x
 Image taken in 2013 in visible light with sunscreen, with sunspots . |
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 Image taken in 2010 in false colors, seen in ultraviolet light (wavelength 30.4 nm) |
|
| Observational data | |
|---|---|
| Dist. average compared to Earth | 1 have 9. 149,600,000 km [1] 8 min 19 s at the speed of light |
| Brightness ( V ) | −26.74 [2] |
| Absolute magnitude | 4.83 [2] |
| Star classification | G2V [3] |
| metalicos | Z = 0.0122 [4] |
| Angular diameter | 31.6–32.7 [5] |
| Orbital features | |
| Average distance from the center of the Milky Way | ≈2.7 × 10 17 km 27,200 light-years |
| Per. galactic | (2.25–2.50) × 10 8 years |
| Speed | ≈ 220 km / s (orbit around the center of the Milky Way) ≈ 20 km / s (relative to the average speed of other stars in the stellar neighborhood) ≈ 370 km / s (relative to background cosmic radiation) [6] |
| Physical characteristics | |
| Equatorial radius | 695,700 km, [7] 696,342 km [8] 109 × Terra [9] |
| The equatorial circumference | 4.379 × 10 6 km [9] 109 × Earth [9] |
| Flatten | 9 × 10 −6 |
| Surface | 6.09 × 10 12 km 2 [9] 12,000 × Earth [9] |
| Volume | 1.41 × 10 18 km 3 [9] 1,300,000 × Earth |
| Table | 1.9884 × 10 30 kg [2] 333,000 × Earth [2] |
| Medium density | 1,408 g/cm3 [2][9][10] 0,255 × Terra[2][9] |
| Equatorial surface gravity | 274 m/s2 [2] 28 × Terra[9] |
| Evacuation speed (from the surface) |
617,7 km/s [9] 55 × Terra[9] |
| Temperature | Center: 15,099,726 ° C [2] The photosphere : 5.498.85 ° C [2] Crown : ≈ 5000 ° C |
| Brightness (L ground ) | 3,828 × 10 26 W [2] ≈ 3.75 × 10 28 lm ≈ 98 lm / W efficiency |
| Average radiance (I soil ) | 2,009 × 107 W·m−2·sr−1 |
| Age | ≈ 4.6 billion years [11] [12] |
| Rotational characteristics | |
| Axial tilt | 7.25 ° [2] (relative to the elliptic plane) 67.23 ° (relative to the galactic plane) |
| Straight ascent of the North Pole [13] |
286.13 ° 19 h 4 min 30 s |
| Declination of the North Pole |
+ 63.87 ° 63 ° 52 'North |
| Sidereal rotation (at the equator) |
25.05 of [2] |
| (16 ° latitude) | 25.38 z [2] 25 z 9 h 7 min 12 s [13] |
| (the police) | 34.4 out of [2] |
| Rotational speed (at the equator) |
7,189 km/h [9] |
| Photosphere composition | |
| Hydrogen | 73.46% [14] |
| Helium | 24.85% |
| Oxygen | 0.77% |
| Carbon | 0.29% |
| Iron | 0.16% |
| Neon | 0.12% |
| Azote | 0.09% |
| Silicon | 0.07% |
| Magnesium | 0.05% |
| Sulfur | 0.04% |
The sun is the star in the center of the Solar System . It is an almost perfect sphere of hot plasma , [15] [16] held by gravity and shaped by a magnetic field . [17] It is by far the most important source of energy for life on Earth . Its diameter is about 1.39 million kilometers (or is 109 times larger than that of the Earth), and its mass is about 330,000 times larger than that of the Earth. It represents approximately 99.86% of the total mass of the Solar System. [18] About three-quarters of the Sun's mass is made up of hydrogen(~ 73%); the rest is mostly helium (~ 25%), with much smaller amounts of heavier elements, including oxygen , carbon , neon and iron . [19]
The sun is a G-type main sequence star (G2V). As such, it is informally and not completely exactly called a yellow dwarf (its light is closer to white than yellow). It formed about 4.6 billion years ago [a] [11] [20] from the gravitational collapse of matter in a region of a large molecular cloud . Most of this matter gathered in the center, while the rest flattened into an orbiting disk that became the Solar System . The central mass became so hot and dense that nuclear fusion finally began in its core. It is believed that almost all stars are formed through this process.
Currently, the Sun fuses about 600 million tons of hydrogen into helium every second, turning 4 million tons of matter into energy. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of light and heat from the Sun.
When the fusion of hydrogen in the nucleus diminishes to the point where the Sun is no longer in hydrostatic equilibrium, its nucleus will undergo a significant increase in density and temperature as its outer layers expand, eventually turning the Sun into a red giant . It has been calculated that the Sun will become large enough to incorporate the current orbits of Mercury and Venus and make the Earth uninhabitable - but after about five billion years. After that, it will overflow its outer layers and become a dense type of cooling star known as the white dwarf. and it will no longer produce energy by fusion, but will continue to shine and release heat from its previous fusion.
The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been considered a deity in some cultures.
The sun is a G-type main-sequence star that comprises about 99.86% of the mass of the Solar System. It has an absolute magnitude of +4.83, and is estimated to be a brighter star than about 85% of the stars in the Milky Way , most of them being red dwarfs . [21] [22] The sun is a population star I or a star rich in heavy elements. [2. 3]
The formation of the Sun could have been triggered by shock waves from one or more nearby supernovae . [24] This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium , compared to the abundances of these elements in Population II stars, poor in heavy elements. Heavy elements could most likely be produced by endothermic nuclear reactions during a supernova or by transmutation by neutron absorption into a second-generation massive star. [2. 3]
The sun is by far the brightest object in the Earth's sky, with an apparent magnitude of -26.74. [25] [26] It is about 13 billion times brighter than its next bright star, Sirius , which has an apparent magnitude of -1.46. An astronomical unit (approximately 150,000,000 km) is defined as the average distance from the center of the Sun to the center of the Earth, although the distance varies as the Earth moves from perihelion in January to aphelion in July. [27]At this average distance, light travels from the Sun's horizon to the Earth's horizon in about 8 minutes and 19 seconds, while light from the nearest points of the Sun and Earth lasts about two seconds less. The energy of this sunlight sustains almost all life [b] on Earth through photosynthesis , [28] and controls the Earth's climate and weather.
The sun does not have a definite boundary, as do the telluric planets, but in its outer layers, the density of the gas decreases exponentially as the distance from its center increases. [29] In order to be measured, 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. [30] By this measure, the Sun is an almost perfect sphere, with a flattening estimated at about 9/1000000, [31] which means that its polar diameter differs from the equator by only 10 kilometers. [32] The tidal effect of the planets is weak and does not significantly affect the shape of the Sun. [33]Because the Sun is made of a plasma that is not a solid, its various parts can rotate at different speeds; this behavior is known as differential rotation. In a star-defined frame of reference, the rotation period is about 25.6 days at the equator and 33.5 days at the poles. However, due to the constant change of the Earth's point of observation as it orbits the Sun, the apparent rotation of the Sun at its equator is 28 days. [34]
The interior of the Sun is not directly observable, and the Sun itself is opaque to electromagnetic radiation . However - just as seismology uses earthquake-generated waves to study the Earth's internal structure - helioseismology uses pressure waves (infrasounds) that pass through the Sun to study and visualize the star's internal structure. [35] Computational modeling is also used as a tool to test the compatibility of theoretical models of its deeper layers with observations.
It is believed that the Sun's core extends from the center to about 20-25% of the sun's rays. [36] It has a density of up to 150 g / cm 3 [37] [38] (about 150 times the density of water) and a temperature of about 15 million degrees Kelvin. [38] In contrast, the Sun's surface temperature is about 5,800 K. Recent analysis of SOHO mission data favors a faster rotational speed in the core than in the radiation zone above. [36] Throughout the life of the Sun, energy was produced by nuclear fusion in the base region through a series of nuclear reactions called the pp chain (proton-proton); this process converts hydrogenin helium . [39] Only 0.8% of the energy generated by the Sun comes from another sequence of fusion reactions called the NOC cycle, although this proportion is expected to increase as the Sun ages. [40]
The nucleus is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated in 24% of the Sun's radius, and at a distance of 30% from the center, the fusion has stopped almost entirely. The rest of the star is heated by heat transferred from the core to the outside. [41] [42]
The sun emits 384.6 jotts (3,846 × 10 26 W) of energy, [2] which corresponds to 9,192 × 10 10 megatons of TNT per second. This energy is equivalent to 4.26 million tons.
The fusion rate in the core is in a state of equilibrium: a slightly higher fusion rate would cause the core to heat up more and expand slightly despite the pressure of the outer layers, and this would reduce the fusion rate, correcting the disturbance. ; and the slightly lower rate would cause the core to cool and contract slightly, leading to an increase in the rate of fusion and a return to the current speed. [43] [44]
In the layers located at more than 25% of the solar ray starting from the center of the Sun, there is practically no nuclear transformation of the elements and heat generation, but only the conduction of heat from the core, which is why the term coating is used in relation to these layers. [45] Solar material is hot and dense enough to allow heat to be transferred from the center to the outside by thermal radiation . [45]
Thermal convection does not occur in this area; although the temperature in this area decreases as the core distance increases (from about 7,000,000 K to 2,000,000 K), the temperature gradient is lower than the adiabatic plasma gradient, not allowing convection to occur. [38] Energy transfer occurs through the interaction of heat radiation with plasma-forming particles; hydrogen and helium ions emit photons that travel only a short distance before being reabsorbed by other ions. [45] In the radial area, the density decreases 100 times, from 20 g / cm 3 to 0.2 g / cm 3 . [45]
The radiative zone and the convective zone are separated by a transition layer called tachocline. This is a region where the movement of the Sun's general rotation (differential rotation due to the gaseous nature) and convection produce a velocity field that forces the plasma to flow between the lines of force of the local magnetic field. [46] This transition between the two distinct rotational regimes is quite abrupt and occurs at the base of the convective zone. Estimation of the thickness of this layer varies between 0.016-0.038 sunlight.
At present, it is considered the hypothesis that a magnetic dynamo in this layer generates the magnetic field of the Sun. [38]
The convection area of the Sun extends from 0.7 sunbeams (500,000 km) to close to the surface. In this layer, the solar plasma is not dense enough or warm enough to transfer thermal energy from the inside to the outside through radiation - in other words, it is not opaque enough. Instead, the plasma density is low enough to allow convective currents to develop and move the Sun's energy outward to its surface. The material heated in tachocline raises the heat that it cannot radiate to the upper layers, expands, which reduces its density and allows it to increase. As a result, an orderly movement of mass develops in the thermal cells that carry most of the heat outward to the photosphere. When the matter cools diffusely and radiates just below the photosphere, its density increases and sinks to the base of the convection zone, where it again raises heat from the top of the radiative zone and the convective cycle repeats. In the photosphere, the temperature drops to 5700 K and the density to 0.2 g / m3 (approximately 1/6000 density of air at sea level). [38]
The columns of hot matter that rise in the convection zone form a trace on the surface of the Sun, giving it a granular appearance called solar granulation at the smallest scale and supergranulation at larger scales.
The turbulent convection in this outer layer of the solar interior creates a dynamic "small-scale" action that generates local magnetic fields with their own northern and southern poles scattered over the entire surface of the Sun. The hot matter columns of the Sun are Bénard cells and take the form of hexagonal prisms. [47]
_as_of_January_20,_2020.jpg)
The surface layer of the Sun, visible to the naked eye, is the photosphere . In this layer, most photons leave the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The photosphere is tens to hundreds of kilometers thick, although this layer is a small part of the star's radius, important phenomena occur in it, and the properties of the matter it builds change significantly. In this layer the temperature drops from 7,610 to 4465 K. Because the upper part of the photosphere is colder than the lower part, the image of the Sun appears brighter in the center than on the edge, in a phenomenon known as edge darkening. [48] The spectrum of sunlight is approximately the spectrum of an absolutely black body radiating at 5,777 K, intersected by atomic absorption lines in the diluted layers above the photosphere.
The photosphere has a particle density of ~ 10 23 m −3 (approximately 0.37% of the number of particles in the volume of the Earth's atmosphere at sea level, ie the photosphere is slightly more transparent than the clean air on Earth). This layer is not completely ionized - the degree of ionization is about 3%, leaving almost all the hydrogen in atomic form. [49]
During early studies of the optical spectrum of the photosphere, several lines of absorption were discovered that did not correspond to any known chemical element on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were caused by a new element he called helium , after the Greek sun god Helios . Twenty-five years later, helium was isolated on Earth. [50]
The parts of the Sun outside the photosphere are collectively called the solar atmosphere. [48] There are five main areas outside the photosphere: the minimum temperature layer, the chromosphere , the transition region , the corona and the heliosphere . [48] These layers can be observed with telescopes operating in the entire electromagnetic spectrum from radio waves, visible light to gamma rays.
The coldest part of the Sun is the minimum temperature layer that extends up to about 500 km above the photosphere, where the temperature drops to about 4,100 K. [48] This part of the Sun is cold enough to allow the existence of simple molecules, such as would be carbon monoxide and water, which can be detected by their absorption spectra. [51]
The chromosphere, the transition region and the corona are much hotter than the surface of the Sun. [48] The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona. [52]
Above the minimum temperature layer is a start of about 2,000 km thick in the spectrum of which the emission and absorption lines dominate. [48] It is called the Greek chromosphere χρώμα ( chroma ), which means "color" because the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses . [45] Chromosphere temperature gradually increases with altitude, to about 20,000 K at the top of the layer. [48] In the upper part of the chromosphere, helium becomes partially ionized. [53]
Above the chromosphere there is a thin transition layer (about 200 km), in which the temperature rises sharply from about 20,000 K in the upper chromosphere to almost 1,000,000 Kelvin. [54] The increase in temperature is facilitated by the complete ionization of helium in the transition region, which significantly reduces the radiation cooling of the plasma. [53] The transition layer does not occur at a strictly defined altitude, but is rather a kind of "halo" around the manifestations of chromosphere activity, such as spikes and protuberances , and is in constant chaotic motion. [45]From the Earth's surface it is difficult to observe the transition layer, but it is easy to observe from space by instruments sensitive to extreme ultraviolet. [55]
The corona is the next layer of the Sun's atmosphere. The low corona, close to the surface of the Sun, has a density of particles around 10 15 m −3 to 10 16 m −3 . [53] The average temperature of the corona and the solar wind is about 1,000,000–2,000,000 K; in the warmest regions it is 8,000,000–20,000,000 K. [54] Although there is no complete theory yet to explain the temperature of the corona, at least some of its heat is generated by magnetic reconnection. [54] [56]The corona is the extended atmosphere of the Sun, which has a much larger volume than the volume contained in the photosphere. The plasma flow from the outer surface of the corona, which propagates randomly and farther from the Sun in interplanetary space, is called the solar wind . [56]
The heliosphere - the thinnest outer region of the Sun's atmosphere - is filled with plasma from the solar wind. Its internal limit is defined by the area where the flow of the solar wind becomes faster than the speed of Alfvén waves, [57] at about 20 sunrays (0.1 au). Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona inside, because information (distortions) can only circulate at the speed of Alfvén waves. The solar wind travels continuously through the heliosphere, [58] [59] forming the Sun's magnetic field in a spiral shape, [56] until it reaches heliopause, at over 50 au from the Sun. In December 2004, the Voyager 1 probehe went through a shock front that is believed to be part of the heliopause. [60] At the end of 2012, Voyager 1 recorded a marked increase in cosmic collisions and a sharp decrease in lower-energy particles from the solar wind, which suggested that the probe went through heliopause and entered the interstellar medium. . [61]
The heliosphere extends far beyond the planets of the Solar System and the orbits of Kuiper belt objects such as Pluto. Heliopause sets the limit of the Sun's influence, beyond which the interstellar environment extends. The Sun's gravitational field dominates a larger area, maintaining the Oort cloud and extending far beyond the heliosphere. [62]
The sun is an active magnetic star, has a strong and changing magnetic field , which varies from year to year and changes polarity around a maximum of eleven years. [17] [64] [65] The Sun's magnetic field causes many effects, which are collectively called solar activity , including sunspots , solar flares, and variations in the solar wind . Both the coronal mass ejections and the high-velocity fluxes of the solar wind carry plasma and magnetic field outward into the Solar System. [66] The impact of solar activity on Earth is manifested, among other things, in the appearance of aurorasat temperate and high latitudes, interruption of radio communications and electricity. Solar activity is believed to have played an important role in the formation and evolution of the Solar System.
Sunspots are regions with intense magnetic activity where convection is inhibited by a strong magnetic field, reducing heat transfer from the hot interior to the surface. As a result, sunspots are slightly colder than the surrounding photosphere, so they look dark. The number of spots visible on the Sun is not constant, it changes in an 11-year cycle, known as the solar cycle. At an ordinary solar minimum , the spots are difficult to see, sometimes not at all, and those that appear appear at high heliographic latitudes. As the solar cycle progresses to its maximum, the number of sunspots increases and they tend to form closer to the solar equator, a phenomenon known as Spörer's law. Sunspots usually occur in pairs with opposite magnetic polarity. The largest sunspots can be tens of thousands of kilometers in diameter. [67]
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The solar cycle has a great impact on both space weather and the Earth's climate, because the Sun's brightness is directly related to magnetic activity. [68] Solar activity minimums appear to be correlated with lower temperatures on Earth, and particularly long solar cycles are correlated with warmer periods. It seems that in the seventeenth century the solar cycle stopped completely for several years. During this era, known as the Maunder Minimum or Little Ice Age , Europe experienced exceptionally low temperatures. [69] A recent theory holds that magnetic instabilities in the Sun's core cause fluctuations over periods of 41,000 or 100,000 years. They can provide an explanation forglaciations as an alternative to the Milankovic cycles. [70] [71]
All matter in the Sun is in the form of gas and plasma at high temperatures. Due to this, the Sun can rotate faster at the equator (about 25 days) than at higher heliographic latitudes (about 35 days near the poles). The differential rotation of the Sun causes a twisting of the lines of its magnetic field , creating loops of the magnetic field that rise from the surface of the Sun and cause sunspots and bumps . This movement drives the solar dynamo and the 11-year cycle of solar activity. [72] [73]
The solar magnetic field extends far beyond the Sun itself. Solar wind plasma carries a magnetic field in interplanetary space, forming what is called an interplanetary magnetic field. [56] In an approximation known as ideal magnetohydrodynamics, plasma particles move only along magnetic field lines. As a result, the solar wind flowing outward extends the interplanetary magnetic field outward, forcing it into an approximately radial structure. Because the northern and southern fields of the solar equator have different poles, with the induction vector facing the sun or vice versa, in the equatorial plane of the sun there is a thin boundary layer along which a weak current flows. [56] At a greater distance, the rotation of the Sun transforms the magnetic field and this layer into the shape of an Archimedes spiral , creating a structure called the Parker spiral. [56] The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field, which on the surface of the photosphere has an induction of 50 to 400 μT and decreases with the inverse of the cube of the distance from the Sun to about 0.1 nT in Earth's orbit. According to space probe observations, the interplanetary magnetic field near Earth has a value of 5 nT. [74] The difference is caused by the magnetic fields generated by the electric currents flowing in the plasma surrounding the Sun.
The sun is now about half of the most stable part of its life. It has not changed dramatically for over 4 billion years, [a] and will remain fairly stable for more than five billion years. However, after the hydrogen fusion in its core stops, the Sun will undergo dramatic changes, both internally and externally.
The sun formed about 4.6 billion years ago as a result of the collapse of part of a huge molecular cloud made up mostly of hydrogen and helium and which probably gave rise to many other stars. [75] This age is estimated using computer models of stellar evolution and nucleocosmoynchronology. [11] The result is consistent with the radiometric dating of the oldest material in the Solar System, with an age of 4.567 billion years. [76] [77]
Research on meteorites has revealed traces of stable daughter isotope, derived from the decomposition of short-lived isotopes, such as iron-60 that can be formed naturally only by the explosion of short-lived stars. This indicates that one or more supernovae must have exploded near where the Sun was born. A shock wave from a nearby supernova would have triggered the formation of the Sun by compressing matter within the molecular cloud and causing certain regions to collapse under their own gravity. [78] The principle of conservation of kinetic momentum caused the faster fragment of the collapsed cloud fragment to rotate. Most of the mass was concentrated in the center, forming aprotostate , while the rest flattened, forming a protoplanetary disk , from which the planets and other satellites of the Sun were later formed . The compression of the matter in the core of the protostar generated large amounts of heat, which increased as the gas accelerated on the surrounding disk, to the fusion reactions in the center. 10 million years after the cloud began to collapse, the Sun was born, a star with a diameter about 1.33 times larger than today and a surface temperature of 4500 K. Intense solar wind removed the remnants of the solar nebula . [79] [80] After about 17 million years, the Sun reached hydrostatic equilibrium and joined the stars in the main sequence., starting the longest stage of its evolution. [79]
.svg)
The sun is almost halfway through its star stage in the main sequence , during which time the nuclear fusion reactions in its nucleus fuse hydrogen into helium. Every second, over four million tons of matter are transformed into energy in the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far transformed about 100 times the mass of the Earth into energy, about 0.03% of the total mass of the Sun. The sun will spend a total of about 10 billion years as a star in the main sequence. [82]
The main sequence is the longest and most stable period in the life of the Sun, but this period is evolving. The changes occur mainly in response to changes in the concentration of components caused by the conversion of hydrogen to helium. The sun is in hydrostatic equilibrium, in which the internal pressure balances the pressure of the outer layers of the star. In a star the size of the Sun, at this stage of its life, the pressure is mainly caused by plasma particles. The core matter behaves like a perfect gas, the pressure depends on the temperature and the number of particles per unit volume. The conversion of hydrogen to helium quadruples the number of particles in the nucleus. This, in turn, increases the density of the nucleus and releases gravitational energy,Virial theorem ).
Higher core temperature and higher energy emission make the outer layers expand easily, and the star becomes brighter. Internally, the changes in the Sun are more significant, but their effects are not very visible from the outside, as long as the star continues to burn hydrogen in the nucleus. [83]
The sun gradually becomes hotter during its time in the main sequence, the nucleus shrinks, allowing the outer layers of the Sun to get closer to the center and experience a stronger gravitational force, according to the law of the inverse square. This stronger force increases the pressure on the core, which is counterbalanced by a gradual increase in the rate at which the fusion occurs. This process accelerates as the core gradually becomes denser. It is estimated that the Sun has become 30% brighter in the last 4.5 billion years. [84] It is currently increasing in brightness by about 1% every 100 million years. [85]
The sun does not have enough mass to explode as a supernova. Instead, it will emerge from the main sequence in about 5 billion years and begin to turn into a red giant . [86] [87] As a red giant, the Sun will rise so large that it will include Mercury , Venus , and probably Earth . [87] [88]
Even before it becomes a red giant, the Sun's brightness will almost double, and the Earth will receive as much sunlight as Venus receives today. Once the hydrogen core is depleted in 5.4 billion years, the Sun will expand in a subdued phase and slowly double in size in about half a billion years. It will then expand faster in about half a billion years until it is over two hundred times larger than it is today and several thousand times brighter. Then the red giant phase will begin, in which the Sun will stay for about a billion years and will lose about a third of its mass. [87]
The sun is composed mainly of two chemical elements : hydrogen and helium ; they represent 74.9% and 23.8% of the Sun's mass. [89] All heavier elements, called metals in astronomy, represent 1.33% by mass: oxygen (about 1% by mass), carbon (0.3%), neon (0.2%) and iron (0, 2 %). [90]
The original chemical composition of the Sun was inherited from the interstellar medium from which it formed. It would initially contain about 71.1% hydrogen, 27.4% helium and 1.5% heavier elements. [89] Hydrogen and most of the Sun's helium were said to have been produced by the Big Bang nucleosynthesis in the first 20 minutes of the Universe, and the heavier elements were produced by stellar nucleosynthesis in generations of stars that completed their stellar evolution. and spread matter in the interstellar medium before the Sun formed. [91]
Since the formation of the Sun, the main fusion process has involved the fusion of hydrogen into helium. Over the last 4.6 billion years, the amount of helium has gradually changed. Inside the nucleus, the proportion of helium increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements settled from the photosphere to the center of the Sun due to gravity . In the current photosphere, the helium fraction is low, and the metallicity is only 84% of what was in the protostellar phase (before nuclear fusion began in the nucleus). In the future, helium will continue to accumulate in the nucleus, and in about 5 billion years this gradual accumulation will cause the Sun to leave the main sequence.and become a red giant . [92]
The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. [93] The content of heavy solar elements is measured by spectroscopy of the Sun's photosphere and compared to the composition of meteorites , which have never been heated to their melting point. [19] These meteorites are believed to retain the composition of the protostellar Sun and are therefore unaffected by the sinking of heavier elements. [19]
In the 1970s, much research focused on the abundance of elements in the Sun's iron group. [94] [95] Although significant research has been done, until 1978 it was difficult to determine the abundance of elements in the iron group (eg cobalt and manganese) by spectrography due to their hyperfine structures. [94] In 1978, the content of individually ionized atoms in the elements of the iron group was determined. [94]
