsonian Institution. pp. 147–148. ISBN 9780815315612.
191. Parnel, C. "Discovery of Helium". University of St Andrews. Archived from the original on 7 November 2015. Retrieved 22 March 2006.
192. Thomson, W. (1862). "On the Age of the Sun's Heat". Macmillan's Magazine. 5: 388–393. Archived from the original on 25 September 2006. Retrieved 25 August 2006.
193. Stacey, Frank D. (2000). "Kelvin's age of the Earth paradox revisited". Journal of Geophysical Research. 105 (B6): 13155–13158. Bibcode:2000JGR...10513155S. doi:10.1029/2000JB900028.
194. Lockyer, J. N. (1890). "The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems". London and New York. Bibcode:1890mhsr.book.....L.
195. Darden, L. (1998). "The Nature of Scientific Inquiry". Archived from the original on 17 August 2012. Retrieved 25 August 2006.
196. Hawking, S. W. (2001). The Universe in a Nutshell. Bantam. p. 12. ISBN 978-0-553-80202-3.
197. "Studying the stars, testing relativity: Sir Arthur Eddington". Space Science. European Space Agency. 2005. Archived from the original on 20 October 2012. Retrieved 1 August 2007.
198. Bethe, H.; Critchfield, C. (1938). "On the Formation of Deuterons by Proton Combination". Physical Review. 54 (10): 862. Bibcode:1938PhRv...54Q.862B. doi:10.1103/PhysRev.54.862.2.
199. Bethe, H. (1939). "Energy Production in Stars". Physical Review. 55 (1): 434–456. Bibcode:1939PhRv...55..434B. doi:10.1103/PhysRev.55.434. PMID 17835673. S2CID 36146598.
200. Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars" (PDF). Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. Archived (PDF) from the original on 23 July 2018. Retrieved 12 April 2020.