One of the major achievements of astronomy in the twentieth. can be considered as establishing the fact, the process of star formation occurs constantly (in our time). Installed, that many of the observed stars are younger than our planet, but some formed more recently, when the Earth already existed people.

Most scientists believe, the stars are formed by condensation of the rarefied gas and dust clouds of the interstellar medium, of which under the action of gravitational forces formed a dense opaque ball of gas. Initially the gas pressure inside this relatively cold ball can't balance the gravitational force, which continue to compress. But as the compression temperature of the star increases subsoil and in the end turns out to be sufficient for, so there began a fusion reaction. The pressure of hot gas inside the future stars balances the gravitational force and compression stops. This entire process lasts a relatively short time — from several million to several hundred million years (depending on the mass of the star).

The light from the star is due to the thermonuclear reactions, flowing in the Central part of the star. The duration of this stage in the life of a star also depends on its mass. The mass determines, how quickly a star uses available stocks of hydrogen, turning into helium. So, hot stars-giants, mass of 10-20 times the mass of the Sun, spend their "nuclear fuel" for a few million years, and our Sun and the other stars of this mass emit steadily for 10-15 billion. years.

However, in the end, in the core of stars hydrogen is no more, the energy release is stopped and gravitational forces start to compress the core. Now, thermonuclear reactions can only go in a relatively thin layer at the boundary of the nucleus. The luminosity of a star and its size should grow. The process of the evolution of stars much faster, and it turns into a red giant. When the temperature of the shrinking helium core reaches 100-150 million. degrees, reaction begins new type: three helium nuclei are formed the nucleus of carbon. Calculations show, what our Sun will become a red giant using 8 billion. years and will remain for several hundred million years. The luminosity of the Sun must increase in hundreds times, and the radius is ten times compared to today.

These giant stars will quickly Deplete the reserves of nuclear fuel, as well as losing a significant portion of its mass or gradually, either as a result of, that shed their outer shell. At the final stage of its development, stars, mass close to solar, turn into white dwarfs. In this case, from the star, only its Central dense part, which has ceased nuclear reactions. Such stars gradually cool down, their radiation is reduced, and they become invisible. Their sizes are less than the size of the Earth, but since the mass comparable to the mass of the Sun, the density of matter millions of times greater than the density of water.

However, not all stars are like this, relatively calm the path of evolution. For some of them characterized by catastrophic changes in their development process. In these cases speak about the outbreak of a supernova, which leads to very significant changes in the structure of stars. In place of the outbursts of such stars detected special nebula (rice. 40.4), all of which are powerful sources of radio emission. When the most powerful flares, the mass of the ejected gas can be several times greater than the mass of the Sun. If the remaining after the flash of the stars has a lot more 1,5 the mass of the Sun, it cannot become a white dwarf. Gravitational forces compress it to much smaller. The diameter of such objects — about 10 km, the average density is about 10¹⁸ kg/m3, t. e. higher than the density of an atomic nucleus. These stars are called neutron stars, because when the density of the substance consists only of neutrons, formed by the merger of protons and electrons.

The theory of such stars was developed in the 30-ies of our century Soviet physicist academician L. D. Landau, but discovered neutron stars were only 1967 g. They were discovered as sources of radio emission with a strictly periodic short (in the order of seconds and fractions of a second) pulses. The reason for the strict periodicity of the radio pulses and pulses, who discovered pulsars in the optical range, is their rapid rotation. The shortest period of optical and radio pulses has a pulsar, opened in the famous crab nebula (rice. 40.4), which is on the site of the Supernova explosion of a star 1054 g. Period this is only 0,033 with.

Even more surprising objects must occur in the last stage of the evolution of stars, if after exhausting of the reserves of nuclear fuel, its mass will exceed the critical, t. e. 1,5— 3 the mass of the Sun. In this case, the pressure of the so-called degenerate gas, making up stars at the last stage of its evolution, unable to hold back the gravitational forces compress it. The star will close, Contracting with great speed. Its mass will remain unchanged, and the speed Vпар, you must have a body, to leave its surface (the so-called parabolic or second space), will grow. Once an object reaches the radius, in which Vпар≈c, its surface will not leave any particles, no radiation. Therefore, such objects have been called black holes. They are not visible, but interact with the external world through gravitational forces. So are their search among the so-called double stars, which are bound by gravitation and revolve around a common center of mass. Scientists believe, one of the components of the double star X Cygni is a black hole, and I hope to find more such objects.

The origin of the planets. Although the planetary systems are not just from the Sun, but the other stars, currently these planets are inaccessible to observation, even in the best telescopes. Therefore, all conclusions about the origin and development of planets have to do based on the study of only one example — Solar system.

The basis for all modern hypotheses about the origin of the Earth and planets based on the idea of forming them from the dust cloud. The majority of scholars is inclined to the conclusion that, the formation of the Sun and planets out of the cloud occurred at the same time. The cloud had the composition, roughly similar to the modern composition of the Sun, and was 98% of hydrogen and helium and only 2% of the remaining elements, formed various compounds and condensed into particles.

Dust particles are gradually concentrated in a single plane, forming a layer of high density. This layer remained homogeneous and was gradually split into separate thickening, facing each other, combined and condensed. Formed in this way a solid body, which also face each other, or disintegrated, either grew at the expense of this fragmented substances. In the end, the largest size reached only 9 germ, became big planets. This idea about the formation of planets by combining rigid bodies and particles put forward by a prominent Soviet scientist, academician Of. Yu. Schmidt. She made a true revolution in planetary cosmogony, replacing the idea of planets condensing from the gas clots. Independently this idea was confirmed by physico-chemical studies the composition and structure of meteorites, conducted by the American geophysicist G. Jüri.

The gas component doplnena clouds was greatly influenced by the solar wind — a powerful stream of particles, which opuscules the Sun in the past even more, than now. Formed near the Sun of the planet Earth are composed mainly of silicates and metals. At large distances from the Sun, there, where he formed Jupiter and Saturn, there still remained a large mass of gases (hydrogen and helium), which entered the composition of these planets. Thus, hypothesis. Yu. Schmidt explains the separation of the planets the physical nature of the two groups.

Evolution of galaxies. Our Galaxy and other galaxies, which are large clusters of stars and interstellar material, as well, like all of the body, included in their composition, over time undergo significant changes.

First, given the above about the formation and evolution of stars, it can be argued, that the amount of interstellar material is gradually reduced.

Secondly, in the process of existence in the form of stars this substance changes its chemical composition: the hydrogen content is reduced, and in this way increases the amount of helium and some other elements, which are formed as the result of thermonuclear processes. The heaviest elements are formed only under special conditions — when a catastrophic supernova explosions of stars. Thus, the next generation of stars is formed from a substance of a different chemical composition.

Observing the distribution of stars with different composition, you can examine the distribution in the galaxy of stars and star clusters of different ages. It turns out, what are the oldest objects in the galaxy form a spherical system. Therefore, the gas cloud, from which the galaxy was formed, had spherical shape. The mass of gas was compressed and flattened, going to the plane, perpendicular to the rotation axis of the galaxy. In the future, the process of star formation occurred in the disk near the plane.

The continued flattening of the disc interfered with the magnetic field, the lines of force which determine the helical structure of the distribution of interstellar hydrogen and formed from stars in the galactic disk.

The spiral branches of the galaxy in some way connected with its core. Nuclei of galaxies, their Central part is not simply a region of high density of distribution of stars. In recent years obtained a lot of facts, speaking about the high activity of galactic nuclei. The first on the special properties of galactic nuclei drew the attention of the eminent Soviet astrophysicist, academician In. And. Hambardzumyan. Observation, conducted in a wide range of spectrum — from radio waves to x-rays, showed, the power of radiation of the nuclei of galaxies changes significantly for a few months or even weeks. Calculations show, this is linked to the processes, taking place in a small volume. As a result of these processes releases energy, far exceeding the, released during the most powerful explosions of stars. Especially strongly manifests itself in the activity of galactic nuclei in the radio, why such galaxies are called radio galaxies. There are other types of galaxies with active nuclei, one of which is shown in figure. 40.5. Gas, thrown away from a Central condensation of about a million years ago, flies jets length 4 thousand. parsec; the total mass of this gas in 5*10⁶ times the mass of the Sun. By all indications, the core of our Galaxy in the past had a high activity.

Thanks to radio astronomy observations in 1963 g. the quasi-star open radiation sources, abbreviated to quasars. The total power of radiation of the quasar reaches 10⁴⁰-10⁴¹W, in thousands and tens of thousands of times greater than the total radiation of the stars, the largest galaxies, while the linear size of the compact core of the quasar, which is the main source of radiation, millions of times smaller than galaxies. For quasars the characteristic variability of the radiation flux and the optical and the radio band. Thanks largely to the strong radio emission from the quasars are detected at the greatest distances, to 3000 IPC. Photos of nearby quasars and radio observations show a complex structure; ejecta extending to tens of thousands of parsecs or faint nebulae. Possible, what are quasars, as well, as galaxy, consist of stars and non-stellar core of nature, which represent a massive plasma body, with a strong magnetic field.