The structure and development of stars briefly. Abstract: Structure, origin and evolution of galaxies and stars

There are two main concepts of the origin of celestial bodies. The first is based on the nebular model of the formation of the solar system, put forward by the French physicist and mathematician Pierre Laplace and developed by the German philosopher Immanuel Kant. In accordance with it, stars and planets were formed from diffuse diffuse matter (cosmic dust) by gradual compression of the original nebula.

The adoption of the Big Bang model and the expanding Universe significantly influenced the models of the formation of celestial bodies and led to Viktor Ambartsumyan's hypothesis about the emergence of galaxies, stars and planetary systems from superdense (consisting of the heaviest elementary particles - hyperons) prestellar matter located in the nuclei of galaxies, by fragmentation.

The interpretation of celestial bodies is determined by which of the two hypotheses is considered true. The discovery by V. Ambartsumyan of stellar associations consisting of very young stars tending to run away from each other was understood by him as confirmation of the hypothesis of the formation of stars from the original superdense matter. Which of the two concepts is closer to the truth will be decided by the subsequent development of natural science.

The model of the expanding universe met with several difficulties that contributed to the progress of astronomy. Scattering after the Big Bang from a point with an infinitely high density, clots of matter should slightly slow down each other by the forces of mutual attraction, and their speed should decrease. But the whole mass of the Universe is not enough for braking. From this objection, in 1939, the hypothesis was born that there are so-called "black holes" in the Universe, which cannot be seen, but which store 9/10 of the mass of the Universe (that is, as much as is missing).

What are "black holes"? If a certain mass of a substance finds itself in a relatively small volume, critical for a given mass, then under the influence of its own gravity, such a substance begins to uncontrollably shrink. A gravitational collapse occurs. As a result of compression, the concentration of mass increases and a moment comes when the gravitational force on the surface becomes so great that in order to overcome it, it would be necessary to develop a speed greater than the speed of light. Therefore, the "black hole" does not let out anything and does not reflect, and therefore it is impossible to detect it. In a black hole, space curves and time slows down. If the compression continues further, then at some stage of it, undamped nuclear reactions begin. The compression stops, and then an anti-collapse explosion occurs, and the "black hole" turns into a "white hole". It is assumed that "black holes" are located in the cores of galaxies, being a super-powerful source of energy.

All celestial bodies can be divided into those that emit energy - stars, and those that do not emit energy - planets, comets, meteorites, cosmic dust. The energy of stars is generated in their depths by nuclear processes at temperatures reaching tens of millions of degrees, which is accompanied by the release of special particles of great penetrating power - neutrinos.

Stars are factories for the production of chemical elements and sources of light and life. This solves several problems at once. Stars move around the center of the galaxy in complex orbits. There may be stars whose brightness and spectrum change - variable stars (Tau Ceti) and non-stationary (young) stars, as well as stellar associations whose age does not exceed 10 million years. It is possible that supernovae are formed from them, during the outbreaks of which a huge amount of energy of non-thermal origin is released and nebulae (gas accumulations) are formed.

There are very large stars - red giants and supergiants, and neutron stars, the mass of which is close to the mass of the Sun, but the radius is 1/50000 of the solar one (10-20 km); they are called so because they consist of a huge bunch of neutrons).

In 1967, pulsars were discovered - cosmic sources of radio, optical, X-ray and gamma radiation that come to Earth in the form of periodically repeating bursts. For radio pulsars (rapidly rotating neutron stars), the pulse periods are 0.03-4 sec, for X-ray pulsars (double stars, where matter flows from a second, ordinary star to a neutron star), the periods are several seconds or more.

Comets are interesting celestial bodies that have often been attributed supernatural significance. Under the influence of solar radiation, gases are released from the nucleus of a comet, forming an extensive head of a comet. The impact of solar radiation and solar wind causes the formation of a tail, sometimes reaching millions of kilometers in length. The emitted gases go into outer space, as a result of which, with each approach to the Sun, the comet loses a significant part of its mass. In this regard, comets live for a relatively short time (millennia and centuries).

The sky only seems calm. Catastrophes constantly occur in it and new and supernova stars are born, during the outbreaks of which the luminosity of the star increases hundreds of thousands of times. These explosions characterize the galactic pulse.

At the end of the evolutionary cycle, when all the hydrogen fuel has been used up, the star shrinks to infinite density (the mass remains the same). An ordinary star turns into a "white dwarf" - a star with a relatively high surface temperature (from 7000 to 30000 ° C) and low luminosity, many times less than the luminosity of the Sun.

It is assumed that one of the stages in the evolution of neutron stars is the formation of a new and supernova, when it increases in volume, sheds its gas shell and releases energy for several days, shining like billions of suns. Then, having exhausted the resources, the star dims, and a gaseous nebula remains in place of the flash.

If the star had super-large dimensions, then at the end of its evolution, particles and rays, having barely left the surface, immediately fall back due to gravitational forces, i.e., a “black hole” is formed, which then turns into a “white hole”.

The structure of the stars. It may seem impossible to know anything about the internal structure of stars. Not only distant stars, but also our Sun seems to be absolutely inaccessible for studying its depths. Nevertheless, we know as much about the structure of stars as we do about the structure of the Earth. The fact is that stars are gas balls, for the most part they are stable, not experiencing either collapse or expansion. Therefore, at any depth, the gas pressure is equal to the weight of the overlying layers, and the radiation flux is proportional to the temperature drop from the inner hot to the outer cold layers. These conditions, formulated in the form of mathematical equations, are sufficient to calculate the structure of a star based on the laws of gas behavior, i.e. change in pressure, temperature and density with depth. At the same time, only the mass, radius, luminosity, and chemical composition of a star need to be known from observations in order to theoretically determine its structure. Calculations show that in the center of the Sun the temperature reaches 16 million K, the density is 160 g/cm 3 , and the pressure is 400 billion atm.

The star is a natural self-regulating system. If, for some reason, the power of energy release in the core of the star cannot compensate for the radiation of energy from the surface, then the star will not be able to resist gravity: it will begin to shrink, this will increase the temperature in its core and increase the intensity of nuclear reactions - thus the energy balance will be restored.

The evolution of the stars. A star begins its life as a cold, rarefied cloud of interstellar gas that contracts under its own gravity. When compressed, the gravitational energy turns into heat, and the temperature of the gas globule increases. In the last century, it was generally believed that the energy released during the compression of a star is sufficient to maintain its luminosity, but geological data came into conflict with this hypothesis: the age of the Earth turned out to be much longer than the time during which the Sun could maintain its radiation due to compression. (about 30 million years).

The contraction of a star leads to an increase in temperature in its core; when it reaches several million degrees, thermonuclear reactions begin and the compression stops. In this state, the star stays for most of its life, being on the main sequence of the Hertzsprung-Russell diagram, until the fuel reserves in its core run out. When all the hydrogen in the center of the star turns into helium, the thermonuclear combustion of hydrogen continues on the periphery of the helium core.

(33.60 Kb)

During this period, the structure of the star begins to noticeably change. Its luminosity grows, the outer layers expand, and the surface temperature decreases - the star becomes a red giant. A star spends much less time on the giant branch than on the main sequence. When the mass of its isothermal helium core becomes significant, it cannot support its own weight and begins to shrink; the temperature rising at the same time stimulates the thermonuclear conversion of helium into heavier elements.

White dwarfs and neutron stars. Shortly after a helium flash, carbon and oxygen "light up"; each of these events causes a strong rearrangement of the star and its rapid movement along the Hertzsprung-Russell diagram. The size of the star's atmosphere increases even more, and it begins to intensively lose gas in the form of expanding stellar wind streams. The fate of the central part of a star depends entirely on its initial mass: the core of a star can end its evolution as a white dwarf, a neutron star (pulsar), or a black hole.

The vast majority of stars, including the Sun, end their evolution by shrinking until the pressure of degenerate electrons balances gravity. In this state, when the size of a star decreases by a factor of a hundred and the density becomes a million times higher than that of water, the star is called white dwarf. It is deprived of sources of energy and, gradually cooling down, becomes dark and invisible.

In stars more massive than the Sun, the pressure of degenerate electrons cannot contain the compression of the core, and it continues until most of the particles turn into neutrons packed so densely that the size of the star is measured in kilometers, and the density is 100 million times greater than density of water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter. see also NEUTRON STAR.

Black holes. In stars more massive than neutron star precursors, the cores experience complete gravitational collapse. As such an object is compressed, the force of gravity on its surface increases so much that no particles and even light can leave it - the object becomes invisible. In its vicinity, the properties of space-time change significantly; they can only be described by the general theory of relativity. Such objects are called black holes.

If the black hole's progenitor was a member of an eclipsing binary, then the black hole would continue to revolve around a nearby normal star. In this case, gas from the star's atmosphere can enter the vicinity of the black hole and fall on it. But before disappearing into the region of invisibility (below the event horizon), it will heat up to a high temperature and become a source of X-rays, which can be observed using special telescopes. When a normal star obscures the black hole, the X-rays should disappear.

Several eclipsing binaries with X-ray sources have already been discovered; they are suspected of having black holes. An example of such a system is the Cygnus X-1 object. Spectral analysis showed that the orbital period of this system is 5.6 days, and X-ray eclipses occur with the same period. There is almost no doubt that there is a black hole there. see also BLACK HOLE.

The duration of the evolution of stars. Apart from some catastrophic episodes in the life of stars, human life is too short to notice the evolutionary changes of each particular star. Therefore, the evolution of stars is judged in the same way as the growth of trees in a forest, i.e. simultaneously observing many instances that are currently at different stages of evolution.

The speed and pattern of a star's evolution is almost entirely determined by its mass; chemical composition also has some effect. A star may be physically young, but already evolutionarily aged in the same sense as a one-month-old mouse is older than a one-year-old baby elephant. The fact is that the intensity of energy release (luminosity) of stars increases very rapidly with the growth of their mass. Therefore, more massive stars burn their fuel much faster than low-mass ones.

Bright, massive stars in the upper main sequence (spectral classes O, B, and A) live much shorter lives than stars like the Sun and even less massive members of the lower main sequence. Therefore, the stars of classes O, B and A born simultaneously with the Sun have long since completed their evolution, and those that are observed now (for example, in the constellation of Orion) should have been born relatively recently.

In the vicinity of the Sun, there are stars of different physical and evolutionary ages. However, in each star cluster, all its members have almost the same physical age. By studying the youngest clusters with an age of ca. 1 million years, we see all of its stars on the main sequence, and some still just approaching it. In older clusters, the brightest stars have already left the main sequence and become red giants. The oldest clusters have only the lower part of the main sequence, but the giant branch and the horizontal branch following it are richly populated with stars.

If we compare the Hertzsprung-Russell diagrams of various open clusters, we can easily understand which one is older. This is judged by the position of the break point of the main sequence, which marks the top of its preserved lower part. At the double cluster h and  Perseus, this point lies much higher than that of the Pleiades and Hyades clusters, therefore, it is much younger than them.

Hertzsprung-Russell diagrams of globular clusters indicate their very old age, close to the age of the Galaxy itself. These clusters consist of stars formed in that distant epoch when the matter of the Galaxy contained almost no heavy elements. Therefore, their evolution does not proceed exactly like that of modern stars, although on the whole it corresponds to it.

In conclusion, we point out that the age of the Sun is about 5 billion years, and at present it is in the middle of its evolutionary path. But if the initial mass of the Sun were only twice as high, then its evolution would have ended long ago, and life on Earth would not have had time to reach its peak in the form of a person. Cm . also ASTRONOMY AND ASTROPHYSICS;GALAXIES; GRAVITATIONAL COLLAPSE; INTERSTELLAR MATTER; SUN.

The content of the article

STARS, hot luminous celestial bodies like the sun. Stars vary in size, temperature, and brightness. In many respects, the Sun is a typical star, although it seems much brighter and larger than all other stars, since it is located much closer to the Earth. Even the nearest star (Proxima Centauri) is 272,000 times farther from the Earth than the Sun, so the stars appear to us as bright points in the sky. Although the stars are scattered throughout the sky, we see them only at night, and during the day they are not visible against the background of bright sunlight scattered in the air.

Living on the surface of the Earth, we are at the bottom of the ocean of air, which is constantly agitated and seething, refracting the rays of starlight, which makes them seem to us blinking and trembling. Astronauts in orbit see the stars as colored, unblinking dots.

Many temples were oriented by the stars. For example, the Great Pyramids at Giza are built in such a way that a narrow corridor in them is directed exactly at the polar star, the role of which was then played by a Dragon. The megalithic structure Stonehenge on the Salisbury Plain in England was built in strict accordance with the seasonal changes in the position of the Sun and Moon.

In our era, stars are often used as bright markers in the sky for telling time and for navigation. As the Earth rotates, each observer notices how the stars alternately cross an imaginary north-zenith-south line (celestial meridian). This phenomenon is used to count sidereal time. For the beginning of a new sidereal day on the whole Earth, the moment of crossing the Greenwich meridian in England by a certain point of the celestial sphere is taken. NAVIGATION.

Star designations.

There are more than 100 billion stars in our Galaxy. In photographs of the sky taken by large telescopes, there are so many stars that it is pointless to even try to name them all, or even count them. About 0.01% of all stars in the Galaxy are cataloged. Thus, the vast majority of stars observed in large telescopes have not yet been labeled and counted.

The brightest stars in each nation got their names. Many of those now used, for example, Aldebaran, Algol, Deneb, Rigel, etc., are of Arabic origin; Arab culture served as a bridge across the intellectual gulf separating the fall of Rome from the Renaissance.

In a beautifully illustrated Uranometry (Uranometria, 1603) by the German astronomer I. Bayer (1572–1625), where the constellations and the legendary figures associated with their names are depicted, the stars were first designated by the letters of the Greek alphabet approximately in descending order of their brightness: a the brightest star in the constellation b- the second in brilliance, etc. When there were not enough letters of the Greek alphabet, Bayer used the Latin. The full designation of the star consisted of the mentioned letter and the Latin name of the constellation. For example, Sirius is the brightest star in the constellation Canis Major, so it is referred to as a Canis Majoris, or for short a CMa; Algol - the second brightest star in Perseus is designated as b Persei, or b per.

J. Flamsteed (1646–1719), the first Astronomer Royal of England, introduced a system for designating stars that was not related to their brightness. In each constellation, he designated the stars by numbers in the order of increasing their right ascension, i.e. in the order in which they cross the meridian. So, Arcturus, he is a Bootes ( b Bootes), designated as 16 Bootes.

Some unusual stars are sometimes named after the astronomers who first described their unique properties. For example, Barnard's star is named after the American astronomer E. Barnard (1857–1923), and Kapteyn's star is named after the Dutch astronomer J. Kapteyn (1851–1922). Modern star charts usually show the ancient proper names of bright stars and Greek letters in Bayer's notation (his Latin letters are rarely used); the remaining stars are designated according to Flamsteed. But there is not always enough space on the maps for these designations, so the designations of the remaining stars must be looked for in star catalogs.

star catalogs.

The most extensive star catalog Bonn Review(Bonner Durchmusterung,BD) was compiled by the German astronomer F. Argelander (1799–1875). It lists the positions of 324,198 stars from the north pole to a declination of -2°. The star designated, for example, as BD +7°1226, is the 1226th star in the order of right ascension in the eighth zone of northern declination. The continuation of this catalog (SBD) to the south to a declination of -23°, containing 133,659 stars, was compiled by the German astronomer E. Schoenfeld (1828–1891). Catalogs covered the rest of the southern sky Cordoba review (Cordoba Durchmusterung, CD) and Cape Photographic Survey (Cape Photographic Durchmusterung, CPD). In total, these catalogs contain more than 1 million stars up to about 10 magnitude.

Significantly more stars in the catalog sky map(carte du ciel, or Astrographic Catalog), containing the positions of several million stars on 44,000 photographic plates taken at observatories around the world. A modern large catalog of the precise positions of 258,997 stars has been created at the Smithsonian Astrophysical Observatory (SAO). An extensive catalog of stellar spectra was created by the American astronomer E. Cannon (1863–1941) and named Henry Draper catalog (Henry Draper Catalog of Stellar Spectra, HD).

There are many special directories. For example, stars with measured proper motions are collected in general directory (General Catalog, GC) and Yale zone catalogs (Yale Zone Catalogues). There are catalogs of stars with measured radial velocities, stars with variable brightness, catalogs of binary stars. The faintest stars are not catalogued, but they can be found on photographic sky maps and their coordinates and brightness can be determined relative to brighter stars. The most complete photographic atlas covering the entire sky is Palomar Review (Palomar Survey), on the maps of which stars up to the 21st magnitude are visible.

variable stars.

Variable stars are designated in the order in which they are found in each constellation. The first is denoted by the letter R, the second by S, then T, and so on. Z is followed by RR, RS, RT, etc. After ZZ comes AA and so on. (The letter J is not used to avoid confusion with I.) When all these combinations are exhausted (there are 334 in total), they continue numbering with the letter V (variable - variable), starting with V335. Examples: S Car, RT Per, V557 Sgr.

Distances to the stars.

The closest star to us is the Sun, approx. 150 million km. The brightest star closest to the Sun is a Centaur, which can only be seen in the Southern Hemisphere, is 42,000 billion km away. But even a little closer to us is its invisible satellite, the star Proxima (“nearest”) Centaur. Just twice as far away is Sirius, the brightest star in our sky.

Since the distances to the stars are so great, it is inconvenient to measure them in kilometers. It is better to use special units; for example, in popular science literature, "light year" is often used, i.e. the distance that a beam of light travels at a speed of about 300,000 km / s per year; its OK. 9460 billion km. Distance to Proxima 4.3 St. years, and to Sirius approx. 8.7 St. of the year.

For the first time, distances to stars were independently measured in 1838 by F. Bessel in Germany (up to the star 61 Cygnus), T. Henderson at the Cape of Good Hope (up to a Centaur) and V. Struve in Russia (before Vega). However, a century and a half earlier, I. Newton was able to estimate the order of distance to the stars. Assuming the Sun to be an ordinary star, he calculated that it would need to be removed by a factor of 250,000 to make the Sun look like an ordinary star in the sky. So Newton introduced a very universal method for determining distances in astronomy. If by any means we know the true luminosity of a star, then it is not difficult to calculate at what distance it will have an observed brightness. The main thing here is to determine the true luminosity of the star. In practice, spectroscopy is used for this: in the spectrum of a star there are several indicators of its luminosity.

nearby stars

NEAREST STARS 1
Star	Parallax (seconds of arc)	Distance (st. years)	Relative luminosity	Color
The sun	–	– 2	1	Yellow
a centaur	0,760	4,3	1,5	Yellow
Barnard's Star	0,552	5,9	0,0006	Red
Wolf 359	0,425	7,7	0,00002	Red
Lalande 21185	0,398	8,2	0,0055	Red
Sirius	0,375	8,6	23	White
Leuthen 726-8	0,368	8,9	0,00006	Red
Ross 154	0,345	9,5	0,00041	Red
Ross 248	0,316	10,2	0,00011	Red
Leuthen 789-6	0,305	10,7	0,00009	Red
e Eridani	0,303	10,8	0,30	Orange
Ross 128	0,301	10,8	0,00054	Red
61 Swans	0,296	11,0	0,084	Orange
e Indian	0,291	11,2	0,14	Orange
Procyon	0,285	11,4	7,3	Yellow
1 Data for principal components of binary and multiple stars only. 2 The distance to the Sun is 150 million km, or 1 astronomical unit.

But the spectroscopic method needs calibration. For some groups of stars, special methods are used to determine distances, such as a statistical method based on the apparent movement of stars across the sky. However, the basic method for determining distances to stars is the trigonometric parallax method.

Parallax.

The parallax method is based on measuring the apparent displacement of nearby stars against the background of more distant ones when observed from different points of the Earth's orbit. The closer the star, the greater its angular displacement. The parallax of a star is the angle at which the radius of the earth's orbit is visible from it, equal to 1 astronomical unit (AU), or 150 million km. It is purely geometric and therefore a very reliable method. Unfortunately, parallaxes can only be measured for a few thousand nearby stars. The distances to them serve as the foundation for determining the distances to more distant stars by spectral methods.

Astronomers of the past, such as T. Brahe (1546–1601), failed to notice the parallactic displacement of the stars, from which they concluded that the Earth is motionless. Indeed, the parallaxes of even the nearest stars do not exceed 1ўў; at this angle, the little finger is visible from a distance of a kilometer. The measurement of such small angles is a great achievement of modern technology. The largest parallax (0.762ўў) has Proxima Centauri - a small satellite of the star a Centaur, located closer to the Sun.

On the basis of trigonometric parallaxes, astronomers introduced the unit of length "parsec" (pc) - the distance to a star whose parallax is 1ўў; 1 pc \u003d 3.26 sv. of the year. The smallest parallaxes that can now be measured are 0.01ўў; this corresponds to a distance of 100 pc or 326 sv. years.

The luminosity of the stars.

The total power of the radiation of a star in the entire range of the electromagnetic spectrum is called the true or bolometric "luminosity". For example, the luminosity of the Sun is 3.86ґ10 26 W. The greater the mass of a normal star, the higher its luminosity; it increases approximately as a cube of mass. This mass-luminosity relation was first found from observations, and later received a theoretical justification.

The flow of energy coming from a star to Earth is called "visible brilliance"; it depends not only on the true luminosity of the star, but also on its distance from the Earth. A low luminosity star located close to the Earth may have more brilliance than a high luminosity star at a great distance.

brightest stars

THE BRIGHTEST STARS
Star	magnitude		Luminosity (Sun=1)	Color index	Color
Star	visible	absolute	Luminosity (Sun=1)	Color index	Color
Sirius	–1,43	+1,4	23	0,00	White
canopus	–0,72	–4,5	1500	0,16	Yellow
a centaur	–0,27	+4,7	1,5	0,68	Yellow
Arcturus	–0,06	–0,1	100	1,24	Orange
Vega	+0,02	+0,5	50	0,00	White
Chapel	+0,05	–0,6	170	0,80	Yellow
Rigel	+0,14	–7,0	40000	–0,04	Blue
Procyon	+0,37	+2,7	7,3	0,41	Yellow
Betelgeuse	+0,50	–5,0	17000	1,87	Red
Achernar	+0,51	–2,0	200	–0,16	Blue
b centaur	+0,63	–4,0	5000	–0,23	Blue
Altair	+0,77	+2,2	9	0,22	White
Aldebaran	+0,86	–0,7	100	1,52	Orange
a Cross	+0,87	–4,0	4000	–0,25	Blue
Spica	+0,96	–3,0	2800	–0,25	Blue
Antares	+1,16	–4,0	3500	1,83	Red
Fomalhaut	+1,16	+1,9	14	0,10	White
Pollux	+1,25	+1,0	45	1,02	Orange
Deneb	+1,28	–7,0	60000	0,09	White
b Cross	+1,36	–4,0	6000	–0,25	Blue
Regulus	+1,48	–0,7	120	–0,12	Blue
Shaula (l sco)	+1,60	–5,0	8000	–0,21	Blue
Adara (e SMa)	+1,64	–3,0	1700	–0,24	Blue
Bellatrix	+1,97	–4,0	2300	–0,23	Blue
Castor		+0,9	27	0,03	White

Star magnitudes.

The brilliance of stars is expressed in special, historically established "star magnitudes". The origin of this system is connected with the peculiarity of our vision: if the strength of the light source changes exponentially, then our sensation from it is only arithmetic. The Greek astronomer Hipparchus (before 161 - after 126 BC) divided all the stars visible to the eye into 6 classes according to brightness. He called the brightest stars of the 1st magnitude, and the weakest - the 6th. Later measurements showed that the flux of light from stars of the 1st magnitude is about 100 times greater than from the stars of the 6th magnitude according to Hipparchus. For definiteness, we decided that a difference of 5 magnitudes exactly corresponds to a ratio of light fluxes of 1:100. Then the difference in brightness by 1 magnitude corresponds to the ratio of brightnesses. For example, a 1st magnitude star is 2.512 times brighter than a 2nd magnitude star, which in turn is 2.512 times brighter than a 3rd magnitude star, and so on. This is a very versatile scale; it is suitable for expressing the illumination created on Earth by any light source.

To compare stars by their true luminosity, "absolute magnitude" is used, which is defined as the apparent magnitude that a given star would have if placed at a standard distance from the Earth of 10 pc. If any star has parallax p and apparent magnitude m, then its absolute value M calculated according to the formula

Stellar magnitudes can describe the radiation of a star in various ranges of the spectrum. For example, a visual value ( mv) expresses the brightness of a star in the yellow-green region of the spectrum, photographic ( m p) - in blue, etc. The difference between the photographic and visual values is called the "color index" (color index)

it is closely related to the temperature and spectrum of the star.

Star sizes.

Stars vary greatly in diameter: white dwarfs are the size of the globe (about 13,000 km), and giant stars exceed the size of the orbit of Mars (455 million km). On average, the size of stars visible in the sky with the naked eye is close to the diameter of the Sun (1,392,000 km).

With rare exceptions, the diameters of stars cannot be measured directly: even in the largest telescopes, stars look like points due to their gigantic distances. Of course, the Sun is an exception: its angular diameter (32º) is easy to measure; for several of the largest and closest stars, with great difficulty, it is possible to measure the angular size and, knowing the distance to them, determine their linear diameter. These data are shown in the table below.

In some cases, it is possible to directly determine the linear diameters of stars in binary systems. If the stars periodically cover each other, then from the duration of the eclipse, by measuring the orbital velocity of the stars from the shift of the spectral lines, one can calculate their diameter.

For the vast majority of stars, the diameters are determined indirectly, on the basis of radiation laws. Having determined the temperature of the star from the form of the spectrum, on the basis of the laws of physics, it is possible to calculate the intensity of radiation from its surface. Knowing the total luminosity, it is already easy to calculate the surface area and diameter of the star. The diameters thus obtained are in good agreement with those measured directly.

The size of a star varies greatly during its lifetime. It begins its evolution as a shrinking gas cloud of enormous size, then remains in the form of a normal star for a long time, and at the end of its life increases tenfold, becoming a giant, sheds its shell and turns into a small “white dwarf” or a very tiny “neutron star” . PULSAR.

Star populations.

In 1944, the German-born American astronomer W. Baade proposed to divide the stars into two types, which he called Population I and Population II. Population I included young stars and their associated interstellar gas and dust, which are observed in the spiral arms of galaxies and open clusters. Population II consists of old stars found in globular clusters, elliptical galaxies, and the central regions of spiral galaxies. The brightest stars in Population I are blue supergiants, which are 100 times brighter than the brightest stars in Population II, the red giants. Population I stars have a much higher abundance of heavy elements. The concept of stellar populations was of great importance for the development of the theory of stellar evolution.

Star movements.

Usually, the motion of a star is characterized from two points of view: as an orbital motion around the center of the Galaxy and as a relative motion in a group of nearby stars. For example, the Sun revolves around the center of the Galaxy at a speed of approx. 240 km / s, and in relation to the stars surrounding it, it moves much more slowly, at a speed of approx. 19 km/s.

The main frame of reference for measuring the motion of stars is the Galaxy as a whole. But for an earthly observer, it is usually more convenient to use a frame of reference associated with the center of the solar system, in fact, with the Sun. In relation to the Sun, the nearest stars move at speeds of 10 km/s and higher. But the distances to the stars are so great that the figures of the constellations change only over many millennia. The movement of stars was first discovered in 1718 by E. Halley, comparing their positions, precisely determined by him at Greenwich, with those indicated in his catalog by Ptolemy (2nd century AD).

The angular movement of a star on the celestial sphere with respect to distant stars is called its "proper motion" and is usually expressed in arc seconds per year. Thus, the proper motion of Arcturus is 2.3ўў/year, and that of Sirius is 1.3ўў/year. Barnard's star has the largest proper motion, 10.3ўў/year.

To calculate the linear velocity of a star in kilometers per second, use the formula T = 4,74 m/p, where T is the tangential velocity (i.e. the component of the total velocity directed across the line of sight), m is the proper motion in seconds of arc per year and p- parallax.

Radial speed.

The speed of a star along the line of sight, which is called the radial velocity, is measured by the Doppler shift of the lines in its spectrum with an accuracy of fractions of a kilometer per second. The shift of the lines to the red side of the spectrum indicates the removal of the star from the Earth, and to the blue - about the approach. The velocities of the stars are not so great as to change the color of the star, but the rapid movement of distant galaxies changes their color quite noticeably. Measuring the Doppler shift of lines is a very delicate operation. In a telescope, simultaneously with the spectrum of a star, the spectrum of a laboratory source with an exactly known position of the lines is photographed on the same plate. Then, using a measuring machine equipped with a powerful microscope, the line shift (D l) in the spectrum of the star relative to the same lines of a laboratory source with a wavelength l. The radial velocity of a star is determined by the formula V = c D l/l, where c is the speed of light. This formula is suitable for normal stellar velocities, but it is not suitable for fast moving galaxies. The accuracy of measuring the radial velocities of stars does not depend on the distance to them, but is entirely determined by the possibility of obtaining good spectra and accurately measuring the position of the lines in them. However, the accuracy of measuring the tangential velocities of stars depends not only on the accuracy of measuring their own motion, but also on their parallax, i.e. from the distance to them: the greater the distance, the lower the accuracy.

Spatial speed.

The radial and tangential velocities are the components of the total spatial velocity of the star relative to the Sun (it can be easily calculated using the Pythagorean theorem). So that the movement of the Sun itself does not "interfere" with this speed, it is usually recalculated in relation to the "local standard of rest" - an artificial coordinate system in which the average movement of circumsolar stars is zero. The speed of a star relative to the local standard of rest is called its "peculiar speed".

Each of the stars orbits around the center of the galaxy. The stars of Population I circulate in almost circular orbits lying in the plane of the galactic disk. The sun and its neighboring stars also move in orbits close to circular, at a speed of about 240 km/s, completing a revolution in 200 million years (a galactic year). The stars of Population II move in elliptical orbits with different eccentricities and inclinations to the plane of the Galaxy, approaching the galactic center in the perigalactia of the orbit and moving away from it in the apogalactia. They spend most of their time in the apogalactic region, where their movement slows down. But in relation to the Sun, their speeds are high, so they are called "high-speed stars."

Double stars.

About half of all stars are part of binary and more complex systems. The center of mass of such a system moves in orbit around the center of the Galaxy, and individual stars revolve around the center of mass of the system. In a binary star, one component revolves around the other in accordance with Kepler's harmonic (third) law:

where m 1 and m 2 – masses of stars in units of solar mass, P- circulation period in years and D is the distance between stars in astronomical units. In this case, both stars revolve around a common center of mass, and their distances from this center are inversely proportional to their masses. Having determined the orbit of each of the components of the binary system relative to the surrounding stars, it is easy to find the ratio of their masses.

Many double stars move so close to each other that it is impossible to see them individually with a telescope; their duality can only be detected from the spectra. As a result of the orbital motion, each of the stars periodically approaches us, then moves away. This causes a Doppler shift of the lines in its spectrum. If the luminosities of both stars are close, then a periodic bifurcation of each spectral line is observed. If one of the stars is much brighter, then only the spectrum of the brighter star is observed, in which all the lines oscillate periodically.

variable stars.

The apparent brightness of a star can change for two reasons: either the luminosity of the star changes, or something blocks it from the observer, for example, the second star in a binary system. Stars with varying luminosity are divided into pulsating and eruptive (i.e. exploding). There are two major types of pulsating variables - Lyrids and Cepheids. The first, RR Lyrae type variables, have approximately the same absolute magnitude and periods shorter than a day. For Cepheids, variables of the type d Cephei, the periods of change in brightness are closely related to their average luminosity. Both types of pulsating variables are very important, because knowing their luminosity allows one to determine distances. The American astronomer H. Shapley used lyrids to measure distances in our Galaxy, and his colleague E. Hubble used Cepheids to determine the distance to the galaxy in Andromeda.

Star colors.

The stars have a variety of colors. Arcturus has a yellow-orange hue, Rigel is white-blue, Antares is bright red. The dominant color in the spectrum of a star depends on the temperature of its surface. The gas envelope of a star behaves almost like an ideal emitter (absolutely black body) and completely obeys the classical laws of radiation by M. Planck (1858–1947), J. Stefan (1835–1893) and V. Wien (1864–1928), which relate body temperature and the nature of its radiation. Planck's law describes the distribution of energy in the spectrum of a body. He indicates that with increasing temperature, the total radiation flux increases, and the maximum in the spectrum shifts towards short waves. The wavelength (in centimeters) that accounts for the maximum radiation is determined by Wien's law: l max = 0.29/ T. It is this law that explains the red color of Antares ( T= 3500 K) and Rigel's bluish color ( T= 18000 K). Stefan's law gives the total radiant flux at all wavelengths (in watts per square meter): E = 5,67ґ10 –8 T 4 .

Spectra of stars.

The study of stellar spectra is the foundation of modern astrophysics. The spectrum can be used to determine the chemical composition, temperature, pressure and velocity of gas in the star's atmosphere. The Doppler shift of the lines is used to measure the speed of the star itself, for example, along the orbit in a binary system.

In the spectra of most stars, absorption lines are visible; narrow gaps in the continuous distribution of radiation. They are also called Fraunhofer or absorption lines. They are formed in the spectrum because the radiation from the hot lower layers of the star's atmosphere, passing through the colder upper layers, is absorbed at certain wavelengths characteristic of certain atoms and molecules.

The absorption spectra of stars vary greatly; however, the intensity of the lines of any chemical element does not always reflect its true amount in the stellar atmosphere: to a much greater extent, the shape of the spectrum depends on the temperature of the stellar surface. For example, iron atoms are found in the atmosphere of most stars. However, the lines of neutral iron are absent in the spectra of hot stars, since all the iron atoms there are ionized. Hydrogen is the main component of all stars. But the optical lines of hydrogen are not visible in the spectra of cold stars, where it is underexcited, and in the spectra of very hot stars, where it is fully ionized. But in the spectra of moderately hot stars with a surface temperature of approx. At 10,000 K, the most powerful absorption lines are the lines of the Balmer series of hydrogen, which are formed during the transitions of atoms from the second energy level.

The gas pressure in the star's atmosphere also has some effect on the spectrum. At the same temperature, the lines of ionized atoms are stronger in low-pressure atmospheres, because there these atoms are less likely to capture electrons and therefore live longer. Atmospheric pressure is closely related to the size and mass, and hence to the luminosity of a star of a given spectral type. Having established the pressure from the spectrum, it is possible to calculate the luminosity of the star and, comparing it with the visible brightness, determine the "distance modulus" ( M - m) and the linear distance to the star. This very useful method is called the method of spectral parallaxes.

Color index.

The spectrum of a star and its temperature are closely related to the color index, i.e. with the ratio of the brightness of the star in the yellow and blue ranges of the spectrum. Planck's law, which describes the distribution of energy in the spectrum, gives an expression for the color index: C.I. = 7200/ T- 0.64. Cold stars have a higher color index than hot ones, i.e. cool stars are relatively brighter in yellow than in blue. Hot (blue) stars appear brighter on conventional photographic plates, while cool stars appear brighter to the eye and special photographic emulsions that are sensitive to yellow rays.

Spectral classification.

All the variety of stellar spectra can be put into a logical system. The Harvard spectral classification was first introduced in Henry Draper's catalog of stellar spectra, prepared under the guidance of E. Pickering (1846–1919). First, the spectra were sorted by line intensities and labeled with letters in alphabetical order. But the physical theory of spectra developed later made it possible to arrange them in a temperature sequence. The letter designation of the spectra has not been changed, and now the order of the main spectral classes from hot to cold stars looks like this: O B A F G K M. 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, indicated by numbers from 0 to 9. For example, the spectrum of type A5 is in the middle between A0 and F0. Additional letters sometimes mark the features of stars: “d” is a dwarf, “D” is a white dwarf, “p” is a peculiar (unusual) spectrum.

The most accurate spectral classification is the MK system created by W. Morgan and F. Keenan at the Yerkes Observatory. This is a two-dimensional system in which the spectra are arranged both by temperature and by the luminosity of stars. Its continuity with the one-dimensional Harvard classification is that the temperature sequence is expressed by the same letters and numbers (A3, K5, G2, etc.). But additional luminosity classes are introduced, marked with Roman numerals: Ia, Ib, II, III, IV, V and VI, respectively, indicating bright supergiants, supergiants, bright giants, normal giants, subgiants, dwarfs (main sequence stars) and subdwarfs. For example, the designation G2 V refers to a star like the Sun, while the designation G2 III indicates that it is a normal giant with a temperature about the same as that of the Sun.

Star sequences.

In 1905–1913, E. Hertzsprung in Denmark and G. Ressel in the USA independently found an empirical relationship between temperature (spectral type) and stellar luminosity. They found that most stars lie along a wide band on a temperature-luminosity diagram. This lane, called the "main sequence" runs from the upper left corner of the diagram, where hot and bright O and B stars are located, to the lower right corner, inhabited by cold and dim K and M dwarfs.

The discovery of the main sequence came as a surprise: it was not clear why stars with a certain surface temperature could not have any size, and therefore luminosity. It turned out that the radius of the star and the temperature of its surface are related to each other.

The second sequence was also found on the Hertzsprung-Russell diagram - a branch of giants, a wide strip extending from the middle of the main sequence (class G, absolute magnitude +1) almost perpendicular to it towards the upper right corner of the diagram (class M, absolute magnitude -1). On the giant branch lie stars of large size and fairly high luminosity, in contrast to the dwarfs that inhabit the main sequence. They are separated by the Hertzsprung Gap.

In the lower left corner of the diagram are white dwarfs - unusual stars with a high surface temperature, but low luminosity, indicating their very small size. In these remnants of the evolution of normal stars, thermonuclear reactions no longer occur, and they slowly cool down.

Several decades after the discovery by Hertzsprung and Russell, it became clear that the temperature–luminosity diagrams differ significantly for different groups of stars. This is especially clear when comparing star clusters, in each of which all stars have the same age. Diagrams of open clusters, such as the Hyades and Pleiades, are generally similar to those of circumsolar stars and differ sharply from diagrams of globular clusters, such as the large cluster in Hercules, where the bright part of the main sequence is absent, and its lower part merges with the giant branch, steeply going up, into the region of high luminosities. Such diagrams turned out to be characteristic of Population II stars, and diagrams of open clusters are typical of Population I stars. Thus, the Hertzsprung-Russell diagram serves as an important tool for elucidating the evolutionary status of stellar populations.

star clusters.

There are three distinct types of stellar groupings known: stellar associations, globular clusters, and open clusters (sometimes referred to as "open" or "galactic"). Star clusters are very valuable for astrophysics, since they are groups of stars equally distant from us and formed simultaneously from the substance of one cloud. Stars within the same cluster differ only in the initial mass, which greatly facilitates the study of their evolution.

star associations.

These are relatively sparse groupings of stars flying apart from a common center where they were probably born. If you trace their trajectories back, it turns out that they "set off" only about a million years ago - quite recently on a stellar scale. Associations are located in the spiral arms of the Galaxy, where the interstellar matter from which stars are formed is concentrated. Less than a hundred associations are known, and they all consist of young, bright and massive stars, mainly of spectral types O and B. Stars of lower mass are also present in associations, but they are more difficult to recognize. When the evolution of O and B stars ends in a few million years, it will become impossible to notice the currently known associations in the sky. Everything suggests that associations are short-lived formations. It is possible that most of the stars in the Galaxy were born precisely as part of associations.

scattered clusters.

Pleiades, Hyades, and Mangers are remarkable representatives of higher order star clusters. If in associations there are usually no more than 100 stars, then in open clusters - about 1000. More densely packed, they can withstand the destructive gravitational influence of the Galaxy much longer; for example, the age of the Pleiades cluster, determined by the form of its Hertzsprung-Russell diagram, ca. 50 million years. Even denser clusters can persist for hundreds of millions of years; one of the oldest open clusters, M 67, is also the densest of them. More than 1,000 open clusters are known, but many thousands more are probably hidden in the remote regions of the Galaxy.

globular clusters.

These clusters differ in many ways from open clusters and associations. About 150 globular clusters have been discovered so far, and that seems to be nearly all there is in the galaxy. It is difficult not to notice them: with a diameter of 40 to 900 St. years, they contain from 10,000 to several million stars. Such "monsters" are visible at great distances. In addition, they do not hide in the dusty disk of the Galaxy, but fill its entire volume, concentrating towards the galactic core.

Photographs of globular clusters such as M 13 in the constellation Hercules are impressive. In the center of the cluster, the stars seem to have merged into a single mess, although in reality the distances between them are not so small and collisions of stars practically do not occur. Each of the stars moves in an orbit around the center of the cluster, and it itself moves in an orbit around the center of the Galaxy.

Due to their large mass and density, globular clusters are very stable; they have existed almost unchanged for billions of years. Their stars were born during the formation of the Galaxy; they contain few heavy elements and belong to Population II. In our era, such stars are no longer formed.

Sources of stellar energy.

When Einstein's theory announced the equivalence of mass ( m) and energy ( E) related by the relation E = mc 2 , where c- the speed of light, it became clear that in order to maintain the radiation of the Sun with a power of 4x10 26 W, it is necessary to convert 4.5 million tons of its mass into radiation every second. By earthly standards, this value looks large, but for the Sun, which has a mass of 2x10 27 tons, such a loss remains imperceptible for billions of years.

The radiation of stars is maintained mainly due to two types of thermonuclear reactions. In massive stars, these are reactions of the carbon-nitrogen cycle, and in low-mass stars like the Sun, these are proton-proton reactions. In the first, carbon plays the role of a catalyst: it is not consumed itself, but contributes to the transformation of other elements, as a result of which 4 hydrogen nuclei are combined into one helium nucleus.

Expressed in atomic units, the masses of hydrogen and helium nuclei are 1.00813 and 4.00389, respectively. The four hydrogen nuclei (i.e., protons) have a mass of 4.03252 and are therefore 0.02863 AU, or 0.7%, greater than the mass of a helium nucleus. This difference turns into energetic gamma quanta, which, after being absorbed and emitted many times, gradually seep to the surface of the star and leave it in the form of light. Similar transformations of matter occur in the proton-proton reaction:

In principle, a great many other thermonuclear reactions are possible, but calculations show that at temperatures prevailing in the cores of stars, it is the reactions of these two cycles that occur most intensively and give the energy output exactly necessary to maintain the observed stellar radiation.

As you can see, a star is a natural setting for controlled thermonuclear reactions. If the same temperature and pressure of plasma are created in the terrestrial laboratory, then the same nuclear reactions will begin in it. But how to keep this plasma within the laboratory? After all, we do not have a material that would withstand the touch of a substance with a temperature of 10–20 million K without evaporating. And the star does not need this: its powerful gravity successfully resists the gigantic pressure of the plasma.

As long as the proton-proton reaction or carbon-nitrogen cycle proceeds in the star, it is on the main sequence, where it spends the bulk of its life. Later, when a helium core is formed at the star and the temperature in it rises, a “helium flash” occurs, i.e. the reactions of converting helium into heavier elements begin, also leading to the release of energy.

The structure of the stars.

It may seem impossible to know anything about the internal structure of stars. Not only distant stars, but also our Sun seems to be absolutely inaccessible for studying its depths. Nevertheless, we know as much about the structure of stars as we do about the structure of the Earth. The fact is that stars are gas balls, for the most part they are stable, not experiencing either collapse or expansion. Therefore, at any depth, the gas pressure is equal to the weight of the overlying layers, and the radiation flux is proportional to the temperature drop from the inner hot to the outer cold layers. These conditions, formulated in the form of mathematical equations, are sufficient to calculate the structure of a star based on the laws of gas behavior, i.e. change in pressure, temperature and density with depth. At the same time, only the mass, radius, luminosity, and chemical composition of a star need to be known from observations in order to theoretically determine its structure. Calculations show that in the center of the Sun the temperature reaches 16 million K, the density is 160 g/cm 3 , and the pressure is 400 billion atm.

The evolution of the stars.

A star begins its life as a cold, rarefied cloud of interstellar gas that contracts under its own gravity. When compressed, the gravitational energy turns into heat, and the temperature of the gas globule increases. In the last century, it was generally believed that the energy released during the compression of a star is sufficient to maintain its luminosity, but geological data came into conflict with this hypothesis: the age of the Earth turned out to be much longer than the time during which the Sun could maintain its radiation due to compression. (about 30 million years).

White dwarfs and neutron stars.

Shortly after a helium flash, carbon and oxygen "light up"; each of these events causes a strong rearrangement of the star and its rapid movement along the Hertzsprung-Russell diagram. The size of the star's atmosphere increases even more, and it begins to intensively lose gas in the form of expanding stellar wind streams. The fate of the central part of a star depends entirely on its initial mass: the core of a star can end its evolution as a white dwarf, a neutron star (pulsar), or a black hole.

Black holes.

In stars more massive than neutron star precursors, the cores experience complete gravitational collapse. As such an object is compressed, the force of gravity on its surface increases so much that no particles and even light can leave it - the object becomes invisible. In its vicinity, the properties of space-time change significantly; they can only be described by the general theory of relativity. Such objects are called black holes.

The duration of the evolution of stars.

Apart from some catastrophic episodes in the life of stars, human life is too short to notice the evolutionary changes of each particular star. Therefore, the evolution of stars is judged in the same way as the growth of trees in a forest, i.e. simultaneously observing many instances that are currently at different stages of evolution.

If we compare the Hertzsprung-Russell diagrams of various open clusters, we can easily understand which one is older. This is judged by the position of the break point of the main sequence, which marks the top of its preserved lower part. At the double cluster h and c Perseus, this point lies much higher than that of the Pleiades and Hyades clusters, therefore, it is much younger than them.

In conclusion, we point out that the age of the Sun is about 5 billion years, and at present it is in the middle of its evolutionary path. But if the initial mass of the Sun were only twice as high, then its evolution would have ended long ago, and life on Earth would not have had time to reach its peak in the form of a person. Cm.also GALAXIES; GRAVITATIONAL COLLAPSE; INTERSTELLAR MATTER; SUN.

Literature:

Theyler R. The structure and evolution of stars. M., 1973
Kaplan S.A. physics of stars. M., 1977
Shklovsky I.S. Stars. Their birth, life and death. M., 1984
Masevich A.G., Tutukov A.V. Evolution of stars: theory and observations. M., 1988
Bisnovaty-Kogan G.S. Physical processes of the theory of stellar evolution. M., 1989
Surdin V.G., Lamzin S.A. Protostars. Where, how and from what stars are formed. M., 1992

In 1948, G. Gamov (1904–1968), who emigrated from the USSR to the USA, put forward the hypothesis of the birth of the Universe as a result of big bang. Now this hypothesis is called hot universe theory. According to this theory, approximately 100 seconds after the Big Bang, which created space, time, matter and initiated the expansion and cooling of the Universe, thermonuclear reactions began to proceed in its rather hot substance containing protons and neutrons at a temperature of 10 9 K primary nucleosynthesis the lightest (not counting hydrogen) nuclei, as a result of which deuterium, tritium and helium nuclei began to form.

One million years after the birth of the Universe, a mixture of hydrogen and helium, obeying the law of universal gravitation, began to gather into clumps, from which the first stars and galaxies subsequently formed. According to G. Gamow's theory, the substance from which they were formed should have consisted of 75% hydrogen and 25% helium. According to modern estimates, the transition from a homogeneous hydrogen-helium Universe to a structural Universe with galaxies and stars lasted from 1 to 3 billion years, and the first stars could appear 200 million years after the birth of the Universe.

According to scientists, the formation of stars and galaxies in the expanding Universe was due to the existence of spatial inhomogeneity of matter, which arose from quantum fluctuations of matter at the birth of the Universe, and the gravitational instability of any uneven distribution of masses (a region of space with a higher density attracts surrounding masses and thus contributes to even greater its compaction).

Gas and dust cosmic clouds, from which stars arise, are unstable: small perturbations in their density can lead to a violation of gravitational equilibrium. Under the influence of the force of universal gravitation, the perturbations will increase, which will lead to the division of the cloud into separate fragments, each of which, under the influence of gravity, will begin to compress, forming protostar. Gradual compression of hydrogen-helium concentrations under the action of its own gravitational force leads to their heating to temperatures sufficient for the occurrence of thermonuclear fusion reactions. Further compression stops at the same time, because. it is now balanced by radiation, a star emerges from the bunch, and the thermonuclear stage of its evolution begins. About 90% of the stars in the visible Universe are at the stage of thermonuclear fusion of helium from hydrogen, because it is this stage of stellar evolution that is the longest in the active "life" of a star.

The birth of a star is usually hidden by cosmic dust that absorbs radiation from the stellar core. At the same time, the shell of dust heats up to hundreds of degrees and, in accordance with this temperature, shines itself in the infrared (IR) range. Therefore, only with the advent of IR photometry and radio astronomy became available for observation and study of phenomena in gas and dust clouds related to the birth of stars.

The matter spent on the formation of stars is partially returned to the interstellar medium during their explosions. Enriched with heavy elements synthesized in the interiors of stars or formed during their explosions, it can again be included in the process of star formation. Stars of different generations are distinguished depending on how many times the interstellar gas included in their composition participated in the formation of stars. Thus, the first stars in the Universe arose from a primordial gas containing only hydrogen (75% by mass) and helium (25% by mass). The stars of subsequent generations were formed from a gas containing the entire set of heavy elements. It is believed that the Sun is a third generation star. So everything in the solar system, including humans, is made up of the ashes of exploding stars. Planets have also been discovered around other stars: more than 100 of them are currently known. Planetary systems could have formed around stars of the second and subsequent generations from matter in which elements heavier than helium were present.

The range of characteristic stellar masses is 0.1M s –100M s (M s is the mass of the Sun). Most stars in the visible universe have a mass less than the Sun. In stars with a mass of M≤0.1M c, thermonuclear combustion of hydrogen is impossible, so they can only shine due to the gradual cooling of their matter. The detection of such stars is complicated by their low luminosity, so it is possible that some of the invisible matter in the Universe ( hidden mass), which can be detected only by their gravitational effect on neighboring objects, lies precisely in them. According to scientists, the matter directly observed in stars and gaseous nebulae is no more than 5% of the total mass of the Universe (while stars account for only 1% of the total mass of the Universe). Stars with M≥100M s are unstable.

The greater the mass of a star, the faster it depletes its nuclear fuel and the faster it ages. Therefore, massive stars with a mass approximately 100 times the mass of the Sun live only about 10 million years; stars with a mass several times greater than the solar mass - hundreds of millions of years; and stars with masses M ~ M c shine for about 10 billion years.

Stars can develop individually or in systems consisting of two or more stars.

A star that radiates by releasing nuclear energy slowly evolves as its chemical composition changes. It spends the most time at the stage when hydrogen burns in its central part. The long duration of this stage is due, in particular, to the fact that hydrogen is the most high-calorie nuclear fuel. When one helium nucleus (alpha particle) is formed from 4 hydrogen nuclei, approximately 26 MeV of energy is released, and when carbon 6 С 12 is formed from 3 alpha particles, only about 7.3 MeV is released, i.e. the release of energy per unit mass is 10 times less.

After the burning of hydrogen in the center of the star and the formation of a helium core, the release of nuclear energy in it stops, and the core begins to contract intensively. Hydrogen continues to burn in a thin shell surrounding the helium core. At the same time, the shell expands, the luminosity of the star increases, the surface temperature decreases, and the star becomes red giant(in the case of less massive stars) or supergiant (red or yellow) for more massive stars. The color of a star is determined by the temperature of its surface: the higher the surface temperature T, the higher the radiation frequency ν according to the formula

where h is Planck's constant and k is Boltzmann's constant. Therefore, red stars are the coldest, and blue stars are the hottest.

The process of subsequent stellar evolution is determined mainly by the mass of the star. The formation of elements heavier than magnesium is possible only in massive stars. The Sun, due to insufficient mass, will end its evolution at the stage of helium combustion. By the end of their lives, stars similar to the Sun shed their shell. (planetary nebula) and turn into white dwarfs, shrinking to the size of the Earth or less. A white dwarf is a hot star, but due to its small size it is almost invisible. In billions of years, the white dwarf should cool and turn into black dwarf that does not emit light. Thus, black dwarfs are the dead remnants of stars.

In massive stars, after the formation of iron, the gravitational contraction of the core is not maintained by the counterpressure of radiation, since as a result of nuclear reactions taking place at this stage, no energy is released. Elements heavier than iron are formed in the interior of stars when free neutrons or protons are captured by nuclei. This is how heavy nuclei are synthesized up to bismuth.

The temperature at the center of red supergiants can reach 10 10 K. At this temperature, the nuclei of atoms fall apart into protons and neutrons, protons absorb electrons, turning into neutrons and emitting neutrinos. As a rule, the evolution of such stars ends with a powerful explosion - a flash supernova. In 1987, scientists observed such an explosion in the galaxy Large Magellanic Cloud, located at a distance of 150 thousand light years from us. As a result of a supernova explosion, the state of a star changes dramatically: it either completely collapses or sheds its outer shell, and its furiously rotating (according to the law of conservation of angular momentum) neutron core turns under the influence of gravitational compression forces into neutron star, whose mass at a size of about 10 km can exceed the mass of the Sun. A neutron star is made up of neutron gas whose internal pressure opposes gravity and stops the star from collapsing. The enormous pressure forces of neutron matter are due to the fact that, according to the Pauli principle, neutrons that are fermions cannot be in the same energy state and therefore repel each other under strong compression.

The idea of the possibility of the existence of neutron stars in the Universe was first put forward by the Soviet physicist L.D. Landau (1908–1968) in 1932 after the discovery of the neutron. Rotating, neutron stars must emit electromagnetic radiation with impulses. Therefore they were called pulsars. In 1967, astronomers discovered the first neutron star located in the center crab nebula, which arose after a supernova explosion in 1054. The star periodically emitted radio waves. Single neutron stars usually manifest themselves as radio pulsars, and neutron stars in binary star systems - as X-ray sources. Losing energy to radiation, a neutron star should gradually slow down its rotation. As follows from theoretical calculations, the mass of a neutron star cannot exceed the mass of the Sun by more than 3-4 times.

The mechanism of the transition of star compression into an explosion, as a result of which the interstellar medium is enriched with heavy elements formed in the interiors of stars and during the explosion itself, is currently not completely clear.

If the mass of the core of a dying contracting star exceeds the mass of the Sun by 3 or more times, no force can stop the process of contraction. This was understood by scientists by the mid-60s of the twentieth century. Having calculated the structure of stars and the course of their evolution, they came to the conclusion that the existence of stable dead stars with a mass M>3M c is impossible. As the contraction progresses, the gravitational field will increase in strength, increasing the curvature of space according to the general theory of relativity and slowing down time near the star. When the star shrinks to gravity radius Rg

R g \u003d 2 GM / c 2 , (2)

where M is the mass of the star, G is the gravitational constant, c is the speed of light in vacuum, it will disappear from the visible universe, leaving only its gravitational field and turning into black hole. The superstrong gravitational pull of a black hole cannot be overcome by any known substance or radiation. Therefore, it is invisible (black).

The German astrophysicist K. Schwarzschild (1873–1916) was the first to find the exact solution of A. Einstein's equations of general relativity, which, as it turned out later, describes the space-time geometry near a black hole. He also calculated the critical radius to which mass must be compressed to become a black hole. This radius became known as the Schwarzschild radius, or gravitational radius. A black hole has no surface, there is only a region of space around it, determined by the gravitational radius and invisible to an external observer. This area is called event horizon. Any body or radiation, being near the event horizon, will only move inside the black hole. It is assumed that in black holes the universe hides most of its matter. If a material object falls into the gravitational field of a black hole, then it heats up to very high temperatures. Therefore, before the final disappearance in it, he throws out intense X-ray radiation into the Universe.

Black holes can be windows to other Universes, spaces and times, Universes can be born from them similarly to the emergence of our Universe from a superdense and hot state of matter. A well-known English scientist, chained by fate to a wheelchair, S. Hawking (b. 1924) put forward a hypothesis that black holes evaporate over time, radiating energy into the surrounding space.

So, according to the modern theory of stellar evolution, when dying, each star becomes either a white dwarf, or a neutron star, or a black hole. White dwarfs have been known for many decades and have long been considered the last stage in the evolution of any star. But then, as noted above, pulsars were discovered, which proved the real existence of neutron stars. Currently, scientists are looking for experimental confirmation of the existence of black holes in the Universe.

5. Search for black holes .

The search for black holes in space is a difficult task, because no information, including light, can escape from the surface of such objects. However, the gravitational field of a black hole exists in the Universe. Black holes absorb light rays passing near it, and deflect rays that travel at a considerable distance. Also, black holes can have a gravitational effect on other space objects: they can hold planets near them or form binary systems with other stars. The matter absorbed by the black hole is heated to very high temperatures and must emit powerful X-rays before disappearing into it.

To search for X-ray sources in space, the American Uhuru satellite was launched into near-Earth orbit in 1970, with the help of which astronomers discovered X-ray sources in many binary star systems. In most of these systems, the mass of the invisible part does not exceed 2 solar masses, i.e. is a neutron star. But there are binary stars with a mass of the invisible part, which is more than 3 times the mass of the Sun. It is assumed that in this case the dark component is a black hole.

The first candidate for black holes was the invisible X-ray source Cygnus-X1, located at a distance of 8000 light-years from Earth. This is a binary star system in which the visible part is a star with a mass of about 30 solar masses, and an invisible object has a mass of more than 6 solar masses.

There is a hypothesis that in the center of many galaxies there are black holes, the masses of which reach tens and hundreds of millions of solar masses. As a result of the fall of matter into a black hole, a huge amount of energy should be released. Astronomers have used the Hubble Space Telescope and NASA's Chandra X-ray Observatory, launched in 1999, to find evidence for black holes in galactic nuclei. As a result of observations of the huge elliptical galaxy M87, located at a distance of 50 million light-years from Earth in the constellation Virgo, it was found that in its center there is an ionized gas disk rotating at a tremendous speed (600 km/s) with a radius of about 3.5 pc (1 pc (parsec) is equal to 3.3 light years). It is assumed that only the gravity of an invisible object with a mass of 2-3 million solar masses could cause the gas to rotate at such a speed.

An X-ray image of the central region of the Milky Way was obtained using the Chandra space observatory. In Sagittarius A, located in this area, the most intense X-ray emission was recorded. During observations, the source of this radiation glowed brightly for several minutes, and then returned to the previous level within 3 hours. Scientists attribute the rapid changes in X-ray power to the fact that the flash was caused by the approach of matter to the black hole.

In addition, stars moving at speeds of more than 1000 km/s have been found in the core of the Milky Way. In a region with a radius of 0.1 pc around Sagittarius A, an increase in the velocities of stars is observed as it approaches the center. Such high speeds can only be explained by the fact that Sagittarius A is a black hole with a mass equal to 2.6 10 6 M s .

The existence of a black hole in the center of our Galaxy does not pose a danger to the Earth because of its great remoteness. But since the black hole feeds on stellar and other matter, it can swallow the entire galaxy. But before it reaches the solar system, it will have to swallow at least 100 billion stars in the Milky Way.

One of the candidates for black holes travels through our galaxy. It was discovered in 2000. Scientists believe that this is a massive binary star system in which a black hole absorbs the matter of a neighboring star. It was possible to determine the orbit of this object. The distance between it and the Sun is now 6000 light years.

In 1999, with the help of the Chandra observatory, a powerful X-ray source was discovered, located at a distance of 2.5 billion light years from Earth in the center of one of the galaxies in the constellation Hydra. It is also believed to be a black hole.

The most powerful sources of electromagnetic radiation in the universe are discovered in 1963 by quasars are quasi-stellar radio sources. They are larger than stars but smaller than galaxies. The diameter of the quasar is approximately several light weeks, and the mass is more than 10 6 M s . Most quasars are located at distances of 10–15 billion light years from the Earth, i.e. at the edge of the visible universe. Therefore, we see them as they were when the Universe was just beginning to form. The luminosity of a quasar can be equivalent to the radiation of dozens of galaxies. Currently, thousands of quasars have been discovered. They are characterized by powerful motions of gas and ejections of jets of matter (jets) at a speed close to the speed of light. There is a hypothesis that quasars are giant black holes with a mass of about 100 million solar masses, located in the dense cores of galaxies. Such massive black holes should destroy and capture stars orbiting in their immediate vicinity. This is confirmed by the change in the luminosity of quasars with a characteristic period of less than one day.

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Vconducting

The overwhelming majority of stars change their main characteristics (luminosity, radius) very slowly. At any given moment, they can be considered as being in a state of equilibrium - a circumstance that we have widely used to clarify the nature of stellar interiors. But the slowness of change does not mean the absence of it. It's all about the timing of evolution, which for stars should be completely inevitable.

The problem of stellar evolution is undoubtedly one of the most fundamental problems of astronomy. Essentially, the question is how stars are born, live, "age" and die. This problem is, by its very nature, complex. It is solved by purposeful studies of representatives of various branches of astronomy - observers and theorists. After all, by studying the stars, it is impossible to immediately say which of them are genetically related. In general, this problem turned out to be very difficult and for several decades did not lend itself to solution at all.

Gradually, the question of the ways of the evolution of stars became clear, although individual details of the problem are still far from being solved. Particular merit in understanding the process of stellar evolution belongs to theoretical astrophysicists, specialists in the internal structure of stars, and above all to the American scientist M. Schwarzschild and his school.

1. The concept of star evolution

evolution star gravitational contraction

The evolution of stars is the change in the physical characteristics, internal structure, and chemical composition of stars over time. The most important tasks of the theory of stellar evolution are the explanation of the formation of stars, changes in their observed characteristics, the study of the genetic relationship of various groups of stars, and the analysis of their final states.

Since in the part of the Universe known to us about 98-99% of the mass of the observed matter is contained in stars or has passed the stage of stars, the explanation of the evolution of stars is one of the most important problems of astrophysics.

A star in a stationary state is a gaseous ball that is in hydrostatic and thermal equilibrium (i.e., the action of gravitational forces is balanced by internal pressure, and energy losses for radiation are compensated by the energy released in the interior of a star. The "birth" of a star is the formation of a hydrostatically equilibrium an object whose radiation is maintained by its own energy sources.The "death" of a star is an irreversible imbalance leading to the destruction of the star or to its catastrophic compression.

To understand the evolution of stars, the question of the sources of their energy is of fundamental importance. Energy losses due to radiation from the surface can be replenished by cooling the interior, releasing gravitational potential energy during compression, and nuclear reactions. Cooling and gravitational contraction are able, for example, to maintain the luminosity of the Sun (mass g, luminosity erg/s) for ~ 10 7 years, stars with masses of 30 and - for ~ 10 5 years, and nuclear reactions, respectively, ~ 10 6 years. Geological evidence suggests that the Sun's luminosity has been virtually unchanged for ~109 years. It follows that only nuclear reactions can be the main source of energy.

The release of gravitational energy can play a decisive role only when the temperature of the interior of the star is insufficient for the nuclear energy release to compensate for energy losses, and the star as a whole or part of it must contract to maintain equilibrium. The illumination of thermal energy becomes important only after the depletion of nuclear energy reserves. Thus, the evolution of stars can be represented as a successive change of energy sources of stars.

The characteristic time of stellar evolution is too long to trace the entire evolution directly. Therefore, the main method for studying the evolution of stars is the construction of sequences of stellar models that describe changes in the internal structure and chemical composition. composition of stars over time. The evolutionary sequences are then compared with the results of observations, for example, with the Hertzsprung-Russell diagram (H.-R.d.), summarizing the observations of a large number of stars at different stages of evolution. Of particular importance is the comparison with G.-R.d. for star clusters, since all cluster stars have the same initial chem. composition and formed almost simultaneously. According to G.-R.d. clusters of different ages, it was possible to establish the direction of stellar evolution. Evolutionary sequences are calculated in detail by numerically solving a system of differential equations describing the distribution of mass, density, temperature, and luminosity over the star, to which are added the equation of state, the laws of energy release and opacity of stellar matter, and equations describing the change in the chemical composition of the star with time.

The evolution of a star depends mainly on its mass and initial chemical composition. The rotation of a star and its magnetic field can play a certain, but not fundamental, role, but the role of these factors in the evolution of stars has not yet been sufficiently studied. The chemical composition of a star depends on the time it was formed and on its position in the galaxy at the time of formation. First-generation stars formed from matter whose composition was determined by cosmological conditions. Apparently, it contained approximately 70% by mass of hydrogen, 30% of helium, and a negligible admixture of deuterium and lithium. During the evolution of the first generation of stars, heavy elements (following helium) were formed, which were thrown into interstellar space as a result of the outflow of matter from stars or during star explosions. The stars of subsequent generations were already formed from matter containing up to 3-4% (by mass) of heavy elements.

The most direct indication that star formation in the Galaxy is still going on at the present time is the existence of massive bright stars of spectral types O and B, whose lifetime cannot exceed ~10 7 years. The rate of star formation in the modern era is estimated at 5 per year.

2. Star formation, stage of gravitational contraction

According to the most common point of view, stars are formed as a result of gravitational condensation of matter in the interstellar medium. The necessary separation of the interstellar medium into two phases - dense cold clouds and a rarefied medium with a higher temperature - can occur under the influence of the Rayleigh-Taylor thermal instability in the interstellar magnetic field. Gas-dust complexes with mass, characteristic size (10-100) pc and particle concentration n~10 2 cm -3 . actually observed due to their emission of radio waves. The compression (collapse) of such clouds requires certain conditions: the gravitational binding energy of the particles of the cloud must exceed the sum of the energy of the thermal motion of the particles, the rotational energy of the cloud as a whole, and the magnetic energy of the cloud (the Jeans criterion). If only the energy of thermal motion is taken into account, then, up to a factor of the order of unity, the Jeans criterion is written as: , where is the mass of the cloud, T is the gas temperature in K, n is the number of particles in 1 cm 3 . At temperatures K typical of modern interstellar clouds, only clouds with a mass no less than that can collapse. The Jeans criterion indicates that for the formation of stars with a really observed mass spectrum, the concentration of particles in collapsing clouds should reach (10 3 -10 6) cm -3 , i.e. 10-1000 times higher than observed in typical clouds. However, such concentrations of particles can be achieved in the depths of clouds that have already begun to collapse. It follows from this that star formation occurs by successive, carried out in several stages, fragmentation of massive clouds. This picture naturally explains the birth of stars in groups - clusters. At the same time, issues related to the heat balance in the cloud, the velocity field in it, and the mechanism that determines the mass spectrum of fragments still remain unclear.

Collapsing stellar mass objects are called protostars. The collapse of a spherically symmetric non-rotating protostar without a magnetic field includes several stages. At the initial moment of time, the cloud is homogeneous and isothermal. It is transparent to the public. radiation, so the collapse occurs with volumetric energy losses, mainly due to the thermal radiation of the dust, to which the gas particles transfer their kinetic energy. In a homogeneous cloud there is no pressure gradient and compression begins in the free fall regime with a characteristic time, where G is the gravitational constant and is the density of the cloud. With the onset of compression, a rarefaction wave arises, moving towards the center at the speed of sound, and since the collapse occurs faster where the density is higher, the protostar is divided into a compact core and an extended shell, in which the matter is distributed according to the law. When the concentration of particles in the core reaches ~ 10 11 cm -3, it becomes opaque for the IR radiation of dust particles. The energy released in the core slowly seeps to the surface due to radiant heat conduction. The temperature begins to increase almost adiabatically, this leads to an increase in pressure, and the core comes into a state of hydrostatic equilibrium. The shell continues to fall on the core, and a shock wave appears on its periphery. The parameters of the core at this time weakly depend on the total mass of the protostar:

As the mass of the nucleus increases due to accretion, its temperature changes almost adiabatically until it reaches 2000 K, when the dissociation of H 2 molecules begins. As a result of energy consumption for dissociation, and not for an increase in the kinetic energy of particles, the value of the adiabatic index becomes less than 4/3, pressure changes are not able to compensate for gravitational forces, and the core collapses again. A new core with parameters is formed, surrounded by a shock front, onto which the remnants of the first core are accreted. A similar rearrangement of the nucleus occurs during the ionization of hydrogen.

Further growth of the core due to the material of the shell continues until all the matter falls on the star or is dispersed under the action of radiation pressure or stellar wind, if the core is massive enough. For protostars with a characteristic accretion time of shell matter t a >t kn, so their luminosity is determined by the energy release of contracting cores.

A star consisting of a core and a shell is observed as an IR source due to the processing of radiation in the shell (the dust of the shell, absorbing photons of UV radiation from the core, radiates in the IR range). When the shell becomes optically thin, the protostar begins to be observed as an ordinary object of stellar nature. In the most massive stars, the shells are preserved until the onset of thermonuclear burning of hydrogen in the center of the star. Radiation pressure limits the mass of stars to a value, probably . Even if more massive stars are formed, they turn out to be pulsationally unstable and can lose a significant part of their mass at the stage of hydrogen combustion in the core. The duration of the stage of collapse and scattering of the protostellar shell is of the same order as the time of free fall for the parent cloud, i.e. 10 5 -10 6 years. The clumps of dark matter of the remnants of the shell illuminated by the core, accelerated by the stellar wind, are identified with Herbig-Haro objects (star-shaped clumps with an emission spectrum). Stars of low mass, when they become visible, are in the G.-R.D. region occupied by T Tauri type stars (dwarf flare stars), more massive ones are in the region where Herbig emission stars (irregular variable stars of early spectral classes with emission lines in the spectra).

The evolutionary tracks of the cores of constant-mass protostars at the stage of hydrostatic compression are shown in Figs. 1. In low-mass stars, at the moment when hydrostatic equilibrium is established, the conditions in the cores are such that the energy in them is transferred by convection. Calculations show that the surface temperature of a fully convective star is almost constant. The radius of the star is continuously decreasing, because. she keeps shrinking. At a constant surface temperature and a decreasing radius, the luminosity of the star should also fall on the G.-R.d. this stage of evolution corresponds to the vertical segments of the tracks.

As the compression continues, the temperature in the interior of the star rises, the matter becomes more transparent, and stars with radiant cores appear, but the shells remain convective. Less massive stars remain fully convective. Their luminosity is regulated by a thin radiant layer in the photosphere. The more massive the star and the higher its effective temperature, the larger its radiant core (in stars with a radiant core, it appears immediately). In the end, almost the entire star (with the exception of the surface convective zone for stars with mass) goes into a state of radiative equilibrium, in which all the energy released in the core is transferred by radiation.

3 . Evolution based on nuclear reactions

At a temperature in the nuclei of ~ 10 6 K, the first nuclear reactions begin - deuterium, lithium, and boron burn out. The primary amount of these elements is so small that their burnout practically does not withstand compression. Compression stops when the temperature in the center of the star reaches ~ 10 6 K and hydrogen ignites, because the energy released during the thermonuclear combustion of hydrogen is sufficient to compensate for radiation losses. Homogeneous stars, in the cores of which hydrogen burns, form on G.-R.d. initial main sequence (NGS). Massive stars reach NGP faster than low-mass stars, because their rate of energy loss per unit mass, and hence the rate of evolution, is higher than that of low-mass stars. From the moment of entering the NGP, the evolution of stars proceeds on the basis of nuclear burning. Nuclear combustion can occur before the formation of elements of the iron group, which have the highest binding energy among all nuclei. Evolutionary tracks of stars on G.-R.d. shown in fig. 2. The evolution of the central values of temperature and density of stars is shown in fig. 3. At K, the main source of energy is the reaction of the hydrogen cycle, at larger T - the reaction of the carbon-nitrogen (CNO) cycle. A side effect of the CNO cycle is the establishment of equilibrium concentrations of nuclides 14 N, 12 C, 13 C - respectively 95%, 4% and 1% by weight. The predominance of nitrogen in the layers where hydrogen combustion occurred is confirmed by the results of Wolf-Rayet observations of stars in which these layers appear on the surface as a result of the loss of ext. layers. Stars, in the center of which the CNO cycle () is realized, have a convective core. The reason for this is the very strong dependence of the energy release on temperature: . The flux of radiant energy is ~ T 4 , therefore, it cannot transfer all the released energy, and convection must arise, which is more efficient than radiative transfer. In the most massive stars, more than 50% of the stellar mass is covered by convection. The significance of the convective core for evolution is determined by the fact that the nuclear fuel is depleted uniformly in a region much larger than the region of effective combustion, while in stars without a convective core it initially burns out only in a small neighborhood of the center, where the temperature is sufficiently high. The burnup time of hydrogen is in the range from ~ 10 10 years for to years for. The time of all subsequent stages of nuclear burning does not exceed 10% of the hydrogen burning time, therefore, stars at the hydrogen burning stage form on the G.-R.d. densely populated area - the main sequence (MS). For stars with a temperature in the center never reaches the values necessary for the ignition of hydrogen, they shrink indefinitely, turning into "black" dwarfs. Hydrogen burnout leads to an increase in avg. the molecular mass of the substance of the nucleus, and therefore, in order to maintain hydrostatic equilibrium, the pressure in the center must increase, which entails an increase in the temperature in the center and the temperature gradient along the star, and, consequently, the luminosity. A decrease in the opacity of a substance with increasing temperature also leads to an increase in luminosity. The core contracts to maintain the conditions for nuclear energy release with a decrease in the hydrogen content, and the shell expands due to the need to transfer the increased energy flux from the core. On G.-R.d. the star moves to the right of the NGP. A decrease in opacity leads to the death of convective cores in all stars, except for the most massive ones. The rate of evolution of massive stars is the highest, and they are the first to leave the MS. The lifetime on the MS is about 10 million years for stars, about 70 million years for c, and about 10 billion years for c.

When the hydrogen content in the core decreases to 1%, the expansion of the stellar shells is replaced by the general compression of the star, which is necessary to maintain the energy release. The compression of the shell causes heating of hydrogen in the layer adjacent to the helium core to the temperature of its thermonuclear combustion, and a layer source of energy release appears. For stars with a mass that depends less on temperature and the region of energy release is not so strongly concentrated towards the center, the general contraction stage is absent.

The evolution of stars after hydrogen burnout depends on their mass. The most important factor influencing the course of evolution of stars with mass is the degeneracy of the electron gas at high densities. In a degenerate gas, due to the high density, the number of low-energy quantum states is limited due to the Pauli principle, and electrons fill quantum levels with high energy, much higher than the energy of their thermal motion. The most important feature of a degenerate gas is that its pressure p depends only on the density: for nonrelativistic degeneracy and for relativistic degeneracy. The electron gas pressure is much greater than the ion pressure. From this follows a fundamental conclusion for the evolution of stars: since the gravitational force acting on a unit volume of a relativistically degenerate gas depends on the density in the same way as the pressure gradient, there must be a limiting mass such that at , the electron pressure cannot counteract gravity and compression begins. Limit mass. The boundary of the region in which the electron gas is degenerate is shown in Fig. 3 . In low-mass stars, degeneracy plays an appreciable role already in the process of formation of helium nuclei.

The second factor that determines the evolution of stars in the later stages is neutrino energy losses. In stellar interiors at T ~ 10 8 K, the main role in the birth of neutrinos is played by: the photoneutrino process, the decay of plasma oscillation quanta (plasmons) into neutrino-antineutrino pairs (), annihilation of electron-positron pairs () and Urca processes. The most important feature of neutrinos is that the matter of the star is practically transparent for them, and neutrinos freely carry away energy from the star.

The helium core, in which the conditions for helium combustion have not yet arisen, is compressed. The temperature in the layered source adjacent to the core increases, and the hydrogen burning rate increases. The need to transfer the increased energy flow leads to the expansion of the shell, for which part of the energy is spent. Since the luminosity of the star does not change, the temperature of its surface drops, and on G.-R.d. the star moves into the region occupied by red giants. The restructuring time of the star is two orders of magnitude shorter than the hydrogen burnout time in the core; therefore, there are few stars between the MS band and the region of red supergiants. With a decrease in the temperature of the shell, its transparency increases, as a result of which an ext. convective zone and the luminosity of the star increases.

The removal of energy from the core through the thermal conduction of degenerate electrons and neutrino losses in stars delays the time of helium ignition. The temperature begins to rise noticeably only when the core becomes almost isothermal. The burning of 4 He determines the evolution of stars from the moment when the energy release exceeds the energy loss through thermal conduction and neutrino emission. The same condition applies to the combustion of all subsequent types of nuclear fuel.

A notable feature of neutrino-cooled stellar cores from degenerate gas is "convergence" - the convergence of tracks that characterize the ratio of density and temperature T c at the center of the star (Fig. 3). The rate of energy release during compression of the core is determined by the rate of attachment of matter to it through a layer source, which depends only on the mass of the core for a given type of fuel. A balance of inflow and outflow of energy must be maintained in the core, so the same distribution of temperature and density is established in the cores of stars. By the time of ignition of 4 He, the mass of the nucleus depends on the content of heavy elements. In degenerate gas nuclei, the ignition of 4 He has the character of a thermal explosion, since the energy released during combustion goes to increase the energy of the thermal motion of electrons, but the pressure almost does not change with increasing temperature until the thermal energy of the electrons is equal to the energy of the degenerate gas of electrons. Then the degeneracy is removed and the core rapidly expands - a helium flash occurs. Helium flashes are probably accompanied by the loss of stellar matter. In globular star clusters, where massive stars have long since completed their evolution and red giants have masses, stars at the helium burning stage are on the horizontal branch of the G.-R.d.

In helium cores of stars with gas is not degenerate, 4 He ignites quietly, but the cores also expand due to the increase in T c . For the most massive stars, 4 He ignites even when they are blue supergiants. The expansion of the core leads to a decrease in T in the region of the hydrogen layer source, and the luminosity of the star after the helium flash decreases. To maintain thermal equilibrium, the shell contracts, and the star leaves the red supergiant region. When 4 He in the core is depleted, the compression of the core and the expansion of the shell begin again, the star again becomes a red supergiant. A layered 4 He combustion source is formed, which dominates the energy release. Outside appears again. convective zone. As helium and hydrogen burn out, the thickness of the layered sources decreases. A thin layer of helium combustion turns out to be thermally unstable, because with a very strong sensitivity of energy release to temperature (), the thermal conductivity of the substance is insufficient to extinguish thermal perturbations in the combustion layer. During thermal flashes, convection occurs in the layer. If it penetrates into layers rich in hydrogen, then as a result of the slow process of neutron capture (s-process), elements with atomic masses from 22 Ne to 209 B are synthesized.

The radiation pressure on the dust and molecules formed in the cold extended shells of red supergiants leads to a continuous loss of matter at a rate of up to a year. The continuous loss of mass can be supplemented by losses due to the instability of stratified combustion or pulsations, which can lead to the ejection of one or more shells. When the amount of matter above the carbon-oxygen core becomes less than a certain limit, the shell to maintain the temperature in the combustion layers is forced to contract as long as the compression is able to support combustion; star on G.-R.d. shifts almost horizontally to the left. At this stage, the instability of the combustion layers can also lead to expansion of the shell and loss of matter. As long as the star is hot enough, it is observed as the core of a planetary nebula with one or more envelopes. When layer sources are displaced to the surface of the star so that the temperature in them becomes below that necessary for nuclear combustion, the star cools, turning into a white dwarf c, which radiates due to the consumption of thermal energy of the ionic component of its substance. The characteristic cooling time for white dwarfs is ~109 years. The lower limit on the masses of single stars turning into white dwarfs is unclear, it is estimated at 3-6 . In stars with electron gas degenerates at the stage of growth of carbon-oxygen (C,O-) stellar cores. As in the helium cores of stars, due to neutrino energy losses there is a "convergence" of conditions in the center and by the time carbon is ignited in the C,O core. The ignition of 12 C under such conditions most likely has the character of an explosion and leads to the complete destruction of the star. Complete destruction may not occur if. Such a density is achievable when the core growth rate is determined by the accretion of the satellite's matter in a close binary system.

4 . Stages of stellar evolution

This process is natural, that is, inevitable. Indeed, the thermal instability of the interstellar medium inevitably leads to its fragmentation, that is, to its division into separate, relatively dense clouds and the intercloud medium. However, its own gravity cannot compress the clouds - for this they are not dense enough and large enough. But this is where the interstellar magnetic field comes into play. In the system of lines of force of this field, fairly deep "pits" are inevitably formed, into which clouds of the interstellar medium "stack". This leads to the formation of huge gas-dust complexes. In such complexes, a layer of cold gas is formed, since the ultraviolet radiation of stars, which ionizes interstellar carbon, is strongly absorbed by cosmic dust located in a dense complex, and neutral carbon atoms greatly cool the interstellar gas and "thermostat" it at a very low temperature - about 5-10 degrees Kelvin. Since the gas pressure in the cold layer is equal to the external pressure of the surrounding hotter gas, the density in this layer is much higher and reaches several thousand atoms per cubic centimeter. Under the influence of its own gravity, the cold layer, after it reaches a thickness of about one parsec, will begin to “fragment” into separate, even denser clumps, which, under the influence of its own gravity, will continue to shrink. In this quite natural way, associations of protostars arise in the interstellar medium. Each such protostar evolves at a rate that depends on its mass.

When a significant part of the gas mass turns into stars, the interstellar magnetic field, which supported the gas-dust complex with its pressure, will naturally not affect the stars and young protostars. Under the influence of the gravitational attraction of the Galaxy, they will begin to fall towards the galactic plane. Thus, young stellar associations must always approach the galactic plane.

Not so long ago, astronomers believed that it took millions of years to form a star from interstellar gas and dust. But in recent years, striking photographs have been taken of a region of the sky that is part of the Great Nebula of Orion, where a small cluster of stars has appeared over the course of several years. Photographs from 1947. a group of three star-like objects was visible at this location. By 1954 some of them became oblong, and by 1959. these oblong formations broke up into separate stars - for the first time in the history of mankind, people observed the birth of stars literally before our eyes. This case showed astronomers that stars can be born in a short period of time, and the arguments that seemed strange earlier that stars usually appear in groups, or star clusters, turned out to be true.

What is the mechanism of their occurrence? Why, after many years of astronomical visual and photographic observations of the sky, is it only now for the first time that it was possible to see the “materialization” of stars? The birth of a star cannot be an exceptional event: in many parts of the sky there are conditions necessary for the appearance of these bodies.

As a result of careful study of photographs of the foggy regions of the Milky Way, it was possible to find small black spots of irregular shape, or globules, which are massive accumulations of dust and gas. They look black because they do not emit their own light and are between us and the bright stars, the light from which they obscure. These gas and dust clouds contain dust particles that absorb light from the stars behind them. The size of the globules is huge - up to several light years in diameter. Despite the fact that the matter in these clusters is very rarefied, their total volume is so large that it is quite enough to form small clusters of stars close in mass to the Sun. In order to imagine how stars arise from globules, we recall that all stars radiate and their radiation exerts pressure. Sensitive instruments have been developed that respond to the pressure of sunlight penetrating through the thickness of the earth's atmosphere. In a black globule, under the influence of radiation pressure emitted by surrounding stars, the matter is compressed and compacted. Inside the globule, the wind “walks”, scattering gas and dust particles in all directions, so that the substance of the globule is in continuous turbulent motion.

A globule can be considered as a turbulent gas-dust mass, which is pressed by radiation from all sides. Under the influence of this pressure, the volume filled with gas and dust will be compressed, becoming smaller and smaller. Such compression proceeds for some time, depending on the sources of radiation surrounding the globule and the intensity of the latter. The gravitational forces arising from the concentration of mass in the center of the globule also tend to compress the globule, causing matter to fall towards its center. Falling, the particles of matter acquire kinetic energy and heat up the gas-dust cloud.

The fall of matter can last hundreds of years. At first, it occurs slowly, unhurriedly, since the gravitational forces that attract particles to the center are still very weak. After some time, when the globule becomes smaller and the gravitational field increases, the fall begins to occur faster. But, as already known, the globule is huge, no less than a light year in diameter. This means that the distance from its outer border to the center can exceed 10 trillion kilometers. If a particle from the edge of the globule starts to fall towards the center at a speed slightly less than 2 km/s, then it will reach the center only after 200,000 years. Observations show that the speed of movement of gas and dust particles is actually much higher, and therefore gravitational compression is much faster.

The fall of matter towards the center is accompanied by very frequent collisions of particles and the transition of their kinetic energy into thermal energy. As a result, the temperature of the globule increases. The globule becomes a protostar and begins to glow, as the energy of the movement of particles turned into heat, heated the dust and gas.

At this stage, the protostar is barely visible, since the main part of its radiation falls on the far infrared region. The star has not yet been born, but its embryo has already appeared. Astronomers don't yet know how long it takes for a protostar to reach the point where it glows like a dim red ball and becomes visible. According to various estimates, this time ranges from thousands to several million years. However, bearing in mind the appearance of stars in the Great Nebula of Orion, it is perhaps worth considering that the estimate that gives the minimum value of time is closest to reality.

Conclusion

The evolution of stars is the change in the physical characteristics, internal structure, and chemical composition of stars over time.

The modern theory of stellar evolution is capable of explaining the general course of stellar development and is in satisfactory qualitative and quantitative agreement with observational data. In the future, the theory should take into account the influence of rotation and magnetic fields, the role of which can be especially important in the process of star formation and at fast stages of evolution, such as, for example, supernova explosions. A special problem is the evolution of stars in close binary systems, where the evolution is affected by the exchange of matter between the components.

Bibliography:

1. Zeldovich, Ya.B. The theory of gravitation and the evolution of stars / Ya.B. Zeldovich, I.D. Novikov. - M.: Nauka, 1971. - 484 p.

2. Kaplan, S.A. Physics of stars / S.A. Kaplan. - 3rd ed., add. and reworked. - M.: Nauka, 1977. - 208 p.

3. At the forefront of astrophysics: Per. from English. / Ed. Y. Evretta. - M.: Mir, 1979. - 576 p.

4. Origin and evolution of galaxies and stars, Ed. S.B. Pikelner. M.: Nauka, 1976. - 408 p.

5. Shklovsky, I.S. Universe, life, mind / I.S. Shklovsky. - M.: Nauka, 1976. - 336 p.

6. Shklovsky, I.S. Stars: their birth, life and death / I.S. Shklovsky. - 3rd ed., revised. - M.: Nauka, Main edition of physical and mathematical literature, 1984. - 384 p.

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The structure and development of stars briefly. Abstract: Structure, origin and evolution of galaxies and stars

Star designations.

star catalogs.

variable stars.

Distances to the stars.

Parallax.

The luminosity of the stars.

Star magnitudes.

Star sizes.

Star populations.

Star movements.

Radial speed.

Spatial speed.

Double stars.

variable stars.

Star colors.

Spectra of stars.

Color index.

Spectral classification.

Star sequences.

star clusters.

star associations.

scattered clusters.

globular clusters.

Sources of stellar energy.

The structure of the stars.

The evolution of the stars.

White dwarfs and neutron stars.

Black holes.

The duration of the evolution of stars.

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