In a calm atmosphere observe the situation. Text assignments (gia in physics)
There are cold and hot air currents in the atmosphere. Where warm layers form over cold layers, air vortices form, under the influence of which light rays are bent, and the position of the star changes.
The brightness of a star changes because rays that deviate incorrectly are unevenly concentrated over the surface of the planet. At the same time, the entire landscape is constantly shifting and changing due to atmospheric phenomena, for example, due to the wind. The observer of the stars finds himself either in a more illuminated area, or, on the contrary, in a more shaded one.
If you want to watch the twinkling of stars, then keep in mind that at the zenith, with a calm atmosphere, you can only occasionally detect this phenomenon. If you shift your gaze to celestial objects that are closer to the horizon, you will find that they flicker much more strongly. This is due to the fact that you look at the stars through a denser layer of air, and, accordingly, pierce a greater number of air currents with your eyes. You won't notice any color changes in stars above 50°. But find frequent color changes in stars below 35°. Sirius flickers very beautifully, shimmering with all the colors of the spectrum, especially in the winter months, low on the horizon.
The strong twinkling of stars proves the heterogeneity of the atmosphere, which is associated with various meteorological phenomena. Therefore, many people think that flickering is related to the weather. Often it gains strength with low atmospheric pressure, lowering the temperature, increasing humidity, etc. But the state of the atmosphere depends on so many different factors that at the moment it is not possible to predict the weather from the twinkling of stars.
This phenomenon keeps its riddles and ambiguities. It is assumed that it intensifies at dusk. This can be both an optical illusion and a consequence of unusual atmospheric changes that often occur at this time of day. It is believed that the twinkling of stars is due to the northern lights. But this is very difficult to explain, given that the northern lights are at an altitude of more than 100 km. In addition, it remains a mystery why white stars twinkle less than red ones.
Stars are suns. The first person to discover this truth was a scientist of Italian origin. Without any exaggeration, his name is known throughout the modern world. This is the legendary Giordano Bruno. He argued that among the stars there are similar to the Sun in size, and in the temperature of their surface, and even in color, which directly depends on temperature. In addition, there are stars that are significantly different from the Sun - giants and supergiants.
Table of ranks
The variety of countless stars in the sky forced astronomers to establish some order among them. To do this, scientists decided to break the stars into the corresponding classes of their luminosity. For example, stars that emit light several thousand times more than the Sun are called giants. In contrast, stars with minimal luminosity are dwarfs. Scientists have found that the Sun, according to this characteristic, is an average star.
shine differently?
For a while, astronomers thought that the stars didn't shine the same way because of their different positions from the Earth. But it is not so. Astronomers have found that even those stars that are located at the same distance from the Earth can have completely different apparent brilliance. This brightness depends not only on the distance, but also on the temperature of the stars themselves. To compare stars by their apparent brilliance, scientists use a specific unit of measurement - absolute magnitude. It allows you to calculate the real radiation of the star. Using this method, scientists have calculated that there are only 20 of the brightest stars in the sky.
Why are the stars different colors?
It was written above that astronomers distinguish stars by their size and their luminosity. However, this is not the whole classification. Along with size and apparent brilliance, all stars are also subdivided according to their own color. The fact is that the light that determines this or that star has wave radiation. These are pretty short. Despite the minimum wavelength of light, even the smallest difference in the size of light waves dramatically changes the color of a star, which directly depends on the temperature of its surface. For example, if you heat it in an iron pan, it will also acquire the corresponding color.
The color spectrum of a star is a kind of passport that determines its most characteristic features. For example, the Sun and Capella (a star similar to the Sun) were singled out by astronomers in the same. Both of them have a yellow-pale color, their surface temperature is 6000°C. Moreover, their spectrum contains the same substances: lines, sodium and iron.
Stars such as Betelgeuse or Antares generally have a characteristic red color. Their surface temperature is 3000°C, titanium oxide is isolated in their composition. Stars such as Sirius and Vega have white color. Their surface temperature is 10000°C. Their spectra have hydrogen lines. There is also a star with a surface temperature of 30,000 ° C - this is a bluish-white Orion.
Have you ever wondered why the stars are not visible in the sky during the daytime? After all, the air is as transparent during the day as it is at night. The whole point here is that in the daytime the atmosphere scatters sunlight.
Imagine that you are in a well-lit room at night. Through the window glass, bright lights located outside are visible quite well. But dimly lit objects are almost impossible to see. However, as soon as the light is turned off in the room, the glass ceases to be an obstacle to our vision.
Something similar happens when observing the sky: during the day the atmosphere above us is brightly lit and the Sun can be seen through it, but the faint light of distant stars cannot penetrate. But after the Sun plunges below the horizon and the sunlight (and with it the light scattered by the air) "turns off", the atmosphere becomes "transparent" and you can observe the stars.
It's different in space. As the spacecraft rises to a height, the dense layers of the atmosphere remain below and the sky gradually darkens.
At an altitude of about 200-300 km, where manned spacecraft usually fly, the sky is completely black. Black is always, even if the Sun is on the visible part of it at the moment.
“The sky is completely black. The stars in this sky look somewhat brighter and are more clearly visible against the background of a black sky,” the first cosmonaut Yu. A. Gagarin described his space impressions.
But still, not all stars are visible from the board of the spacecraft on the day side of the sky, but only the brightest ones. The blinding light of the Sun and the light of the Earth interferes with the eye.
If we look at the sky from Earth, we can clearly see that all the stars twinkle. They seem to fade, then flare up, shimmering with different colors. And the lower the star is above the horizon, the stronger the twinkling.
The twinkling of stars is also due to the presence of an atmosphere. Before reaching our eyes, the light emitted by a star passes through the atmosphere. In the atmosphere there are always masses of warmer and colder air. The density of the air depends on the temperature of the air in a particular area. Passing from one area to another, light rays experience refraction. The direction of their propagation changes. Due to this, in some places above the earth's surface they are concentrated, in others they are relatively rare. As a result of the constant movement of air masses, these zones are constantly shifting, and the observer sees either an increase or a decrease in the brightness of the stars. But since different colored rays are not refracted in the same way, the moments of amplification and weakening of different colors do not occur simultaneously.
In addition, other, more complex optical effects can play a certain role in the twinkling of stars.
The presence of warm and cold layers of air, intensive movements of air masses also affect the quality of telescopic images.
Where are the best conditions for astronomical observations: in mountainous regions or on the plain, on the seashore or in the depths of the mainland, in the forest or in the desert? And in general, what is better for astronomers - ten cloudless nights for a month or just one clear night, but one when the air is perfectly transparent and calm?
This is only a small part of the issues that have to be addressed when choosing a site for the construction of observatories and the installation of large telescopes. A special field of science deals with similar problems - astro-climatology.
Of course, the best conditions for astronomical observations are outside the dense layers of the atmosphere, in space. By the way, the stars here do not twinkle, but burn with a cold calm light.
Familiar constellations look exactly the same in space as they do on Earth. The stars are at great distances from us, and a few hundred kilometers away from the earth's surface can change nothing in their apparent mutual arrangement. Even when viewed from Pluto, the outlines of the constellations would be exactly the same.
During one orbit from the board of a spacecraft moving in near-Earth orbit, in principle, one can see all the constellations of the earth's sky. The observation of stars from space is of twofold interest: astronomical and navigational. In particular, it is very important to observe starlight unaltered by the atmosphere.
Equally important in space is navigation through the stars. Observing pre-selected "reference" stars, one can not only orient the ship, but also determine its position in space.
For a long time, astronomers have dreamed of future observatories on the surface of the moon. It seemed that the complete absence of the atmosphere should create ideal conditions for astronomical observations on the natural satellite of the Earth, both during the lunar night and during the lunar day.
Passing through the earth's atmosphere, the rays of light change the rectilinear direction. Due to the increase in the density of the atmosphere, the refraction of light rays increases as it approaches the surface of the Earth. As a result, the observer sees the celestial bodies as if elevated above the horizon by an angle called astronomical refraction.
Refraction is one of the main sources of both systematic and random observational errors. In 1906 Newcomb wrote that there is no such branch of practical astronomy that has been written about as much as refraction, and which would be in such an unsatisfactory state. Until the middle of the 20th century, astronomers reduced their observations to refraction tables compiled in the 19th century. The main shortcoming of all the old theories was an inaccurate understanding of the structure of the earth's atmosphere.
Let us take the Earth's surface AB as a sphere of radius OA = R, and represent the Earth's atmosphere in the form of layers concentric with it aw, a 1 into 1, a 2 into 2... with densities that increase as the layers approach the earth's surface (Fig. 2.7). Then the ray SA from some very distant star, refracted in the atmosphere, will come to point A in the direction S¢A, deviating from its original position SA or from the direction S²A parallel to it by some angle S¢AS²= r called astronomical refraction. All elements of the curvilinear ray SA and its final apparent direction AS¢ will lie in the same vertical plane ZAOS. Consequently, astronomical refraction only increases the true direction to the star in the vertical plane passing through it.
The angular elevation of the luminary above the horizon in astronomy is called the height of the luminary. Angle S¢AH = h¢ will be the apparent height of the star, and the angle S²AH = h = h¢ - r is its true height. Injection z is the true zenith distance of the star, and z¢ is its visible value.
The value of refraction depends on many factors and can change in every place on Earth, even during the day. For average conditions, an approximate refraction formula was obtained:
Dh=-0.9666ctg h¢. (2.1)
The coefficient 0.9666 corresponds to the density of the atmosphere at a temperature of +10°C and a pressure of 760 mm Hg. If the characteristics of the atmosphere are different, then the correction for refraction calculated by formula (2.1) must be corrected with corrections for temperature and pressure.
Fig. 2.7. Astronomical refraction
To take into account astronomical refraction in zenithal methods of astronomical determinations, during the observation of the zenithal distances of the luminaries, the temperature and air pressure are measured. In precise methods of astronomical determinations, the zenith distances of the luminaries are measured in the range from 10° to 60°. The upper limit is due to instrumental errors, the lower limit is due to errors in refraction tables.
The zenith distance of the star, corrected by the correction for refraction, is calculated by the formula:
Average (normal at a temperature of +10°C and a pressure of 760 mm Hg. Art.) refraction, calculated by z¢;
Coefficient taking into account the air temperature, calculated from the temperature value;
B- coefficient taking into account air pressure.
The theory of refraction was studied by many scientists. Initially, the initial assumption was that the density of the various layers of the atmosphere decreases with an increase in the height of these layers in an arithmetic progression (Bouguer). But this assumption was soon recognized as unsatisfactory in all respects, since it led to too little refraction and to a too rapid decrease in temperature with height above the earth's surface.
Newton hypothesized that the density of the atmosphere decreases with height according to the law of geometric progression. And this hypothesis turned out to be unsatisfactory. According to this hypothesis, it turned out that the temperature in all layers of the atmosphere should remain constant and equal to the temperature on the Earth's surface.
The most ingenious was Laplace's hypothesis, intermediate between the two above. On this hypothesis of Laplace were based tables of refraction, which were annually placed in the French astronomical calendar.
The Earth's atmosphere with its instability (turbulence, refraction variations) imposes a limit on the accuracy of astronomical observations from the Earth.
When choosing a site for the installation of large astronomical instruments, the astroclimate of the region is first comprehensively studied, which is understood as a set of factors that distort the shape of the wave front of the radiation of celestial objects passing through the atmosphere. If the wave front reaches the device undistorted, then the device in this case can operate with maximum efficiency (with a resolution approaching the theoretical one).
As it turned out, the quality of the telescopic image is reduced mainly due to interference introduced by the surface layer of the atmosphere. The earth, due to its own thermal radiation, cools significantly at night and cools the layer of air adjacent to it. A change in air temperature by 1°C changes its refractive index by 10 -6 . On isolated mountain peaks, the thickness of the surface layer of air with a significant difference (gradient) in temperature can reach several tens of meters. In valleys and flat areas at night, this layer is much thicker and can be hundreds of meters. This explains the choice of sites for astronomical observatories on the spurs of the ridges and on isolated peaks, from where denser cold air can drain into the valleys. The height of the telescope tower is chosen such that the instrument is above the main region of temperature inhomogeneities.
An important factor in the astroclimate is the wind in the surface layer of the atmosphere. By mixing layers of cold and warm air, it causes the appearance of density inhomogeneities in the air column above the device. Irregularities smaller than the telescope diameter lead to image defocusing. Larger density fluctuations (several meters or larger) do not cause sharp distortions of the wave front and lead mainly to a shift rather than to defocusing of the image.
In the upper layers of the atmosphere (in the tropopause), fluctuations in the density and refractive index of air are also observed. But perturbations in the tropopause do not noticeably affect the quality of images given by optical instruments, since the temperature gradients there are much smaller than in the surface layer. These layers do not cause trembling, but the twinkling of stars.
In astroclimatic research, a relationship is established between the number of clear days recorded by the meteorological service and the number of nights suitable for astronomical observations. The most advantageous regions, according to the astroclimatic analysis of the territory of the former USSR, are some mountainous regions of the Central Asian states.
Earth refraction
Rays from ground objects, if they travel a long enough path in the atmosphere, also experience refraction. The trajectory of the rays under the influence of refraction is bent, and we see them in the wrong places or in the wrong direction, where they actually are. Under certain conditions, as a result of terrestrial refraction, mirages arise - false images of distant objects.
The angle of the earth's refraction a is the angle between the direction to the visible and the actual position of the observed object (Fig. 2.8). The value of the angle a depends on the distance to the observed object and on the vertical temperature gradient in the surface layer of the atmosphere, in which rays propagate from ground objects.
Fig.2.8. The manifestation of the earth's refraction when sighting:
a) - from bottom to top, b) - from top to bottom, a - the angle of the earth's refraction
The geodetic (geometric) visibility range is associated with terrestrial refraction (Fig. 2.9). Let us assume that the observer is at point A at a certain height h H above the earth's surface and observes the horizon in the direction of point B. The NAS plane - a horizontal plane passing through point A perpendicular to the radius of the globe, is called the plane of the mathematical horizon. If the rays of light propagated in the atmosphere in a straight line, then the farthest point on Earth that an observer from point A can see would be point B. The distance to this point (tangent AB to the globe) is the geodesic (or geometric) visibility range D 0 . The circular line on the earth's surface BB is the geodesic (or geometric) horizon of the observer. The value of D 0 is determined only by geometric parameters: the radius of the Earth R and the height h H of the observer and is equal to D o ≈ √ 2Rh H = 3.57√ h H, which follows from Figure 2.9.
Fig.2.9. Terrestrial refraction: mathematical (HH) and geodetic (BB) horizons, geodetic visibility range (AB = D 0)
If the observer observes some object located at a height h pr above the Earth's surface, then the geodetic range will be the distance AC \u003d 3.57 (√ h H + √ h pr). These statements would be true if light propagated in the atmosphere in a straight line. But it's not. With a normal distribution of temperature and air density in the surface layer, the curved line depicting the trajectory of the light beam faces the Earth with its concave side. Therefore, the farthest point that an observer from A will see will not be B, but B¢. The geodetic visibility range AB¢, taking into account refraction, will be on average 6-7% larger and instead of a coefficient of 3.57 in the formulas there will be a coefficient of 3.82. Geodetic range is calculated by the formulas
, h - in m, D - in km, R - 6378 km
where h n and h pr - in meters, D- in kilometers.
For a person of average height, the range of the horizon on Earth is about 5 km. For cosmonauts V.A. Shatalov and A.S. Eliseev, who flew on the Soyuz-8 spacecraft, the horizon range at perigee (altitude 205 km) was 1730 km, and at apogee (altitude 223 km) - 1800 km.
For radio waves, refraction is almost independent of wavelength, but in addition to temperature and pressure, it also depends on the content of water vapor in the air. Under the same conditions of change in temperature and pressure, radio waves are refracted more strongly than light waves, especially at high humidity.
Therefore, in the formulas for determining the range of the horizon or detecting an object by a radar beam, there will be a factor of 4.08 in front of the root. Therefore, the horizon of the radar system is about 11% further away.
Radio waves are well reflected from the earth's surface and from the lower boundary of the inversion or a layer of low humidity. In such a peculiar waveguide formed by the earth's surface and the base of the inversion, radio waves can propagate over very long distances. These features of radio wave propagation are successfully used in radar.
The air temperature in the surface layer, especially in its lower part, does not always drop with height. It may decrease at different rates, it may not change in height (isothermia), and it may increase with height (inversion). Depending on the magnitude and sign of the temperature gradient, refraction can affect the range of the visible horizon in different ways.
The vertical temperature gradient in a homogeneous atmosphere in which the air density does not change with height, g 0 = 3.42°C/100m. Consider what will be the trajectory of the beam AB at different temperature gradients near the Earth's surface.
Let , i.e. air temperature decreases with height. Under this condition, the refractive index also decreases with height. The trajectory of the light beam in this case will be turned to the earth's surface with its concave side (in Fig. 2.9, the trajectory AB¢). Such refraction is called positive. farthest point AT¢ The observer will see in the direction of the last tangent to the ray path. This tangent, i.e. the horizon visible due to refraction is equal to the mathematical horizon NAS angle D, smaller angle d. Injection d is the angle between the mathematical and geometric horizon without refraction. Thus, the visible horizon has risen by an angle ( d- D) and expanded as D > D0.
Now let's imagine that g gradually decreases, i.e. temperature decreases more and more slowly with height. There will come a moment when the temperature gradient becomes equal to zero (isotherm), and then the temperature gradient becomes negative. The temperature no longer decreases, but grows with height, i.e. temperature inversion is observed. With a decrease in the temperature gradient and its transition through zero, the visible horizon will rise higher and higher, and there will come a moment when D becomes equal to zero. The visible geodetic horizon will rise to the mathematical one. The earth's surface, as it were, straightened, became flat. The geodetic visibility range is infinitely large. The radius of curvature of the beam became equal to the radius of the globe.
With an even stronger temperature inversion, D becomes negative. The visible horizon has risen above the mathematical one. To the observer at point A it will seem that he is at the bottom of a huge basin. Because of the horizon, objects that are far beyond the geodetic horizon rise and become visible (as if floating in the air) (Fig. 2.10).
Such phenomena can be observed in polar countries. So, from the Canadian coast of America through the Smith Strait, one can sometimes see the coast of Greenland with all the buildings on it. The distance to the Greenland coast is about 70 km, while the geodetic visibility range is no more than 20 km. Another example. From the English side of the Pas de Calais, from Hastings, I have seen the French coast lying across the strait at a distance of about 75 km.
Fig.2.10. The phenomenon of unusual refraction in polar countries
Now let's assume that g=g 0 , therefore, the air density does not change with height (homogeneous atmosphere), there is no refraction and D=D 0 .
At g > g 0, the refractive index and air density increase with height. In this case, the trajectory of light rays faces the earth's surface with its convex side. This refraction is called negative. The last point on Earth that an observer at A sees will be B². The visible horizon AB² narrowed and sank to an angle (D - d).
From the above, we can formulate the following rule: if the air density (and, hence, the refractive index) changes along the propagation of a light beam in the atmosphere, then the light beam will bend so that its trajectory is always convex in the direction of decreasing density (and refractive index) of air .
Refraction and mirages
The word mirage is of French origin and has two meanings: "reflection" and "deceptive vision." Both meanings of this word well reflect the essence of the phenomenon. A mirage is an image of an object that really exists on Earth, often enlarged and greatly distorted. There are several types of mirages depending on where the image is located in relation to the subject: upper, lower, lateral and complex. The most commonly observed are superior and inferior mirages, which occur when there is an unusual distribution of density (and, therefore, refractive index) along the height, when at a certain height or near the very surface of the Earth there is a relatively thin layer of very warm air (with a low refractive index), in which rays coming from ground objects experience total internal reflection. This happens when rays fall on this layer at an angle greater than the angle of total internal reflection. This warmer layer of air plays the role of an air mirror that reflects the rays falling into it.
Superior mirages (Fig. 2.11) occur in the presence of strong temperature inversions, when the air density and refractive index rapidly decrease with height. In superior mirages, the image is located above the subject.
Fig.2.11. superior mirage
The trajectories of light rays are shown in Figure (2.11). Let us assume that the earth's surface is flat and layers of equal density are parallel to it. Since the density decreases with height, then . The warm layer, which plays the role of a mirror, lies at a height. In this layer, when the angle of incidence of the rays becomes equal to the refractive index (), the rays turn back to the earth's surface. The observer can simultaneously see the object itself (if it is not beyond the horizon) and one or more images above it - straight and inverted.
Fig.2.12. Complex superior mirage
On fig. 2.12 shows a diagram of the emergence of a complex upper mirage. The object itself is visible ab, above it is its direct image a¢b¢, inverted in²b² and straight again a²¢b²¢. Such a mirage can occur if the air density decreases with height, first slowly, then quickly, and again slowly. The image is inverted if the rays coming from the extreme points of the object intersect. If the object is far away (beyond the horizon), then the object itself may not be visible, and its images, raised high into the air, are visible from great distances.
The city of Lomonosov is located on the coast of the Gulf of Finland, 40 km from St. Petersburg. Usually from Lomonosov St. Petersburg is not visible at all or is visible very poorly. Sometimes St. Petersburg is visible "at a glance". This is one of the examples of superior mirages.
Apparently, at least a part of the so-called ghostly Earths, which were searched for in the Arctic for decades and never found, should be attributed to the number of upper mirages. The search for Sannikov Land was especially long.
Yakov Sannikov was a hunter, engaged in fur trade. In 1811 he set off on dogs across the ice to the group of New Siberian Islands and from the northern tip of Kotelny Island saw an unknown island in the ocean. He could not reach it, but announced the discovery of a new island to the government. In August 1886 E.V. Tol, during his expedition to the New Siberian Islands, also saw Sannikov Island and made an entry in his diary: “The horizon is completely clear. In the direction to the northeast, 14-18 degrees, we clearly saw the contours of four mesas, which in the east connected with low-lying land. Thus, Sannikov's message was fully confirmed. We have the right, therefore, to draw a dotted line on the map in an appropriate place and inscribe on it: "Sannikov Land".
Tol spent 16 years of his life searching for Sannikov Land. He organized and led three expeditions to the area of the New Siberian Islands. During the last expedition on the schooner "Zarya" (1900-1902), Tolya's expedition perished without finding Sannikov Land. No one else has seen Sannikov Land. Perhaps it was a mirage that appears in the same place at certain times of the year. Both Sannikov and Tol saw the mirage of the same island located in this direction, only much further in the ocean. Maybe it was one of the De Long Islands. Perhaps it was a huge iceberg - a whole ice island. Such ice mountains, up to 100 km2 in area, travel across the ocean for several decades.
The mirage did not always deceive people. English polar explorer Robert Scott in 1902. in Antarctica I saw mountains, as if hanging in the air. Scott guessed that there was a mountain range further over the horizon. And, indeed, the mountain range was later discovered by the Norwegian polar explorer Raoul Amundsen exactly where Scott had supposed it to be.
Fig.2.13. inferior mirage
Inferior mirages (Fig. 2.13) occur with a very rapid decrease in temperature with height, i.e. at very large temperature gradients. The role of the air mirror is played by the thin surface warmest layer of air. The mirage is called the lower one, since the image of the object is placed under the object. In the lower mirages, it seems as if there is a water surface under the object and all objects are reflected in it.
In calm water, all objects standing on the shore are well reflected. Reflection in a thin layer of air heated from the earth's surface is completely analogous to reflection in water, only the air itself plays the role of a mirror. The state of air in which inferior mirages occur is extremely unstable. After all, below, near the ground, lies strongly heated, and therefore lighter air, and above it - colder and heavier. Jets of hot air rising from the ground penetrate layers of cold air. Due to this, the mirage changes before our eyes, the surface of the “water” seems to be waving. A small gust of wind or a push is enough and a collapse will occur, i.e. reversal of air layers. Heavy air will rush down, destroying the air mirror, and the mirage will disappear. Favorable conditions for the occurrence of inferior mirages are a homogeneous, even underlying surface of the Earth, which takes place in the steppes and deserts, and sunny calm weather.
If a mirage is an image of a real-life object, then the question arises - the image of what water surface do travelers in the desert see? After all, there is no water in the desert. The fact is that the apparent water surface or lake visible in a mirage is in fact an image not of a water surface, but of the sky. Parts of the sky are reflected in the air mirror and create a complete illusion of a brilliant water surface. Such a mirage can be seen not only in the desert or in the steppe. They arise even in St. Petersburg and its environs on sunny days over asphalt roads or a flat sandy beach.
Fig.2.14. side mirage
Lateral mirages occur when air layers of the same density are located in the atmosphere not horizontally, as usual, but obliquely and even vertically (Fig. 2.14). Such conditions are created in the summer, in the morning shortly after sunrise near the rocky shores of the sea or lake, when the shore is already illuminated by the Sun, and the surface of the water and the air above it are still cold. Lateral mirages have been repeatedly observed on Lake Geneva. A side mirage can appear at the stone wall of a house heated by the Sun, and even to the side of a heated stove.
Mirages of a complex type, or Fata Morgana, occur when there are conditions for the appearance of both an upper and lower mirage at the same time, for example, with a significant temperature inversion at a certain height above a relatively warm sea. Air density first increases with height (air temperature decreases), and then also rapidly decreases (air temperature rises). With such a distribution of air density, the state of the atmosphere is very unstable and subject to sudden changes. Therefore, the appearance of the mirage is changing before our eyes. The most ordinary rocks and houses, due to repeated distortions and magnification, turn into the wonderful castles of Fairy Morgana before our eyes. Fata Morgana is observed off the coast of Italy, Sicily. But it can also occur at high latitudes. This is how the well-known explorer of Siberia F.P. Wrangel described the fata morgana he saw in Nizhnekolymsk: “The action of horizontal refraction produced the genus fata morgana. The mountains lying to the south seemed to us in various distorted forms and hanging in the air. The distant mountains seemed to be overturned peaks. The river narrowed to the point that the opposite bank seemed to be located almost at our huts.
Astronomers call flashes "sporadic" - they are sudden and unpredictable. Moreover, it is known from observations that very intense flare activity is inherent in red dwarfs. They are less massive stars than our Sun, and are also considered suitable for the role of "cradles of life." Recently, scientists have found out the reason for this phenomenon.
Interest in the phenomenon of flares in red dwarfs is quite natural - the fact is that such a powerful flare can be fatal for an emerging or developed biota. But red dwarfs have planets, some of which have quite normal conditions for the existence of life.
Against the background of giant stars, red dwarfs look like faintly luminous stars, so their observations are made in a limited near area. In our Galaxy, in the constellation Ursa Major, there is a binary star system consisting of two red dwarfs - they are separated by a distance of 190 astronomical units. On the scale of the solar system, this is four times the distance from the Sun to Pluto.
This star system is called Gliese 412 and has been studied quite thoroughly. Its stars, red dwarfs, are as follows: the first - Gliese 412 A in mass reaches half the mass of the Sun, and glows much weaker - it reaches only 2 percent of the luminosity of our star. The second star Gliese 412 B is much less massive and does not have a constant luminosity. This is a very dim M6 class star, a hundred times fainter than its neighbor Gliese 412 A! But the brightest moments of stellar flares are detected by such variable stars, this is truly their "star moment" - the strongest burst of luminosity brightness is found in observations.
The stellar flare theory explains these phenomena as transformations in the complex hierarchy of stellar magnetic fields that govern stellar activity. This is clearly visible on the Sun: a new activity complex with spots is formed, it grows and changes, and when a new strong magnetic flux emerges, the lines of force are reconnected, and in the conducting plasma medium a powerful energy transformation occurs on the Sun, which is seen as a flash. This ejection has gigantic kinetic energy and flies away from the Sun at speeds of more than 1000 km/s. Giant flares occur on red dwarfs, the convective plasma medium of these stars generates flare activity according to the same electric discharge scheme.
Vakhtang Tamazyan, professor at the University de Santiago de Compostela (Galicia, Spain), with a group of colleagues from Spain and Armenia, identified and studied an exceptionally powerful example of such a flare process: the variable star WX UMa increased its brightness by 15 times in 160 s. The temperature of its surface, equal to 2800 K, in the region of the flare event reached 18000 K - such is the surface temperature of blue giants of spectral class B! But blue giants feed their monstrous luminosity with a constant influx of energy from the depths of the star. In the case of a red dwarf, this temperature reveals the heating of a coronal flare loop, an active formation in the upper atmosphere of a red dwarf, the luminosity of which is initiated by the realized energy of the magnetic field.
A similar change in the brightness of the coronal loop on the Sun was discovered in the Koronas-F space experiment at IZMIRAN named after V.I. N. V. Pushkov RAS, the discovery was awarded the State Prize. Usually, the corona of the Sun is heated to about 2 million degrees; in the Koronas-F experiment, heating up to 20 million degrees was observed. On red dwarfs, typical flare stars, the instabilities of their complex magnetic fields are realized in this way. It is not easy to register these phenomena due to the low luminosity, since red dwarfs cannot be observed further than 60 light years from the Earth, this is the limit of modern technical capabilities.
The stellar pair, which includes the star WX UMa, gives researchers a unique opportunity to "investigate whether the frequency of flares and the relative position of a pair of luminaries rotating around each other are related," Vakhtang Tamazyan emphasizes. The study of a binary system, where red dwarfs interact with each other gravitationally, makes it possible to investigate the question of the connectivity of flare processes and expand our understanding of the physical nature of unique flares on red dwarfs.
Simultaneously with the observation of the star WX UMa, a team of astronomers studied four additional binary systems with red dwarfs, observing their flare activity. No powerful outbursts were observed, but nevertheless, three more dwarfs became brighter during outbursts, and only one of them did not show such activity during the observation period. So, as it turned out, the flare characteristics of red dwarfs do not have a revealed periodicity. As a result, scientists suggested that since a large number of flares were recorded in binary systems in such a short time, then, apparently, they appear due to the influence of a companion star.
It should be noted that red dwarfs raging with flashes are not like our much more stable Sun in this respect. The flare activity of the Sun originates on the growth branch of each 11-year cycle, reaches its apogee at the maximum of the cycle, falling to the minimum manifestations at the minimum of solar activity. Although exceptions to the general trends have already been observed: in 2003, shortly before the minimum, a series of powerful solar flares took place, which attracted great attention from specialists.
Such strong flares on the Sun are called x-ray flares, M and X points. Studies of flares, as the most energetic manifestations of solar and stellar activity, are carefully recorded and analyzed according to modern space observatories. Their nature is becoming more and more clear to scientists, but the forecast of flare events is still only probabilistic and not accurate. But it is quite possible that as knowledge improves, such a forecast may appear ...
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