A message on the topic of the physical nature of stars. Abstract: Evolution and structure of the galaxy

Federal Agency for Education
State educational institution of higher professional education
"Chelyabinsk State Pedagogical University" (GOU VPO "ChGPU")

ABSTRACT ON THE CONCEPT OF MODERN NATURAL SCIENCE

Topic: Physical nature of stars

Completed by: Rapokhina T.I.
543 group
Checked by: Barkova V.V.

Chelyabinsk – 2012
CONTENT
Introduction………………………………………………………………3
Chapter 1. What is a star………………………………………………………4

The essence of the stars…………………………………………………………….. .4
The Birth of Stars……………………………………………………………7

1.2 Evolution of stars……………… ……………………………………………………… 10
1.3 The end of a star……………………………………………………………… .14
Chapter 2. Physical nature of stars…………………………………………..24
2.1 Luminosity …………………………………………………………… ………………….24
2.2 Temperature…………………………………………………………………… …………..…26
2.3 Spectra and chemical composition of stars…………………………….…… ……27
2.4 Average densities of stars…………………………………………….28
2.5 Radius of stars……………………………………………………………….39
2.6 Mass of stars……………………………………………………………… 30
Conclusion……………………………………………………………………..32
References………………………………………………………………………………33
Appendix……………………………………………………………………34

INTRODUCTION

There is nothing simpler than a star...
(A.S. Eddington)

From time immemorial, Man tried to give names to the objects and phenomena that surrounded him. This also applies to celestial bodies. First, the brightest, clearly visible stars were given names, and over time, others were given names.
The discovery of stars whose apparent brightness changes over time led to special designations. They are designated by capital Latin letters, followed by the name of the constellation in the genitive case. But the first variable star discovered in a certain constellation is not designated by the letter A. The countdown is from the letter R. The next star is designated by the letter S, and so on. When all the letters of the alphabet are exhausted, a new circle begins, that is, after Z, A is used again. In this case, letters can be doubled, for example “RR”. "R Leo" means it is the first variable star discovered in the constellation Leo.
Stars are very interesting to me, so I decided to write an essay on this topic.
Stars are distant suns, therefore, while studying the nature of stars, we will compare their physical characteristics with the physical characteristics of the Sun.

Chapter 1. WHAT IS A STAR
1.1 ESSENCE OF STARS
When examined carefully, the star appears as a luminous point, sometimes with diverging rays. The phenomenon of rays is associated with a feature of vision and has nothing to do with the physical nature of the star.
Any star is the sun distant from us. The nearest star, Proxima, is 270,000 times farther from us than the Sun. The brightest star in the sky, Sirius in the constellation Canis Major, located at a distance of 8x1013 km, has about the same brightness as a 100-watt light bulb at a distance of 8 km (if you do not take into account the attenuation of light in the atmosphere). But in order for the light bulb to be visible from the same angle at which the disk of distant Sirius is visible, its diameter must be 1 mm!
With good visibility and normal vision, about 2,500 stars can be seen above the horizon at the same time. 275 stars have their own names, for example, Algol, Aldebaran, Antares, Altair, Arcturus, Betelgeuse, Vega, Gemma, Dubhe, Canopus (the second brightest star), Capella, Mizar, Polaris (guiding star), Regulus, Rigel, Sirius, Spica, Heart of Karl, Taygeta, Fomalhaut, Sheat, Etamin, Electra, etc.
The question of how many stars are in a given constellation is meaningless, since it lacks specificity. To answer, you need to know the visual acuity of the observer, the time when observations are made (the brightness of the sky depends on this), the height of the constellation (it is difficult to detect a faint star near the horizon due to atmospheric attenuation of light), the observation location (in the mountains the atmosphere is cleaner, more transparent - therefore it is visible more stars), etc. On average, there are approximately 60 stars per constellation that are visible to the naked eye (the Milky Way and large constellations have the most). For example, in the constellation Cygnus you can count up to 150 stars (Milky Way region); and in the constellation Leo - only 70. In the small constellation Triangle, only 15 stars are visible.
If we take into account stars up to 100 times fainter than the faintest stars still visible to a keen observer, then on average there will be about 10,000 stars per constellation.
Stars vary not only in their brightness, but also in color. For example, Aldebaran (Taurus), Antares (Scorpio), Betelgeuse (Orion) and Arcturus (Bootes) are red, and Vega (Lyra), Regulus (Leo), Spica (Virgo) and Sirius (Canis Major) are white and bluish. .
The stars are twinkling. This phenomenon is clearly visible near the horizon. The cause of flickering is the optical inhomogeneity of the atmosphere. Before reaching the observer's eye, starlight crosses many small irregularities in the atmosphere. In their optical properties, they are similar to lenses that concentrate or scatter light. The continuous movement of such lenses is what causes flicker.
The reason for the color change during flickering is explained in Fig. 6, from which it can be seen that blue (c) and red (k) light from the same star travels unequal paths in the atmosphere before entering the eye of the observer (O). This is a consequence of unequal refraction of blue and red light in the atmosphere. Inconsistency in brightness fluctuations (caused by various inhomogeneities) leads to unbalanced colors.

Fig.6.
Unlike general flickering, color flickering can only be seen in stars close to the horizon.
For some stars, called variable stars, changes in brightness occur much more slowly and smoothly than during scintillation, Fig. 7. For example, the star Algol (Devil) in the constellation Perseus changes its brightness with a period of 2.867 days. The reasons for the “variability” of stars are diverse. If two stars revolve around a common center of mass, then one of them can periodically cover the other (Algol case). In addition, some stars change brightness as they pulsate. Other stars change brightness during explosions on the surface. Sometimes the entire star explodes (then a supernova is observed, the luminosity of which is billions of times greater than that of the sun).

Fig.7.
The movements of stars relative to each other at speeds of tens of kilometers per second lead to a gradual change in star patterns in the sky. However, human life expectancy is too short for such changes to be noticed with the naked eye.

1.2 THE BIRTH OF STARS

Modern astronomy has a large number of arguments in favor of the assertion that stars are formed by the condensation of clouds of gas and dust in the interstellar medium. The process of star formation from this environment continues to this day. Clarification of this circumstance is one of the greatest achievements of modern astronomy. Until relatively recently, it was believed that all stars were formed almost simultaneously many billions of years ago. The collapse of these metaphysical ideas was facilitated, first of all, by the progress of observational astronomy and the development of the theory of the structure and evolution of stars. As a result, it became clear that many of the observed stars are relatively young objects, and some of them arose when man was already on Earth.
An important argument in favor of the conclusion that stars are formed from the interstellar gas-dust medium is the location of groups of obviously young stars (the so-called “associations”) in the spiral arms of the Galaxy. The fact is that, according to radio astronomical observations, interstellar gas is concentrated mainly in the spiral arms of galaxies. In particular, this occurs in our Galaxy. Moreover, from detailed “radio images” of some galaxies close to us, it follows that the highest density of interstellar gas is observed on the inner (relative to the center of the corresponding galaxy) edges of the spiral, which has a natural explanation, the details of which we will not dwell on here. But it is precisely in these parts of the spirals that “HH zones”, i.e., clouds of ionized interstellar gas, are observed by optical astronomy methods. The reason for the ionization of such clouds can only be ultraviolet radiation from massive hot stars - obviously young objects.
Central to the problem of the evolution of stars is the question of the sources of their energy. In the last century and at the beginning of this century, various hypotheses were proposed about the nature of the energy sources of the Sun and stars. Some scientists, for example, believed that the source of solar energy was the continuous fall of meteors on its surface, others looked for the source in the continuous compression of the Sun. The potential energy released during such a process could, under certain conditions, turn into radiation. As we will see below, this source can be quite effective at an early stage of stellar evolution, but it cannot provide radiation from the Sun for the required time.
Advances in nuclear physics made it possible to solve the problem of sources of stellar energy back in the late thirties of our century. Such a source is thermonuclear fusion reactions occurring in the depths of stars at the very high temperature prevailing there (on the order of ten million degrees).
As a result of these reactions, the speed of which strongly depends on temperature, protons turn into helium nuclei, and the released energy slowly “leaks” through the depths of stars and, in the end, significantly transformed, is emitted into outer space. This is an extremely powerful source. If we assume that initially the Sun consisted only of hydrogen, which as a result of thermonuclear reactions will completely turn into helium, then the amount of energy released will be approximately 10 52 erg. Thus, to maintain radiation at the observed level for billions of years, it is enough for the Sun to “use up” no more than 10% of its initial supply of hydrogen.
Now we can imagine the evolution of a star as follows. For some reasons (several of them can be specified), a cloud of interstellar gas-dust medium began to condense. Quite soon (of course, on an astronomical scale!) under the influence of forces universal gravity from this cloud a relatively dense opaque gas ball is formed. Strictly speaking, this ball cannot yet be called a star, since in its central regions the temperature is not sufficient for thermonuclear reactions to begin. The gas pressure inside the ball is not yet able to balance the forces of attraction of its individual parts, so it will continuously compress. Some astronomers previously believed that such protostars were observed in individual nebulae in the form of very dark compact formations, the so-called globules. The successes of radio astronomy, however, forced us to abandon this rather naive point of view. Usually, not one protostar is formed at the same time, but a more or less numerous group of them. Subsequently, these groups become stellar associations and clusters, well known to astronomers. It is very likely that at this very early stage in the evolution of a star, clumps with a lower mass are formed around it, which then gradually turn into planets.
When a protostar contracts, its temperature rises and a significant part of the released potential energy is radiated into the surrounding space. Since the dimensions of the collapsing gas ball are very large, the radiation per unit of its surface will be insignificant. Since the radiation flux per unit surface is proportional to the fourth power of temperature (Stefan-Boltzmann law), the temperature of the surface layers of the star is relatively low, while its luminosity is almost the same as that of an ordinary star with the same mass. Therefore, on the spectrum-luminosity diagram, such stars will be located to the right of the main sequence, i.e., they will fall into the region of red giants or red dwarfs, depending on the values ​​of their initial masses.
Subsequently, the protostar continues to contract. Its dimensions become smaller, and the surface temperature increases, as a result of which the spectrum becomes increasingly earlier. Thus, moving along the spectrum-luminosity diagram, the protostar will rather quickly “sit down” on the main sequence. During this period, the temperature of the stellar interior is already sufficient for thermonuclear reactions to begin there. In this case, the gas pressure inside the future star balances the attraction and the gas ball stops compressing. A protostar becomes a star.

Magnificent columns of mostly hydrogen gas and dust give rise to newborn stars inside the Eagle Nebula.

Photo: NASA, ESA, STcI, J Hester and P Scowen (Arizona State University)

1.3 EVOLUTION OF STARS
It takes relatively little time for protostars to go through the earliest stages of their evolution. If, for example, the mass of the protostar is greater than the solar one, it takes only a few million years, if less, several hundred million years. Since the evolutionary time of protostars is relatively short, this earliest phase of star development is difficult to detect. Nevertheless, stars in such a stage are apparently observed. We mean very interesting stars type T Tauri, usually immersed in dark nebulae.
In 5966, quite unexpectedly, it became possible to observe protostars in the early stages of their evolution. Radio astronomers were greatly surprised when, when surveying the sky at a wavelength of 18 cm, corresponding to the OH radio line, bright, extremely compact (i.e., having small angular dimensions) sources were discovered. This was so unexpected that at first they refused to even believe that such bright radio lines could belong to a hydroxyl molecule. It was hypothesized that these lines belonged to some unknown substance, which was immediately given the “appropriate” name “mysterium”. However, the “mysterium” very soon shared the fate of its optical “brothers” - “nebulia” and “crown”. The fact is that for many decades the bright lines of nebulae and the solar corona could not be identified with any known spectral lines. Therefore, they were attributed to certain hypothetical elements unknown on earth - “nebulium” and “crown”. In 1939-1941. It was convincingly shown that the mysterious "coronium" lines belong to multiply ionized atoms of iron, nickel and calcium.
If it took decades to “debunk” “nebulium” and “coronia,” then within a few weeks after the discovery it became clear that the “mysterium” lines belong to ordinary hydroxyl, but only under unusual conditions.
So, the sources of “mysterium” are giant, natural cosmic masers operating at the wave of the hydroxyl line, the length of which is 18 cm. It is in masers (and at optical and infrared frequencies - in lasers) that enormous brightness in the line is achieved, and its spectral width is small . As is known, amplification of radiation in lines due to this effect is possible when the medium in which the radiation propagates is “activated” in some way. This means that some “external” energy source (the so-called “pumping”) makes the concentration of atoms or molecules at the initial (upper) level abnormally high. Without a constantly operating "pumping" a maser or laser is impossible. The question of the nature of the mechanism for “pumping” cosmic masers has not yet been finally resolved. However, most likely the “pumping” is provided by fairly powerful infrared radiation. To others possible mechanism"pumping" may be some chemical reaction.
The mechanism of “pumping” these masers is not yet entirely clear, but one can still get a rough idea of ​​the physical conditions in the clouds emitting the 18 cm line using the maser mechanism. First of all, it turns out that these clouds are quite dense: in a cubic centimeter there are at least at least 10 8 -10 9 particles, and a significant (and perhaps most) part of them are molecules. The temperature is unlikely to exceed two thousand degrees, most likely it is about 1000 degrees. These properties are sharply different from the properties of even the densest clouds of interstellar gas. Considering the relatively small size of the clouds, we involuntarily come to the conclusion that they are more likely to resemble the extended, rather cold atmospheres of supergiant stars. It is very likely that these clouds are nothing more than an early stage in the development of protostars, immediately following their condensation from the interstellar medium. Other facts also support this statement (which the author of this book expressed back in 1966). In nebulae where cosmic masers are observed, young, hot stars are visible. Consequently, the star formation process there recently ended and, most likely, continues at the present time. Perhaps the most curious thing is that, as radio astronomy observations show, cosmic masers of this type are, as it were, “immersed” in small, very dense clouds of ionized hydrogen. These clouds contain a lot of cosmic dust, which makes them unobservable in the optical range. Such "cocoons" are ionized by the young, hot star located inside them. Infrared astronomy has proven to be very useful in studying star formation processes. Indeed, for infrared rays, interstellar absorption of light is not so significant.
We can now imagine the following picture: from the cloud of the interstellar medium, through its condensation, several clumps of different masses are formed, evolving into protostars. The rate of evolution is different: for more massive clumps it will be greater. Therefore, the most massive clump will turn into a hot star first, while the rest will linger more or less long at the protostar stage. We observe them as sources of maser radiation in the immediate vicinity of a “newborn” hot star, ionizing the “cocoon” hydrogen that has not condensed into clumps. Of course, this rough scheme will be further refined, and, of course, significant changes will be made to it. But the fact remains: it unexpectedly turned out that for some time (most likely a relatively short time) newborn protostars, figuratively speaking, “scream” about their birth, using the latest methods of quantum radio physics (i.e., masers).
Once on the main sequence and having stopped burning, the star radiates for a long time, practically without changing its position on the spectrum-luminosity diagram. Its radiation is supported by thermonuclear reactions occurring in the central regions. Thus, the main sequence is, as it were, a geometric location of points on the spectrum-luminosity diagram where a star (depending on its mass) can emit for a long time and steadily due to thermonuclear reactions. A star's place on the main sequence is determined by its mass. It should be noted that there is one more parameter that determines the position of the equilibrium emitting star on the spectrum-luminosity diagram. This parameter is the initial chemical composition of the star. If the relative abundance of heavy elements decreases, the star will "fall" in the diagram below. It is this circumstance that explains the presence of a sequence of subdwarfs. As mentioned above, the relative abundance of heavy elements in these stars is tens of times less than in main sequence stars.
The time a star stays on the main sequence is determined by its initial mass. If the mass is large, the star’s radiation has enormous power and it quickly uses up its reserves of hydrogen “fuel”. For example, main sequence stars with a mass several tens of times greater than the Sun (these are hot blue giants of spectral class O) can emit steadily while remaining on this sequence for only a few million years, while stars with a mass close to solar, have been on the main sequence for 10-15 billion years.
The “burning out” of hydrogen (i.e., its transformation into helium during thermonuclear reactions) occurs only in the central regions of the star. This is explained by the fact that stellar matter is mixed only in the central regions of the star, where nuclear reactions take place, while the outer regions keep the relative hydrogen content unchanged. Since the amount of hydrogen in the central regions of the star is limited, sooner or later (depending on the mass of the star) almost all of it will “burn out” there. Calculations show that the mass and radius of its central region, in which nuclear reactions take place, gradually decrease, while the star slowly moves to the right on the spectrum-luminosity diagram. This process occurs much faster in relatively massive stars.
What will happen to a star when all (or almost all) of the hydrogen in its core “burns out”? Since the release of energy in the central regions of the star ceases, the temperature and pressure there cannot be maintained at the level necessary to counteract the gravitational force compressing the star. The star's core will begin to contract, and its temperature will increase. A very dense hot region is formed, consisting of helium (which hydrogen has turned into) with a small admixture of heavier elements. A gas in this state is called “degenerate”. It has a number of interesting properties. In this dense hot region, nuclear reactions will not occur, but they will proceed quite intensely at the periphery of the nucleus, in a relatively thin layer. The star, as it were, “swells” and begins to “descend” from the main sequence, moving into the region of red giants. Further, it turns out that giant stars with a lower content of heavy elements will have a higher luminosity for the same size.

Evolution of a class G star using the example of the Sun:

1.4 END OF A STAR
What will happen to stars when the helium-carbon reaction in the central regions exhausts itself, as well as the hydrogen reaction in the thin layer surrounding the hot dense core? What stage of evolution will come after the red giant stage?

White dwarfs

The totality of observational data, as well as a number of theoretical considerations, indicate that at this stage of evolution, stars whose mass is less than 1.2 solar masses “shed” a significant part of their mass, forming their outer shell. We observe such a process, apparently, as the formation of so-called “planetary nebulae.” After the outer shell separates from the star at a relatively low speed, its inner, very hot layers will be “exposed.” In this case, the separated shell will expand, moving further and further from the star.
Powerful ultraviolet radiation from the star - the core of the planetary nebula - will ionize the atoms in the shell, exciting them to glow. After a few tens of thousands of years, the shell will dissipate and only a small, very hot, dense star will remain. Gradually, rather slowly cooling, it will turn into a white dwarf.
Thus, white dwarfs seem to “ripen” inside stars - red giants - and “come into being” after the outer layers of giant stars separate. In other cases, the shedding of the outer layers may occur not through the formation of planetary nebulae, but through the gradual outflow of atoms. One way or another, white dwarfs, in which all the hydrogen has “burned out” and nuclear reactions have stopped, apparently represent the final stage in the evolution of most stars. The logical conclusion from this is the recognition of a genetic connection between the most recent stages of the evolution of stars and white dwarfs.

White dwarfs with carbon atmospheres

At a distance of 500 light years from Earth in the constellation Aquarius there is a dying star like the Sun. Over the past few thousand years, this star has given birth to the Helix Nebula, a well-studied nearby planetary nebula. A planetary nebula is the usual final stage of evolution for stars of this type. This image of the Helix Nebula from the Infrared Space Observatory shows radiation coming primarily from expanding shells of molecular hydrogen. The dust that is usually present in such nebulae should also emit intense radiation in the infrared. However, it seems to be missing from this nebula. The reason may lie in the central star itself - a white dwarf. This small but very hot star emits energy in the short-wave ultraviolet and is therefore not visible in the infrared image. Astronomers believe that over time, this intense ultraviolet radiation may have broken down the dust. The Sun is also expected to go through a planetary nebula stage within 5 billion years.

At first glance, the Helix Nebula (or NGC 7293) has a simple round shape. However, it is now clear that this well-studied planetary nebula, created by a Sun-like star approaching the end of its life, has a surprisingly complex structure. Its extensive loops and comet-like clumps of gas and dust were examined in images taken by the Hubble Space Telescope. However, this clear image of the Helix Nebula was taken with a telescope with a lens diameter of only 16 inches (40.6 cm), equipped with a camera and an array of broadband and narrowband filters. The color composite image reveals intriguing structural details, including blue-green radial stripes, or spokes, ~1 light-year long that make the nebula resemble a cosmic bicycle wheel. The presence of spokes appears to indicate that the Helix Nebula itself is an old, evolved planetary nebula. The nebula is located just 700 light years from Earth in the constellation Aquarius.

Black dwarfs

Gradually cooling, they emit less and less, turning into invisible “black” dwarfs. These are dead, cold stars of very high density, millions of times denser than water. Their sizes are smaller globe, although the masses are comparable to the solar one. The cooling process of white dwarfs lasts many hundreds of millions of years. This is how most stars end their existence. However, the final life of relatively massive stars can be much more dramatic.

Neutron stars

If the mass of a collapsing star exceeds the mass of the Sun by more than 1.4 times, then such a star, having reached the white dwarf stage, will not stop there. The gravitational forces in this case are very strong, so that electrons are pressed inside the atomic nuclei. As a result, isotopes turn into neutrons capable of flying to each other without any gaps. The density of neutron stars exceeds even that of white dwarfs; but if the mass of the material does not exceed 3 solar masses, neutrons, like electrons, can themselves prevent further compression. A typical neutron star is only 10 to 15 km across, and one cubic centimeter of its material weighs about a billion tons. In addition to their incredibly high density, neutron stars have two other special properties that make them detectable despite their small size: rapid rotation and a strong magnetic field. In general, all stars rotate, but when a star contracts, its rotation speed increases - just as a figure skater on ice rotates much faster when he presses his hands towards himself. A neutron star rotates several times per second. Along with this exceptionally fast rotation, neutron stars have a magnetic field millions of times stronger than Earth's.

Hubble saw a single neutron star in space.

Pulsars

The first pulsars were discovered in 1968, when radio astronomers discovered regular signals coming to us from four points in the Galaxy. Scientists were amazed by the fact that some natural objects could emit radio pulses in such a regular and fast rhythm. At first, however, for a short time, astronomers suspected the participation of some thinking creatures living in the depths of the Galaxy. But a natural explanation was soon found. In the powerful magnetic field of a neutron star, spiraling electrons generate radio waves that are emitted in a narrow beam, like a spotlight. The star rotates rapidly, and the radio beam crosses our line of observation like a beacon. Some pulsars emit not only radio waves, but also light, x-rays and gamma rays. The period of the slowest pulsars is about four seconds, and the fastest - thousandths of a second. The rotation of these neutron stars was for some reason even more accelerated; perhaps they are part of binary systems.
Thanks to the distributed computing project Einstein@Home, 63 pulsars were found in 2012.

Dark Pulsar

Supernovae

Stars whose masses do not reach 1.4 solar die quietly and serenely. What happens to more massive stars? How do neutron stars and black holes arise? A catastrophic explosion that ends the life of a massive star is a truly spectacular event. This is the most powerful of the natural phenomena occurring in the stars. In an instant, more energy is released than our Sun emits in 10 billion years. The luminous flux emitted by one dying star is equivalent to an entire galaxy, and visible light makes up only a small fraction of the total energy. The remains of the exploding star fly away at speeds of up to 20,000 km per second.
Such enormous stellar explosions are called supernovae. Supernovae are a fairly rare phenomenon. Every year, 20 to 30 supernovae are discovered in other galaxies, mainly as a result of systematic searches. Over a century, each galaxy can have from one to four. However, supernovae have not been observed in our own Galaxy since 1604. They may have existed, but remained invisible due to the large amount of dust in the Milky Way.

Supernova explosion.

Black holes

FROM a star with a mass greater than three solar masses and a radius greater than 8.85 kilometers, light will no longer be able to escape from it into space. The beam leaving the surface is bent in the gravity field so strongly that it returns back to the surface. Quanta of light
etc.................

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Test

on the topic: “The nature of stars”

group student

Mataev Boris Nikolaevich

Tyumen 2010

Nature of stars

“There is nothing simpler than a star” (A. Eddington, 1926)

The basis of this topic is information on astrophysics (solar physics, heliobiology, stellar physics, theoretical astrophysics), celestial mechanics, cosmogony and cosmology.

Introduction

Chapter 1. Stars. Types of stars.

1.1 Normal stars

1.2 Giants and dwarfs

1.3 Life cycle stars

1.4 Pulsating variable stars

1.5 Irregular variable stars

1.6 Flare stars

1.7 Double stars

1.8 Discovery of double stars

1.9 Close binary stars

1.10 The star overflows

1.11 Neutron stars

1.12 Crab Nebula

1.13 Name of Supernovae

Chapter 2. Physical nature of stars.

2.1 Color and temperature of stars

2.2 Spectra and chemical composition of stars

2.3 Star luminosities

2.4 Radii of stars

2.5 Stellar masses

2.6 Average densities of stars

Conclusion

List of sources used

Glossary

Introduction

From the point of view of modern astronomy, stars are celestial bodies similar to the Sun. They are vast distances away from us and therefore are perceived by us as tiny dots visible in the night sky. Stars vary in their brightness and size. Some of them have the same size and brightness as our Sun, others are very different from them in these parameters. There is a complex theory of internal processes in stellar matter, and astronomers claim that they can use it to explain in detail the origin, history and death of stars.

Chapter 1. Stars. Types of stars

3 stars are newborn, young, middle-aged and old. New stars are constantly being formed, and old ones are constantly dying.

The youngest, called T Tauri stars (after one of the stars in the constellation Taurus), are similar to the Sun, but much younger than it. In fact, they are still in the process of formation and are examples of protostars (primary stars).

These are variable stars, their luminosity changes because they have not yet reached a stationary mode of existence. Many T Tauri stars have rotating disks of material around them; Powerful winds emanate from such stars. The energy of the matter that falls on the protostar under the influence of gravity is converted into heat. As a result, the temperature inside the protostar is increasing all the time. When its central part becomes so hot that nuclear fusion begins, the protostar turns into a normal star. Once nuclear reactions begin, the star has a source of energy that can support its existence for a very long time. How long depends on the size of the star at the start of the process, but a star the size of our Sun would have enough fuel to sustain itself for about 10 billion years.

However, it happens that stars much more massive than the Sun last only a few million years; the reason is that they compress their nuclear fuel at a much faster rate.

1.1 Normal stars

All stars are fundamentally similar to our Sun: they are huge balls of very hot glowing gas, in the very depths of which nuclear energy is generated. But not all stars are exactly like the Sun. The most obvious difference is the color. There are stars that are reddish or bluish, not yellow.

In addition, stars differ in both brightness and brilliance. How bright a star appears in the sky depends not only on its true luminosity, but also on the distance separating it from us. Taking into account distances, the brightness of stars varies over a wide range: from one ten thousandth the brightness of the Sun to the brightness of more than E million Suns. The vast majority of stars appear to be located closer to the dim end of this scale. The Sun, which is in many ways a typical star, is much more luminous than most other stars. A very small number of inherently faint stars can be seen with the naked eye. In the constellations of our sky, the main attention is drawn to the “signal lights” of unusual stars, those that have a very high luminosity. universe star evolution

Why do stars vary so much in their brightness? It turns out that this does not depend on the mass of the star.

The amount of matter contained in a particular star determines its color and brightness, as well as how the brightness changes over time. The minimum amount of mass required for a star to be a star is about one-twelfth the mass of the Sun.

1.2 Giants and dwarfs

The most massive stars are also the hottest and the brightest. They appear white or bluish. Despite their enormous size, these stars produce such colossal amounts of energy that all their nuclear fuel reserves burn out in just a few million years.

In contrast, stars with low mass are always dim and their color is reddish. They can exist for many billions of years.

However, among the very bright stars in our sky there are red and orange ones. These include Aldebaran - the eye of the bull in the constellation Taurus, and Antares in Scorpio. How can these cool stars with faintly luminous surfaces compete with white-hot stars like Sirius and Vega? The answer is that these stars have expanded enormously and are now much larger in size than normal red stars. For this reason they are called giants, or even supergiants.

Due to their enormous surface area, giants emit immeasurably more energy than normal stars like the Sun, despite the fact that their surface temperatures are much lower. The diameter of a red supergiant - for example, Betelgeuse in Orion - is several hundred times greater than the diameter of the Sun. In contrast, the size of a normal red star is typically no more than one-tenth the size of the Sun. In contrast to the giants, they are called “dwarfs”.

Stars become giants and dwarfs at different stages of their lives, and a giant may eventually become a dwarf when it reaches “old age.”

1.3 Life cycle of a star

An ordinary star, such as the Sun, releases energy by converting hydrogen into helium in a nuclear furnace located at its very core. The sun and stars change in a regular (correct) way - a section of their graph over a period of time of a certain length (period) is repeated again and again. Other stars change completely unpredictably.

Regular variable stars include pulsating stars and double stars. The amount of light changes because stars pulsate or emit clouds of material. But there is another group of variable stars that are double (binary).

When we see a change in the brightness of binary stars, it means that one of several possible phenomena has occurred. Both stars can be in our line of sight, since, moving along their orbits, they can pass directly in front of one another. Such systems are called eclipsing binary stars. The most famous example of this kind is the star Algol in the constellation Perseus. In a closely spaced pair, material can rush from one star to the other, often with dramatic consequences.

1.4 Pulsating variable stars

Some of the most regular variable stars pulsate, contracting and expanding again - as if vibrating at a certain frequency, much like a string does. musical instrument. The most famous type of such stars are Cepheids, named after the star Delta Cephei, which is typical example. These are supergiant stars, their mass exceeds the mass of the Sun by 3 - 10 times, and their luminosity is hundreds and even thousands of times higher than that of the Sun. The Cepheid pulsation period is measured in days. As a Cepheid pulsates, both the area and temperature of its surface change, causing an overall change in its brightness.

Mira, the first variable star described, and other stars like it owe their variability to pulsations. These are cool red giants last stage of their existence, they are about to completely shed their outer layers, like a shell, and create a planetary nebula. Most red supergiants, like Betelgeuse in Orion, vary only within certain limits.

Using special observation equipment, astronomers discovered large dark spots on the surface of Betelgeuse.

RR Lyrae stars represent another important group of pulsating stars. These are old stars with about the same mass as the Sun. Many of them are found in globular star clusters. As a rule, they change their brightness by one magnitude in about a day. Their properties, like those of Cepheids, are used to calculate astronomical distances.

1.5 Irregular variable stars

R Corona Nord and stars like it behave in completely unpredictable ways. This star can usually be seen with the naked eye. Every few years, its brightness drops to about eighth magnitude, and then gradually increases, returning to its previous level. Apparently, the reason for this is that this supergiant star throws off clouds of carbon, which condenses into grains, forming something like soot. If one of these thick black clouds passes between us and a star, it blocks the star's light until the cloud dissipates into space.

Stars of this type produce thick dust, which is important in regions where stars form.

1.6 Flare stars

Magnetic phenomena on the Sun cause sunspots and solar flares, but they cannot significantly affect the brightness of the Sun. For some stars - red dwarfs - this is not the case: on them such flares reach enormous proportions, and as a result, light radiation can increase by a whole stellar magnitude, or even more. The closest star to the Sun, Proxima Centauri, is one such flare star. These bursts of light cannot be predicted in advance and last only a few minutes.

1.7 Double stars

About half of all the stars in our Galaxy belong to binary systems, so binary stars orbiting one another are a very common phenomenon.

Belonging to a binary system greatly influences the entire life of a star, especially when partners are close to each other. Streams of material rushing from one star to another lead to dramatic explosions such as novae and supernovae.

Binary stars are held together by mutual gravity. Both stars of the binary system rotate in elliptical orbits around a certain point lying between them and called the center of gravity of these stars. This can be imagined as a fulcrum if you imagine the stars sitting on a children's swing: each at its own end of a board placed on a log. The farther the stars are from each other, the longer their orbital paths last. Most double stars (or simply double stars) are too close to each other to be distinguished individually even with the most powerful telescopes. If the distance between the partners is large enough, the orbital period can be measured in years, and sometimes as much as a century or even longer.

Double stars that you can see separately are called visible binaries.

1.8 Discovery of double stars

Most often, double stars are identified either by the unusual motion of the brighter of the two, or by their combined spectrum. If any star makes regular fluctuations in the sky, this means that it has an invisible partner. It is then said to be an astrometric double star, discovered through measurements of its position.

Spectroscopic double stars are detected by changes and special characteristics in their spectra. The spectrum of an ordinary star like the Sun is like a continuous rainbow, intersected by numerous narrow Neles - the so-called absorption lines. The exact colors these lines are on change as the star moves towards or away from us. This phenomenon is called the Doppler effect. When the stars of a binary system move in their orbits, they alternately approach us and then move away. As a result, the lines of their spectra move in some part of the rainbow. Such moving lines in the spectrum indicate that the star is double.

If both members of a binary system have approximately the same brightness, two sets of lines can be seen in the spectrum. If one star is much brighter than the other, its light will dominate, but regular shifts in spectral lines will still reveal its true binary nature.

Measuring the velocities of stars in a binary system and applying legal gravity are an important method for determining stellar masses. Studying binary stars is the only direct way to calculate stellar masses. However, it is not so easy to get an exact answer in each specific case.

1.9 Close binary stars

In a system of closely spaced double stars, mutual gravitational forces tend to stretch each of them, giving it the shape of a pear. If gravity is strong enough, a critical moment comes when matter begins to flow away from one star and fall onto another. Around these two stars there is a certain region in the shape of a three-dimensional figure eight, the surface of which represents the critical boundary.

These two pear-shaped figures, each around a different star, are called Roche lobes. If one of the stars grows so large that it fills its Roche lobe, then matter from it rushes to the other star at the point where the cavities touch. Often, stellar material does not fall directly onto the star, but is first swirled into a vortex, forming what is called an accretion disk. If both stars have expanded so much that they have filled their Roche lobes, then a contact binary star appears. The material from both stars mixes and merges into a ball around the two stellar cores. Since all stars will eventually swell into giants, and many stars are binaries, interacting binary systems are not uncommon.

1.10 The star overflows

One of the striking results of mass transfer in binary stars is the so-called nova burst.

One star expands so much that it fills its Roche lobe; this means inflating the outer layers of a star to the point where its material begins to be captured by another star, subject to its gravity. This second star is a white dwarf. Suddenly the brightness increases by about ten magnitudes - a nova flares up. What happens is nothing more than a gigantic release of energy in a very short time, a powerful nuclear explosion on the surface of the white dwarf. As material from the bloated star rushes towards the dwarf, the pressure in the downward flow of matter increases sharply, and the temperature under the new layer increases to a million degrees. There have been cases where, after tens or hundreds of years, outbreaks of new ones were repeated. Other explosions have only been observed once, but they could happen again thousands of years from now. Another type of star produces less dramatic outbursts—dwarf novae—that repeat after days and months.

When a star's nuclear fuel is used up and energy production in its depths ceases, the star begins to shrink toward the center. The inward gravitational force is no longer balanced by the buoyant force of the hot gas.

The further development of events depends on the mass of the compressed material. If this mass does not exceed the solar mass by more than 1.4 times, the star stabilizes, becoming a white dwarf. Catastrophic compression does not occur due to the basic property of electrons. There is a degree of compression at which they begin to repel, although there is no longer any source of thermal energy. True, this only happens when electrons and atomic nuclei are compressed incredibly tightly, forming extremely dense matter.

A white dwarf with the mass of the Sun is approximately equal in volume to Earth.

Just a cup of white dwarf material would weigh a hundred tons on Earth. Interestingly, the more massive white dwarfs are, the smaller their volume. It is very difficult to imagine what the interior of a white dwarf looks like. Most likely, it is something like a single giant crystal that gradually cools, becoming increasingly dull and red. In fact, although astronomers call a whole group of stars white dwarfs, only the hottest of them, with a surface temperature of about 10,000 C, are actually white. Ultimately, each white dwarf will turn into a dark ball of radioactive ash, the completely dead remains of a star. White dwarfs are so small that even the hottest ones emit very little light and can be difficult to detect. However, the number of known white dwarfs now numbers in the hundreds; According to astronomers, at least a tenth of all the stars in the Galaxy are white dwarfs. Sirius, the brightest star in our sky, is a member of a binary system, and its companion is a white dwarf called Sirius B.

1.11 Neutron stars

If the mass of a collapsing star exceeds the mass of the Sun by more than 1.4 times, then such a star, having reached the white dwarf stage, will not stop at an atom. In this case, the gravitational forces are so strong that the electrons are pressed into the atomic nuclei. As a result, isotopes turn into neutrons that can adhere to each other without any gaps. The density of neutron stars exceeds even that of white dwarfs; but if the mass of the material does not exceed 3 solar masses, neutrons, like electrons, can themselves prevent further compression. A typical neutron star is only 10 to 15 km across, and one cubic centimeter of its material weighs about a billion tons. In addition to their incredible density, neutron stars have two other special properties that make them detectable despite their small size: fast rotation and a strong magnetic field. In general, all stars rotate, but when a star contracts, its rotation speed increases - just as a figure skater on ice rotates much faster when he presses his hands towards himself.

1.12 Crab Nebula

One of the most famous supernova remnants, the Crab Nebula owes its name to William Parsons, third Earl of Ross, who first observed it in 1844. Its impressive name doesn't quite do justice to this strange object. We now know that the nebula is the remnant of a supernova, which was observed and described in 1054 by Chinese astronomers. Its age was established in 1928 by Edwin Hubble, who measured the rate of its expansion and drew attention to the coincidence of its position in the sky with ancient Chinese records. It has the shape of an oval with uneven edges; reddish and greenish filaments of luminous gas are visible against the background of a dull white spot. THREADS OF GLOWING gas resemble a net thrown over a hole. White light comes from electrons racing in spirals in a strong magnetic field. The nebula is also an intense source of radio waves and X-rays. When astronomers realized that pulsars are the neutrons of supernovae, it became clear to them that they needed to look for pulsars in remnants like the Crab Nebula. In 1969, it was discovered that one of the stars near the center of the nebula periodically emits radio pulses, as well as X-ray signals every 33 thousandths of a second. This is a very high frequency even for a pulsar, but it gradually decreases. Those pulsars that rotate much more slowly are much older than the Crab Nebula pulsar.

1.13 Name of Supernovae

Although modern astronomers have not witnessed a supernova in our Galaxy, they have observed at least the second most interesting event - a supernova in 1987 in the Large Magellanic Cloud, a nearby galaxy visible in the southern hemisphere. The supernova was named YAH 1987A. Supernovae are named by the year of discovery, followed by a capital letter in alphabetical order according to the sequence of discoveries, BH is an abbreviation for ~supernova~. (If more than 26 of them are open for a td, the designations AA, BB, etc. follow.)

Chapter 2. Physical nature of stars

We already know that stars are distant suns, so when studying the nature of stars, we will compare their physical characteristics with the physical characteristics of the Sun.

Stars are spatially isolated, gravitationally bound, radiation-opaque masses of matter in the range from 10 29 to 10 32 kg (0.005-100 M¤), in the depths of which thermonuclear reactions of converting hydrogen into helium have occurred, are occurring, or will occur on a significant scale .

The classification of stars depending on their main physical characteristics is shown in Table 1.

Table 1

Star classes		Dimensions R¤	Density g/cm 3	Luminosity L¤	Life time, years	% of total stars	Peculiarities
Brightest supergiants							Gravity is described by Newton's laws of classical mechanics; gas pressure is described by the basic equations of molecular kinetic theory; energy release depends on the temperature in the zone of thermonuclear reactions of the proton-proton and nitrogen-carbon cycles
Supergiants
Bright giants
Normal giants
Subgiants
Normal stars
Reds
White dwarfs							The final stages of the evolution of normal stars. Pressure is determined by the density of the electron gas; energy release does not depend on temperature
Neutron stars		8-15 km (up to 50 km)					The final stages of the evolution of giant and subgiant stars. Gravity is described by the laws of general relativity, pressure is non-classical

The sizes of stars vary within a very wide range from 10 4 m to 10 12 m. The garnet star m Cephei has a diameter of 1.6 billion km; the red supergiant E Aurigae A has dimensions of 2700 R¤ - 5.7 billion km! The Leuthen and Wolf-475 stars are smaller than the Earth, and neutron stars have sizes of 10 - 15 km (Fig. 1).

Rice. 1. Relative sizes of some stars, the Earth and the Sun

Rapid rotation around its axis and the attraction of nearby massive cosmic bodies disrupts the spherical shape of stars, “flattening” them: the star R Cassiopeia has the shape of an ellipse, its polar diameter is 0.75 equatorial; in the close binary system W of Ursa Major, the components acquired an ovoid shape.

2.1 Color and temperature of stars

While observing the starry sky, you may have noticed that the colors of the stars are different. Just as one can judge its temperature by the color of a hot metal, so the color of a star indicates the temperature of its photosphere. Do you know what's in between maximum length radiation waves and temperature there is a certain dependence; for different stars, the maximum radiation occurs at different wavelengths. For example, our Sun is a yellow star. The same color is Capella, whose temperature is about 6000 o K. Stars with a temperature of 3500-4000 o K are reddish in color (Aldebaran). The temperature of red stars (Betelgeuse) is approximately 3000 o K. The coldest currently known stars have a temperature of less than 2000 o K. Such stars can be observed in the infrared part of the spectrum.

There are many known stars hotter than the Sun. These include, for example, white stars (Spica, Sirius, Vega). Their temperature is about 10 4 - 2x10 4 K. Less common are bluish-white ones, the temperature of the photosphere of which is 3x10 4 -5x10 4 K. In the depths of stars, the temperature is at least 10 7 K.

The visible surface temperature of stars ranges from 3000 K to 100,000 K. The recently discovered star HD 93129A in the constellation Puppis has a surface temperature of 220,000 K! The coldest ones - Garnet star (m Cephei) and Mira (o Ceti) have a temperature of 2300K, e Aurigae A - 1600 K.

2.2 Spectra and chemical composition of stars

Astronomers obtain the most important information about the nature of stars by deciphering their spectra. The spectra of most stars, like the spectrum of the Sun, are absorption spectra: dark lines are visible against the background of a continuous spectrum.

The spectra of stars that are similar to each other are grouped into seven main spectral classes. They are designated by capital letters of the Latin alphabet:

O-B-A-F-G-K-M

and are located in such a sequence that when moving from left to right, the color of the star changes from close to blue (class O), white (class A), yellow (class O), red (class M). Consequently, the temperature of stars decreases in the same direction from class to class.

Thus, the sequence of spectral classes reflects the difference in the color and temperature of stars. Within each class there is a division into ten more subclasses. For example, spectral class F has the following subclasses:

F0-F1-F2-F3-F4-F5-Fb-F7-F8-F9

The Sun belongs to the spectral class G2.

Basically, the atmospheres of stars have a similar chemical composition: the most common elements in them, as in the Sun, are hydrogen and helium. The diversity of stellar spectra is explained primarily by the fact that stars have different temperatures. The physical state in which the atoms of matter are located in stellar atmospheres depends on the type of spectrum; at low temperatures (red stars), neutral atoms and even the simplest molecular compounds (C 2, CN, TiO, ZrO, etc.) can exist in the atmospheres of stars. . The atmospheres of very hot stars are dominated by ionized atoms.

In addition to temperature, the type of spectrum of a star is determined by the pressure and density of the gas of its photosphere, the presence magnetic field, features of the chemical composition.

Rice. 35. Main spectral types of stars

Spectral analysis of stellar radiation indicates the similarity of their composition with the chemical composition of the Sun and the absence of chemical elements unknown on Earth. Differences in appearance The spectra of different classes of stars indicate differences in their physical characteristics. The temperature, presence and speed of rotation, magnetic field strength and chemical composition of stars are determined based on direct spectral observations. The laws of physics allow us to draw conclusions about the mass of stars, their age, internal structure and energy, consider in detail all stages of the evolution of stars.

Almost all stellar spectra are absorption spectra. The relative abundance of chemical elements is a function of temperature.

Currently accepted in astrophysics unified classification stellar spectra (Table 2). Based on the characteristics of the spectra: the presence and intensity of atomic spectral lines and molecular bands, the color of the star and the temperature of its emitting surface, stars are divided into classes, designated by letters of the Latin alphabet:

W - O - B - F - G - K - M

Each class of stars is divided into ten subclasses (A0...A9).

Spectral classes from O0 to F0 are called "early"; from F to M9 - “late”. Some scientists classify stars of classes R and N as class G. A number of stellar characteristics are indicated by additional small letters: for giant stars the letter “g” is placed before the class indication, for dwarf stars - the letter “d”, for supergiants - “c”, stars with emission lines in their spectrum have the letter “e”, stars with unusual spectra have the letter “p”, etc. Modern star catalogs contain the spectral characteristics of hundreds of thousands of stars and their systems.

W * O * B * A * F * G * K * M ......... R ... N .... S

Table 2. Spectral classification of stars

	Temperature, K		Characteristic spectral lines	Typical stars
			Wolf-Rayet stars with emission lines in their spectrum	S Golden Fish
		bluish-white	Absorption lines He +, N +, He, Mg +, Si ++, Si +++ (the + sign means the degree of ionization of the atoms of a given chemical element)	z Poop, l Orion, l Perseus
		white and blue	The absorption lines of He +, He, H, O +, Si ++ are enhanced towards class A; weak lines H, Ca + are noticeable	e Orion, a Virgo, g Orion
			The absorption lines of H, Ca + are intense and intensify towards class F, weak lines of metals appear	a Canis Major, a Lyra, g Gemini
		yellowish	The absorption lines of Ca + , H, Fe + of calcium and metals intensify towards class G. The calcium line 4226A and the hydrocarbon band appear and intensify	d Gemini, a Canis Minor, a Perseus
			The absorption lines of calcium H and Ca + are intense; the 4226A line and the iron line are quite intense; numerous lines of metals; the hydrogen lines weaken; intense G band	Sun, a Auriga
		orange	The absorption lines of metals, Ca +, 4226A are intense; hydrogen lines are barely noticeable. From subclass K5 absorption bands of titanium oxide TiO are observed	a Bootes, b Gemini, a Taurus
		Absorption lines of Ca +, many metals and absorption bands of carbon molecules	R Northern Crown
		Powerful absorption bands of zirconium oxide (ZrO) molecules
		Absorption bands of carbon molecules C 2 and cyanide CN
			Powerful absorption bands of titanium oxide molecules TiO, VO and other molecular compounds. The absorption lines of the metals Ca +, 4226A are noticeable; G band weakens	a Orion, a Scorpio, o Ceti, Proxima Centauri
		Planetary nebulae
		New stars

Table 3. Average characteristics of stars of the main spectral classes located on the main sequence (Arabic numerals - decimal divisions within the class): S p - spectral class, M b - absolute bolometric magnitude, T eff - effective temperature, M, L, R - respectively, the mass, luminosity, radius of stars in solar units, t m ​​- the lifetime of stars on the main sequence:

2.3 Star luminosities

The luminosity of stars - the amount of energy emitted by their surface per unit time - depends on the rate of energy release and is determined by the laws of thermal conductivity, the size and temperature of the star's surface. The difference in luminosity can reach 250000000000 times! Stars of high luminosity are called giant stars, stars of low luminosity are called dwarf stars. The blue supergiant star Pistol in the constellation Sagittarius has the greatest luminosity - 10,000,000 L¤! The luminosity of the red dwarf Proxima Centauri is about 0.000055 L¤.

Stars, like the Sun, emit energy in the range of all wavelengths of electromagnetic oscillations. You know that luminosity (L) characterizes the total radiation power of a star and represents one of its most important characteristics. Luminosity is proportional to the surface area (photosphere) of the star (or the square of the radius R) and the fourth power of the effective temperature of the photosphere (T), i.e.

L = 4ПR 2 оT 4. (45)

The formula connecting the absolute magnitudes and luminosities of stars is similar to the relationship you know between the brightness of a star and its apparent magnitude, i.e.

L 1 / L 2 = 2.512 (M 2 - M 1),

where L 1 and L 2 are the luminosities of two stars, and M 1 and M 2 are their absolute magnitudes.

If we choose the Sun as one of the stars, then

L/L o = 2.512 (Mo - M),

where letters without indices refer to any star, and with an o sign to the Sun.

Taking the luminosity of the Sun as unity (Lo = 1), we obtain:

L = 2.512 (Mo - M)

log L = 0.4 (Mo - M). (47)

Using formula (47), one can calculate the luminosity of any star whose absolute magnitude is known.

Stars have different luminosities. There are known stars whose luminosities are hundreds and thousands of times greater than the luminosities of the Sun. For example, the luminosity of a Taurus (Aldebaran) is almost 160 times greater than the luminosity of the Sun (L = 160Lo); luminosity of Rigel (in Orion) L = 80000Lo

The vast majority of stars have luminosities comparable to or less than the luminosity of the Sun, for example, the luminosity of the star known as Kruger 60A, L = 0.006 Lo.

2.4 Star radii

Using the most modern technology astronomical observations, it has now been possible to directly measure the angular diameters (and from them, knowing the distance, and linear dimensions) of only a few stars. Basically, astronomers determine the radii of stars by other methods. One of them is given by formula (45). If the luminosity L and effective temperature T of the star are known, then using formula (45), we can calculate the radius of the star R, its volume and the area of ​​the photosphere.

Having determined the radii of many stars, astronomers became convinced that there are stars whose sizes differ sharply from the size of the Sun. Supergiants have the largest sizes. Their radii are hundreds of times greater than the radius of the Sun. For example, the radius of the star a Scorpii (Antares) is no less than 750 times greater than the solar one. Stars whose radii are tens of times greater than the radius of the Sun are called giants. Stars that are close in size to the Sun or smaller than the Sun are classified as dwarfs. Among dwarfs there are stars that are smaller than the Earth or even the Moon. Even smaller stars have been discovered.

2.5 Masses of stars

The mass of a star is one of its most important characteristics. The masses of stars are different. However, in contrast to luminosity and size, the masses of stars lie within relatively narrow limits: the most massive stars are usually only tens of times larger than the Sun, and the smallest stellar masses are on the order of 0.06 Mo. The main method for determining stellar masses comes from the study of double stars; a relationship between luminosity and star mass was discovered.

2.6 Average stellar densities

The average densities of stars vary in the range from 10 -6 g/cm 3 to 10 14 g/cm 3 - 10 20 times! Since the sizes of stars vary much more than their masses, the average densities of stars differ greatly from each other. Giants and supergiants have very low densities. For example, the density of Betelgeuse is about 10 -3 kg/m 3. At the same time, there are extremely dense stars. These include small white dwarfs (their color is due to high temperature). For example, the density of the white dwarf Sirius B is more than 4x10 7 kg/m 3. Currently, much denser white dwarfs are known (10 10 - 10 11 kg/m 3). The enormous densities of white dwarfs are explained by the special properties of the matter of these stars, which consists of atomic nuclei and electrons torn from them. The distances between atomic nuclei in the matter of white dwarfs should be tens and even hundreds of times smaller than in ordinary solid and liquid bodies that we encounter under terrestrial conditions. State of aggregation, in which this substance is located, can not be called either liquid or solid, since the atoms of white dwarfs are destroyed. This substance bears little resemblance to gas or plasma. And yet it is generally considered to be a “gas,” given that the distance between particles even in dense white dwarfs is many times greater than the nuclei of atoms or electrons themselves.

Conclusion

1. Stars are a separate independent type of cosmic bodies, qualitatively different from other cosmic objects.

2. Stars are one of the most common (perhaps the most common) type of cosmic bodies.

3. Stars concentrate up to 90% of the visible matter in the part of the Universe in which we live and which is accessible to our research.

4. All the main characteristics of stars (size, luminosity, energy, “lifetime” and final stages of evolution) are interdependent and are determined by the value of the mass of stars.

5. Stars consist almost entirely of hydrogen (70-80%) and helium (20-30%); the share of all other chemical elements ranges from 0.1% to 4%.

6. Thermonuclear reactions occur in the depths of stars.

7. The existence of stars is due to the balance of gravitational forces and radiation (gas) pressure.

8. The laws of physics make it possible to calculate all the basic physical characteristics of stars based on the results of astronomical observations.

9. The main, most productive method for studying stars is spectral analysis of their radiation.

Bibliography

1. E. P. Levitan. Textbook of Astronomy for 11th grade, 1998

2. Materials from the site http://goldref.ru/

Glossary

Telescopes designed for photographic observations are called astrographs. Advantages of astrophotography over visual observations: integrity - the ability of a photographic emulsion to gradually accumulate light energy; immediacy; panoramic views; objectivity - it is not influenced personal characteristics observer. Conventional photographic emulsion is more sensitive to blue-violet radiation, but at present, when photographing space objects, astronomers use photographic materials that are sensitive to various parts of the spectrum of electromagnetic waves, not only visible, but also infrared and ultraviolet rays. The sensitivity of modern photographic emulsions is tens of thousands of ISO units. Wide Application received filming, video recording, and the use of television.

Astrophotometry is one of the main methods of astrophysical research that determines the energy characteristics of objects by measuring the energy of their electromagnetic radiation. The main concepts of astrophotometry are:

The brilliance of a celestial body is the illumination created by it at the observation point:

where L is the total radiation power (luminosity) of the star; r is the distance from the star to the Earth.

To measure brightness in astronomy, a special unit of measurement is used - stellar magnitude. Formula for the transition from stellar magnitudes to illumination units accepted in physics:

where m is the apparent magnitude of the star.

Magnitude (m) is the conventional (dimensionless) magnitude of the emitted luminous flux, characterizing the brightness of a celestial body, chosen in such a way that an interval of 5 magnitudes corresponds to a change in brightness by a factor of 100. One magnitude differs by 2.512 times. Pogson's formula relates the brilliance of luminaries to their magnitudes:

The determined stellar magnitude depends on the spectral sensitivity of the radiation receiver: visual (m v) is determined by direct observations and corresponds to the spectral sensitivity of the human eye; photographic (m p) is determined by measuring the illumination of the luminary on a photographic plate sensitive to blue-violet and ultraviolet rays; bolometric (m in) corresponds to the total radiation power of the luminary, summed over the entire radiation spectrum. For extended objects with large angular dimensions, the integral (total) magnitude is determined, equal to the sum of the brightness of its parts.

To compare the energy characteristics of space objects located at different distances from the Earth, the concept of absolute magnitude was introduced.

Absolute magnitude (M) is the magnitude that a star would have at a distance of 10 parsecs from the Earth: , where p is the parallax of the star, r is the distance from the star. 10 pc = 3.086H 10 17 m.

The absolute magnitude of the brightest supergiant stars is about -10 m.

The absolute magnitude of the Sun is + 4.96 m.

Luminosity (L) is the amount of energy emitted by the surface of a star per unit time. The luminosity of stars is expressed in absolute (energy) units or in comparison with the luminosity of the Sun (L¤ or LD). L ¤ = 3.86H 10 33 erg/s.

The luminosity of luminaries depends on their size and the temperature of the emitting surface. Depending on the radiation receivers, visual, photographic and bolometric luminosity of luminaries are distinguished. Luminosity is related to the apparent and absolute magnitude of the luminaries:

The coefficient A(r) takes into account the absorption of light in the interstellar medium.

The luminosity of cosmic bodies can be judged by the width of the spectral lines.

The luminosity of space objects is closely related to their temperature: , where R * is the radius of the star, s is the Stefan-Boltzmann constant, s = 5.67H 10 -8 W/m 2H K 4 .

Since the surface area of ​​the ball, and according to the Stefan-Boltzmann equation, .

Based on the luminosity of stars, their sizes can be determined:

Based on the luminosity of stars, the mass of stars can be determined:

A protostar is a star in the earliest stage of formation, when densification occurs in the interstellar cloud, but nuclear reactions within it have not yet begun.

Stellar magnitude is a characteristic of the visible brightness of stars. Apparent magnitude has nothing to do with the size of the star. This term has historical origins and characterizes only the brightness of a star. The brightest stars have zero or even negative magnitude. For example, stars such as Vega and Capella have approximately zero magnitude, and the brightest star in our sky, Sirius, has a magnitude of minus 1.5.

A galaxy is a huge rotating star system.

Periastron is the point of closest approach of both stars of the binary system.

A spectrogram is a permanent recording of a spectrum obtained photographically or digitally using an electronic detector.

Effective temperature is a measure of the energy release of an object (specifically a star), defined as the temperature of a black body that has the same total luminosity as the object being observed. Effective temperature is one of the physical characteristics of a star. Since the spectrum of a normal star is similar to that of a blackbody, the effective temperature is a good indicator of the temperature of its photosphere.

The Small Magellanic Cloud (SMC) is one of the satellites of our Galaxy.

Parsec is a unit of distance used in professional astronomy. It is defined as the distance at which an object would have a yearly parallax equal to one arcsecond. One parsec is equivalent to 3.0857 * 10 13 km, 3.2616 light years or 206265 AU.

Parallax is the change in the relative position of an object when viewed from different perspectives.

A globular star cluster is a dense collection of hundreds of thousands or even millions of stars, the shape of which is close to spherical.

The Michelson Stellar Interferometer is a series of interferometric instruments built by A.A. Michelson (1852-1931) to measure the diameters of stars that could not be measured directly using ground-based telescopes.

Right ascension (RA) is one of the coordinates used in the equatorial system to determine the position of objects on celestial sphere. It is the equivalent of longitude on Earth, but measured in hours, minutes and seconds of time eastward from the zero point, which is the intersection of the celestial equator and the ecliptic, known as the first point of Aries. One hour of right ascension is equivalent to 15 degrees of arc; This is the apparent angle that, due to the rotation of the Earth, the celestial sphere passes in one hour of sidereal time.

Pulsating (P) star-shaped (S) (source) of radio emission (R).

Declination (DEC) is one of the coordinates that determines the position on the celestial sphere in the equatorial coordinate system. Declination is the equivalent of latitude on Earth. This is the angular distance, measured in degrees, north or south of the celestial equator. The northern declination is positive, and the southern declination is negative.

A Roche lobe is a region of space in binary star systems bounded by an hourglass-shaped surface on which lie points where the gravitational forces of both components acting on small particles of matter are equal.

Lagrange points are points in the orbital plane of two massive objects rotating around a common center of gravity, where a particle with negligible mass can remain in an equilibrium position, i.e. motionless. For two bodies in circular orbits, there are five such points, but three of them are unstable to small disturbances. The remaining two, located in the orbit of a less massive body at an angular distance of 60° on either side of it, are stable.

Precession is a uniform periodic movement of the axis of rotation of a freely rotating body when it is acted upon by a torque arising due to external gravitational influences.

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Stars are celestial bodies that, like our Sun, glow from within. The structure of stars, its dependence on mass. The compression of a star, which leads to an increase in temperature in its core. The lifespan of a star, its evolution. Nuclear reactions of hydrogen combustion.

PHYSICAL NATURE OF STARS

• Color and temperature of stars.

• Spectra and chemical composition of stars

• Star luminosities

• Radii of stars.

• Masses of stars

• Average densities of stars.

• Spectrum-luminosity diagram

• General information about the SUN.

• SUN data

Spectra and chemical composition of stars

• Astronomers obtain the most important information about the nature of stars by deciphering their spectra. The spectra of most stars, like the spectrum of the SUN, are absorption spectra. The spectra of stars that are similar to each other are grouped into seven main spectral classes. They are designated by capital letters of the Latin alphabet:

O-B-A-F-G-K-M and are arranged in such a sequence that when moving from left to right, the color of the star changes from close to blue (class O), white (class A), yellow (class G), red (class M). Consequently, in the same direction, the temperature of stars decreases from class to class. Within each class there is a division into 10 subclasses. The SUN belongs to the spectral class G2.

• Basically, the atmospheres of stars have a similar chemical composition: the most common elements in them, as in the SUN, turned out to be hydrogen and helium.

Star luminosities

• Stars, like the SUN, emit energy in the range of all wavelengths of electromagnetic oscillations. Luminosity (L) characterizes the total radiation power of a star and represents one of its most important characteristics. Luminosity is proportional to the surface area of ​​the star (or the square of the radius) and the fourth power of the effective temperature of the photosphere.

• L=4πR^2T^4

RADIUS OF STARS.

The radii of stars can be determined from the formula for determining the luminosity of stars. Having determined the radii of many many stars, astronomers were convinced that there are stars whose dimensions differ sharply from the sizes of the SUN.. Supergiants have the largest sizes. Their radii are hundreds of times greater than the radius of the SUN. Stars whose radii are tens of times greater than the radius of the SUN are called giants. Stars that are close in size to the SUN or smaller than the SUN are classified as dwarfs. Among the dwarfs there are stars that less EARTH or even the MOON. Even smaller stars have been discovered.

Masses of stars.

• The mass of a star is one of its most important characteristics. The masses of stars are different. However, in contrast to luminosity and size, the masses of stars lie within relatively narrow limits: the most massive stars are usually only tens of times larger than the SUN, and the smallest stellar masses are on the order of 0.06 MΘ.

Average densities of stars.

Since the sizes of stars differ much more than their masses, the average densities of stars differ greatly from each other. Giants and supergiants have very low densities. At the same time, there are extremely dense stars. These include small white dwarfs. The enormous densities of white dwarfs are explained by the special properties of the matter of these stars, which consists of atomic nuclei and electrons torn from them. The distances between atomic nuclei in the matter of white dwarfs should be tens of times and even hundreds of times smaller than in ordinary solid and liquid bodies. The state of aggregation in which this substance is found cannot be called either liquid or solid, since the atoms of white dwarfs are destroyed. This substance bears little resemblance to gas or plasma. And yet it is generally considered to be “gas”.

Spectrum-luminosity diagram

At the beginning of this century, the Dutch astronomer E. Hertzsprung (1873-1967) and the American astronomer G. Russell (1877-1957) independently discovered that there is a connection between the spectra of stars and their luminosities. This dependence, obtained by comparing observational data, is presented in a diagram. Each star has a corresponding point on the diagram, called the spectrum-luminosity diagram or Hertzsprung-Russell diagram. The vast majority of stars belong to the main sequence, ranging from hot supergiants to cool red dwarfs. Looking at the main sequence, you can see that the hotter the stars belonging to it, the greater their luminosity. From the main sequence, giants, supergiants and white dwarfs are grouped in different parts of the diagram.

GENERAL INFORMATION ABOUT THE SUN

• THE SUN plays an exceptional role in the life of the Earth. The entire organic world of our planet owes its existence to the SUN.

• The SUN is the only star in the solar system, the source of energy on Earth. This is a fairly ordinary star in the Universe, which is not unique in its physical characteristics (mass, size, temperature, chemical composition).

• THE SUN - emits energy in various ranges of electromagnetic waves.

• The source of energy for the SUN and stars is thermonuclear reactions occurring in their depths.

SUN DATA

• Horizontal parallax – 8.794 sec

• Average distance from the EARTH 1,496*10^8 km

• Linear diameter 1.39*10^6 km

• Weight 2*10^30 kg

• Average density 1.4*10^3 kg/m^3

• Gravity acceleration 274 m/s

• Luminosity 3.8*10^26 W

• Apparent magnitude -26.8^m

• Absolute magnitude +4.8^m

• Spectral class G2

• Distance from the SUN to the center of the GALAXY 10^4 pc

LET'S REMEMBER V. KHODASEVICH'S POEM

• A STAR IS BURNING, THE ether is trembling, the night is HIDDEN IN THE FLYING ARCHES, HOW CAN YOU NOT LOVE THIS WHOLE WORLD, YOUR INCREDIBLE GIFT?

• YOU GAVE ME FIVE WRONG FEELINGS

• YOU GAVE ME TIME AND SPACE

• PLAYING IN THE HAZARD OF ARTS

• MY SOUL IS INCONSTANT.

• AND I CREATE OUT OF NOTHING

• YOUR SEA, DESERT, MOUNTAINS,

• ALL THE GLORY OF YOUR SUN,

• SO DAMNING TO THE EYES.

• AND I DESTROY SUDDENLY JOKINGLY

• ALL THIS LUXURY RIDICULOUSNESS,

• HOW A SMALL CHILD IS RUINED

• A FORTRESS BUILT FROM CARDS.

The nature of stars. While observing the starry sky, you may notice that the colors of the stars are different. The color of the hot metal can be used to judge the temperature of its photosphere. The sun is a yellow star. Stars with a temperature of 3500-4000K are reddish in color. The spectra of most stars are absorption spectra: dark lines are visible against the background of a continuous spectrum. The sequence of spectral types reflects differences in the color and temperature of stars. The diversity of stellar spectra is explained by the fact that stars have different temperatures. In addition to temperature, the type of spectrum of a star is determined by the pressure and density of the gas in its photosphere, the presence of a magnetic field, and the characteristics of its chemical composition.

Slide 5 from the presentation "Astronomy as a Science".

The size of the archive with the presentation is 391 KB.

Astronomy 11th grade summary

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“The Origin of the Universe” - Galaxies consist of hundreds of billions of stars. Gorchakova Tatyana 11 “A” class. How did the Universe come into being? Creation of the Universe by God in 6 days. The Universe is an expanding space filled with a sponge-like ragged structure. The Big Bang Theory. Theories of the origin of the Universe: Stars are mainly composed of hydrogen. The structure of the Universe. Age of the Universe. An endlessly pulsating Universe. Creationism. Target:<- Силикатные соединения. Железное ядро. Уран. Ледяная кора Каллисто имеет очень большую толщину. Левитан Е. П. 2000 год. Интерес вызывает Миранда.

“Planets” - About Mars. Pluto is the god of the underworld in Greek mythology. The composition of the Earth is dominated by: iron (34.6%), oxygen (29.5%), silicon (15.2%), magnesium (12.7%). Photos of Mercury. About the Earth. Thanks to the greenhouse effect, Venus is extremely hot. Satellites of Mercury. Mercury. Saturn, the sixth planet from the Sun, has an amazing ring system. Characteristics of Venus.

“Hypotheses of the origin of the Solar System” - But Laplace knew and spoke critically about the assumptions of his compatriot Buffon. Buffon's hypothesis. Petrova Regina, 11th class. All other development of the World occurs without the participation of the Creator. What is the solar system? Solar system. Kant's hypothesis. This is how the first condensations of matter appeared in Chaos.

“Planets and their satellites” - Saturn’s satellite Phoebus orbits the planet in the opposite direction. Only the visible part of the Moon is visible from Earth. On the left is a 1.5-kilogram basalt from one of the lunar maria. The thickness of the middle mantle is about 600 km. The surface of the satellite is light and reflects about 80% of the incident solar rays. Iapetus. The density of the satellite is quite high - 3.04 g/cm3. The rocks on the Moon became solid about 4.4 billion years ago. The rings of Uranus are almost black: the albedo is 0.03.

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White dwarf, the hottest known, and the planetary nebula NGC 2440, 05/07/2006 Physical nature of stars

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Spectrum λ = 380 ∻ 470 nm – violet, blue; λ = 470 ∻ 500 nm – blue-green; λ = 500 ∻ 560 nm – green; λ = 560 ∻ 590 nm – yellow-orange λ = 590 ∻ 760 nm – red. Distribution of colors in the spectrum = K O F Z G S F Remember, for example: How Once Jacques the City Beller Broke the Lantern. In 1859, G.R. Kirchhoff (1824-1887, Germany) and R.W. Bunsen (1811-1899, Germany) discovered spectral analysis: gases absorb the same wavelengths that they emit when heated. Stars have dark (Fraunhofer) lines against the background of continuous spectra - these are absorption spectra. In 1665, Isaac Newton (1643-1727) obtained spectra of solar radiation and explained their nature, showing that color is an intrinsic property of light. In 1814, Joseph von Fraunhofer (1787-1826, Germany) discovered, identified and by 1817 described in detail 754 lines in the solar spectrum (named after him), creating in 1814 an instrument for observing spectra - a spectroscope. Kirchhoff-Bunsen spectroscope

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Spectra of stars Spectra of stars are their passport with a description of all stellar patterns. From the spectrum of a star, you can find out its luminosity, distance to the star, temperature. The study of stellar spectra is the foundation of modern astrophysics. Spectrogram of the Hyades open cluster. William HEGGINS (1824-1910, England), an astronomer who was the first to use a spectrograph, began the spectroscopy of stars. In 1863 he showed that the spectra of the Sun and stars have much in common and that their observed radiation is emitted by hot matter and passes through overlying layers of cooler absorbing gases. Combined emission spectrum of a star. Above is “natural” (visible in a spectroscope), below is the dependence of intensity on wavelength. size, chemical composition of its atmosphere, speed of rotation around its axis, features of movement around the common center of gravity.

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Chemical composition The chemical composition is determined by the spectrum (intensity of Fraunhofer lines), which also depends on temperature, pressure and density of the photosphere, and the presence of a magnetic field. Stars are composed of the same chemical elements known on Earth, but mainly hydrogen and helium (95-98% of mass) and other ionized atoms, while cool stars have neutral atoms and even molecules in their atmosphere. As the temperature increases, the composition of particles capable of existing in the stellar atmosphere becomes simpler. Spectral analysis of stars of classes O, B, A (T from 50,000 to 10,0000C) shows in their atmospheres lines of ionized hydrogen, helium and metal ions, in class K (50000C) radicals are already detected, and in class M (38000C) molecules oxides The chemical composition of a star reflects the influence of factors: the nature of the interstellar medium and those nuclear reactions that develop in the star during its life. The initial composition of the star is close to the composition of the interstellar matter from which the star arose. Supernova remnant NGC 6995 is hot, glowing gas formed after the star exploded 20-30 thousand years ago. Such explosions actively enriched space with heavy elements from which planets and stars of the next generation were subsequently formed.

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Color of stars In 1903-1907. Einar Hertzsprung (1873-1967, Denmark) was the first to determine the colors of hundreds of bright stars. Stars come in a variety of colors. Arcturus has a yellow-orange hue, Rigel is white-blue, Antares is bright red. The dominant color in a star's spectrum depends on its surface temperature. The gas shell of a star behaves almost like an ideal emitter (absolutely black body) and is completely subject to the classical laws of radiation by M. Planck (1858–1947), J. Stefan (1835–1893) and V. Wien (1864–1928), relating body temperature and the nature of its radiation. Planck's law describes the distribution of energy in the spectrum of a body and indicates that with increasing temperature, the total flux of radiation increases, and the maximum in the spectrum shifts towards shorter waves. During observations of the starry sky, one might notice that the color (the property of light to cause a certain visual sensation) of stars is different. The color and spectrum of stars is related to their temperature. Light of different wavelengths excites different color sensations. The eye is sensitive to the wavelength that carries the maximum energy λmax = b/T (Wien's law, 1896). Like precious stones, the stars of the open cluster NGC 290 shimmer in different colors. Photo by CT named after. Hubble, April 2006

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Temperature of stars The temperature of stars is directly related to color and spectrum. The first measurement of the temperature of stars was made in 1909 by the German astronomer Julius Scheiner (1858-1913), having carried out absolute photometry of 109 stars. The temperature is determined from the spectra using Wien's law λmax.T=b, where b=0.289782.107Å.K is Wien's constant. Betelgeuse (Hubble Telescope image). In such cool stars with T = 3000K, radiation in the red region of the spectrum predominates. The spectra of such stars contain many lines of metals and molecules. Most stars have temperatures of 2500K<Т< 50000К Звезда HD 93129A (созв. Корма) самая горячая – Т= 220000 К! Самые холодные - Гранатовая звезда (m Цефея), Мира (o Кита) – Т= 2300К e Возничего А - 1600 К.

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Spectral classification In 1866, Angelo Secchi (1818-1878, Italy) gave the first spectral classification of stars by color: White, Yellowish, Red. The Harvard spectral classification was first presented in the Catalog of Stellar Spectra of Henry Draper (1837-1882, USA), prepared under the direction of E. Pickering (1846-1919) by 1884. All spectra were arranged according to line intensities (later in temperature sequence) and designated by letters in alphabetical order from hot to cold stars: O B A F G K M. By 1924, it was finally established by Anna Cannon (1863-1941, USA) and published in a catalog of 9 volumes on 225330 stars - HD catalogue.

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Modern spectral classification The most accurate spectral classification is represented by the MK system, created by W. Morgan and F. Keenan at the Yerkes Observatory in 1943, where the spectra are arranged both by temperature and luminosity of stars. Luminosity classes were additionally introduced, marked with Roman numerals: Ia, Ib, II, III, IV, V and VI, respectively indicating the size of the stars. Additional classes R, N and S denote spectra similar to K and M, but with a different chemical composition. Between each two classes, subclasses are introduced, designated by numbers from 0 to 9. For example, the spectrum of type A5 is halfway between A0 and F0. Additional letters sometimes mark the features of stars: “d” – dwarf, “D” – white dwarf, “p” – peculiar (unusual) spectrum. Our Sun belongs to the spectral class G2 V

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Luminosity of stars In 1856, Norman Pogson (1829-1891, England) established a formula for luminosities in terms of absolute M magnitudes (i.e. from a distance of 10 pc). L1/L2=2.512 M2-M1. The Pleiades open cluster contains many hot and bright stars that were formed at the same time from a cloud of gas and dust. The blue haze accompanying the Pleiades is scattered dust reflecting the light of the stars. Some stars shine brighter, others weaker. Luminosity is the radiation power of a star - the total energy emitted by a star in 1 second. [J/s=W] Stars emit energy over the entire range of wavelengths L = 3.846.1026 W/s Comparing the star with the Sun, we get L/L=2.512 M-M, or logL=0.4 (M -M ) Star luminosity: 1.3.10-5L

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The sizes of stars are determined: 1) Direct measurement of the angular diameter of the star (for bright ≥2.5m, close stars, >50 measured) using a Michelson interferometer. For the first time on December 3, 1920, the angular diameter of the star Betelgeuse (α Orionis) was measured = A. Michelson (1852-1931, USA) and F. Pease (1881-1938, USA). 2) Through the luminosity of the star L=4πR2σT4 in comparison with the Sun. Stars, with rare exceptions, are observed as point sources of light. Even the largest telescopes cannot see their disks. According to their sizes, stars have been divided since 1953 into: Supergiants (I) Bright giants (II) Giants (III) Subgiants (IV) Main sequence dwarfs (V) Subdwarfs (VI) White dwarfs (VII) The names dwarfs, giants and supergiants were introduced Henry Russell in 1913, and they were discovered in 1905 by Einar Hertzsprung, introducing the name “white dwarf”. Sizes of stars 10 km

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Mass of stars One of the most important characteristics of stars, indicating its evolution, is the determination of the star’s life path. Determination methods: 1. Mass-luminosity relationship L≈m3.9 2. Kepler’s 3rd refined law in physically binary systems Theoretically, the mass of stars is 0.005M

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