Classification of galaxies by their shapes and appearance. What are the galaxies

In modern astronomy, the very first classification of galaxies is most widely used, proposed by Edwin Powell Hubble in 1926, and subsequently modified by him, and then by Gerard de Vaucouleurs and Alan Sandage.

This classification is based on the shape of the known galaxies. According to her, all galaxies are divided into 5 main types:

Elliptical (E);

Spiral (S);

Barred Spiral Galaxies (SB);

Incorrect (Irr);

The galaxies are too faint to be classified, Hubble labeled them with the symbol Q.

In addition, the designations of galaxies in this classification use numbers to indicate how flatten an elliptical galaxy is, and letters to indicate how tightly the arms of spiral galaxies are adjacent to the core.

Graphically, this classification is presented as a series, which is called the Hubble sequence (or Hubble tuning fork because of the similarity of the scheme with this instrument).

Elliptical galaxies (type E) constitute 13% of the total galaxies. They look like a circle or ellipse, the brightness of which decreases rapidly from the center to the periphery. Elliptical galaxies are very diverse in shape: they can be both spherical and very flattened. In this regard, they are subdivided into 8 subclasses - from E0 (spherical shape, no compression) to E7 (greatest compression).

Elliptical galaxies are the simplest in structure. They are composed mainly of old red and yellow giants, red, yellow and white dwarfs. There is no dusty matter in them. The formation of stars in galaxies of this type has not been going on for several billion years. There is almost no cold gas and cosmic dust in them. Rotation was found only in the most compressed of the elliptical galaxies.

Spiral galaxies- the most numerous type: they make up about 50% of all observed galaxies. Most of the stars in the spiral galaxy are located within the galactic disk. The galactic disk has a spiral pattern of two or more twisted in one direction branches or arms emerging from the center of the galaxy.

There are two types of spirals. In the first type, designated SA or S, the spiral branches extend directly from the central seal. In the second, they begin at the ends of the oblong formation, in the center of which there is an oval seal. The impression is that the two spiral branches are connected by a bridge, which is why such galaxies are called crossed spirals; they are designated by the SB symbol.

Spiral galaxies differ in the degree of development of their spiral structure, which in the classification is marked by the addition of letters a, b, c to the symbols S (or SA) and SB.

The arms of spiral galaxies are bluish in color, as they contain many young giant stars. All spiral galaxies rotate at significant speeds, so stars, dust and gases are concentrated in their narrow disk (Population I stars). In the overwhelming majority of cases, rotation occurs in the direction of twisting of the spiral branches.

Each spiral galaxy has a central cluster. The color of the clusters of spiral galaxies is reddish-yellow, indicating that they are composed mainly of stars of spectral classes G, K, and M (i.e. the smallest and coldest).

The abundance of gas and dust clouds and the presence of bright blue giants of spectral types O and B indicate active star formation processes occurring in the spiral arms of these galaxies.

The disk of spiral galaxies is immersed in a rarefied weakly luminous cloud of stars - a halo. The halo is composed of young Population II stars that form numerous globular clusters.

In some galaxies, the central part is spherical and glows brightly. This part is called the bulge (from the English bulge - thickening, swelling). The bulge is made up of old Population II stars and, often, a supermassive black hole at its center. In other galaxies in the central part there is a "stellar bar" - a bar.

The most famous spiral galaxies are our Milky Way Galaxy and the Andromeda Nebula.

Lenticular galaxy(type S0) is an intermediate type between spiral and elliptical galaxies. In galaxies of this type, the bright central cluster (bulge) is strongly compressed and looks like a lens, and the branches are absent or very weakly traced.

Lenticular galaxies consist of old giant stars, and therefore their color is reddish. Two-thirds of lenticular galaxies, like elliptical ones, do not contain gas; in one-third the gas content is the same as in spiral galaxies. Therefore, star formation processes are very slow pace... Dust in lenticular galaxies is concentrated near the galactic core. About 10% of known galaxies belong to lenticular galaxies.

For irregular or irregular galaxies (Ir) an irregular, clumpy shape is characteristic. Irregular galaxies are characterized by the absence of central condensation and symmetrical structure, as well as low luminosity. Such galaxies contain a lot of gas (mainly neutral hydrogen) - up to 50% of them total mass... This type includes about 25% of all star systems.

Irregular galaxies are divided into 2 large groups. The first of them, designated as Irr I, includes galaxies with a hint of a certain structure. The division of Irr I is not final: for example, if a similarity of spiral arms is found in the studied galaxy (typical for S-type galaxies), the galaxy is designated Sm or SBm (it has a bar in its structure); if similar phenomenon not observed - notation Im.

The second group of irregular galaxies (Irr II) includes all other galaxies with a chaotic structure.

There is also a third group of irregular galaxies - dwarf ones, denoted as dI or dIrrs. It is believed that dwarf irregular galaxies are similar to the earliest galactic formations that existed in the universe. Some are small spiral galaxies destroyed by the tidal forces of their more massive companions.

Typical representatives of such galaxies are the Large and Small Magellanic Clouds. In the past, the Large and Small Magellanic Clouds were thought to be irregular galaxies. However, they were later discovered to have a spiral structure with a bar. Therefore, these galaxies were reclassified as SBm, the fourth type of bar spiral galaxies.

Galaxies that have certain individual characteristics that do not allow them to be attributed to any of the above classes are called peculiar.

An example of a peculiar galaxy is the radio galaxy Centaurus A (NGC 5128).

The Hubble classification is on this moment the most common, but not the only one. In particular, the de Vaucouleur System, which is a more extended and revised version of the Hubble classification, and the Yerkes system, in which galaxies are grouped according to their spectra, shape and degree of concentration towards the center, are widely used.

GALAXIES, "extragalactic nebulae" or "island universes," are gigantic stellar systems that also contain interstellar gas and dust. The solar system is part of our Galaxy - the Milky Way. All outer space to the limits where the most powerful telescopes can penetrate is filled with galaxies. Astronomers count at least a billion. The nearest galaxy is located from us at a distance of about 1 million sv. years (10 19 km), and to the most distant galaxies recorded by telescopes - billions of light years. The exploration of galaxies is one of the most daunting tasks of astronomy.

Historical reference. The brightest and nearest outer galaxies - the Magellanic Clouds - are visible to the naked eye in the southern hemisphere of the sky and were known to the Arabs as early as the 11th century, as well as the brightest galaxy in the northern hemisphere - the Great Nebula in Andromeda. With the rediscovery of this nebula in 1612 with the help of a telescope by the German astronomer S. Marius (1570-1624), the scientific study of galaxies, nebulae and star clusters began. Many nebulae were discovered by various astronomers in the 17th and 18th centuries; then they were considered to be clouds of glowing gas.

The concept of stellar systems outside the Galaxy was first discussed by philosophers and astronomers of the 18th century: E. Swedenborg (1688–1772) in Sweden, T. Wright (1711–1786) in England, I. Kant (1724–1804) in Prussia, And . Lambert (1728-1777) in Alsace and W. Herschel (1738-1822) in England. However, only in the first quarter of the 20th century. the existence of "island universes" was unequivocally proved mainly thanks to the works of American astronomers G. Curtis (1872-1942) and E. Hubble (1889-1953). They proved that the distances to the brightest, and hence the nearest "white nebula" are much larger than the size of our Galaxy. During the period from 1924 to 1936, Hubble pushed the frontier of galaxy exploration from the nearest systems to the limit of the capabilities of the 2.5-meter telescope at Mount Wilson Observatory, i.e. up to several hundred million light years.

In 1929, Hubble discovered the relationship between the distance to the galaxy and the speed of its movement. This dependence, Hubble's law, has become the observational basis of modern cosmology. After the end of World War II, an active study of galaxies began with the help of new large telescopes with electronic light amplifiers, automatic measuring machines and computers. The detection of radio emission from our and other galaxies gave new opportunity for the study of the Universe and led to the discovery of radio galaxies, quasars and other manifestations of activity in the nuclei of galaxies. Extra-atmospheric observations from geophysical rockets and satellites have made it possible to detect X-rays from the cores of active galaxies and galaxy clusters.

Rice. 1. Classification of galaxies according to Hubble

The first catalog of "nebulae" was published in 1782 by the French astronomer Charles Messier (1730-1817). This list includes both star clusters and gas nebulae in our Galaxy, as well as extragalactic objects. Messier object numbers are still used today; for example, Messier 31 (M 31) is the famous Andromeda Nebula, the closest large galaxy observed in the constellation Andromeda.

A systematic survey of the sky, begun by W. Herschel in 1783, led him to the discovery of several thousand nebulae in the northern sky. This work was continued by his son J. Herschel (1792-1871), who made observations in the Southern Hemisphere at the Cape of Good Hope (1834-1838) and published in 1864 General catalog 5 thousand nebulae and star clusters. In the second half of the 19th century. newly discovered objects were added to these objects, and J. Dreyer (1852-1926) in 1888 published New general catalog (New General Catalog - NGC), including 7814 objects. With the publication in 1895 and 1908 of two additional Index-catalogs(IC) the number of detected nebulae and star clusters has exceeded 13 thousand. The designation according to the NGC and IC catalogs has since become generally accepted. Thus, the Andromeda Nebula is designated either M 31 or NGC 224. A separate list of 1249 galaxies brighter than 13th magnitude, based on a photographic survey of the sky, was compiled by H. Shepley and A. Ames from the Harvard Observatory in 1932.

This work was substantially expanded by the first (1964), second (1976) and third (1991) editions. Abstract catalog of bright galaxies J. de Vaucouleurs with co-workers. More extensive, but less detailed catalogs based on viewing photographic plates of the sky survey were published in the 1960s by F. Zwicky (1898-1974) in the USA and B.A. Vorontsov-Velyaminov (1904-1994) in the USSR. They contain approx. 30 thousand galaxies up to 15th magnitude. A similar survey of the southern sky was recently completed with the 1 m Schmidt camera of the European Southern Observatory in Chile and the British 1.2 m Schmidt camera in Australia.

There are too many galaxies fainter than magnitude 15 to list. In 1967, the results of counting galaxies brighter than the 19th magnitude (to the north of declination 20), made by C. Shein and K. Virtanen, were published on the 50-cm plates of the Lick observatory astrograph. There were approx. 2 million, not counting those that are hidden from us by the wide dust strip of the Milky Way. And back in 1936, Hubble at the Mount Wilson Observatory counted the number of galaxies up to magnitude 21 in several small areas evenly distributed over the celestial sphere (north of the 30 declination). According to these data, over 20 million galaxies are brighter than 21st magnitude in the entire sky.

Classification. There are galaxies of various shapes, sizes and luminosities; some of them are isolated, but most have neighbors or satellites that exert gravitational influence on them. As a rule, galaxies are calm, but there are often active ones as well. In 1925, Hubble proposed a classification of galaxies based on their appearance. Later it was refined by Hubble and Shepley, then by Sandage and finally by Vaucouleur. All galaxies in it are divided into 4 types: elliptical, lenticular, spiral and irregular.

Elliptical(E) galaxies are elliptical in photographs without sharp boundaries and clear details. Their brightness increases towards the center. These are rotating ellipsoids made up of old stars; their apparent shape depends on the orientation to the line of sight of the observer. When viewed from the edge, the ratio of the lengths of the short and long axes of the ellipse reaches  5/10 (denoted E5).

Rice. 2. Elliptical Galaxy ESO 325-G004

Lenticular(L or S 0), the galaxies are similar to ellipticals, but apart from the spheroidal component, they have a thin, rapidly rotating equatorial disk, sometimes with ring-like structures like the rings of Saturn. Edge-on lenticular galaxies look more compressed than elliptical ones: their axes ratio reaches 2/10.

Rice. 2. Spindle Galaxy (NGC 5866), a lenticular galaxy in the constellation Draco.

Spiral(S) galaxies also consist of two components - spheroidal and flat, but with a more or less developed spiral structure in the disk. Along a sequence of subtypes Sa, Sb, Sc, Sd(from "early" spirals to "late") spiral arms become thicker, more complex and less curled, and the spheroid (central condensation, or bulge) decreases. Edge-on spiral galaxies do not show spiral arms, but the type of galaxy can be determined from the relative brightness of the bulge and disk.

Rice. 2. An example of a spiral galaxy, Pinwheel Galaxy (Messier 101 or NGC 5457)

Wrong(I) galaxies are of two main types: the Magellanic type, i.e. of the Magellanic Clouds type, continuing the sequence of spirals from Sm before Im, and non-magellanic type I 0, with chaotic dark dust lanes over a spheroidal or disc structure such as a lenticular or early spiral.

Rice. 2. NGC 1427A, an example of an irregular galaxy.

Types L and S fall into two families and two types depending on the presence or absence of a passing through the center and crossing the disk linear structure (bar), as well as a centrally symmetric ring.

Rice. 2. Computer model of the Milky Way galaxy.

Rice. 1. NGC 1300, an example of a barred spiral galaxy.

Rice. 1. THREE-DIMENSIONAL CLASSIFICATION OF GALAXIES... Basic types: E, L, S, I are located sequentially from E before Im; families of common A and crossed B; of the kind s and r... The circular diagrams below are a cross-section of the main configuration in the region of spiral and lenticular galaxies.

Rice. 2. MAIN FAMILIES AND SPIRALS on the section of the main configuration in the area Sb.

There are other schemes for the classification of galaxies based on finer morphological details, but an objective classification based on photometric, kinematic and radio measurements has not yet been developed.

Composition... Two structural components- spheroid and disk - reflect the difference in the stellar population of galaxies, discovered in 1944 by the German astronomer W. Baade (1893-1960).

Population I present in irregular galaxies and spiral arms, contains blue giants and supergiants of spectral types O and B, red supergiants of classes K and M, as well as interstellar gas and dust with bright regions of ionized hydrogen. It also contains low-mass main sequence stars, which are visible near the Sun, but are indistinguishable in distant galaxies.

Population II present in elliptical and lenticular galaxies, as well as in the central regions of spirals and in globular clusters, contains red giants ranging from G5 to K5, subgiants, and probably subdwarfs; it contains planetary nebulae and outbursts of new ones (Fig. 3). In fig. 4 shows the relationship between spectral types (or colors) of stars and their luminosity in different populations.

Rice. 3. STAR POPULATIONS... The photograph of the spiral galaxy of the Andromeda Nebula shows that blue giants and supergiants of Population I are concentrated in its disk, and the central part consists of red stars of Population II. The satellites of the Andromeda Nebula are also visible: the galaxy NGC 205 ( at the bottom) and M 32 ( top left). The brightest stars in this photo are from our Galaxy.

Rice. 4. DIAGRAM HERZSPRUNG - RESEL, which shows the relationship between spectral type (or color) and luminosity in stars different types... I: Young Population I stars, typical of spiral arms. II: the aged stars of Population I; III: old Population II stars, typical of globular clusters and elliptical galaxies.

Originally, elliptical galaxies were thought to contain only Population II, and irregular ones only Population I. However, it turned out that galaxies usually contain a mixture of two stellar populations in different proportions. A detailed analysis of populations is possible only for a few nearby galaxies, but measurements of the color and spectrum of distant systems show that the difference in their stellar populations may be greater than Baade thought.

Distance... Measurement of distances to distant galaxies is based on the absolute scale of distances to stars in our Galaxy. It is established by several methods. The most fundamental is the trigonometric parallax method, which operates up to distances of 300 sv. years. The rest of the methods are indirect and statistical; they are based on the study of proper motions, radial velocities, brightness, color and spectrum of stars. On their basis, the absolute values ​​of New and variables of the RR Lyrae type and  Cephei, which become the primary indicators of the distance to the nearest galaxies where they are visible. Globular clusters, brightest stars and emission nebulae of these galaxies become secondary indicators and make it possible to determine distances to more distant galaxies. Finally, the diameters and luminosities of the galaxies themselves are used as tertiary indicators. Astronomers usually use the difference between the apparent magnitude of an object as a measure of distance m and its absolute stellar magnitude M; this value ( m - M) is called the "visible distance unit". To find out the true distance, it must be corrected to account for the absorption of light by interstellar dust. In this case, the error usually reaches 10–20%.

The extragalactic distance scale is revised from time to time, which means that other parameters of galaxies, depending on the distance, also change. Table 1 shows the most accurate distances to the nearest groups of galaxies today. To more distant galaxies, billions of light years away, distances are estimated with low accuracy from their redshift ( see below: The nature of redshift).

Luminosity. Measuring the surface brightness of a galaxy gives the total luminosity of its stars per unit area. The change in surface luminosity with distance from the center characterizes the structure of the galaxy. Elliptical systems, as the most regular and symmetric ones, have been studied in more detail than others; in general, they are described by a single luminosity law (Fig. 5, a):

Rice. 5. LUMINANCE DISTRIBUTION IN GALAXIES. a- elliptical galaxies (shown is the logarithm of the surface brightness depending on the fourth root of the reduced radius ( r / r e) 1/4, where r Is the distance from the center, and r e is the effective radius, which contains half of the total luminosity of the galaxy); b- lenticular galaxy NGC 1553; v- three normal spiral galaxies (the outer part of each of lines straight, which indicates an exponential dependence of luminosity on distance).

The data on lenticular systems are not so complete. Their luminosity profiles (Fig. 5, b) differ from the profiles of elliptical galaxies and have three main regions: the core, lens and envelope. These systems appear to be intermediate between elliptical and spiral.

The spirals are very diverse, their structure is complex, and there is no single law for the distribution of their luminosity. However, it seems that for simple spirals far from the core, the surface luminosity of the disk decreases exponentially toward the periphery. Measurements show that the luminosity of the spiral arms is not as great as it seems when looking at photographs of galaxies. The arms add no more than 20% to the luminosity of the disk in blue and much less in red. The contribution to luminosity from the bulge decreases from Sa To Sd(fig. 5, v).

Measuring the apparent magnitude of the galaxy m and determining its distance modulus ( m - M), calculate the absolute value M... The brightest galaxies, excluding quasars, M 22, i.e. their luminosity is almost 100 billion times greater than that of the Sun. And the smallest galaxies M10, i.e. luminosity approx. 10 6 solar. Distribution of the number of galaxies over M called the "luminosity function" is an important characteristic of the galactic population of the universe, but it is not easy to accurately determine it.

For galaxies selected to a certain limiting apparent magnitude, the luminosity function of each type separately from E before Sc almost Gaussian (bell-shaped) with an average absolute value in blue rays M m= 18.5 and variance  0.8 (Fig. 6). But galaxies of later types from Sd before Im and elliptical dwarfs are weaker.

For a complete sample of galaxies in a given volume of space, for example, in a cluster, the luminosity function increases steeply with decreasing luminosity, i.e. the number of dwarf galaxies is many times greater than the number of giant

Rice. 6. GALAXY LUMINANCE FUNCTION. a- the sample is brighter than a certain limiting visible value; b- a complete sample in a certain large amount of space. Pay attention to the overwhelming number of dwarf systems with M B< -16.

The size... Since the stellar density and luminosity of galaxies gradually decrease outward, the question of their size actually rests on the capabilities of the telescope, in its ability to distinguish the faint glow of the outer regions of the galaxy against the background of the glow of the night sky. Modern technology allows registering regions of galaxies with brightness less than 1% of the brightness of the sky; this is about a million times lower than the brightness of galactic nuclei. According to this isophote (lines of equal brightness), the diameters of galaxies range from several thousand light years in dwarf systems to hundreds of thousands in giant ones. As a rule, the diameters of galaxies correlate well with their absolute luminosity.

Spectral grade and color. The first spectrogram of the galaxy - the Andromeda Nebula, obtained at the Potsdam Observatory in 1899 by Yu Scheiner (1858–1913), with its absorption lines resembles the spectrum of the Sun. The mass study of the spectra of galaxies began with the creation of “fast” spectrographs with low dispersion (200–400 / mm); later, the use of electronic image intensifiers made it possible to increase the dispersion to 20–100 / mm. Morgan's observations at the Yerkes Observatory showed that, despite the complex stellar composition of galaxies, their spectra are usually close to the spectra of stars of a certain class from A before K, and there is a noticeable correlation between the spectrum and the morphological type of the galaxy. Typically the range of the class A have irregular galaxies Im and spirals Sm and Sd... Class Spectra A – F at the spirals Sd and Sc... Transfer from Sc To Sb accompanied by a change in the spectrum from F To F – G and the spirals Sb and Sa, lenticular and elliptical systems have spectra G and K... True, it later turned out that the radiation of galaxies of the spectral type A actually consists of a mixture of light from giant stars of spectral types B and K.

In addition to absorption lines, many galaxies show emission lines, like the emission nebulae of the Milky Way. Usually these are the hydrogen lines of the Balmer series, for example, H  on  6563, doublets of ionized nitrogen (N II) on  6548 and 6583 and sulfur (S II) on  6717 and 6731, ionized oxygen (O II) on  3726 and 3729 and doubly ionized oxygen (O III) on  4959 and 5007. The intensity of emission lines usually correlates with the amount of gas and supergiant stars in the disks of galaxies: these lines are absent or very weak in elliptical and lenticular galaxies, but intensified in spiral and irregular galaxies - from Sa To Im... In addition, the intensity of emission lines of elements heavier than hydrogen (N, O, S) and, probably, the relative abundance of these elements decreases from the core to the periphery of disk galaxies. Some galaxies have unusually strong emission lines in their cores. In 1943, K. Seifert discovered a special type of galaxies with very broad hydrogen lines in their cores, indicating their high activity. The luminosity of these nuclei and their spectra change over time. In general, the nuclei of Seyfert galaxies are similar to quasars, although not as powerful.

The integral index of their color changes along the morphological sequence of galaxies ( B - V), i.e. the difference between the magnitude of the galaxy in blue B and yellow V rays. The average color index of the main types of galaxies is as follows:

On this scale, 0.0 corresponds to white, 0.5 to yellowish, 1.0 to reddish.

Detailed photometry usually reveals that the color of the galaxy changes from the core to the edge, which indicates a change in the stellar composition. Most galaxies are bluer in the outer regions than in the core; in spirals this is much more pronounced than in ellipticals, since there are many young blue stars in their disks. Irregular galaxies, usually devoid of a nucleus, are often bluer in the center than at the edge.

Rotation and mass. The rotation of the galaxy around the axis passing through the center leads to a change in the wavelength of the lines in its spectrum: lines from the regions of the galaxy approaching us are shifted to the violet part of the spectrum, and from those receding, to the red (Fig. 7). According to the Doppler formula, the relative change in the line wavelength is   / = V r / c, where c Is the speed of light, and V r Is the radial velocity, i.e. component of the source velocity along the line of sight. The orbital periods of stars around the centers of galaxies are hundreds of millions of years, and their orbital velocities reach 300 km / s. Usually, the rotation speed of the disk reaches its maximum value ( V M) at some distance from the center ( r M), and then decreases (Fig. 8). Our Galaxy V M= 230 km / s at a distance r M= 40 thousand light years from the center:

Rice. 7. SPECTRAL LINES OF THE GALAXY rotating around the axis N, when the spectrograph slit is oriented along the axis ab... The line from the receding edge of the galaxy ( b) is deflected towards the red side (R), and from the approaching edge ( a) - to ultraviolet (UV).

Rice. 8. CURVE OF ROTATION OF THE GALAXY... Rotational speed V r reaches its maximum value V M at a distance R M from the center of the galaxy and then slowly decreases.

The absorption and emission lines in the spectra of galaxies have the same shape; therefore, the stars and gas in the disk rotate at the same speed in the same direction. When the location of the dark dust lanes in the disk makes it possible to understand which edge of the galaxy is closer to us, we can find out the direction of the twisting of the spiral arms: in all studied galaxies they are lagging, i.e., moving away from the center, the arm is bent in the direction opposite to the direction rotation.

Analysis of the rotation curve allows you to determine the mass of the galaxy. In the simplest case, equating the force of gravity to the centrifugal force, we get the mass of the galaxy inside the orbit of the star: M = rV r 2 /G, where G- constant gravitation. Analysis of the motion of peripheral stars allows us to estimate the total mass. Our Galaxy has a mass of approx. 210 11 solar masses, the Andromeda Nebula 410 11, and the Large Magellanic Cloud - 1510 9. The masses of disk galaxies are approximately proportional to their luminosity ( L), so the ratio M / L they have almost the same and for the luminosity in blue rays equal M / L 5 in units of mass and luminosity of the Sun.

The mass of a spheroidal galaxy can be estimated in the same way, taking the speed of chaotic motion of stars in the galaxy instead of the speed of rotation of the disk (  v), which is measured by the width of the spectral lines and is called the velocity dispersion: M  R v 2 /G, where R Is the radius of the galaxy (virial theorem). The dispersion of stellar velocities in elliptical galaxies is usually from 50 to 300 km / s, and masses from 10 9 solar masses in dwarf systems to 10 12 in giant ones.

Radio emission The Milky Way was discovered by K. Yansky in 1931. The first radio map of the Milky Way was received by G. Reber in 1945. This radiation comes in a wide range of wavelengths  or frequencies  = c/ , from several megahertz (   100 m) up to tens of gigahertz (   1 cm), and is called “continuous”. Several physical processes are responsible for it, the most important of which is the synchrotron radiation of interstellar electrons moving at almost the speed of light in a weak interstellar magnetic field. In 1950, continuous radiation at a wavelength of 1.9 m was discovered by R. Brown and K. Hazard (Jodrell Bank, England) from the Andromeda Nebula, and then from many other galaxies. Normal galaxies like ours or M 31 are weak sources of radio waves. They emit in the radio frequency range barely one millionth of their optical power. But in some unusual galaxies, this radiation is much stronger. The nearest "radio galaxies" Virgo A (M 87), Centaur A (NGC 5128) and Perseus A (NGC 1275) have a radio luminosity of 10 –4 10 –3 from optical. And for rare objects, such as the Cygnus A radio galaxy, this ratio is close to unity. Only a few years after the discovery of this powerful radio source was it possible to find a faint galaxy associated with it. Many faint radio sources, probably associated with distant galaxies, have not yet been identified with optical objects.

Bound by the forces of gravitational interaction. The number of stars and the size of galaxies can vary. Typically, galaxies contain from a few million to several trillion (1,000,000,000,000) stars. In addition to ordinary stars and the interstellar medium, galaxies also contain various nebulae. Galaxies range in size from several thousand to several hundred thousand light years. And the distance between galaxies reaches millions of light years.

About 90% of the mass of galaxies is made up of dark matter and energy. The nature of these invisible components has not yet been studied. There is evidence that there are supermassive galaxies at the center of many galaxies. The space between galaxies contains practically no matter and has an average density of less than one atom per cubic meter... Presumably, there are about 100 billion galaxies in the visible part of the universe.

According to the classification proposed by astronomer Edwin Hubble, in 1925 there are several types of galaxies:

• elliptical (E),
• lenticular (S0),
• conventional spiral (S),
• crossed spiral (SB),
• wrong (Ir).

Elliptical galaxies are a class of galaxies with a pronounced spherical structure and brightness decreasing towards the edges. They rotate relatively slowly; noticeable rotation is observed only in galaxies with significant compression. In such galaxies, there is no dusty matter, which in those galaxies in which it is present is visible as dark stripes against a continuous background of galaxy stars. Therefore, outwardly elliptical galaxies differ from each other mainly in one feature - more or less compression.

The share of elliptical galaxies in the total number of galaxies in the observable part of the universe is about 25%.

Spiral The galaxies are so named because they have bright stellar arms within the disk that extend almost logarithmically from the bulge (an almost spherical thickening at the center of the galaxy). Spiral galaxies have a central cluster and several spiral arms, or arms, which are bluish in color, as they contain many young giant stars. These stars excite the glow of diffuse gas nebulae scattered with dust clouds along the spiral branches. The disk of a spiral galaxy is usually surrounded by a large spheroidal halo (a ring of light around an object; an optical phenomenon) made up of old second-generation stars. All spiral galaxies rotate at significant speeds, so stars, dust and gases are concentrated in a narrow disk. The abundance of gas and dust clouds and the presence of bright blue giants indicate active star formation processes taking place in the spiral arms of these galaxies.

Many spiral galaxies have a bar in the center, from the ends of which spiral arms extend. Our Galaxy also belongs to the barred spiral galaxies.

Lenticular galaxies are an intermediate type between spiral and elliptical. They have a bulge, a halo, and a disk, but no spiral arms. There are about 20% of all stellar systems. In these galaxies, the bright main body, the lens, is surrounded by a faint halo. Sometimes the lens has a ring around it.

Wrong galaxies are galaxies that exhibit neither spiral nor elliptical structure. Most often, such galaxies have a chaotic shape without a pronounced nucleus and spiral branches. In percentage terms, they make up one quarter of all galaxies. Most irregular galaxies in the past were spiral or elliptical, but were deformed by gravitational forces.

Evolution of galaxies

The formation of galaxies is considered as a natural stage of evolution, occurring under the influence of gravitational forces... As scientists assume, about 14 billion years ago there was a big bang, after which the universe was the same everywhere. Then the particles of dust and gas began to group, combine, collide, and thus clumps appeared, which later turned into galaxies. The variety of forms of galaxies is associated with a variety of initial conditions for the formation of galaxies. The accumulation of hydrogen gas within such clumps became the first stars.

From the moment of its inception, the galaxy begins to shrink. The compression of the galaxy lasts about 3 billion years. During this time, the gas cloud transforms into a stellar system. Stars are formed by the gravitational compression of clouds of gas. When densities and temperatures are reached in the center of the compressed cloud sufficient for effective thermonuclear reactions to occur, a star is born. Thermonuclear fusion occurs in the bowels of massive stars chemical elements heavier than helium. These elements enter the primary hydrogen-helium medium during explosions of stars or during a quiet outflow of matter with stars. Elements heavier than iron are formed in grandiose supernova explosions. Thus, first generation stars enrich the primary gas with chemical elements heavier than helium. These stars are the oldest and consist of hydrogen, helium and a very small admixture of heavy elements. V second generation stars the admixture of heavy elements is more noticeable, since they are formed from the primary gas already enriched in heavy elements.

The process of star birth occurs with the continuing compression of the galaxy, so the formation of stars occurs closer and closer to the center of the system, and the closer to the center, the more heavy elements should be in the stars. This conclusion is in good agreement with the data on the abundance of chemical elements in the stars of the halo of our Galaxy and elliptical galaxies. In a rotating galaxy, the stars of the future halo are formed at an earlier stage of compression, when the rotation has not yet affected the overall shape of the galaxy. Globular star clusters are evidence of this era in our Galaxy.

When the compression of the protogalaxy stops, the kinetic energy of the formed disk stars is equal to the energy of the collective gravitational interaction. At this time, conditions are created for the formation of a spiral structure, and the birth of stars occurs already in the spiral branches, in which the gas is quite dense. it third generation stars... Ours belongs to them.

The reserves of interstellar gas are gradually depleted, and the birth of stars is becoming less intense. In a few billion years, when all gas reserves are exhausted, the spiral galaxy will turn into a lenticular galaxy consisting of faint red stars. Elliptical galaxies are already at this stage: all the gas in them was consumed 10-15 billion years ago.

The age of galaxies is roughly the age of the universe. One of the secrets of astronomy is the question of what the nuclei of galaxies are. Highly important discovery it turned out that some galactic nuclei are active. This discovery was unexpected. It used to be thought that the galactic core is nothing more than a cluster of hundreds of millions of stars. It turned out that both the optical and radio emission of some galactic nuclei can change over several months. This means that within a short time, a huge amount of energy is released from the nuclei, hundreds of times higher than that which is released in a supernova explosion. Such nuclei are called "active", and the processes occurring in them, "activity".

In 1963, objects of a new type were discovered, located outside the aisles of our galaxy. These objects are star-shaped. Over time, they found out that their luminosity is many tens of times greater than the luminosity of galaxies! The most amazing thing is that their brightness changes. Their radiation power is thousands of times higher than the radiation power of active nuclei. These objects were named. It is now believed that the nuclei of some galaxies are quasars.

Hubble classification

There are three main types of galaxies: elliptical, spiral, and irregular (irregular). Two of these three types are subdivided and subdivided into systems, and general classification now known as the Hubble tuning fork. When Hubble first created this scheme, he believed that it was an evolutionary sequence, as well as their classification.

However, today, scientists adhere to the following morphological classification, detailed in the table

Modern classification of galaxies according to the Herschel and Spitzer infrared telescopes.

In this diagram, 61 close objects captured by the Herschel and Spitzer space telescopes. They are located about 10-100 million light-years from Earth and have been photographed as part of research programs.

In images of galaxies, instead of stars, interstellar dust is visible, which is heated by hot young stars, visible only with infrared telescopes such as Herschel and Spitzer.

Each individual image is tricolor and shows warm dust ( blue color), detected by Spitzer at a wavelength of 24 microns, and cooler dust filmed by Herschel in the range of 100 microns (green) and 250 microns (red).

Elliptical - have the shape of a spheroid or elongated sphere. In the sky, where we can only see two of three dimensions, these stellar islands are oval and disc-shaped. Their surface brightness decreases away from the center. The larger the number in the classification of elliptical galaxies, the larger the shape of an ellipse they have. So, for example, according to the classification, E0 is perfectly round, and E7 is in the form of an oval. The elliptical scale ranges from E0 to E7.

Spiral

Spirals are made up of three main components: bulge, disc, and halo. The bulge (bulge) is at the center of the galaxy. It contains mostly old stars. The disk is composed of dust, gas, and young stars. The disc forms a series of structures. Our Sun, for example, is in the hand of Orion. Halos are loose, spherical structures located around the bulge. The halo contains old star clusters known as globular clusters.

S0 type

S0 is an intermediate type between E7 and spiral Sa. They differ from ellipticals because they have a bulge and a thin disc, but differ from Sa because they do not have a spiral structure. S0 galaxies are also known as lenticular galaxies.

Wrong

Variety of galaxies

Galaxies are large stellar systems in which stars are bound together by gravitational forces. There are galaxies that include trillions of stars. Our Galaxy - the Milky Way - is also quite large, with over 200 billion stars. The smallest galaxies contain a million times smaller stars and are more like globular clusters in the Milky Way, only much larger in size. In addition to ordinary stars, galaxies include interstellar gas, dust, and various "exotic" objects: white dwarfs, neutron stars, black holes. Gas in galaxies is not only scattered between the stars, but also forms huge clouds, bright nebulae around hot stars, dense and cold gas and dust nebulae. Large stellar systems have masses of hundreds of billions of solar masses. The smallest of the dwarf galaxies "weigh" only 100,000 times the Sun. Thus, the mass interval of galaxies is much wider than that of stars: the "heaviest" and "lightest" stars differ in mass by less than 1000 times.

Star Islands - a variety of galaxies

The appearance and structure of stellar systems are very different, and in accordance with this they are divided into morphological types.

The nearest and brightest galaxies in the sky are the Magellanic Clouds. When studying the sky with modern telescopes, many galaxies similar to the Magellanic Clouds have been discovered. They are characterized by an irregular, clumpy shape. Such galaxies contain a lot of gas - up to 50% of their total mass. This type is called wrong galaxies and stand for Ir (from the English irregular - "wrong").

Elliptical galaxies it is customary to denote by the letter E (from the English elliptical - "elliptical"), to which a number from 0 to 6 is added, corresponding to the degree of flattening of the system (E0 - "spherical" galaxies, E6 - the most "oblate"). Elliptical galaxies are reddish in color, as they are composed primarily of old stars. There is almost no cold gas in such systems, but the most massive of them are filled with very rarefied hot gas with a temperature of over a million degrees.

Spiral galaxies on the galactic disk, a spiral pattern of two or more (up to ten) twisted in one direction branches, or arms, emerging from the center of the galaxy is noticeable. The disk is immersed in a rarefied weakly luminous spheroidal cloud of stars - a halo. Spiral galaxies are designated by the letter S. According to the degree of structure (development) of the spiral branches and their general shape, they are subdivided into types called Hubble types - after the American astronomer Edwin Hubble, who proposed the classification of galaxies. Systems with smooth, tightly twisted spiral arms are referred to as Sa. In them, the central spherical part (bulge) is bright and extended, and the arms are indistinct and blurred. If the spirals are more powerful and distinct, and the central part is less prominent, then such galaxies belong to the Sb type. Galaxies with a developed clumpy spiral structure, the bulge of which is poorly visible against the general background, belong to the Sc type.

Some spiral systems in the central part have an almost straight stellar bar - a bar.

Leo A, a dwarf irregular galaxy, is one of the most numerous type of galaxies in the universe, and is arguably the building blocks of more massive galaxies.

NGC 205 is one of the members of the family of dwarf elliptical galaxies. NGC 205 is one of the moons of the Andromeda Galaxy.

In this case, B is added to their designation after the letter S (for example SBc).
Lenticular galaxies is an intermediate type between spiral and elliptical. They have a bulge, a halo, and a disk, but no spiral arms. Such galaxies are designated SO.

Found among galaxies and dwarf that do not fit into the Hubble classification. The life path of these stellar systems is so peculiar that it leaves an imprint on the properties of stars inside galaxies, and on the properties of galaxies in general. The discovery of the family of dwarf galaxies began in the 1930s. XX century At that time, the American astronomer Harlow Shapley discovered two faint, barely noticeable clusters of stars in the constellations Sculptor and Fornax. Their nature remained unclear until the distances to them were measured. Faint clusters of stars turned out to be extragalactic objects, independent dwarf systems of very low density. This sparked interest in faint galaxies with low surface brightness, and after a while many dwarf galaxies were known. Dwarf galaxies are designated by the letter d (from the English dwarf - "dwarf"). They can be divided into dwarf elliptical dE, dwarf spheroidal dSph (Sph is an abbreviation for the English sphere - "ball"), dwarf irregular dIr and dwarf blue compact galaxies dBCG (here BCG - blue compact galaxies).

Dwarf dE differ from normal elliptical galaxies mainly in size and mass. These are actually the same elliptical galaxies, only with fewer stars. They consist mainly of old stars of small mass, contain very little gas and dust. Dwarf spheroidal galaxies are in many ways similar to elliptical dwarf galaxies, but much more rarefied. They are formed by old hydrogen-helium stars with very low concentrations of heavy chemical elements. The latter circumstance leaves an imprint on the physical properties of these stars: they are hotter, bluer, and their evolution proceeds somewhat differently than that of stars with a "solar" chemical composition.

Other types of dwarf galaxies - dIr and dBCG - are small, shapeless systems that are very rich in gas. The main difference between the two is that intense star formation is often observed in dBCG and a large number of blue massive stars are born. This makes the galaxies appear brighter, more compact, and colored blue. There are no galaxies with well-developed spiral arms among dwarfs. Most likely, a massive stellar disk is needed to form spirals.

There is also a class of large spiral stellar systems whose surface brightness is much less than normal. Unusual in them is the low density of the stellar disk. They are called low brightness anemic or spiral galaxies.

Subsystems in the galaxy (bulge, disk, halo) interact gravitationally with each other, making up a single whole. Until now, galaxies "complete" themselves from the inside, forming stars and star clusters. Gas serves as "food" for this. Elliptical galaxies have long since used up their gas supply, and there are no young stars in them. In other galaxies, where there is still gas, stars continue to be born. They arise in large groups - huge regions up to several thousand light years in size are covered by star formation. The most massive stars, quickly passing their life path explode like supernovae. Supernova explosions cause powerful compression waves in the surrounding interstellar medium, and this in turn stimulates an "epidemic" of star formation in neighboring regions of the galaxy.

The "social status" of a galaxy depends on its mass. Massive, large ones are surrounded by a large retinue of smaller galaxies. Small galaxies, when passing through large ones, sometimes "pay tribute", giving them part or all of their building material - gas. If two galaxies pass close enough to each other, then their gravitational fields actively influence the movement of stars and gas in these systems. As a result, the appearance of galaxies can undergo noticeable changes.

Spiral galaxies

In 1845, the British astronomer Lord Ross (William Parsons), using a telescope with a 180-centimeter metal mirror, discovered a whole class of "spiral nebulae", the most striking example of which was the nebula in the constellation Canine Hounds (M 51, cataloged by Messier Messier). The nature of these nebulae was established only in the first half of the 20th century. At that time, intensive research was carried out to determine the size of our Galaxy - the Milky Way - and the distances to some nebulae that could be decomposed into stars. The conclusions were contradictory both in the estimates of the distances to the nebulae and in the determination of the scales. everything fell into place when in the 20s. Cepheids were discovered in the nearest spiral nebulae, which made it possible to estimate the distance to them. Back in 1908, Harvard Observatory astronomer Henrietta Leavitt discovered a relationship between the period of brightness variation of variable Cepheid stars and their luminosity. This made it possible, by the magnitude of the period, to find out the luminosity of the star, by the luminosity - the distance to it, and, consequently, to the stellar system where it enters. This method made it possible to determine the distance to the Andromeda nebula at 900 thousand light years. This estimate turned out to be underestimated. Thus, the latest evidence was obtained that spiral nebulae are huge stellar systems,

The large beautiful barred spiral galaxy NGC 1300 lies about 70 million light-years away in the constellation Eridanus. NGC 1300 spans over 100,000 light years.

Spiral galaxy M66, shown in the picture, spans 100,000 light years and lies 35 million light years from the Sun. It is the largest galaxy in the Leo triplet.

comparable to our Galaxy. Since then, they have been called galaxies.

Spiral galaxies are flat, disc-shaped, due to rotation. During the formation of the galaxy, centrifugal forces prevented the compression of a protogalactic cloud or a system of gas clouds in a direction perpendicular to the axis of rotation. As a result, the gas was concentrated to a certain plane - this is how the rotating disks of spiral galaxies were formed. The disc does not rotate as one solid(for example, a wheel): the orbital period of stars at the edges of the disk is much longer than in the inner parts.

Astronomers had to work hard to understand the reason for other observed properties of spiral galaxies. Russian science has made a significant contribution to the study of their nature. This is how the nature of the spiral arms of galaxies is imagined today. All stars that inhabit the galaxy interact gravitationally, resulting in a common gravitational field of the galaxy.

There are several known reasons why, when a massive disk rotates, regular compaction of matter occurs, propagating like waves on the surface of water. In galaxies, they are in the form of spirals, which is associated with the nature of the rotation of the disk. An increase in the density of both stars and interstellar matter — dust and gas — is observed in the spiral arms. The increased gas density accelerates the formation and subsequent compression of gas clouds and thus stimulates the birth of new stars. Therefore, the spiral branches are the site of intense star formation.

Spiral branches are density waves traveling along a rotating disk. Therefore, after a while, a star born in a spiral turns out to be outside it. The brightest and most massive stars have very short term life, they burn before leaving the spiral branch. Less massive stars live long and survive in the interspiral space of the disk. The low-mass yellow and red stars that make up the bulge (a spherical "bulge" in the center of the galaxy) are much older than the stars concentrated in the spiral arms. These stars were born even before the galactic disk formed. Having arisen in the center of the protogalactic cloud, they could no longer be involved in compression to the plane of the galaxy and therefore form a spherical structure.

Consider spiral galaxies using the example of M 51, called the Whirlpool. This galaxy has a small satellite galaxy at the end of one of its spiral arms. It revolves around the parent galaxy. It was possible to build a computer model of the formation of this system. It is assumed that a small galaxy, flying near a large one, led to strong gravitational disturbances of its disk. As a result, a spiral-shaped density wave is created in the disk of a large galaxy. The stars that are born in the spiral branches make these branches bright and clear.

The bulge and disk of the galaxy are immersed in a massive halo. Some researchers suggest that the bulk of the halo is not contained in stars, but in non-luminous (hidden) matter, consisting either of bodies with a mass intermediate between the masses of stars and planets, or of elementary particles, the existence of which theorists predict, but which have yet to be discovered ... The problem of the nature of this substance - latent mass - now occupies the minds of many scientists, and its solution may provide a key to the nature of matter in the Universe as a whole.

Galaxies with active nuclei

All but the smallest galaxies have a bright central part called the nucleus. In normal galaxies such as ours, the brighter core is due to the high concentration of stars. But still, the total number of stars in the core is only a few percent of their total number in the galaxy.

There are galaxies with particularly bright nuclei. Moreover, in these cores, in addition to stars, there is a bright star-like source in the center and a glowing gas moving at tremendous speeds - thousands of kilometers per second. Galaxies with active nuclei were discovered by the American astronomer Karl Seyfert in 1943 and later called Seyfert galaxies. Thousands of similar objects are now known. Seyfert galaxies (or simply Seyferts) are gigantic

Active galaxy Centaurus A, in the center swirls a mixture of young blue star clusters, giant glowing gas clouds and interspersed dark dust streaks.

Artistic depiction of jets around a massive black hole with an acretion disk. Jets are jets of matter.

spiral star systems. Among them, the proportion of crossed spirals is increased; galaxies with a bar (SB). Seyferts are more likely than normal galaxies to form pairs or groups, but avoid large clusters. Seyfert discovered 12 galaxies with active nuclei, but for 15 years they have hardly been studied. In 1958, Soviet astrophysicist Viktor Amazaspovich Ambartsumyan attracted the attention of astronomy.

The forms of manifestation of the activity of nuclei are not the same in different galaxies. This can be a very high power of radiation in the optical, X-ray or infrared region of the spectrum, and it changes noticeably over several years, months or even days. In some cases, a very fast movement of gas in the core is observed - at speeds of thousands of kilometers per second. Sometimes the gas forms long straight-line emissions. In some galaxies, nuclei are sources of high-energy elementary particles. These particle streams often leave the galaxy forever in the form of radio emissions, or radio jets. Active nuclei of any type are characterized by a very high luminosity in the entire range of the electromagnetic spectrum. The radiation power of Seyfert galaxies sometimes reaches 10 35 W, which is not much inferior to the luminosity of our entire galaxy. But this huge energy is released in an area with a diameter of about 1 pc - less than the distance from the Sun to the nearest star! Light emission power (optical luminosity) is much lower. Most of the energy is usually emitted in the infrared range.

What is the source of energy for such a violent activity? What kind of "reactor" with less than 1 pc is producing so much energy? Nobody knows the final answer yet, but as a result of long-term work of theorists and observers, several most probable models have been developed. The first hypothesis was put forward that there is a dense massive cluster of young stars in the center of the galaxy. Supernova explosions should often occur in such a cluster. These explosions can explain both the observed ejections of matter from the nuclei and the variability of radiation. The second model was proposed in the late 60s. partly by analogy with the then only discovered pulsars. According to this version, a supermassive star-like object with a powerful magnetic field- the so-called magnetoid. The third model is associated with such a mysterious object as a black hole. It is assumed that there is a black hole with a mass of tens or hundreds of millions of solar masses in the center of the galaxy. As a result of accretion (falling) of matter onto a black hole, a huge amount of energy is released. When falling in the gravitational field of a black hole, matter accelerates to speeds close to the speed of light. Then on collision gas masses near the black hole, the energy of motion is converted into radiation of electromagnetic waves.

Spectral observations with the Hubble Space Telescope and large ground-based telescopes have confirmed the presence of large masses of non-luminous matter in the cores of a number of galaxies. This is in good agreement with the assumption that massive black holes are responsible for the nuclear activity. A significant proportion of galaxies may have black holes weighing more than a million times the mass of the Sun. There is observational evidence for the existence of black holes in the cores of our Galaxy and the Andromeda nebula. But since their mass is relatively small, the activity of the nuclei is weak.

Interacting galaxies

In the middle of the 20th century, large telescopes allowed astronomers to study the positions and shapes of tens of thousands of faint galaxies. Attention was drawn to the fact that some of the galaxies (5-10%) have a very strange, distorted appearance, so that it is sometimes difficult to attribute them to some morphological type. Some of them look very asymmetrical, as if rumpled. Sometimes two galaxies are surrounded by a common luminous stellar fog or are connected by a stellar or gas bridge. And in some cases, they move away from galaxies long tails stretching for hundreds of thousands of light years into intergalactic space. Some systems differ in the nature of the internal motions of interstellar gas, which are not reduced to a simple circulation of matter around the center. Such non-circular motions cannot last long; they must decay in one or two revolutions of the disk. This means that they have arisen relatively recently. Perhaps we are observing young, not yet fully formed galaxies? No, the analysis of the star composition showed that they are as old as most of the others.

Most often, these unusual star systems are members of pairs or close groups, and this suggests that all these features are the result of the influence of galaxies on each other. The famous Soviet astronomer Boris Aleksandrovich Vorontsov-Velyaminov, who was the first to begin the study of such objects, gave them the name "interacting galaxies". He described and cataloged thousands of interacting systems, including the rarest in structure and form.

Research on Arp 230 has shown that the lonely-looking spiral galaxy is actually the result of a recent collision of two spiral galaxies.

Centaurus A appears to be the product of a collision of two galaxies, the debris of which continues to be swallowed up by the black hole.

galaxies whose unusual appearance still puzzles astronomers. Statistical studies have led to the conclusion that most of the interacting galaxies are not random wanderers in the Universe, but relatives linked by a common origin. In their movement, they either approach or move away from each other. Gravitational fields nearby star systems create tidal forces sufficient to distort the shape of galaxies OR change their internal structure. It is rather difficult to describe this process theoretically. The construction of computer models played a very important role in his research. Those processes, which in nature take hundreds of millions of years, unfold on the screen literally before our eyes. When stellar systems approach each other, their shape is distorted, powerful spiral branches appear, and bridges are born between galaxies. Later, as galaxies begin to move away from each other, long tails of gas and stars are ejected from one or both of them. Strong interaction leads to irreversible changes in the size, shape, and even morphological type of galaxies.

The nature of the interaction depends on many factors. For example, it depends on whether a galaxy has a stellar disk, how much interstellar gas is in it, how far a neighboring galaxy approaches it, in what direction and with what speed it moves, how its orbit is oriented. Therefore, the forms of interacting systems are so diverse. But one general prediction can be made: if galaxies did not accidentally meet in space, but form a system, then their interaction, sooner or later, should lead to close approach and subsequent merger. This process can take over a billion years. Such merging systems have indeed been found among known galaxies. They contain double, less often multiple nuclei, light jets of matter once ejected into intergalactic space, or unusually extended stellar "crowns".

Interaction plays a very important role in the evolution of stellar systems. Many galaxies should have experienced strong interactions, culminating in merging, in the distant past. Now their appearance can be completely normal, and only special studies allow one to suspect the turbulent processes they once experienced. So, the nearest radio galaxy Centaurus A is considered to be the result of the merger of an elliptical system with a disk system, the interstellar gas of which formed a giant gas and dust disk. It is located edge to us and therefore is visible in the photographs as a dark stripe crossing the galaxy. It can be assumed that billions of years ago, interaction and merging of galaxies occurred much more often - after all, many galaxies have already merged into unified systems by now. Indeed, observations of distant and faint galaxies carried out at the Hubble Space Telescope, the light from which flew towards us for billions of years, have shown that among them the proportion of distorted, interacting systems is increased.

The interaction of galaxies is not limited to simple changes in their structure or type. The influence of even relatively distant galaxies on each other often leads to a burst of star formation in one or both of them. The tidal interaction of galaxies contributes to the formation of massive clouds of gas. In addition, the relative speeds of the clouds increase and they collide with each other more often. It is these processes that largely determine the intensity of the birth of stars. Finally, among the interacting galaxies, there are quite a few systems with active nuclei. According to modern concepts, the activity of the nucleus requires a massive compact object in the center of the galaxy and gas that can freely fall on it.