Photosphere is the main part of the solar atmosphere in which visible radiation is formed, which is continuous. Thus, it emits almost all of the solar energy that comes to us.

The photosphere is a thin layer of gas several hundred kilometers long, quite opaque.

The photosphere is visible when directly observing the Sun in white light in the form of its apparent “surface”.

The photosphere strongly emits, and therefore absorbs, radiation throughout the entire visible continuous spectrum.

For each layer of the photosphere located at a certain depth, its temperature can be found. The temperature in the photosphere increases with depth and is on average 6000 K.

The length of the photosphere is several hundred km.

The density of the photosphere substance is 10 -7 g/cm 3 .

1 cm 3 of the photosphere contains about 10 16 hydrogen atoms. This corresponds to a pressure of 0.1 atm.

Under these conditions everything chemical elements with low ionization potentials they become ionized. Hydrogen remains in a neutral state.

The photosphere is the only region of neutral hydrogen on the Sun.

Visual and photographic observations of the photosphere reveal its fine structure, reminiscent of closely spaced cumulus clouds. Light round formations are called granules, and the entire structure is called granulation. The angular dimensions of the granules are no more than 1” arc, which corresponds to 700 km. Each individual granule exists for 5-10 minutes, after which it disintegrates and new granules form in its place. The granules are surrounded by dark spaces. The substance rises in the granules and falls around them. The speed of these movements is 1-2 km/s.

Granulation is a manifestation of the convective zone located under the photosphere. In the convective zone, mixing of matter occurs as a result of the rise and fall of individual masses of gas.

The reason for the occurrence of convection in the outer layers of the Sun is two important circumstances. On the one hand, the temperature directly below the photosphere increases very quickly in depth and radiation cannot ensure the release of radiation from deeper hot layers. Therefore, energy is transferred by the moving inhomogeneities themselves. On the other hand, these inhomogeneities turn out to be tenacious if the gas in them is not completely, but only partially ionized.

When passing into the lower layers of the photosphere, the gas is neutralized and is not able to form stable inhomogeneities. therefore, in the very upper parts of the convective zone, convective movements are slowed down and convection suddenly stops.

Oscillations and disturbances in the photosphere generate acoustic waves.

The outer layers of the convective zone represent a kind of resonator in which 5-minute oscillations are excited in the form of standing waves.

17.5 Outer layers of the solar atmosphere: chromosphere and corona. Causes and mechanism of heating of the chromosphere and corona.

The density of matter in the photosphere quickly decreases with height and the outer layers turn out to be very rarefied. In the outer layers of the photosphere, the temperature reaches 4500 K, and then begins to rise again.

There is a slow increase in temperature to several tens of thousands of degrees, accompanied by the ionization of hydrogen and helium. This part of the atmosphere is called chromosphere.

In the upper layers of the chromosphere, the density of the substance reaches 10 -15 g/cm 3 .

1 cm 3 of these layers of the chromosphere contains about 10 9 atoms, but the temperature increases to a million degrees. This is where the outermost part of the Sun's atmosphere, called the solar corona, begins.

The reason for the heating of the outermost layers of the solar atmosphere is the energy of acoustic waves arising in the photosphere. As they propagate upward into lower-density layers, these waves increase their amplitude to several kilometers and turn into shock waves. As a result of the occurrence of shock waves, wave dissipation occurs, which increases the chaotic velocities of particle movement and an increase in temperature occurs.

The integral brightness of the chromosphere is hundreds of times less than the brightness of the photosphere. Therefore, to observe the chromosphere it is necessary to use special methods, making it possible to isolate its weak radiation from the powerful flux of photospheric radiation.

The most convenient methods are observations during eclipses.

The length of the chromosphere is 12 - 15,000 km.

When studying photographs of the chromosphere, inhomogeneities are visible, the smallest ones are called spicules. The spicules are oblong in shape, elongated in the radial direction. Their length is several thousand km, thickness is about 1,000 km. At speeds of several tens of km/s, spicules rise from the chromosphere into the corona and dissolve in it. Through spicules, the substance of the chromosphere is exchanged with the overlying corona. Spicules form a larger structure, called a chromospheric network, generated by wave motions caused by much larger and deeper elements of the subphotospheric convective zone than granules.

Crown has very low brightness, so it can only be observed during the total phase of solar eclipses. Outside of eclipses, it is observed using coronagraphs. The crown does not have sharp outlines and has irregular shape, changing greatly over time.

The brightest part of the corona, removed from the limb no more than 0.2 - 0.3 radii of the Sun, is usually called the inner corona, and the remaining, very extended part is called the outer corona.

An important feature of the crown is its radiant structure. The rays come in different lengths, up to a dozen or more solar radii.

The inner crown is rich structural formations, resembling arcs, helmets, individual clouds.

Corona radiation is scattered light from the photosphere. This light is highly polarized. Such polarization can only be caused by free electrons.

1 cm 3 of corona matter contains about 10 8 free electrons. The appearance of such a number of free electrons must be caused by ionization. This means that 1 cm 3 of the corona contains about 10 8 ions. The total concentration of the substance should be 2 . 10 8 .

The solar corona is a rarefied plasma with a temperature of about a million Kelvin. A consequence of high temperature is the large extent of the corona. The length of the corona is hundreds of times greater than the thickness of the photosphere and amounts to hundreds of thousands of kilometers.

18. Internal structure of the Sun.

>What is the Sun made of?

Find out, what is the sun made of: description of the structure and composition of the star, listing of chemical elements, number and characteristics of layers with photos, diagram.

From Earth, the Sun appears as a smooth ball of fire, and before the Galileo spacecraft's discovery of sunspots, many astronomers believed that it was perfectly shaped without defects. Now we know that The sun consists from several layers, like the Earth, each of which performs its own function. This massive furnace-like structure of the Sun is the supplier of all the energy on Earth needed for terrestrial life.

What elements does the Sun consist of?

If you could take the star apart and compare its constituent elements, you would realize that the composition is 74% hydrogen and 24% helium. Also, the Sun consists of 1% oxygen, and the remaining 1% are chemical elements of the periodic table such as chromium, calcium, neon, carbon, magnesium, sulfur, silicon, nickel, iron. Astronomers believe that an element heavier than helium is a metal.

How did all these elements of the Sun come into being? The Big Bang produced hydrogen and helium. At the beginning of the formation of the Universe, the first element, hydrogen, emerged from elementary particles. Due to the high temperature and pressure, the conditions in the Universe were similar to those in the core of a star. Later, hydrogen was fused into helium while the universe had the high temperature required for the fusion reaction to occur. The existing proportions of hydrogen and helium that are in the Universe now developed after the Big Bang and have not changed.

The remaining elements of the Sun are created in other stars. In the cores of stars, the process of synthesis of hydrogen into helium constantly occurs. After producing all the oxygen in the core, they switch to nuclear fusion of heavier elements such as lithium, oxygen, helium. Many of the heavy metals found in the Sun were formed in other stars at the end of their lives.

The heaviest elements, gold and uranium, were formed when stars many times larger than our Sun detonated. In the split second of the black hole's formation, the elements collided at high speed and the heaviest elements were formed. The explosion scattered these elements throughout the Universe, where they helped form new stars.

Our Sun has collected elements created by the Big Bang, elements from dying stars, and particles created as a result of new star detonations.

What layers does the Sun consist of?

At first glance, the Sun is just a ball made of helium and hydrogen, but upon deeper study it is clear that it consists of different layers. When moving towards the core, temperature and pressure increase, as a result of which layers were created, since under different conditions hydrogen and helium have different characteristics.

solar core

Let's begin our movement through the layers from the core to the outer layer of the Sun's composition. In the inner layer of the Sun - the core, the temperature and pressure are very high, facilitating the occurrence of nuclear fusion. The sun creates helium atoms from hydrogen, as a result of this reaction, light and heat are formed, which reach. It is generally accepted that the temperature on the Sun is about 13,600,000 degrees Kelvin, and the density of the core is 150 times higher than the density of water.

Scientists and astronomers believe that the Sun's core reaches about 20% of the length of the solar radius. And inside the core, high temperature and pressure cause hydrogen atoms to break apart into protons, neutrons and electrons. The sun converts them into helium atoms, despite their free-floating state.

This reaction is called exothermic. When this reaction occurs, it releases a large number of heat equal to 389 x 10 31 J. per second.

Radiation zone of the Sun

This zone originates at the core boundary (20% of the solar radius), and reaches a length of up to 70% of the solar radius. Inside this zone there is solar matter, which in its composition is quite dense and hot, so thermal radiation passes through it without losing heat.

Nuclear fusion reaction occurs inside the solar core - the creation of helium atoms as a result of the fusion of protons. This reaction produces a large amount of gamma radiation. In this process, photons of energy are emitted, then absorbed in the radiation zone and emitted again by various particles.

The trajectory of a photon is usually called a “random walk.” Instead of moving in a straight path to the surface of the Sun, the photon moves in a zigzag pattern. As a result, each photon takes approximately 200,000 years to overcome the radiation zone of the Sun. When moving from one particle to another particle, the photon loses energy. This is good for the Earth, because we could only receive gamma radiation coming from the Sun. A photon entering space needs 8 minutes to travel to Earth.

A large number of stars have radiation zones, and their sizes directly depend on the scale of the star. The smaller the star, the smaller the zones will be, most of which will be occupied by the convective zone. The smallest stars may lack radiation zones, and the convective zone will reach the distance to the core. At the most big stars the situation is the opposite, the radiation zone extends to the surface.

Convective zone

The convective zone is outside the radiative zone, where the sun's internal heat flows through columns of hot gas.

Almost all stars have such a zone. For our Sun, it extends from 70% of the Sun's radius to the surface (photosphere). The gas in the depths of the star, near the very core, heats up and rises to the surface, like bubbles of wax in a lamp. Upon reaching the surface of the star, heat loss occurs; as it cools, the gas sinks back toward the center, recovering thermal energy. As an example, you can bring a pan of boiling water on fire.

The surface of the Sun is like loose soil. These irregularities are columns of hot gas that carry heat to the surface of the Sun. Their width reaches 1000 km, and the dispersal time reaches 8-20 minutes.

Astronomers believe that low-mass stars, such as red dwarfs, have only a convective zone that extends to the core. They have no radiation zone, which cannot be said about the Sun.

Photosphere

The only layer of the Sun visible from Earth is . Below this layer, the Sun becomes opaque, and astronomers use other methods to study the interior of our star. Surface temperatures reach 6000 Kelvin and glow yellow-white, visible from Earth.

The atmosphere of the Sun is located behind the photosphere. The part of the Sun that is visible during a solar eclipse is called.

Structure of the Sun in the diagram

NASA specially developed for educational needs schematic representation of the structure and composition of the Sun, indicating the temperature for each layer:

• (Visible, IR and UV radiation) – these are visible radiation, infrared radiation and ultraviolet radiation. Visible radiation is the light that we see coming from the Sun. Infrared radiation is the heat we feel. Ultraviolet radiation is the radiation that gives us a tan. The sun produces these radiations simultaneously.
• (Photosphere 6000 K) – The photosphere is the upper layer of the Sun, its surface. A temperature of 6000 Kelvin is equal to 5700 degrees Celsius.
• Radio emissions (trans. Radio emission) - In addition to visible radiation, infrared radiation and ultraviolet radiation, the Sun sends out radio emissions that astronomers have detected using a radio telescope. Depending on the number of sunspots, this emission increases and decreases.
• Coronal Hole - These are places on the Sun where the corona has a low plasma density, as a result it is darker and colder.
• 2100000 K (2100000 Kelvin) – The radiation zone of the Sun has this temperature.
• Convective zone/Turbulent convection (trans. Convective zone/Turbulent convection) – These are places on the Sun where the thermal energy of the core is transferred by convection. Columns of plasma reach the surface, give up their heat, and again rush down to heat up again.
• Coronal loops (trans. Coronal loops) are loops consisting of plasma in the solar atmosphere, moving along magnetic lines. They look like huge arches extending from the surface for tens of thousands of kilometers.
• Core (trans. Core) is the solar heart in which nuclear fusion occurs using high temperature and pressure. All solar energy comes from the core.
• 14,500,000 K (per. 14,500,000 Kelvin) – Temperature of the solar core.
• Radiative Zone (trans. Radiation zone) - A layer of the Sun where energy is transmitted using radiation. The photon overcomes the radiation zone beyond 200,000 and goes into outer space.
• Neutrinos (trans. Neutrino) are negligibly small particles emanating from the Sun as a result of a nuclear fusion reaction. Hundreds of thousands of neutrinos pass through the human body every second, but they do not cause us any harm, we do not feel them.
• Chromospheric Flare (translated as Chromospheric Flare) - The magnetic field of our star can twist and then abruptly break into various forms. As a result of breaks in magnetic fields, powerful X-ray flares appear from the surface of the Sun.
• Magnetic Field Loop - The Sun's magnetic field is above the photosphere and is visible as hot plasma moves along magnetic lines in the Sun's atmosphere.
• Spot – A sunspot (trans. Sun spots) – These are places on the surface of the Sun where magnetic fields pass through the surface of the Sun, and the temperature is lower, often in the form of a loop.
• Energetic particles (trans. Energetic particles) - They come from the surface of the Sun, resulting in the creation of the solar wind. In solar storms their speed reaches the speed of light.
• X-rays (translated as X-rays) are rays invisible to the human eye that are formed during solar flares.
• Bright spots and short-lived magnetic regions (trans. Bright spots and short-lived magnetic regions) - Due to temperature differences, bright and dim spots appear on the surface of the Sun.

Atmosphere of the Sun

Layer name	Height of the upper boundary of the layer, km	Density, kg/m 3	Temperature, K
Photosphere
Chromosphere
	Several tens of solar radii

Sunspots ( dark formations on the solar disk, due to the fact that their temperature is ~ 1500 K lower than the temperature of the photosphere) consist of a dark oval - the shadow of a spot, surrounded by a lighter fibrous penumbra. The smallest sunspots (pores) have diameters of ~1000 km; the diameters of the largest sunspots observed exceeded 100,000 km. Small spots often exist for less than 2 days, developed ones for 10-20 days, the largest ones can last up to 100 days.

Chromosphere spicules (isolated gas columns) have a diameter of ~1000 km, a height of up to ~8000 km, ascent and descent speeds of ~20 km/s, a temperature of ~15,000 K, and a lifetime of several minutes.

Prominences (relatively cold, dense clouds in the corona) extend up to 1/3 the radius of the Sun. The most common are “quiet” prominences, having a lifetime of up to 1 year, a length of ~200 thousand km, a thickness of ~10 thousand km, and a height of ~30 thousand km. Fast eruptive prominences are usually ejected upward at speeds of 100-1000 km/s after flares.

During a total solar eclipse, the brightness of the sky around the Sun is 1.6 10 -9 of the average brightness of the Sun.

The brightness of the Moon during a total solar eclipse in the light reflected from the Earth is 1.1 10 -10 of the average brightness of the Sun.

Photosphere

The photosphere (the layer that emits light) forms the visible surface of the Sun. Its thickness corresponds to an optical thickness of approximately 2/3 units. In absolute terms, the photosphere reaches a thickness, according to various estimates, from 100 to 400 km. The main part of the optical (visible) radiation of the Sun comes from the photosphere, but radiation from deeper layers no longer reaches us. The temperature, as it approaches the outer edge of the photosphere, decreases from 6600 K to 4400 K. The effective temperature of the photosphere as a whole is 5778 K. It can be calculated according to the Stefan-Boltzmann law, according to which the radiation power of an absolutely black body is directly proportional to the fourth power of the body temperature. Hydrogen under such conditions remains almost completely neutral. The photosphere forms the visible surface of the Sun, from which the size of the Sun, distance from the Sun, etc. are determined. Since the gas in the photosphere is relatively rarefied, its rotation speed is much less than the rotation speed solids. At the same time, gas in the equatorial and polar regions moves unevenly - at the equator it makes a revolution in 24 days, at the poles - in 30 days.

Chromosphere

The chromosphere is the outer shell of the Sun, about 2000 km thick, surrounding the photosphere. The origin of the name of this part of the solar atmosphere is associated with its reddish color, caused by the fact that the red H-alpha emission line of hydrogen from the Balmer series dominates the visible spectrum of the chromosphere. The upper boundary of the chromosphere does not have a distinct smooth surface; hot emissions called spicules constantly occur from it. The number of spicules observed simultaneously is on average 60-70 thousand. Because of this, at the end of the 19th century, the Italian astronomer Secchi, observing the chromosphere through a telescope, compared it to burning prairies. The temperature of the chromosphere increases with altitude from 4000 to 20,000 K (the temperature range above 10,000 K is relatively small).

The density of the chromosphere is low, so the brightness is insufficient for observation under normal conditions. But during a total solar eclipse, when the Moon covers the bright photosphere, the chromosphere located above it becomes visible and glows red. It can also be observed at any time using special narrow-band optical filters. In addition to the already mentioned H-alpha line with a wavelength of 656.3 nm, the filter can also be tuned to the Ca II K (393.4 nm) and Ca II H (396.8 nm) lines. The main chromospheric structures that are visible in these lines are:

· chromospheric network covering the entire surface of the Sun and consisting of lines surrounding supergranulation cells up to 30 thousand km in diameter;

· flocculi - light cloud-like formations, most often confined to areas with strong magnetic fields- active areas, often surrounded by sunspots;

· fibers and fibers (fibrils) - dark lines of varying widths and lengths, like flocculi, are often found in active areas.

Crown

The corona is the last outer shell of the Sun. The corona is mainly composed of prominences and energetic eruptions that emanate and erupt several hundred thousand and even more than a million kilometers into space, forming the solar wind. The average coronal temperature is from 1 to 2 million K, and the maximum, in some areas, is from 8 to 20 million K. Despite such a high temperature, it is visible naked eye only during a total solar eclipse, since the density of matter in the corona is low, and therefore its brightness is low. The unusually intense heating of this layer is apparently caused by the effect of magnetic reconnection and the influence of shock waves (see The problem of heating the corona). The shape of the crown changes depending on the phase of the cycle solar activity: during periods of maximum activity it has a round shape, and at minimum it is elongated along the solar equator. Since the temperature of the corona is very high, it emits intense radiation in the ultraviolet and x-ray ranges. These radiations do not pass through the earth's atmosphere, but recently it has become possible to study them using spacecraft. Radiation in different areas of the corona occurs unevenly. There are hot active and quiet regions, as well as coronal holes with a relatively low temperature of 600,000 K, from which magnetic field lines emerge into space. This (“open”) magnetic configuration allows particles to escape the Sun unhindered, so the solar wind is emitted mainly from coronal holes.

The visible spectrum of the solar corona consists of three different components, called the L, K and F components (or, respectively, the L-corona, K-corona and F-corona; another name for the L-components is the E-corona. The K-component is continuous spectrum of the corona. Against its background, up to a height of 9-10′ from the visible edge of the Sun, the emission L-component is visible. Starting from a height of about 3′ (the angular diameter of the Sun is about 30′) and higher, a Fraunhofer spectrum is visible, the same as the spectrum of the photosphere. It constitutes the F-component of the solar corona. At a height of 20′, the F-component dominates the spectrum of the corona. The height of 9-10′ is taken as the boundary separating the inner corona from the outer. Radiation from the Sun with a wavelength of less than 20 nm comes entirely from the corona. This means that, for example, in common photographs of the Sun at wavelengths of 17.1 nm (171 Å), 19.3 nm (193 Å), 19.5 nm (195 Å), only the solar corona with its elements is visible, and the chromosphere and the photosphere are not visible. Two coronal holes, almost always existing near the northern and south poles The sun, as well as others that temporarily appear on its visible surface, emit virtually no X-ray radiation.

sunny wind

From the outer part of the solar corona, the solar wind flows out - a stream of ionized particles (mainly protons, electrons and α-particles), spreading with a gradual decrease in its density to the boundaries of the heliosphere. The solar wind is divided into two components - the slow solar wind and the fast solar wind. The slow solar wind has a speed of about 400 km/s and a temperature of 1.4–1.6·10 6 K and is closely similar in composition to the corona. The fast solar wind has a speed of about 750 km/s, a temperature of 8·10 5 K, and is similar in composition to the substance of the photosphere. The slow solar wind is twice as dense and less constant as the fast one. The slow solar wind has a more complex structure with regions of turbulence.

On average, the Sun emits about 1.3·10 36 particles per second with the wind. Consequently, the total loss of mass by the Sun (for this type of radiation) is 2-3·10 −14 solar masses per year. The loss over 150 million years is equivalent to the Earth's mass. Many natural phenomena on Earth are associated with disturbances in the solar wind, including geomagnetic storms and auroras.

First direct performance measurements solar wind were carried out in January 1959 by the Soviet Luna-1 station. Observations were carried out using a scintillation counter and a gas ionization detector. Three years later, the same measurements were carried out by American scientists using the Mariner 2 station. In the late 1990s, using the Ultraviolet Coronal Spectrometer.Ultraviolet Coronal Spectrometer ( UVCS) ) on board the SOHO satellite, observations of areas where fast solar wind occurs at the solar poles were carried out.

§ 43. sun

The Sun is a star, the thermonuclear reaction in the core of which provides us with the energy necessary for life.

The Sun is the closest star to Earth. It provides light and warmth, without which life on Earth would be impossible. Part of the solar energy falling on the Earth is absorbed and scattered by the atmosphere. If this were not the case, then the radiation power received by each square meter of the Earth’s surface from vertically falling sun rays, was about 1.4 kW/m2. This quantity is called solar constant. Knowing the average distance from the Earth to the Sun and the solar constant, we can find the total radiation power of the Sun, called its luminosity and equal to approximately 4. 10 26 W.

The Sun is a huge hot ball, consisting mainly of hydrogen (70% of the mass of the Sun) and helium (28%), rotating around an axis (revolution in 25-30 Earth days). The diameter of the Sun is 109 times greater than that of the Earth. The apparent surface of the Sun, its photosphere- the lowest and densest layer of the Sun’s atmosphere, from which bó most of the energy it emits. The thickness of the photosphere is about 300 km, and the average temperature is 6000 K. Dark spots are often visible on the Sun ( sunspots), existing for several days and sometimes months (Fig. 43 A). The layer of the Sun's atmosphere 12-15 thousand km thick, located above the photosphere, is called chromosphere. Solar corona- the outer layer of the Sun's atmosphere, extending to distances of several of its diameters. The brightness of the chromosphere and the solar corona is very low, and they can only be seen during a total solar eclipse (Fig. 43 b).

As you approach the center of the Sun, the temperature and pressure increase and near it they are about 15× 10 6 K and 2.3 10 16 Pa, respectively. At such a high temperature, solar matter becomes plasma– a gas consisting of atomic nuclei and electrons. High temperature and pressure in core of the Sun with a radius of about 1/3 of the radius of the Sun (Fig. 43 V) create conditions for reactions to occur between nuclei, as a result of which nuclei are formed and enormous energy is released.

Nuclear reactions in which heavier nuclei are produced from light nuclei are called thermonuclear(from lat. thermo - heat), because they can only go at very high temperatures. Energy output thermonuclear reaction may be several times greater than during the fission of the same mass of uranium. The source of the Sun's energy is thermonuclear reactions occurring in its core. The high pressure of the outer layers of the Sun not only creates conditions for a thermonuclear reaction to occur, but also keeps its core from exploding.

The energy of a thermonuclear reaction is released in the form of gamma radiation, which, leaving the core of the Sun, enters a spherical layer called radiant zone, thickness about 1/3 of the radius of the Sun (Fig. 43 V). Matter located in the radiant zone absorbs gamma radiation coming from the nucleus and emits its own, but at a lower frequency. Therefore, as radiation quanta move from inside to outside, their energy and frequency decrease, and gamma radiation is gradually converted into ultraviolet, visible and infrared.

The outer shell of the Sun is called convective zone, in which mixing of the substance occurs ( convection), and energy transfer is carried out by the movement of the substance itself (Fig. 43 V). A decrease in convection leads to a decrease in temperature by 1-2 thousand degrees and the appearance of a sunspot. At the same time, convection intensifies near the sunspot, and hotter matter is carried to the surface of the Sun, and in the chromosphere, prominences– ejections of matter at distances up to ½ the radius of the Sun. The appearance of spots is often accompanied solar flares – bright glow of the chromosphere, X-ray radiation and a flow of fast charged particles. It has been established that all these phenomena, called solar activity, occur more often, the more sunspots there are. The number of sunspots varies on average with a period of 11 years.

Review questions:

· Why is equal to the solar constant, and what is called the luminosity of the Sun?

· What is the internal structure of the Sun?

· Why does thermonuclear reaction occur only in the core of the Sun?

· List the phenomena of solar activity?

Rice. 43. ( A) – sunspots; ( b) – solar corona during a solar eclipse; ( V) – structure of the Sun ( 1 - core, 2 – radiant zone, 3 – convective zone).

Internal structure of the Sun

© Vladimir Kalanov
Knowledge is power

What is visible on the Sun?

Everyone probably knows that you cannot look at the Sun with the naked eye, much less through a telescope without special, very dark filters or other devices that attenuate the light. By neglecting this prohibition, the observer risks getting severe eye burns. The easiest way to view the Sun is to project its image onto a white screen. Using even a small amateur telescope, you can get a magnified image of the solar disk. What can you see in this image? First of all, the sharpness of the sunny edge attracts attention. The sun is a gas ball that does not have a clear boundary, its density decreases gradually. Why, then, do we see it sharply outlined? The fact is that almost all visible radiation from the Sun comes from a very thin layer, which has a special name - the photosphere. (Greek: “sphere of light”). The thickness of the photosphere does not exceed 300 km. It is this thin luminous layer that creates the illusion for the observer that the Sun has a “surface”.

Internal structure of the Sun

Photosphere

The atmosphere 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 of which we perceive, 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 of the type H, OH, and CH preserved. A special role in the solar atmosphere is played by a substance not found in terrestrial nature. negative hydrogen ion, 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 supplied 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.

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 that results complex interaction with solar magnetic fields 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 thousand times stronger than on Earth. Ionized plasma is a good conductor; it cannot move 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

Above the photosphere is the chromosphere, a heterogeneous layer in which the temperature ranges from 6,000 to 20,000 K. The chromosphere (Greek for “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 heats up 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 -. 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 the 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 power of solar radiation in this short-wavelength 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.