65. Distribution of stars in the Galaxy. Clusters. General structure of the Galaxy.
end of form beginning of form Knowing the distances to the stars allows us to approach the study of their distribution in space, and hence the structure of the Galaxy. In order to characterize the number of stars in different parts of the Galaxy, the concept of stellar density is introduced, which is analogous to the concept of the concentration of molecules. Stellar density is the number of stars in a unit volume of space. The unit of volume is usually taken to be 1 cubic parsec. In the vicinity of the Sun, the stellar density is about 0.12 stars per cubic parsec, in other words, each star has an average volume of over 8 ps3; the average distance between the stars is about 2 ps. To find out how the stellar density changes in different directions, the number of stars per unit area (for example, 1 square degree) in different parts of the sky is counted.
The first thing that catches your eye in such calculations is an unusually strong increase in the concentration of stars as you approach the Milky Way band, the middle line of which forms a large circle in the sky. On the contrary, as we approach the pole of this circle, the concentration of stars decreases rapidly. This fact is already at the end of the 18th century. allowed V. Herschel to draw the correct conclusion that our star system has an oblate shape, and the Sun should be close to the plane of symmetry of this formation. spherical sector, the radius of which is determined by the formula
lg r m =1 + 0.2 (m * M)
end of form beginning of form To characterize how many stars of different luminosities are contained in a given region of space, the luminosity function j (M) is introduced, which shows what proportion of the total number of stars has a given value of absolute stellar magnitude, say, from M to M + 1.
end of form beginning of form Clusters of galaxies are gravitationally bound systems galaxies, one of the largest structures in universe. The sizes of clusters of galaxies can reach 10 8 light years.
Accumulations are conditionally divided into two types:
regular - clusters of a regular spherical shape, in which elliptical and lenticular galaxies, with a clearly defined central part. At the centers of such clusters are giant elliptical galaxies. An example of a regular cluster - Cluster of Veronica's Hair.
irregular - clusters without a definite shape, inferior to regular ones in the number of galaxies. Clusters of this species are dominated by spiral galaxies. Example - Virgo Cluster.
Cluster masses vary from 10 13 to 10 15 solar masses.
The structure of the galaxy
The distribution of stars in the Galaxy has two pronounced features: firstly, a very high concentration of stars in the galactic plane, and secondly, a large concentration in the center of the Galaxy. So, if in the vicinity of the Sun, in the disk, one star falls on 16 cubic parsecs, then in the center of the Galaxy there are 10,000 stars in one cubic parsec. In the plane of the Galaxy, in addition to an increased concentration of stars, there is also an increased concentration of dust and gas.
Dimensions of the Galaxy: - the diameter of the disk of the Galaxy is about 30 kpc (100,000 light years), - the thickness is about 1000 light years.
The Sun is located very far from the nucleus of the Galaxy - at a distance of 8 kpc (about 26,000 light years).
The center of the Galaxy is located in the constellation Sagittarius in the direction of? = 17h46.1m, ? = –28°51′.
The galaxy consists of a disk, a halo and a corona. The central, most compact region of the Galaxy is called the nucleus. There is a high concentration of stars in the core: there are thousands of stars in every cubic parsec. If we lived on a planet near a star located near the core of the Galaxy, then dozens of stars would be visible in the sky, comparable in brightness to the Moon. A massive black hole is assumed to exist at the center of the Galaxy. Almost all molecular matter of the interstellar medium is concentrated in the annular region of the galactic disk (3–7 kpc); there is the largest number of pulsars, supernova remnants and sources of infrared radiation. The visible radiation of the central regions of the Galaxy is completely hidden from us by powerful layers of absorbing matter.
The galaxy contains two main subsystems (two components), nested one into the other and gravitationally bound to each other. The first is called spherical - a halo, its stars are concentrated towards the center of the galaxy, and the density of matter, which is high in the center of the galaxy, decreases rather quickly with distance from it. The central, densest part of the halo within a few thousand light-years of the center of the Galaxy is called the bulge. The second subsystem is a massive stellar disk. It looks like two plates folded at the edges. The concentration of stars in the disk is much greater than in the halo. The stars inside the disk move in circular paths around the center of the Galaxy. The Sun is located in the stellar disk between the spiral arms.
The stars of the galactic disk were called population type I, the stars of the halo - population type II. The disk, the flat component of the Galaxy, includes stars of the early spectral classes O and B, stars in open clusters, and dark dusty nebulae. Halos, on the contrary, are made up of objects that arose in the early stages of the evolution of the Galaxy: stars of globular clusters, stars of the RR Lyrae type. The stars of the flat component, compared with the stars of the spherical component, are distinguished by a high abundance of heavy elements. The age of the population of the spherical component exceeds 12 billion years. It is usually taken as the age of the Galaxy itself.
Compared to the halo, the disk rotates noticeably faster. The disk rotation speed is not the same at different distances from the center. The mass of the disk is estimated at 150 billion M. There are spiral branches (sleeves) in the disk. Young stars and star formation centers are located mainly along the arms.
The disk and the halo surrounding it are immersed in the corona. It is currently believed that the size of the corona of the Galaxy is 10 times larger than the size of the disk.
where H¾ Hubble constant. In relation (6.12) V expressed in km/s, a r¾ in Mps.
This law is called Hubble law . Hubble constant is currently taken to be H = 72 km/(s∙Mpc).
Hubble's law allows us to say that The universe is expanding. However, this does not mean at all that our Galaxy is the center from which the expansion proceeds. At any point in the Universe, the observer will see the same picture: all galaxies have a redshift proportional to their distance. Therefore, it is sometimes said that space itself is expanding. This, of course, should be understood conditionally: galaxies, stars, planets, and we are not expanding.
Knowing the value of the redshift , for example, for some galaxy, we can determine the distance to it with great accuracy using the relation for the Doppler effect (6.3) and the Hubble law. But for z ³ 0.1, the usual Doppler formula is no longer applicable. In such cases, use the formula from the special theory of relativity:
| . | (6.13) |
Galaxies are very rarely single. Usually galaxies are found in small groups containing ten members, often combined into vast clusters of hundreds and thousands of galaxies. Our galaxy is part of the so-called local group, which includes three giant spiral galaxies (our Galaxy, the Andromeda nebula and the galaxy in the constellation Triangulum), as well as several dozen dwarf elliptical and irregular galaxies, the largest of which are several megaparsecs long. . They are divided into irregular and regular clusters. Irregular clusters do not have a regular shape and have blurred outlines. The galaxies are the Magellanic Clouds.
On average, the sizes of halo clusters in them are very weakly concentrated towards the center. An example of a giant open cluster is the nearest cluster of galaxies in the constellation Virgo. In the sky, it occupies about 120 square meters. degrees and contains several thousand predominantly spiral galaxies. The distance to the center of this cluster is about 15 Mps.
Regular clusters of galaxies are more compact and symmetrical. Their members are noticeably concentrated towards the center. An example of a spherical cluster is the cluster of galaxies in the constellation Coma Berenices, which contains a very large number of elliptical and lenticular galaxies. It contains about 30,000 galaxies brighter than 19 photographic magnitude. The distance to the center of the cluster is about 100 Mps.
Powerful extended X-ray sources are associated with many clusters containing a large number of galaxies.
There is reason to believe that clusters of galaxies, in turn, are also unevenly distributed. According to some studies, the clusters and groups of galaxies surrounding us form a grandiose system - supergalaxy or Local supercluster. In this case, individual galaxies apparently concentrate towards a certain plane, which can be called the equatorial plane of the Supergalaxy. The cluster of galaxies just discussed in the constellation Virgo is at the center of such a gigantic system. The cluster in Veronica's Hair is the center of another, nearby supercluster.
The observable part of the universe is usually called Metagalaxy . A metagalaxy is made up of various observable structural elements: galaxies, stars, supernovae, quasars, etc. The dimensions of the Metagalaxy are limited by our observational possibilities and are currently taken equal to 10 26 m. It is clear that the concept of the dimensions of the Universe is very arbitrary: the real Universe is unlimited and does not end anywhere.
Long-term studies of the Metagalaxy revealed two main properties that make up basic cosmological postulate:
1. The metagalaxy is homogeneous and isotropic in large volumes.
2. The metagalaxy is not stationary.
Usually galaxies are found in small groups containing ten members, often combined into vast clusters of hundreds and thousands of galaxies. Our Galaxy is part of the so-called Local Group, which includes three giant spiral galaxies (our Galaxy, the Andromeda nebula and the nebula in the constellation Triangulum), as well as more than 15 dwarf elliptical and irregular galaxies, the largest of which are the Magellanic Clouds. The average size of galaxy clusters is about 3 Mpc. In some cases, their diameter can exceed 10–20 Mpc. They are divided into scattered (irregular) and spherical (regular) clusters. Open clusters do not have a regular shape and have blurred outlines. The galaxies in them are very weakly concentrated towards the center. An example of a giant open cluster is the closest cluster of galaxies to us in the constellation Virgo (241). In the sky, it occupies about 120 square meters. degrees and contains several thousand predominantly spiral galaxies. The distance to the center of this cluster is about 11 Mpc. Spherical clusters of galaxies are more compact than open ones and have spherical symmetry. Their members are noticeably concentrated towards the center. An example of a spherical cluster is the cluster of galaxies in the constellation Coma Berenices, which contains a large number of elliptical and lenticular galaxies (242). Its diameter is almost 12 degrees. It contains about 30,000 galaxies brighter than 19 photographic magnitude. The distance to the center of the cluster is about 70 Mpc. Many rich clusters of galaxies are associated with powerful extended X-ray sources, the nature of which is most likely associated with the presence of hot intergalactic gas, similar to the coronas of individual galaxies. There is reason to believe that clusters of galaxies, in turn, are also unevenly distributed. According to some studies, the clusters and groups of galaxies surrounding us form a grandiose system - the Supergalaxy. In this case, individual galaxies apparently concentrate towards a certain plane, which can be called the equatorial plane of the Supergalaxy. The cluster of galaxies just discussed in the constellation Virgo is at the center of such a gigantic system. The mass of our Supergalaxy should be about 1015 solar masses, and its diameter should be about 50 Mpc. However, the reality of the existence of such clusters of second-order galaxies currently remains controversial. If they exist, then only as a weakly expressed inhomogeneity in the distribution of galaxies in the Universe, since the distances between them can slightly exceed their sizes.
The most striking feature of the spatial distribution of globular clusters in the Galaxy is a strong concentration towards its center. On fig. 8-8 shows the distribution of globular clusters over the entire celestial sphere, here the center of the Galaxy is in the center of the figure, the north pole of the Galaxy is at the top. There is no visible zone of avoidance along the plane of the Galaxy, so interstellar extinction in the disk does not hide a significant number of clusters from us.
On fig. 8-9 shows the distribution of globular clusters along the distance from the center of the Galaxy. There is a strong concentration towards the center - most globular clusters are located in a sphere with a radius of ≈ 10 kpc. It is within this radius that almost all globular clusters formed from matter are located. single protogalactic cloud and formed subsystems of the thick disk (clusters with > -1.0) and halo proper (less metallic clusters with extreme blue horizontal branches). Metal-poor clusters with horizontal branches anomalously red for their metallicity form a spheroidal subsystem accreted halo radius ≈ 20 kpc. About a dozen more distant clusters belong to the same subsystem (see Fig. 8-9), among which there are several objects with anomalously high metal contents.
Clusters of the accreted halo are believed to be selected by the gravitational field of the Galaxy from satellite galaxies. On fig. 8-10 schematically shows this structure according to Borkova and Marsakov from the Southern Federal University. Here, the letter C denotes the center of the Galaxy, S is the approximate position of the Sun. At the same time, accumulations with a high content of metals belong to the oblate subsystem. We will dwell on a more detailed substantiation of the division of globular clusters into subsystems in § 11.3 and § 14.3.
Globular clusters are also common in other galaxies, and their spatial distribution in spiral galaxies resembles the distribution in our Galaxy. Noticeably different from the Galactic clusters of the Magellanic Clouds. The main difference is that along with old objects, the same as in our Galaxy, young clusters are also observed in the Magellanic Clouds - the so-called blue globular clusters. Probably, in the Magellanic Clouds, the epoch of the formation of globular clusters either continues or ended relatively recently. It seems that there are no young globular clusters in our Galaxy similar to the blue clusters of the Magellanic Clouds, so the era of the formation of globular clusters in our Galaxy ended a very long time ago.
Globular clusters are evolving objects that gradually lose stars in the process. dynamic evolution . Thus, all clusters for which it was possible to obtain a high-quality optical image showed traces of tidal interaction with the Galaxy in the form of extended deformations (tidal tails). Currently, such lost stars are also observed in the form of increases in stellar density along the galactic orbits of clusters. Some clusters that orbit near the galactic center are destroyed by its tidal action. At the same time, the galactic orbits of clusters also evolve due to dynamic friction.
On fig. 8-11 is a dependency diagram masses of globular clusters
from their galactocentric positions. Dashed lines mark the region of slow evolution of globular clusters. The upper line corresponds to the critical value of the mass that is stable for effects of dynamic friction
, leading to a slowdown of a massive star cluster and its fall to the center of the Galaxy, and the lower one - for dissipation effects
taking into account tidal clusters during the flight through the galactic plane. The reason for dynamic friction is external: a massive globular cluster moving through the stars of the field attracts the stars it meets on its way and forces them to fly around itself behind along a hyperbolic trajectory, due to which an increased density of stars is formed behind it, creating a decelerating acceleration. As a result, the cluster slows down and begins to approach the galactic center along a spiral trajectory until it falls on it in a finite time. The greater the mass of the cluster, the shorter this time. Dissipation (evaporation) of globular clusters occurs due to the internal mechanism of stellar-stellar relaxation that is constantly operating in the cluster, distributing stars according to velocities according to Maxwell's law. As a result, the stars that have received the largest increments of speed leave the system. This process is significantly accelerated by the passage of the cluster near the galactic core and through the galactic disk. Thus, with a high probability we can say that the clusters lying on the diagram outside the area bounded by these two lines are already ending their life path.
It's interesting that accreted globular clusters discover the dependence of their masses on their position in the Galaxy. The solid lines in the figure represent direct regressions for genetically related (black dots) and accreted (open circles) globular clusters. It can be seen that genetically related clusters show no change in the average mass with increasing distance from the galactic center. On the other hand, there is a clear anticorrelation for accreted clusters. Thus, the question that needs to be answered arises, why is there an increasing deficit of massive globular clusters in the outer halo with increasing galactocentric distance (almost empty upper right corner in the diagram)?
Usually galaxies are found in small groups containing ten members, often combined into vast clusters of hundreds and thousands of galaxies. Our Galaxy is part of the so-called Local Group, which includes three giant spiral galaxies (our Galaxy, the Andromeda nebula and the nebula in the constellation Triangulum), as well as more than 15 dwarf elliptical and irregular galaxies, the largest of which are the Magellanic Clouds. The average size of galaxy clusters is about 3 Mpc. In some cases, their diameter can exceed 10–20 Mps. They are divided into scattered (irregular) and spherical (regular) clusters. Open clusters do not have a regular shape and have blurred outlines. The galaxies in them are very weakly concentrated towards the center. An example of a giant open cluster is the nearest cluster of galaxies in the constellation Virgo. In the sky, it occupies about 120 square meters. degrees and contains several thousand predominantly spiral galaxies. The distance to the center of this cluster is about 11 Mpc. Spherical clusters of galaxies are more compact than open ones and have spherical symmetry. Their members are noticeably concentrated towards the center. An example of a spherical cluster is the cluster of galaxies in the constellation Coma Berenices, which contains a lot of elliptical and lenticular galaxies (Fig. 242). Its diameter is almost 12 degrees. It contains about 30,000 galaxies brighter than 19 photographic magnitude. The distance to the center of the cluster is about 70 Mpc. Many rich clusters of galaxies are associated with powerful extended X-ray sources, the nature of which is most likely associated with the presence of hot intergalactic gas, similar to the coronas of individual galaxies.
There is reason to believe that clusters of galaxies, in turn, are also unevenly distributed. According to some studies, the clusters and groups of galaxies surrounding us form a grandiose system - the Supergalaxy. In this case, individual galaxies apparently concentrate towards a certain plane, which can be called the equatorial plane of the Supergalaxy. The cluster of galaxies just discussed in the constellation Virgo is at the center of such a gigantic system. The mass of our Supergalaxy should be about 1015 solar masses, and its diameter is about 50 Mpc. However, the reality of the existence of such clusters of second-order galaxies currently remains controversial. If they exist, then only as a weakly expressed inhomogeneity of the distribution of galaxies in the Universe, since the distances between them can slightly exceed their sizes. On the evolution of galaxies The ratio of the total amount of stellar and interstellar matter in the Galaxy changes with time, since stars form from interstellar diffuse matter, and at the end of their evolutionary path they return only part of the matter to interstellar space; some of it remains in white dwarfs. Thus, the amount of interstellar matter in our Galaxy should decrease with time. The same should happen in other galaxies. Being processed in the stellar depths, the matter of the Galaxy gradually changes its chemical composition, being enriched with helium and heavy elements. It is assumed that the Galaxy was formed from a gas cloud, which consisted mainly of hydrogen. It is even possible that, apart from hydrogen, it did not contain any other elements. Helium and heavy elements were formed in this case as a result of thermonuclear reactions inside stars. The formation of heavy elements begins with the triple helium reaction 3He4 ® C 12, then C 12 combines with a-particles, protons and neutrons, the products of these reactions undergo further transformations, and thus more and more complex nuclei appear. However, the formation of the heaviest nuclei, such as uranium and thorium, cannot be explained by a gradual increase. In this case, one would inevitably have to go through the stage of unstable radioactive isotopes, which would decay faster than they could capture the next nucleon. Therefore, it is assumed that the heaviest elements at the end of the periodic table are formed during supernova explosions. A supernova explosion is the result of the rapid contraction of a star. At the same time, the temperature rises catastrophically, chain thermonuclear reactions take place in the contracting atmosphere, and powerful neutron fluxes arise. The intensity of neutron fluxes can be so high that intermediate unstable nuclei do not have time to collapse. Before that happens, they capture new neutrons and become stable. As already mentioned, the abundance of heavy elements in the stars of the spherical component is much less than in the stars of the flat subsystem. This is apparently due to the fact that the stars of the spherical component were formed in the very initial stage of the evolution of the Galaxy, when the interstellar gas was still poor in heavy elements. At that time, the interstellar gas was an almost spherical cloud, the concentration of which increased towards the center. The stars of the spherical component that formed in this epoch also retained the same distribution. As a result of collisions of clouds of interstellar gas, their speed gradually decreased, kinetic energy turned into thermal energy, and the general shape and size of the gas cloud changed. Calculations show that in the case of rapid rotation, such a cloud should have taken the form of an oblate disk, which is what we observe in our Galaxy. Stars formed at a later time therefore form a flat subsystem. By the time the interstellar gas formed into a flat disk, it had been processed in the stellar interior, the abundance of heavy elements had increased significantly and the stars of the flat component were therefore also rich in heavy elements. Often the stars of the flat component are called second generation stars, and the stars of the spherical component are called first generation stars, to emphasize the fact that the stars of the flat component were formed from matter that had already been in the stellar interior. The evolution of other spiral galaxies probably proceeds in a similar way. The shape of the spiral arms, in which the interstellar gas is concentrated, is apparently determined by the direction of the lines of force of the general galactic magnetic field. The elasticity of the magnetic field, to which the interstellar gas is "glued", limits the flattening of the gaseous disk. If only gravity acted on the interstellar gas, its compression would continue indefinitely. In this case, due to its high density, it would quickly condense into stars and practically disappear. There is reason to believe that the rate of star formation is approximately proportional to the square of the density of the interstellar gas.
If the galaxy rotates slowly, then the interstellar gas is collected by gravity in the center. Apparently, in such galaxies the magnetic field is weaker and hinders the compression of the interstellar gas less than in rapidly rotating ones. The high density of interstellar gas in the central region leads to the fact that it is quickly consumed, turning into stars. As a result, slowly rotating galaxies should have an approximately spherical shape with a sharp increase in stellar density in the center. We know that elliptical galaxies have just such characteristics. Apparently, the reason for their difference from the spiral ones lies in the slower rotation. From what has been said above, it is also clear why there are few stars of early classes and little interstellar gas in elliptical galaxies.
Thus, the evolution of galaxies can be traced from the stage of a gaseous cloud of approximately spherical shape. The cloud consists of hydrogen, it is not uniform. Separate clumps of gas, moving, collide with each other - the loss of kinetic energy leads to cloud compression. If it rotates quickly, a spiral galaxy is obtained, if it rotates slowly, an elliptical one. It is natural to ask why the matter in the Universe broke up into separate gas clouds, which later became galaxies, why we observe the expansion of these galaxies, in what form the matter in the Universe was before the formation of galaxies.
