Sun (astro. ☉) – the only star Solar system. Other objects of this system revolve around the Sun: planets and their satellites, dwarf planets and their satellites, asteroids, meteoroids, comets and cosmic dust.
Internal structure of the Sun
Our Sun is a huge glowing ball of gas, inside of which flows complex processes and as a result, energy is continuously released. The interior volume of the Sun can be divided into several regions; the substance in them differs in its properties, and energy is distributed through different physical mechanisms. Let's get to know them, starting from the very center.
In the central part of the Sun is the source of its energy, or, in other words, figurative language, that “stove” that heats it and does not allow it to cool down. This area is called the core. Under the weight of the outer layers, the matter inside the Sun is compressed, and the deeper, the stronger. Its density increases towards the center along with increasing pressure and temperature. In the core, where the temperature reaches 15 million kelvins, energy is released.
This energy is released as a result of the fusion of atoms of light chemical elements into atoms of heavier ones. In the depths of the Sun, one helium atom is formed from four hydrogen atoms. It is this terrible energy that people have learned to release during an explosion. hydrogen bomb. There is hope that in the near future people will be able to learn to use it for peaceful purposes (in 2005 news feeds reported about the start of construction of the first international fusion reactor in France).
The core has a radius of no more than a quarter of the total radius of the Sun. However, half of the solar mass is concentrated in its volume and almost all the energy that supports the glow of the Sun is released. But the energy of the hot core must somehow escape outward, to the surface of the Sun. There are different ways to transfer energy depending on physical conditions environment, namely: radiative transfer, convection and thermal conductivity. Thermal conductivity does not play a big role in energy processes in the Sun and stars, while radiative and convective transfers are very important.
Immediately around the nucleus, a zone of radiative energy transfer begins, where it spreads through the absorption and emission of a portion of light by the substance - quanta. Density, temperature and pressure decrease as you move away from the core, and energy flows in the same direction. Overall, this process is extremely slow. It takes many thousands of years for quanta to get from the center of the Sun to the photosphere: after all, when re-emitted, quanta constantly change direction, moving backward almost as often as forward.
Gamma quanta are born in the center of the Sun. Their energy is millions of times greater than the energy of visible light quanta, and their wavelength is very short. Along the way, quanta undergo amazing transformations. A separate quantum is first absorbed by some atom, but is immediately re-emitted again; Most often, in this case, not one previous quantum appears, but two or more. According to the law of conservation of energy, their total energy is conserved, and therefore the energy of each of them decreases. This is how quanta of lower and lower energies arise. Powerful gamma rays seem to be split into less energetic quanta - first X-ray, then ultraviolet and
finally visible and infrared rays. In the end greatest number The sun emits energy in visible light, and it is no coincidence that our eyes are sensitive to it.
As we have already said, it takes a very long time for a quantum to penetrate through the dense solar matter to the outside. So if the “stove” inside the Sun suddenly went out, we would only know about it millions of years later. On its way through the inner solar layers, the energy flow encounters a region where the opacity of the gas greatly increases. This is the convective zone of the Sun. Here energy is transferred not by radiation, but by convection.
What is convection?
When the liquid boils, it is stirred. Gas can behave the same way. Huge streams of hot gas rise upward, where they give off their heat environment, and the cooled solar gas descends. The solar matter appears to be boiling and stirring. The convective zone begins at approximately 0.7 radius from the center and extends almost to the most visible surface of the Sun (photosphere), where the transfer of the main energy flow again becomes radiant. However, due to inertia, hot flows from deeper, convective layers still penetrate here. The pattern of granulation on the surface of the Sun, well known to observers, is a visible manifestation of convection.
Convective zone of the Sun
The radioactive zone is about 2/3 of the internal diameter of the Sun, and the radius is about 140 thousand km. Moving away from the center, photons lose their energy under the influence of collision. This phenomenon is called the convection phenomenon. This is reminiscent of the process that occurs in a boiling kettle: the energy coming from the heating element is much moreover the amount of heat removed by conduction. Hot water close to the fire rises, and colder water sinks. This process is called convention. The meaning of convection is that denser gas is distributed over the surface, cools and again goes to the center. The mixing process in the convective zone of the Sun is carried out continuously. Looking through a telescope at the surface of the Sun, you can see its granular structure - granulations. It feels like it's made of granules! This is due to convection occurring beneath the photosphere.
Photosphere of the Sun
A thin layer (400 km) - the photosphere of the Sun, is located directly behind convective zone and represents the “real solar surface” visible from Earth. Granules in the photosphere were first photographed by the Frenchman Janssen in 1885. The average granule has a size of 1000 km, moves at a speed of 1 km/sec and exists for approximately 15 minutes. Dark formations in the photosphere can be observed in the equatorial part, and then they shift. Strong magnetic fields are a distinctive feature of such spots. A dark color is obtained due to the lower temperature relative to the surrounding photosphere.
Chromosphere of the Sun
Chromosphere of the Sun (colored sphere) – dense layer (10,000 km) solar atmosphere, which is located just beyond the photosphere. The chromosphere is quite problematic to observe due to its close location to the photosphere. It is best seen when the Moon covers the photosphere, i.e. during solar eclipses.
Solar prominences are huge emissions of hydrogen, resembling long luminous filaments. Prominences rise to huge distance, reaching the diameter of the Sun (1.4 mm km), move at a speed of about 300 km/sec, and the temperature reaches 10,000 degrees.
Solar corona
The solar corona is the outer and extended layers of the Sun's atmosphere, originating above the chromosphere. The length of the solar corona is very long and reaches values of several solar diameters. Scientists have not yet received a clear answer to the question of where exactly it ends.
The composition of the solar corona is a rarefied, highly ionized plasma. It contains heavy ions, electrons with a helium core, and protons. The temperature of the corona reaches from 1 to 2 million degrees K, relative to the surface of the Sun.
The solar wind is a continuous outflow of matter (plasma) from the outer shell of the solar atmosphere. It consists of protons, atomic nuclei and electrons. The speed of the solar wind can vary from 300 km/sec to 1500 km/sec, in accordance with the processes occurring on the Sun. The solar wind spreads throughout the solar system and, interacting with the Earth's magnetic field, causes various phenomena, one of which is the northern lights.
Radiation from the Sun
The sun emits its energy in all wavelengths, but in different ways. Approximately 44% of the radiation energy comes from visible part spectrum, and the maximum corresponds to the yellow-green color. About 48% of the energy lost by the Sun is carried away by near and far infrared rays. Gamma rays, X-rays, ultraviolet and radio radiation account for only about 8%.
The visible part of solar radiation, when studied using spectrum-analyzing instruments, turns out to be inhomogeneous - absorption lines first described by J. Fraunhofer in 1814 are observed in the spectrum. These lines arise when photons of certain wavelengths are absorbed by atoms of various chemical elements in the upper, relatively cold, layers of the Sun's atmosphere. Spectral analysis allows us to obtain information about the composition of the Sun, since a certain set of spectral lines characterizes a chemical element extremely accurately. For example, using observations of the spectrum of the Sun, the discovery of helium was predicted, which was isolated later on Earth.
Types of radiation
During observations, scientists found that the Sun is a powerful source of radio emission. Radio waves penetrate into interplanetary space, which are emitted by the chromosphere (centimeter waves) and the corona (decimeter and meter waves). Radio emission from the Sun has two components – constant and variable (bursts, “noise storms”). During strong solar flares The radio emission of the Sun increases thousands and even millions of times compared to the radio emission of the quiet Sun. This radio emission is non-thermal in nature.
X-rays come mainly from upper layers chromosphere and corona. Radiation is especially strong during peak years. solar activity.
The sun emits not only light, heat and all other types electromagnetic radiation. It is also a source of a constant flow of particles - corpuscles. Neutrinos, electrons, protons, alpha particles, and heavier atomic nuclei all together make up corpuscular radiation Sun. A significant part of this radiation is a more or less continuous outflow of plasma - solar wind, which is a continuation of the outer layers of the solar atmosphere - the solar corona. Against the background of this constantly blowing plasma wind, individual regions on the Sun are sources of more directed, enhanced, so-called corpuscular flows. Most likely, they are associated with special regions of the solar corona - coronal holes, and also, possibly, with long-lived active regions on the Sun. Finally, the most powerful short-term fluxes of particles, mainly electrons and protons, are associated with solar flares. As a result, most powerful flashes particles can acquire speeds that are a noticeable fraction of the speed of light. Particles with such high energies are called solar cosmic rays.
Solar corpuscular radiation has strong influence on the Earth, and primarily on the upper layers of its atmosphere and magnetic field, causing many geophysical phenomena. From harmful influence The radiation from the sun protects us from the magnetosphere and atmosphere of the Earth.
Solar radiation intensity
Having extremely high temperatures, the Sun is a very strong source of radiation. The visible range of solar radiation has the highest radiation intensity. At the same time, it also reaches the Earth large number invisible spectrum. Processes take place inside the Sun in which helium atoms are synthesized from hydrogen atoms. These processes are called processes nuclear fusion, they are accompanied by the release huge amount energy. This energy causes the Sun to heat up to a temperature of 15 million degrees Celsius (in its inner part).
On the surface of the Sun (photosphere) the temperature reaches 5500 °C. On this surface, the Sun emits energy of 63 MW/m². Only a small part of this radiation reaches the surface of the Earth, which allows humanity to exist comfortably on our planet. The average radiation intensity on the Earth's atmosphere is approximately 1367 W/m². This value can fluctuate in the range of 5% due to the fact that, moving along an elliptical orbit, the Earth moves away from the Sun at different distances throughout the year. The value of 1367 W/m² is called the solar constant.
Solar energy on the surface of the Earth
The Earth's atmosphere does not allow all solar energy. The Earth's surface reaches no more than 1000 W/m2. Some of the energy is absorbed, some is reflected in the layers of the atmosphere and in the clouds. A large amount of radiation is scattered in the layers of the atmosphere, resulting in the formation of scattered radiation (diffuse). On the surface of the Earth, part of the radiation is also reflected and turns into scattered radiation. The sum of diffuse and direct radiation is called total solar radiation. Scattered radiation can range from 20 to 60%.
The amount of energy reaching the Earth's surface is also affected by geographic latitude and time of year. The axis of our planet, passing through the poles, is tilted by 23.5° relative to its orbit around the Sun. Between March
until September, more sunlight falls on Northern Hemisphere, the rest of the time – Yuzhnoe. Therefore, the length of the day in summer and winter is different. The latitude of the area affects the duration daylight hours. The further north, the longer summer time and vice versa.
Evolution of the Sun
It is assumed that the Sun was born in a compressed gas and dust nebula. There are at least two theories as to what triggered the initial contraction of the nebula. According to one of them, it is assumed that one of the spiral arms of our galaxy passed through our region of space approximately 5 billion years ago. This could cause slight compression and lead to the formation of centers of gravity in the gas-dust cloud. Indeed, we now see quite a large number of young stars and glowing gas clouds along the spiral arms. Another theory suggests that somewhere nearby (on the scale of the Universe, of course) an ancient massive supernova exploded. The resulting shock wave could be strong enough to initiate star formation in “our” gas-dust nebula. This theory is supported by the fact that scientists studying meteorites have discovered quite a lot of elements that could have been formed during a supernova explosion.
Further, when such a colossal mass (2 * 1030 kg) was compressed under the influence of gravitational forces, it strongly heated itself with internal pressure to temperatures at which thermonuclear reactions could begin in its center. In the central part, the temperature on the Sun is 15,000,000K, and the pressure reaches hundreds of billions of atmospheres. This is how a newborn star was lit (not to be confused with new stars).
The Sun at the beginning of its life consisted mainly of hydrogen. It is hydrogen during thermonuclear reactions turns into helium, releasing energy emitted by the Sun. The Sun belongs to a type of star called yellow dwarf. It's a star main sequence and belongs to the spectral class G2. The mass of a lone star quite clearly determines its fate. During its lifetime (~5 billion years), in the center of our star, where the temperature is quite high, about half of all the hydrogen there was burned. About the same amount of time, 5 billion years, the Sun has left to live in the form to which we are accustomed.
After the hydrogen in the center of the star runs out, the Sun will increase in size and become a red giant. This will have a dramatic impact on Earth: temperatures will rise, the oceans will boil, life will become impossible. Then, having exhausted the “fuel” completely and no longer having the strength to hold the outer layers of the red giant, our star will end its life as a white dwarf, delighting the unknown extraterrestrial astronomers of the future nova planetary nebula, the shape of which can turn out to be very bizarre due to the influence of the planets.
Death of the Sun by time
- In just 1.1 billion years, the star will increase its brightness by 10%, which will lead to strong heating of the Earth.
- In 3.5 billion years, the brightness will increase by 40%. The oceans will begin to evaporate and all life on Earth will end.
- After 5.4 billion years, the star's core will run out of fuel - hydrogen. The sun will begin to increase in size due to the rarefaction of the outer shell and heating of the core.
- In 7.7 billion years, our star will turn into a red giant, because increase by 200 times because of this the planet Mercury will be absorbed.
- At the end, after 7.9 billion years, the outer layers of the star will be so thin that they will disintegrate into a nebula, and in the center of the former Sun there will be a small object - a white dwarf. This is how our existence will end solar system. All building elements remaining after the collapse will not be lost; they will become the basis for the birth of new stars and planets.
- The most common stars in the universe are red dwarfs. This is largely due to their low mass, which allows them to live for a very long time before becoming white dwarfs.
- Almost all stars in the universe have the same chemical composition and the nuclear fusion reaction occurs in every star and is almost identical, determined only by the supply of fuel.
- As we know, like a white dwarf, neutron stars are one of the final processes of stellar evolution, largely arising after a supernova explosion. Previously, it was often difficult to distinguish a white dwarf from a neutron star, but now scientists using telescopes have found differences in them. A neutron star gathers around itself more light and it is easy to see with infrared telescopes. Eighth place among interesting facts about the stars.
- Thanks to its incredible mass, according to general theory According to Einstein's relativity, a black hole is actually a bend in space such that everything within its gravitational field is pushed towards it. The gravitational field of a black hole is so strong that not even light can escape it.
- As far as we know, when a star runs out of fuel, the star can grow in size by more than 1000 times, then it turns into a white dwarf, and due to the speed of the reaction, it explodes. This reaction is better known as a supernova. Scientists suggest that in connection with this long process, such mysterious black holes are formed.
- Many of the stars we see in the night sky can appear as just one glimpse of light. However, this is not always the case. Most of the stars we see in the sky are actually two star systems, or binary star systems. They are simply unimaginably far away and it seems to us that we see only one speck of light.
- The stars that have the shortest lifespans are the most massive. They represent a high mass chemicals and typically burn their fuel much faster.
- Despite the fact that sometimes it seems to us that the Sun and stars are twinkling, in reality this is not the case. The flickering effect is only the light from the star, which at this time passes through the Earth's atmosphere but has not yet reached our eyes. Third place among the most interesting facts about stars.
- The distances involved in estimating how far away a star is are unimaginably huge. Let's consider an example: The closest star to earth is approximately 4.2 light years away, and to get to it, even on our fastest ship, will take about 70,000 years.
- The coldest famous star, this is a brown dwarf "CFBDSIR 1458+10B" with a temperature of only about 100 °C. The hottest known star, a blue supergiant in the Milky Way called Zeta Puppis, has a temperature of over 42,000°C.
The energy of the Sun is the source of life on our planet. The sun heats the atmosphere and surface of the Earth. Thanks to solar energy, winds blow, the water cycle occurs in nature, seas and oceans heat up, plants develop, and animals have food (see Fig. 1.1). It is thanks to solar radiation that fossil fuels exist on Earth.
Figure 1.1 – The influence of solar radiation on the Earth
Solar energy can be converted into heat or cold, driving force and electricity. The main source of energy for almost all natural processes occurring on the Earth's surface and in the atmosphere is the energy coming to the Earth from the Sun in the form of solar radiation.
Figure 1.2 presents a classification scheme that reflects the processes that occur on the Earth's surface and in its atmosphere under the influence of solar radiation.
The results of direct solar activity are the thermal effect and the photoelectric effect, as a result of which the Earth receives thermal energy and light. The results of the indirect activity of the Sun are corresponding effects in the atmosphere, hydrosphere and geosphere, which cause the appearance of wind and waves, determine the flow of rivers, and create conditions for preserving the internal heat of the Earth.
Figure 1.2 - Classification of renewable energy sources
The Sun is a gas ball with a radius of 695,300 km, 109 times greater than radius Earth, with a radiating surface temperature of about 6000°C. The temperature inside the Sun reaches 40 million °C.
Figure 1.3 shows a diagram of the structure of the Sun. The sun is a giant “thermonuclear reactor” that runs on hydrogen and converts 564 million tons of hydrogen into 560 million tons of helium every second by melting. The loss of four million tons of mass is equal to 9:1-10 9 GW 
h of energy (1 GW equals 1 million kW). In one second, more energy is produced than six billion nuclear power plants could produce in a year. Thanks to the protective shell of the atmosphere, only part of this energy reaches the Earth's surface.
The distance between the centers of the Earth and the Sun is on average 1.496 * 10 8 km.
Annually Sun sends about 1.6 to Earth 
10 18 kW 
h of radiant energy or 1.3 * 10 24 cal heat. This is 20 thousand times more than current global energy consumption. Contribution Sun in the energy balance of the globe is 5000 times greater than the total contribution of all other sources.
This amount of heat would be enough to melt the 35 m thick layer of ice covering earth's surface at 0°C.
Compared to solar radiation, all other sources of energy reaching the Earth are negligible. Thus, the energy of stars is one hundred millionth of the solar energy; cosmic radiation - two parts per billion. The internal heat coming from the depths of the Earth to its surface is one ten-thousandth of solar energy.
Figure 1.3 – Diagram of the structure of the Sun
Thus. The sun is virtually the only source of thermal energy on Earth.
At the center of the Sun is the solar core (see Fig. 1.4). The photosphere is the visible surface of the Sun, which is the main source of radiation. The sun is surrounded by a solar corona, which has a very high temperature, however, it is extremely rarefied, so it is visible to the naked eye only during periods of total solar eclipse.
The visible surface of the Sun that emits radiation is called the photosphere (sphere of light). It consists of hot vapors of various chemical elements in an ionized state.
Above the photosphere is the luminous, almost transparent atmosphere of the Sun, consisting of rarefied gases, which is called the chromosphere.
Above the chromosphere is the outer shell of the Sun, called the corona.
The gases that form the Sun are in a state of continuous violent (intense) movement, which causes the appearance of the so-called sunspots, torches and prominences.
Sunspots are large funnels formed as a result of vortex movements of gas masses, the speed of which reaches 1-2 km/s. The temperature of the spots is 1500°C lower than the temperature of the Sun and is about 4500°C. The number of sunspots varies from year to year with a period of about 11 years.
Figure 1.4 - Structure of the Sun
Solar torches are emissions of solar energy, and prominences are colossal explosions in the chromosphere of the Sun, reaching altitudes of up to 2 million km.
Observations have shown that with an increase in the number of sunspots, the number of faculae and prominences increases and, accordingly, solar activity increases.
With increasing solar activity on Earth, magnetic storms, which have a negative impact on telephone, telegraph and radio communications, as well as on living conditions. An increase in auroras is associated with the same phenomenon.
It should be noted that during the period of increasing sunspots, the intensity of solar radiation first increases, which is associated with a general increase in solar activity in initial period, and then solar radiation decreases, as the area of sunspots increases, having a temperature 1500° lower than the temperature of the photosphere.
The part of meteorology that studies the effects of solar radiation on Earth and in the atmosphere is called actinometry.
When performing actinometric work, it is necessary to know the position of the Sun in the firmament. This position is determined by the altitude or azimuth of the Sun.
Height of the Sun he is called the angular distance from the Sun to the horizon, that is, the angle between the direction to the Sun and the plane of the horizon.
The angular distance of the Sun from the zenith, that is, from its vertical direction is called azimuth or zenith distance.
There is a relationship between height and zenith distance
(1.1)
The azimuth of the Sun is rarely determined, only for special work.
The height of the Sun above the horizon is determined by the formula:
Where 
-
latitude of the observation site;
-
the declination of the Sun is the arc of the circle of declination from the equator to the Sun, which is calculated depending on the position of the Sun on both sides of the equator from 0 to ±90°;
t - hour angle of the Sun or true solar time in degrees.
The value of the declination of the Sun for each day is given in astronomical reference books over a long period.
Using formula (1.2) you can calculate for any time t height of the sun he or at a given height hc determine the time when the Sun is at a given height.
The maximum height of the Sun at noon for various days of the year is calculated by the formula:
(1.3)
Life-giving rays.
The sun emits three types of ultraviolet rays. Each of these types affects the skin differently.
Most of us feel healthier and healthier after spending time at the beach. full of life. Thanks to the life-giving rays, vitamin D is formed in the skin, which is necessary for the complete absorption of calcium. But they only have a beneficial effect on the body small doses solar radiation.
But heavily tanned skin is still damaged skin and, as a consequence, premature aging and high risk development of skin cancer.
Sunlight is electromagnetic radiation. In addition to the visible spectrum of radiation, it contains ultraviolet radiation, which is actually responsible for tanning. Ultraviolet light stimulates the ability of melanocyte pigment cells to produce more melanin, which performs a protective function.
Types of UV rays.
There are three types of ultraviolet rays, which differ in wavelength. Ultraviolet radiation is able to penetrate through the epidermis of the skin into deeper layers. This activates the production of new cells and keratin, resulting in tighter, rougher skin. Sun rays penetrating the dermis destroy collagen and lead to changes in the thickness and texture of the skin.
Ultraviolet rays A.
These rays have the most low level radiation. Previously, it was generally believed that they were harmless, however, it has now been proven that this is not the case. The level of these rays remains almost constant throughout the day and year. They even penetrate glass.
UV A rays penetrate through the layers of the skin, reaching the dermis, damaging the base and structure of the skin, destroying collagen and elastin fibers.
A-rays promote the appearance of wrinkles, reduce skin elasticity, accelerate the appearance of signs of premature aging, and weaken the skin's defense system, making it more susceptible to infections and possibly cancer.
Ultraviolet rays B.
Rays of this type are emitted by the sun only at certain times years and hours of the day. Depending on the air temperature and geographical latitude they usually enter the atmosphere between 10 a.m. and 4 p.m.
UVB rays cause more serious damage to the skin because they interact with DNA molecules found in skin cells. B rays damage the epidermis, leading to sunburn. B rays damage the epidermis, leading to sunburn. This type of radiation increases the activity of free radicals, which weaken the skin's natural defense system.
Ultraviolet B rays promote tanning and cause sunburn, lead to premature aging and the appearance of dark pigment spots, make the skin rough and rough, accelerate the appearance of wrinkles, and can provoke the development of precancerous diseases and skin cancer.
Candidate of Physical and Mathematical Sciences E. LOZOVSKAYA.
With the onset of warm weather summer days We are just drawn to bask in the sun. Sunlight improves mood, stimulates the formation of vital vitamin D in the skin, but at the same time, unfortunately, contributes to the appearance of wrinkles and increases the risk of developing skin cancer. A significant part of both beneficial and harmful effects is associated with that part of solar radiation that is invisible to the human eye - ultraviolet.
Spectrum of electromagnetic radiation and spectrum of the sun. The boundary between ultraviolet B and C corresponds to the transmission of the earth's atmosphere.
Ultraviolet radiation causes various damage to DNA molecules in living organisms.
The intensity of ultraviolet B varies with latitude and time of year.
Cotton clothing provides good UV protection.
The sun serves as the main source of energy for our planet, and this energy comes in the form of radiation - infrared, visible and ultraviolet. The ultraviolet region is located beyond the short-wavelength limit of the visible spectrum. When we're talking about on the effect on living organisms, three regions are usually distinguished in the ultraviolet spectrum of the sun: ultraviolet A (UV-A; 320-400 nanometers), ultraviolet B (UV-B; 290-320 nm) and ultraviolet C (UV-C; 200- 290 nm). This division is quite arbitrary: the boundary between UV-B and UV-C was chosen based on the considerations that light with a wavelength of less than 290 nm does not reach the Earth’s surface, since the earth’s atmosphere, thanks to oxygen and ozone, acts as an effective natural light filter. The boundary between UVB and UVA is based on the fact that radiation shorter than 320 nm causes much more severe erythema (reddening of the skin) than light in the 320-400 nm range.
Spectral composition sunlight largely depends on the time of year, weather, latitude and altitude above sea level. For example, the further from the equator, the more the short-wave boundary shifts to the side long waves, since in this case the light hits the surface at an oblique angle and travels a greater distance in the atmosphere, which means it is more strongly absorbed. The position of the short-wave boundary is also affected by the thickness of the ozone layer, therefore, under " ozone holes"More ultraviolet radiation reaches the Earth's surface.
At noon, the intensity of radiation at a wavelength of 300 nm is 10 times higher than three hours before or three hours later. Clouds scatter ultraviolet light, but only dark clouds can block it completely. Ultraviolet rays are reflected well from sand (up to 25%) and snow (up to 80%), worse from water (less than 7%). The ultraviolet flux increases with altitude, by approximately 6% with every kilometer. Accordingly, in places located below sea level (for example, off the coast Dead Sea), the radiation intensity is lower.
LIFE UNDER THE SUN
Without light, life on Earth could not exist. Plants use solar energy, store it through photosynthesis, and provide energy through food to all other living things. Light provides humans and other animals with the ability to see. the world around us, regulates biological rhythms body.
This cheerful picture is a little complicated by ultraviolet light, since its energy is enough to cause serious damage to DNA. Scientists count more than two dozen different diseases that arise or are aggravated by sunlight, including xeroderma pigmentosum, squamous cell skin cancer, basal cell carcinoma, melanoma, and cataracts.
Of course, in the process of evolution, our body has developed mechanisms to protect against ultraviolet radiation. The first barrier that potentially blocks dangerous radiation access to the body - skin. Almost all ultraviolet radiation is absorbed in the epidermis, the outer layer of skin 0.07-0.12 mm thick. Sensitivity to light is largely determined by the body's inherited ability to produce melanin, a dark pigment that absorbs light in the epidermis and thereby protects the deeper layers of the skin from photodamage. Melanin is produced by special skin cells - melanocytes. Ultraviolet irradiation stimulates the production of melanin. This biological pigment is most intensively formed during irradiation UV-B light range. True, the effect does not appear immediately, but 2-3 days after exposure to the sun, but it persists for 2-3 weeks. At the same time, the division of melanocytes accelerates, the number of melanosomes (granules containing melanin) increases, and their size increases. UV-A light can also cause tanning, but it is weaker and less persistent, since the number of melanosomes does not increase, but only photochemical oxidation of the melanin precursor into melanin occurs.
There are six skin types based on sensitivity to sunlight. Type I skin is very light, it burns easily and does not tan at all. Type II skin burns easily and develops a slight tan. Leather type III tans quickly and burns to a lesser extent. Type IV skin is even more resistant to sun damage. Skin types V and VI are naturally dark (for example, among the indigenous people of Australia and Africa) and are almost not subject to the damaging effects of the sun. Representatives of the Negroid race have a 100-fold lower risk of developing non-melanoma skin cancer, and a 10-fold lower risk of melanoma compared to Europeans.
People with very fair skin are most vulnerable to ultraviolet radiation. In them, even short-term exposure to bright sun causes erythema - redness of the skin. Mainly responsible for the occurrence of erythema UV-B radiation. As a measure of the effect of ultraviolet radiation on the body, a concept such as the minimum erythemal dose (MED) is often used, that is, one at which slight redness is noticeable to the eye. In fact, the MED value varies not only between different people, but also within one person in different parts of the body. For example, for the skin of the abdomen of a white, untanned person, the MED value is about 200 J/m 2, and on the legs it is more than three times higher. Erythema usually occurs several hours after irradiation. In severe cases, a true sunburn with blisters develops.
What substances in the epidermis, besides melanin, absorb ultraviolet radiation? Nucleic acids, amino acids tryptophan and tyrosine, urocanic acid. The most dangerous damage to the body nucleic acids. Under the influence of light in the UV-B range, dimers are formed due to covalent bonds between adjacent pyrimidine (cytosine or thymine) bases. Since pyrimidine dimers do not fit into the double helix, this part of the DNA loses the ability to perform its functions. If the damage is small, special enzymes cut out the defective area (and this is another fairly effective defense mechanism). However, if the damage is greater than the cell's ability to repair, the cell dies. Outwardly, this manifests itself in the fact that the burned skin “peeles off.” DNA damage can lead to mutations and, as a consequence, to cancer diseases. Other damage to molecules also occurs, for example, DNA crosslinks with proteins are formed. By the way, visible light helps to heal damage to nucleic acids (this phenomenon is called photoreactivation). Prevent dangerous consequences Photochemical reactions are helped by antioxidants contained in the body.
Another consequence ultraviolet irradiation- suppression of immunity. This reaction may be intended to reduce the inflammation caused by sunburn, but it may also reduce resistance to infection. The signal for immune suppression is the photochemical reactions of urocanic acid and DNA.
FASHION FOR TANING IS A SYMBOL OF INDUSTRIAL SOCIETY
For a long time, white skin was considered distinctive feature noble and rich: it was immediately clear that its owners did not have to work in the field from morning to night. But in the twentieth century everything changed, the poor now spent whole days in factories, and the rich could afford to relax on fresh air, by the sea, showing off a beautiful golden tan. After World War II, the fashion for tanning became widespread; Tanned skin began to be considered a sign of not only wealth, but also excellent health. The tourism industry has grown, offering holidays by the sea at any time of the year. But some time passed, and doctors sounded the alarm: it turned out that the incidence of skin cancer among tanners has increased several times. And as a life-saving remedy, everyone, without exception, was asked to use sunscreens and lotions that contain substances that reflect or absorb ultraviolet radiation.
It is known that even in the time of Columbus, the Indians used to paint themselves red to protect themselves from the sun. Perhaps the ancient Greeks and Romans used a mixture of sand and vegetable oil because the sand reflected the sun's rays. The use of chemical sunscreens began in the 1920s when para-aminobenzoic acid (PABA) was patented as a sunscreen. However, it dissolved in water, so the protective effect disappeared after swimming, and also irritated the skin. In the 1970s, PABA was replaced by its esters, which are almost insoluble in water and do not cause severe irritation. The real boom in the field of sunscreen cosmetics began in the 1980s. Ultraviolet-absorbing substances (in cosmetology they are called “UV filters”) began to be added not only to special “beach” creams, but also to almost all cosmetic products intended for daytime use: cream, liquid powder, lipstick.
Based on their operating principle, UV filters can be divided into two groups: light reflecting (“physical”) and absorbing (“chemical”). Reflective means include various kinds mineral pigments, primarily titanium dioxide, zinc oxide, magnesium silicate. The principle of their action is simple: they scatter ultraviolet radiation, preventing it from penetrating the skin. Zinc oxide covers the wavelength region from 290 to 380 nm, the rest - somewhat less. The main disadvantage of reflective products is that they are powder, opaque and give the skin a white color.
Naturally, cosmetics manufacturers were more attracted to transparent and highly soluble “chemical” UV filters (known in photochemistry as UV absorbers). These include the already mentioned PABA and its esters (nowadays they are almost not used, since there is information that they decompose to form mutagens), salicylates, cinnamic acid derivatives (cinnamates), anthranil esters, hydroxybenzophenones. The principle of operation of a UV absorber is that, having absorbed an ultraviolet quantum, its molecule changes its internal structure and converts light energy into heat. The most efficient and light-resistant UV absorbers operate via the intramolecular proton transfer cycle.
Most UV absorbers only absorb light in the UV-B region. Typically, sunscreens contain not one UV filter, but several, both physical and chemical. General content UV filters can exceed 15 percent.
To characterize the protective effectiveness of creams, lotions and other cosmetic products, the so-called sun protection factor (in English “sun protection factor”, or SPF) began to be used. The idea of SPF was first proposed in 1962 by Austrian scientist Franz Greiter and adopted by representatives of the cosmetics and pharmaceutical industries. The sun protection factor is defined as the ratio of the minimum dose of ultraviolet radiation required to cause erythema when exposed to protected skin to the dose that causes the same effect on unprotected skin. A popular interpretation has become widespread: if without protection you burn in 20 minutes, then by smearing your skin with a cream with a protective factor of, say, 15, you will only get a sunburn after being in the sun 15 times longer, that is, after 5 hours.
A FALSE SENSE OF PROTECTION
It would seem that a solution to the ultraviolet problem has been found. But in reality everything is not so simple. Reports began to appear in the scientific literature that in people who regularly use sunscreens, the incidence of such types of skin cancer as melanoma and basal cell carcinoma not only did not decrease, but actually increased. Several explanations have been proposed for this disconcerting fact.
First, scientists suggested that consumers were using sunscreens incorrectly. When testing creams, it is customary to apply 2 mg of cream per 1 cm 2 to the skin. But, as studies have shown, people often apply a thinner layer, 2-4 times less, and the protection factor decreases accordingly. In addition, creams and lotions are partially washed off with water, for example during bathing.
There was another explanation. As noted, most chemical UV absorbers (and these are the ones most widely used in cosmetics) absorb light only in the UV-B region, preventing the development of sunburn. But, according to some data, melanoma occurs under the influence of UV-A radiation. By blocking UV-B radiation, sunscreens block the natural warning signal of skin redness, slow down the formation of a protective tan, and as a result, a person receives an excess dose in the UVA region, which can cause cancer.
Survey results show that those who use creams with a higher sun protection factor spend more time in the sun, which means they unknowingly put themselves at greater risk.
We must not forget that the mixture of chemicals that are part of protective creams, with prolonged exposure to ultraviolet radiation, can become a source of free radicals - initiators of the oxidation of biomolecules. Some of the UV filters are potentially toxic or cause allergies.
"SUNSHINE" VITAMIN
It's time to remember that in addition to the many negative effects of ultraviolet radiation, there are also positive ones. And the most shining example- photosynthesis of vitamin D 3.
The epidermis contains quite a lot of 7-dihydrocholesterol, a precursor of vitamin D 3 . Irradiation with UV-B light triggers a chain of reactions, which results in the production of cholecalciferol (vitamin D 3), which is not yet active. This substance binds to one of the blood proteins and is transported to the kidneys. There it turns into active form vitamin D 3 - 1, 25-dihydroxycholecalciferol. Vitamin D 3 is necessary for the absorption of calcium into small intestine, normal phosphorus-calcium metabolism and bone formation; with its deficiency, children develop serious illness- rickets.
After whole body irradiation at a dose of 1 MED, the concentration of vitamin D 3 in the blood increases 10 times and returns to its previous level after a week. The use of sunscreens inhibits the synthesis of vitamin D 3 in the skin. The doses required for its synthesis are small. It is considered sufficient to spend about 15 minutes in the sun every day, exposing your face and hands to the sun's rays. The total annual dose required to maintain vitamin D 3 levels is 55 MED.
Chronic deficiency of vitamin D 3 leads to weakening bone tissue. The risk group includes black children living in northern countries, and older people who spend little time outdoors. Some researchers believe that the increase in cancer incidence when using sunscreens is due to blocking the synthesis of vitamin D 3 . It is possible that its deficiency leads to an increased risk of colon and breast cancer.
Other beneficial effects of ultraviolet light are mainly related to medicine. Ultraviolet light is used to treat diseases such as psoriasis, eczema, and pityriasis rosea. Danish physician Niels Finsen received his award in 1903 Nobel Prize for the use of ultraviolet radiation in the treatment of lupus tuberculosis of the skin. The method of irradiating blood with ultraviolet light is now successfully used to treat inflammatory and other diseases.
STRAW SUN HAT
The question of whether ultraviolet light is beneficial or harmful does not have a clear answer: yes and no. Much depends on the dose, spectral composition and characteristics of the organism. Excess ultraviolet radiation is certainly dangerous, but you cannot completely rely on protective creams. More research is needed to determine the extent to which sunscreen use may contribute to the development of cancer.
The best way to protect your skin from sunburn, premature aging, and at the same time reduce the risk of cancer is clothing. Regular summer clothing is characterized by protective factors above 10. Good protective properties Cotton has this effect, although in dry form (when wet, it transmits more ultraviolet radiation). Don't forget a wide-brimmed hat and sunglasses.
The recommendations are quite simple. Avoid being in the sun during the hottest hours. Be especially careful with the sun if you are taking medications that have photosensitizing properties: sulfonamides, tetracyclines, phenothiazines, fluoroquinolones, non-steroidal anti-inflammatory drugs and some others. Photosensitizers are also included in some plants, for example St. John's wort (see "Science and Life" No. 3, 2002). The effect of light can be enhanced by aromatic substances contained in cosmetics and perfumes.
Given that scientists have doubts about the effectiveness and safety of sunscreens and lotions, do not use them (as well as daytime cosmetics with a high content of UV filters) unless absolutely necessary. If such a need arises, give preference to those products that provide protection in a wide spectrum - from 280 to 400 nm. Typically, these creams and lotions contain zinc oxide or other mineral pigments, so it makes sense to carefully read the ingredients on the label.
Sun protection should be individual, depending on where you live, season and skin type.
Scientists from the USA and Israel have discovered that the intensity of gamma radiation from the Sun depends on its activity and the position of the source on the surface, which contradicts all existing theoretical models.
To do this, the researchers analyzed data from the Fermi Gamma-ray Space Telescope collected in 2008–2018. The article was published in Physical Review Letters, Physics briefly reports on it, and a preprint of the work is posted on the website arXiv.org. An extended version of the work was published in Physical Review D (preprint).
Although most of the sun's radiation comes from the visible (44 percent) and infrared (48 percent) regions of the spectrum, our star is also a bright source of gamma rays. The energy of gamma radiation photons (gamma quanta) exceeds 100 kiloelectronvolts, which is approximately one hundred thousand times more than the energy of visible light photons. Currently, scientists are considering two fundamentally different mechanisms for the formation of such high-energy photons. On the one hand, photons can be accelerated in the solar halo due to inverse Compton scattering by cosmic ray electrons. This effect is quite well studied in practice and theory; at the same time, it only works during solar flares and does not provide energy of more than four gigaelectronvolts.
On the other hand, gamma rays can be born inside the Sun when cosmic ray protons accelerated to near-light speeds crash into solar molecules. This process is not tied to solar flares and allows one to obtain photons with energies of about 100 gigaelectronvolts. However, scientists still poorly understand the physics of this process. The only one theoretical model, which explains the emission of gamma rays from the solar disk, the SSG model (Seckel, Stanev & Gaisser), was developed in 1991 and does not agree well with observational data.
In 2014, a team led by Kenny Ng analyzed data from the Fermi Space Telescope, which observed the Sun for six years, and discovered several properties of solar gamma rays that could not be explained by the SSG model. Firstly, the intensity of radiation from the solar disk was more than 50 times higher than the intensity of radiation from the corona (at an energy of the order of 10 gigaelectronvolts).
Secondly, the photon energy reached 100 gigaelectronvolts. Thirdly, the intensity of gamma radiation turned out to be negatively correlated with solar activity - in other words, the flux of gamma rays was maximum when the intensity of solar flares and the number of sunspots were minimal. The SSG model predicts much lower radiation intensity and also cannot explain seasonal variations intensity. Unfortunately, the collected data was not enough to develop a correct theory, and therefore scientists continued their observations.
Now the researchers have presented the results of a similar analysis - however, this time the observations covered almost the entire 11-year cycle of solar activity (from 2008 to 2018) and were of higher quality (that is, they had greater spatial and energy resolution) due to changes in the data processing algorithm. This allowed scientists to identify several more features of solar gamma radiation.
It turned out that the intensity of the radiation depends not only on the phase of the cycle, but also on the position of the point on the surface of the Sun - in other words, in the radiation one can distinguish polar and equatorial components, which change differently over time. The polar component is almost constant during solar cycle, and its spectrum ends abruptly after 100 gigaelectronvolts. At the same time, the equatorial component increases sharply at solar activity minima (in this case, in 2009) and is negligible at other time intervals, and its spectrum extends up to 200 gigaelectronvolts. In total, over the entire observation period, astronomers recorded nine photons with energies of more than 100 gigaelectronvolts - all of them came from equatorial regions, eight of them were emitted in 2009 (the previous minimum) and another at the beginning of 2018 (the beginning of a new minimum). In addition, on December 13, 2008, researchers recorded one “double” event - two almost simultaneous flares with an energy of more than 100 gigaelectronvolts (the flares were separated by a time interval of about 3.5 hours). Scientists note that these flares may be associated with a coronal mass ejection, which began on December 12.
Of course, these dependencies cannot be explained within the framework of the SSG model, since it predicts that the intensity of radiation does not depend on time and the position of a point on the surface of the Sun. Therefore, scientists looked at several alternative models- for example, the focusing or capture of cosmic rays by the magnetic fields of the Sun - but none of them has been able to reproduce the observed dependencies. Nevertheless, the authors of the article continue to observe the Sun and hope that in the future a correct model will be developed.
Since 2008 space telescope Fermi was launched into orbit and managed to make several major discoveries. For example, in November 2015, the telescope discovered the most powerful gamma-ray pulsar, the luminosity of which was twenty times higher than the luminosity of the previous record holder. In June 2016, it recorded a gamma-ray burst, total energy which is equivalent to the mass of complete annihilation of solar matter (~2.5?1054 erg). In October 2017, Fermi for the first time in history detected gamma radiation, which arrived almost simultaneously with gravitational waves from merging neutron stars.
In addition, using a telescope, scientists were able to see a flare on the far side of the Sun and show that dark matter not involved in excess gamma radiation emanating from the center milky way. You can read more about the work of the Fermi telescope in articles by astrophysicist Boris Stern dedicated to the tenth anniversary of the mission.
Since cosmic rays are absorbed by the matter of the Sun, in the vicinity of the star their intensity drops sharply - it turns out that they cast a characteristic “shadow” in the light of gamma radiation. By measuring how this shadow shifts throughout the year, this January, The Tibet AS? estimated the magnitude of the interplanetary magnetic field and showed that the observational results diverge by almost one and a half times from the theory of the potential magnetic field. This indicates that some of the approximations necessary for the theory to work do not hold in practice.
