A kind of “passport” of each star, including the Sun, is its spectrum. More than 30,000 lines belonging to 72 chemical elements have been recorded in the solar spectrum. Of course, the other 20 elements are also “present” on the Sun. It’s just that their lines are very weak and it’s not easy to notice them against the general background. The Sun currently consists of approximately 75% hydrogen and 25% helium by mass (92.1% hydrogen and 7.8% helium by number of atoms); all other chemical elements (the so-called "metals") contain only 0.2% of the total mass. This ratio changes slowly over time as hydrogen turns into helium in the Sun's core.
Internal structure of the Sun
The sun is a spherically symmetrical body in equilibrium. Everywhere at the same distances from the center of this ball, the physical conditions are the same, but they change noticeably as you approach the center. . The sun can be divided into several concentric layers, gradually transforming into each other (Fig. 3). At the center of the Sun, temperature and density reach their highest values. Conditions in the solar core (which occupies approximately 25% of its radius) are extremely extreme. The temperature reaches 15.6 million degrees Kelvin, and the pressure reaches 250 billion atmospheres. The gas at the core is more than 150 times denser than water. Nuclear reactions and the accompanying energy release occur most intensely near the very center of the Sun. As you move away from the center of the Sun, the temperature and density become lower, the release of energy quickly stops, and up to a distance of 0.2-0.3 radii from the center. At a distance from the center greater than 0.3 radius, the temperature becomes less than 5 million degrees. As a result, nuclear reactions practically do not occur here. These layers only transmit radiation that originates at greater depths outward, absorbed and re-emitted by the overlying layers. The last 20% of the way to the surface, energy is transferred by convection rather than radiation. Convection is the movement of matter as a whole, in currents or bubbles, similar to how boiling water behaves. Huge streams of hot gas rise upward, where they give up their heat to the environment, and cooled solar gas falls down.
Atmosphere of the Sun
All the layers of the Sun discussed above are actually not observable. Above the convective zone are the directly observable layers of the Sun, called its atmosphere. The solar atmosphere also consists of several different layers. In the structure of the outer layers of the Sun, the photosphere (“sphere of light”, translated from Greek), the chromosphere (“sphere of light”) and the corona are distinguished.
Photosphere
Visible solar surface - photosphere- This is a layer of gas about 700 km thick, in which the solar radiation coming to the Earth is formed. It is precisely through the middle of this layer that the conditional surface of our star is “drawn”, used for various calculations, specifically for measuring heights (up) and depths (down). In the outer, cooler, rarefied layers of the photosphere, Fraunhofer absorption lines appear against the background of a continuous spectrum. By analyzing the solar spectrum, which contains over 300 thousand absorption lines, they establish the chemical composition not of the photosphere, but of the layers located above it. Propagating into the upper layers of the solar atmosphere, waves that arise in the convective zone and in the photosphere transfer to them part of the mechanical energy of convective movements and produce heating of the gases of subsequent layers of the atmosphere - the chromosphere and corona. As a result, the upper layers of the photosphere with a temperature of about 4500K turn out to be the “coldest” on the Sun. Both deep into and upward from them, the temperature of the gases increases rapidly.
To get acquainted with the internal structure of the Sun, let us now take an imaginary journey from the center of the star to its surface. But how will we determine the temperature and density of the solar globe at different depths? How can we find out what processes take place inside the Sun?
It turns out that most of the physical parameters of stars (our Sun is also a star!) are not measured, but are calculated theoretically using computers. The starting points for such calculations are only some general characteristics of the star, for example its mass, radius, as well as the physical conditions prevailing on its surface: temperature, extent and density of the atmosphere, and the like. The chemical composition of a star (in particular, the Sun) is determined spectrally. And based on these data, a theoretical astrophysicist will create a mathematical model of the Sun. If such a model corresponds to the observational results, then it can be considered a fairly good approximation to reality. And we, relying on such a model, will try to imagine all the exotic depths of the great star.
The central part of the Sun is called its core. The matter inside the solar core is extremely compressed. Its radius is approximately 1/4 the radius of the Sun, and its volume is 1/45 (a little more than 2%) of the total volume of the Sun. Nevertheless, almost half of the solar mass is packed into the core of the star. This became possible due to the very high degree of ionization of solar matter. The conditions there are exactly the same as those needed for the operation of a thermonuclear reactor. The Core is a giant controlled power station where solar energy is generated.
Having moved from the center of the Sun to approximately 1/4 of its radius, we enter the so-called radiation energy transfer zone. This most extensive inner region of the Sun can be imagined as like the walls of a nuclear boiler, through which solar energy slowly leaks out. But the closer to the surface of the Sun, the lower the temperature and pressure. As a result, vortex mixing of the substance occurs and energy transfer occurs predominantly by the substance itself. This method of energy transfer is called convection, and the subsurface layer of the Sun where it occurs is called the convective zone. Solar researchers believe that its role in the physics of solar processes is exceptionally great. After all, it is here that various movements of solar matter and magnetic fields originate.
Finally we are at the visible surface of the Sun. Since our Sun is a star, a hot plasma ball, it, unlike the Earth, Moon, Mars and similar planets, cannot have a real surface, understood in the full sense of the word. And if we are talking about the surface of the Sun, then this concept is conditional.
The visible luminous surface of the Sun, located directly above the convective zone, is called the photosphere, which is translated from Greek as “sphere of light.”
The photosphere is a 300-kilometer layer. This is where solar radiation comes to us. And when we look at the Sun from Earth, the photosphere is precisely the layer that penetrates our vision. Radiation from deeper layers no longer reaches us, and it is impossible to see them.
The temperature in the photosphere increases with depth and is estimated on average at 5800 K.
The bulk of the optical (visible) radiation of the Sun comes from the photosphere. Here, the average gas density is less than 1/1000 the density of the air we breathe, and the temperature decreases to 4800 K as we approach the outer edge of the photosphere. Hydrogen under such conditions remains almost completely neutral.
Astrophysicists take the base of the photosphere as the surface of the great star. They consider the photosphere itself to be the lowest (inner) layer of the solar atmosphere. Above it are two more layers that form the outer layers of the solar atmosphere - the chromosphere and the corona. And although there are no sharp boundaries between these three layers, let’s get acquainted with their main distinguishing features.
The yellow-white light of the photosphere has a continuous spectrum, that is, it looks like a continuous rainbow stripe with a gradual transition of colors from red to violet. But in the lower layers of the rarefied chromosphere, in the region of the so-called temperature minimum, where the temperature drops to 4200 K, sunlight experiences absorption, due to which narrow absorption lines are formed in the solar spectrum. They are called Fraunhofer lines, named after the German optician Joseph Frau and Gopher, who carefully measured the wavelengths of 754 lines in 1816.
To date, over 26 thousand dark lines of varying intensity have been recorded in the spectrum of the Sun, arising due to the absorption of light by “cold” atoms. And since each chemical element has its own characteristic set of absorption lines, this makes it possible to determine its presence in the outer layers of the solar atmosphere.
The chemical composition of the Sun's atmosphere is similar to that of most stars formed within the last few billion years (called second-generation stars). Compared to old celestial bodies (stars of the first generation), they contain tens of times more heavy elements, that is, elements that are heavier than helium. Astrophysicists believe that heavy elements first appeared as a result of nuclear reactions that occurred during the explosions of stars, and perhaps even during the explosions of galaxies. During the formation of the Sun, the interstellar medium was already quite well enriched with heavy elements (the Sun itself does not yet produce elements heavier than helium). But our Earth and other planets condensed, apparently, from the same gas and dust cloud as the Sun. Therefore, it is possible that, by studying the chemical composition of our daylight, we are also studying the composition of the primary protoplanetary matter.
Since the temperature in the solar atmosphere varies with altitude, absorption lines at different levels are created by atoms of different chemical elements. This makes it possible to study the various atmospheric layers of the great star and determine their extent.
Above the photosphere is a rarer syllable! atmosphere of the Sun, which is called the chromosphere, which means "colored sphere". Its brightness is many times less than the brightness of the photosphere, so the chromosphere is visible only during short minutes of total solar eclipses, like a pink ring around the dark disk of the Moon. The reddish color of the chromosphere is caused by hydrogen radiation. This gas has the most intense spectral line - Ha - in the red region of the spectrum, and there is especially a lot of hydrogen in the chromosphere.
From the spectra obtained during solar eclipses, it is clear that the red line of hydrogen disappears at an altitude of approximately 12 thousand km above the photosphere, and the lines of ionized calcium cease to be visible at an altitude of 14 thousand km. This height is considered as the upper boundary of the chromosphere. As the temperature rises, the temperature increases, reaching 50,000 K in the upper layers of the chromosphere. With increasing temperature, the ionization of hydrogen and then helium increases.
The increase in temperature in the chromosphere is quite understandable. As is known, the density of the solar atmosphere quickly decreases with height, and a rarefied medium emits less energy than a dense one. Therefore, the energy coming from the Sun heats up the upper chromosphere and the corona lying above it.
Currently, heliophysicists using special instruments observe the chromosphere not only during solar eclipses, but also on any clear day. During a total solar eclipse, you can see the outermost layer of the solar atmosphere - the corona - a delicate pearly-silver glow extending around the eclipsed Sun. The total brightness of the corona is about one millionth the light of the Sun or half the light of the full Moon.
The solar corona is a highly rarefied plasma with a temperature close to 2 million K. The density of coronal matter is hundreds of billions of times less than the density of air near the Earth's surface. Under such conditions, atoms of chemical elements cannot be in a neutral state: their speed is so high that during mutual collisions they lose almost all their electrons and are ionized many times. This is why the solar corona consists mainly of protons (hydrogen atomic nuclei), helium nuclei and free electrons.
The exceptionally high temperature of the corona causes its material to become a powerful source of ultraviolet and X-ray radiation. For observations in these ranges of the electromagnetic spectrum, as is known, special ultraviolet and X-ray telescopes installed on spacecraft and orbital scientific stations are used.
Using radio methods (the solar corona intensely emits decimeter and meter radio waves), coronal rays are “viewed” up to distances of 30 solar radii from the edge of the solar disk. With distance from the Sun, the density of the corona decreases very slowly, and its uppermost layer flows into outer space. This is how solar wind is formed.
Only due to the volatilization of corpuscles, the mass of the Sun decreases every second by no less than 400 thousand tons.
The solar wind blows across the entire space of our planetary system. By then the initial speed reaches more than 1000 km/s, but then it slowly decreases. The Earth's orbit has an average wind speed of about 400 km/s. Ohm sweeps away in its path all the gases emitted by planets and comets, the smallest meteoric dust particles and even particles of low-energy galactic cosmic rays, carrying all this “garbage” to the outskirts of the planetary system. Figuratively speaking, we seem to be bathing in the crown of a great star...
A spectral analysis of solar rays showed that our star contains the most hydrogen (73% of the star’s mass) and helium (25%). The remaining elements (iron, oxygen, nickel, nitrogen, silicon, sulfur, carbon, magnesium, neon, chromium, calcium, sodium) account for only 2%. All substances discovered on the Sun are found on Earth and on other planets, which indicates their common origin. The average density of the Sun's matter is 1.4 g/cm3.
How the Sun is studied
The sun is a “” with many layers that have different composition and density, and different processes take place in them. Observing a star in the spectrum familiar to the human eye is impossible, but telescopes, radio telescopes and other instruments have now been created that record ultraviolet, infrared, and X-ray radiation from the Sun. From Earth, observation is most effective during a solar eclipse. During this short period, astronomers around the world study the corona, prominences, chromosphere and various phenomena occurring on the only star available for such detailed study.
Structure of the Sun
The corona is the outer shell of the Sun. It has a very low density, which is why it is visible only during an eclipse. The thickness of the outer atmosphere is uneven, so holes appear in it from time to time. Through these holes, the solar wind rushes into space at a speed of 300-1200 m/s - a powerful flow of energy, which on earth causes northern lights and magnetic storms.
The chromosphere is a layer of gases reaching a thickness of 16 thousand km. Convection of hot gases occurs in it, which, from the surface of the lower layer (photosphere), fall back again. They are the ones who “burn through” the corona and form solar wind streams up to 150 thousand km long.
The photosphere is a dense opaque layer 500-1,500 km thick, in which severe firestorms with a diameter of up to 1 thousand km occur. The temperature of the photosphere gases is 6,000 oC. They absorb energy from the underlying layer and release it as heat and light. The structure of the photosphere resembles granules. Gaps in the layer are perceived as sunspots.
The convective zone, 125-200 thousand km thick, is the solar shell in which gases constantly exchange energy with the radiation zone, heating up, rising to the photosphere and, cooling, descending again for a new portion of energy.
The radiation zone is 500 thousand km thick and has a very high density. Here, the substance is bombarded with gamma rays, which are converted into less radioactive ultraviolet (UV) and x-rays (X) rays.
The crust, or core, is the solar “boiler”, where proton-proton thermonuclear reactions constantly occur, thanks to which the star receives energy. Hydrogen atoms transform into helium at a temperature of 14 x 10 °C. Here, titanic pressure is a trillion kg per cubic cm. Every second, 4.26 million tons of hydrogen are converted into helium.
Prominences
The surface of the Sun that we see is known as the photosphere. This is the area where light from the core finally reaches the surface. The photosphere has a temperature of about 6000 K and glows white.
Just above the photosphere, the atmosphere extends for several hundred thousand kilometers. Let's take a closer look at the structure of the Sun's atmosphere.
The first layer in the atmosphere has a minimum temperature, and is located at a distance of about 500 km above the surface of the photosphere, with a temperature of about 4000 K. For a star, this is quite cool.
Chromosphere
The next layer is known as the chromosphere. It is located at a distance of only about 10,000 km from the surface. In the upper part of the chromosphere, temperatures can reach 20,000 K. The chromosphere is invisible without special equipment that uses narrow-band optical filters. Giant solar prominences can rise in the chromosphere to a height of 150,000 km.
Above the chromosphere there is a transition layer. Below this layer, gravity is the dominant force. Above the transition region, the temperature rises quickly because helium becomes fully ionized.
Solar corona
The next layer is the corona, and it extends from the Sun millions of kilometers into space. You can see the corona during a total eclipse, when the disk of the luminary is covered by the Moon. The temperature of the corona is about 200 times hotter than the surface.
Atmosphere
The Earth's atmosphere is the air that we breathe, the gaseous shell of the Earth that is familiar to us. Other planets also have such shells. Stars are made entirely of gas, but their outer layers are also called atmospheres. In this case, those layers from which at least part of the radiation can freely escape into the surrounding space without being absorbed by the overlying layers are considered external.
Photosphere
The photosphere of the Sun begins 200-300 km deeper than the visible edge of the solar disk. These deepest layers of the atmosphere are called the photosphere. Since their thickness is no more than one three-thousandth of the solar radius, the photosphere is sometimes conventionally called the surface of the Sun.
The density of gases in the photosphere is approximately the same as in the Earth's stratosphere, and hundreds of times less than at the Earth's surface. The temperature of the photosphere decreases from 8000 K at a depth of 300 km to 4000 K in the uppermost layers. The temperature of the middle layer, the radiation from which we perceive, is about 6000 K.
Under such conditions, almost all gas molecules disintegrate into individual atoms. Only in the uppermost layers of the photosphere are relatively few simple molecules and radicals such as H 2, OH, CH preserved.
A special role in the solar atmosphere is played by the negative hydrogen ion, which is not found in earthly nature, which is a proton with two electrons. This unusual compound occurs in the thin outer, “coldest” layer of the photosphere when negatively charged free electrons, which are delivered by easily ionized atoms of calcium, sodium, magnesium, iron and other metals, “stick” to neutral hydrogen atoms. When generated, negative hydrogen ions emit most of the visible light. The ions greedily absorb this same light, which is why the opacity of the atmosphere quickly increases with depth. Therefore, the visible edge of the Sun seems very sharp to us.
Almost all of our knowledge about the Sun is based on the study of its spectrum - a narrow multi-colored strip of the same nature as a rainbow. For the first time, placing a prism in the path of a solar ray, Newton received such a stripe and exclaimed:
“Spectrum!” (Latin spectrum - “vision”). Later, dark lines were noticed in the spectrum of the Sun and considered to be the boundaries of colors. In 1815, the German physicist Joseph Fraunhofer gave the first detailed description of such lines in the solar spectrum, and they began to be called after him. It turned out that Fraunhofer lines correspond to certain parts of the spectrum that are strongly absorbed by atoms of various substances (see the article “Analysis of Visible Light”). In a telescope with high magnification, you can observe subtle details of the photosphere: it all seems strewn with small bright grains - granules, separated by a network of narrow dark paths. Granulation is the result of the mixing of warmer gas flows rising and colder ones descending. The temperature difference between them in the outer layers is relatively small (200-300 K), but deeper, in the convective zone, it is greater, and mixing occurs much more intensely. Convection in the outer layers of the Sun plays a huge role in determining the overall structure of the atmosphere.
Ultimately, it is convection, as a result of a complex interaction with solar magnetic fields, that is the cause of all the diverse manifestations of solar activity. Magnetic fields are involved in all processes on the Sun. At times, concentrated magnetic fields arise in a small region of the solar atmosphere, several times stronger than on Earth. Ionized plasma is a good conductor; it cannot mix across the magnetic induction lines of a strong magnetic field. Therefore, in such places, the mixing and rise of hot gases from below is inhibited, and a dark area appears - a sunspot. Against the background of the dazzling photosphere, it appears completely black, although in reality its brightness is only ten times weaker.
Over time, the size and shape of the spots change greatly. Having appeared in the form of a barely noticeable point - a pore, the spot gradually increases its size to several tens of thousands of kilometers. Large spots, as a rule, consist of a dark part (core) and a less dark part - penumbra, the structure of which gives the spot the appearance of a vortex. The spots are surrounded by brighter areas of the photosphere, called faculae or flare fields.
The photosphere gradually passes into the more rarefied outer layers of the solar atmosphere - the chromosphere and corona.
Chromosphere
The chromosphere (Greek: “sphere of color”) is so named for its reddish-violet color. It is visible during total solar eclipses as a ragged, bright ring around the black disk of the Moon, which has just eclipsed the Sun. The chromosphere is very heterogeneous and consists mainly of elongated elongated tongues (spicules), giving it the appearance of burning grass. The temperature of these chromospheric jets is two to three times higher than in the photosphere, and the density is hundreds of thousands of times less. The total length of the chromosphere is 10-15 thousand kilometers.
The increase in temperature in the chromosphere is explained by the propagation of waves and magnetic fields penetrating into it from the convective zone. The substance is heated in much the same way as if it were in a giant microwave oven. The speed of thermal motion of particles increases, collisions between them become more frequent, and atoms lose their outer electrons: the substance becomes a hot ionized plasma. These same physical processes also maintain the unusually high temperature of the outermost layers of the solar atmosphere, which are located above the chromosphere.
Often during eclipses (and with the help of special spectral instruments - and without waiting for eclipses) above the surface of the Sun one can observe bizarrely shaped “fountains”, “clouds”, “funnels”, “bushes”, “arches” and other brightly luminous formations from the chromospheric substances. They can be stationary or slowly changing, surrounded by smooth curved jets that flow into or out of the chromosphere, rising tens and hundreds of thousands of kilometers. These are the most ambitious formations of the solar atmosphere - prominences. When observed in the red spectral line emitted by hydrogen atoms, they appear against the background of the solar disk as dark, long and curved filaments.
Prominences have approximately the same density and temperature as the Chromosphere. But they are above it and surrounded by higher, highly rarefied upper layers of the solar atmosphere. Prominences do not fall into the chromosphere because their matter is supported by the magnetic fields of active regions of the Sun.
For the first time, the spectrum of a prominence outside an eclipse was observed by the French astronomer Pierre Jansen and his English colleague Joseph Lockyer in 1868. The spectroscope slit is positioned so that it intersects the edge of the Sun, and if a prominence is located near it, then its radiation spectrum can be seen. By directing the slit at different parts of the prominence or chromosphere, it is possible to study them in parts. The spectrum of prominences, like the chromosphere, consists of bright lines, mainly hydrogen, helium and calcium. Emission lines from other chemical elements are also present, but they are much weaker.
Some prominences, having remained for a long time without noticeable changes, suddenly seem to explode, and their matter is thrown into interplanetary space at a speed of hundreds of kilometers per second. The appearance of the chromosphere also changes frequently, indicating the continuous movement of its constituent gases.
Sometimes something similar to explosions occurs in very small areas of the Sun's atmosphere. These are so-called chromospheric flares. They usually last several tens of minutes. During flares in the spectral lines of hydrogen, helium, ionized calcium and some other elements, the glow of a separate section of the chromosphere suddenly increases tens of times. Ultraviolet and X-ray radiation increases especially strongly: sometimes its power is several times higher than the total radiation power of the Sun in this short-wave region of the spectrum before the flare.
Spots, torches, prominences, chromospheric flares - all these are manifestations of solar activity. With increasing activity, the number of these formations on the Sun increases.
Crown
Unlike the photosphere and chromosphere, the outermost part of the Sun's atmosphere - the corona - has a huge extent: it extends over millions of kilometers, which corresponds to several solar radii, and its weak extension goes even further.
The density of matter in the solar corona decreases with height much more slowly than the density of air in the earth's atmosphere. The decrease in air density as it rises is determined by the gravity of the Earth. On the surface of the Sun, the force of gravity is much greater, and it would seem that its atmosphere should not be high. In reality it is extraordinarily extensive. Consequently, there are some forces acting against the attraction of the Sun. These forces are associated with the enormous speeds of movement of atoms and electrons in the corona, heated to a temperature of 1 - 2 million degrees!
The corona is best observed during the total phase of a solar eclipse. True, in the few minutes that it lasts, it is very difficult to sketch not only individual details, but even the general appearance of the crown. The observer's eye is just beginning to get used to the sudden twilight, and a bright ray of the Sun emerging from behind the edge of the Moon already announces the end of the eclipse. Therefore, sketches of the corona made by experienced observers during the same eclipse were often very different. It was not even possible to accurately determine its color.
The invention of photography gave astronomers an objective and documentary method of research. However, getting a good shot of the crown is also not easy. The fact is that its part closest to the Sun, the so-called inner corona, is relatively bright, while the far-reaching outer corona appears to be a very pale glow. Therefore, if the outer crown is clearly visible in photographs, the inner one turns out to be overexposed, and in photographs where the details of the inner crown are visible, the outer one is completely invisible. To overcome this difficulty, during an eclipse they usually try to take several photographs of the corona at once - with long and short shutter speeds. Or the corona is photographed by placing a special “radial” filter in front of the photographic plate, which weakens the annular zones of the bright inner parts of the corona. In such photographs, its structure can be traced to distances of many solar radii.
