How are galaxies distributed in space?
It turned out that this distribution is extremely uneven. Most of them are part of clusters. Galaxy clusters are as diverse in their properties as the galaxies themselves. To bring at least some order to their description, astronomers have come up with several classifications of them. As always in such cases, no classification can be considered complete. For our purposes, it is enough to say that clusters can be divided into two types - regular and irregular.
Regular clusters are often enormous in mass. They are spherical in shape and contain tens of thousands of galaxies. As a rule, all these galaxies are elliptical or lenticular. At the center are one or two giant elliptical galaxies. The closest regular cluster to us is in the direction of the constellation Coma Berenices at a distance of about three hundred million light years and is more than ten million light years across. The galaxies in this cluster move relative to each other at speeds of about a thousand kilometers per second.
Irregular clusters are much more modest in mass. The number of galaxies included in them is tens of times less than in regular clusters, and these are galaxies of all types. Their shape is irregular; there are separate clusters of galaxies within the cluster.
Irregular clusters can be very small, down to small groups consisting of several galaxies.
IN Lately Research by Estonian astrophysicists J. Einasto, A. Saar, M. Jõevaer and others, American specialists P. Peebles, O. Gregory, L. Thompson showed that the largest-scale inhomogeneities in the distribution of galaxies are “cellular” in nature. There are many galaxies and their clusters in the “walls of cells”, but inside there is emptiness. The dimensions of the cells are about 300 million light years, the thickness of the walls is 10 million light years. Large clusters of galaxies are located at the nodes of this cellular structure. Individual fragments of cellular
structures I call superclusters. Superclusters often have a highly elongated shape, like threads or noodles. And even further?
Here we are faced with a new circumstance. So far we have met more and more complex systems: small systems formed a large system, these large systems, in turn, united into an even larger one, and so on. That is, the Universe resembled a Russian nesting doll. A small nesting doll is inside a big one, which is inside an even bigger one. It turned out that there is the largest nesting doll in the Universe! The large-scale structure in the form of “noodles” and “cells” is no longer assembled into larger systems, but evenly, on average, fills the space of the Universe. The Universe on the largest scales (more than three hundred million light years) turns out to be identical in its properties - homogeneous. This is very important property and one of the mysteries of the Universe. For some reason, on a relatively small scale there are huge clumps of matter - celestial bodies, their systems are becoming more and more complex, up to superclusters of galaxies, and on very large scales the structure disappears. Like sand on a beach. Looking close, we see individual grains of sand; looking from a great distance and covering a large area with our gaze, we see a homogeneous mass of sand.
What The universe is homogeneous, managed to trace down to the distances ten billion light years!
We will return to solving the riddle of homogeneity later, but for now let’s turn to the question that probably arose in the reader’s mind. How is it possible to measure such enormous distances to galaxies and their systems, and confidently speak about their masses and the speeds of galaxy movement?
Novikov I.D.
Where H¾ Hubble constant. In relation (6.12) V expressed in km/s, A r¾ V Mps.
This law was called Hubble's law . Hubble constant is currently accepted as equal 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 expansion occurs. An observer anywhere in the Universe will see the same picture: all galaxies have a redshift proportional to their distance. That's why they sometimes say that space itself is expanding. This, of course, should be understood conditionally: galaxies, stars, planets and you and I are not expanding.
Knowing the redshift value, for example, for a galaxy, we can determine the distance to it with great accuracy using the relation for the Doppler effect (6.3) and Hubble's law. But for z ³ 0.1 the usual Doppler formula is no longer applicable. In such cases, use the formula from special theory relativity:
| . | (6.13) |
Galaxies are very rarely single. Typically, galaxies occur in small groups containing a dozen members, often combining 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 . They are divided into irregular And regular clusters. Irregular clusters do not have a regular shape and have blurred outlines. Galaxies are Magellanic Clouds.
On average, the sizes of gala clusters 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. In the sky it occupies approximately 120 square meters. degrees and contains several thousand predominantly spiral galaxies. The distance to the center of this cluster is about 15 Mps.
Regular galaxy clusters 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 many elliptical and lenticular galaxies. It contains about 30,000 galaxies brighter than photographic magnitude 19. The distance to the center of the cluster is about 100 Mps.
Many clusters containing a large number of galaxies are associated with powerful, extended sources of X-ray radiation.
There is reason to believe that galaxy clusters, 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 examined in the constellation Virgo is at the center of such giant system. The Coma cluster is the center of another, neighboring 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 observation capabilities and are currently accepted as equal to 10 26 m. It is clear that the concept of the size of the Universe is very arbitrary: the real Universe is limitless and does not end anywhere.
Long-term studies of the Metagalaxy have 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.
65.Distribution of stars in the Galaxy. Clusters. General structure Galaxies.
end of form beginning of form Knowing the distances to stars allows us to approach the study of their distribution in space, and, consequently, 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 similar to the concept of the concentration of molecules. Stellar density is the number of stars located 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 ps 3 ; the average distance between stars is about 2 ps. To find out how stellar density changes in different directions, count the number of stars per unit area (for example, per 1 square degree) in different parts of the sky.
The first thing that catches your eye in such calculations is the unusually strong increase in the concentration of stars as you approach the strip of the Milky Way, the middle line of which forms a large circle in the sky. On the contrary, as one approaches the pole of this circle, the concentration of stars quickly decreases. This fact already at the end of the 18th century. allowed V. Herschel to draw the correct conclusion that our stellar system has an oblate shape, and the Sun should be located not far from the plane of symmetry of this formation. end of form beginning of form All stars with an apparent magnitude less than or equal to m, projected onto a certain area of the sky, are located inside spherical sector, the radius of which is determined by the formula
log r m =1 + 0.2 (m * M)
end of shape beginning of shape To characterize how many stars of different luminosities are contained in a given region of space, a luminosity function j (M) is introduced, which shows what proportion of the total number of stars has a given absolute magnitude, say, from M to M + 1.
end of form beginning of form Galaxy clusters - gravitationally bound systems galaxies, one of the largest structures in universe. The sizes of galaxy clusters can reach 10 8 light years.
Clusters are conventionally divided into two types:
regular - clusters of 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 is Coma Cluster.
irregular - clusters without a definite shape, inferior in number of galaxies to regular ones. Clusters of this species are dominated by spiral galaxies. Example - Virgo cluster.
Cluster masses vary from 10 13 to 10 15 mass of the Sun.
Structure of the galaxy
The distribution of stars in the Galaxy has two distinct 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, there is one star per 16 cubic parsecs, then in the center of the Galaxy there are 10,000 stars in one cubic parsec. In addition to the increased concentration of stars, in the plane of the Galaxy there is also an increased concentration of dust and gas.
Dimensions of the Galaxy: – diameter of the Galaxy’s disk is about 30 kpc (100,000 light years), – thickness – about 1000 light years.
The Sun is located very far from the galactic core - 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 core. The core has a high concentration of stars, with 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 suspected to exist at the center of the Galaxy. Almost all the molecular matter of the interstellar medium is concentrated in the annular region of the galactic disk (3–7 kpc); it contains the largest number of pulsars, supernova remnants and sources of infrared radiation. The visible radiation from the central regions of the Galaxy is completely hidden from us by thick layers of absorbing matter.
The galaxy contains two main subsystems (two components), nested one inside the other and gravitationally connected to each other. The first is called spherical - halo, its stars are concentrated towards the center of the galaxy, and the density of matter, high in the center of the galaxy, falls quite quickly with distance from it. The central, densest part of the halo within several thousand light years from 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 trajectories 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 early spectral types O and B, stars of 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. Stars with a flat component, compared to stars with a spherical component, are distinguished by a higher content of heavy elements. The age of the population of the spherical component exceeds 12 billion years. It is usually taken to be the age of the Galaxy itself.
Compared to a halo, the disk rotates noticeably faster. The rotation speed of the disk is not the same at different distances from the center. The mass of the disk is estimated at 150 billion M. The disk contains spiral branches (sleeves). Young stars and centers of star formation are located mainly along the arms.
The disk and surrounding halo are embedded in the corona. It is currently believed that the size of the Galaxy's corona is 10 times larger than the size of the disk.
We will begin our quick review with a brief discussion current state The Universe (more precisely, its observable part).
1.2.1. Homogeneity and isotropy
On large scales, the visible part modern universe homogeneous and isotropic. The sizes of the largest structures in the Universe - superclusters of galaxies and giant "voids" (voids) - reach tens of megaparsecs). Regions of the Universe with a size of 100 Mpc or more all look the same (homogeneity), while there are no distinguished directions in the Universe (isotropy). These facts are now firmly established as a result of in-depth surveys in which hundreds of thousands of galaxies have been observed.
More than 20 superclusters are known. The Local Group is part of a supercluster centered in the Virgo cluster. The size of the supercluster is about 40 Mpc, and in addition to the Virgo cluster, it includes clusters from the constellations Hydra and Centaurus. These largest structures are already very “loose”: the density of galaxies in them is only 2 times higher than the average. The center of the next supercluster, located in the constellation Coma Berenices, is about a hundred megaparsecs away.
Currently, work is underway to compile the largest catalog of galaxies and quasars - the SDSS (Sloan Digital Sky Survey) catalogue. It is based on data obtained using a 2.5-meter telescope, capable of simultaneously measuring the spectra of 640 objects in 5 frequency ranges (light wavelengths $\lambda = 3800-9200 A$, visible range). This telescope was supposed to measure the position and luminosity of more than two hundred million astronomical objects and determine the distances to more than $10^6$ galaxies and more than $10^5$ quasars. The total observation area amounted to almost a quarter of the celestial sphere. Currently processed most of experimental data, which made it possible to determine the spectra of about 675 thousand galaxies and more than 90 thousand quasars. The results are illustrated in Fig. 1.1, which shows early SDSS data: the positions of 40 thousand galaxies and 4 thousand quasars discovered in an area of \u200b\u200bthe celestial sphere with an area of 500 square degrees. Clusters of galaxies and voids are clearly visible, the isotropy and homogeneity of the Universe begins to appear on scales of the order of 100 Mpc and larger. The color of the dot determines the type of object. The dominance of one type or another is determined, generally speaking, by the processes of formation and evolution of structures - this asymmetry is temporal, not spatial.
Indeed, from a distance of 1.5 Gpc, which is the maximum in the distribution of bright red elliptical galaxies (red dots in Fig. 1.1), light traveled to Earth for about 5 billion years. Then the Universe was different (for example, the Solar system did not yet exist).
This temporal evolution becomes noticeable at large spatial scales. Another reason for choosing observation objects is the presence of a sensitivity threshold in recording instruments: at large distances only bright objects are recorded, and the brightest constantly emitting light objects in the Universe are quasars.
Rice. 1.1. Spatial distribution of galaxies and quasars according to SDSS data. Green dots indicate all galaxies (in a given solid angle) with brightness exceeding a certain value. The red dots indicate the most luminous galaxies from distant clusters, forming a fairly homogeneous population; in the accompanying reference frame, their spectrum is shifted to the red region compared to ordinary galaxies. The light blue and blue dots show the locations of regular quasars. The h parameter is approximately 0.7
1.2.1. Extension
The Universe is expanding: galaxies are moving away from each other (Of course, this does not apply to galaxies located in the same cluster and gravitationally connected to each other; we are talking about galaxies that are sufficiently distant from each other). Figuratively speaking, space, while remaining homogeneous and isotropic, is stretched, as a result of which all distances increase.
To describe this expansion, the concept of a scale factor $a(t)$ is introduced, which increases over time. The distance between two distant objects in the Universe is proportional to $a(t)$, and the particle density decreases as $^(-3)$. The rate of expansion of the Universe, i.e. relative increase in distances per unit time, characterized by the Hubble parameter $$ H(t)=\frac(\dot(a)(t))(a(t)) $$
The Hubble parameter depends on time; for its modern meaning we use, as usual, the notation $H_0$.
Due to the expansion of the Universe, the wavelength of a photon emitted in the distant past also increases. Like all distances, the wavelength increases in proportion to $a(t).$ As a result, the photon experiences a red shift. Quantitatively, the red shift z is related to the ratio of photon wavelengths at the moment of emission and at the moment of absorption $$ \frac(\lambda_(abs))(\lambda_(em))=1+z,\,\,\,\,\, \,\,\,\,\,\,\,\,\,\, (1.3) $$ where $_(abs)$ is absorption, $_(em)$ is emission.
Of course, this ratio depends on when the photon was emitted (assuming that it is absorbed on Earth today), i.e. on the distance between the source and the Earth. Red shift is a directly measurable quantity: the wavelength at the moment of emission is determined by the physics of the process (for example, this is the wavelength of the photon emitted during the transition of a hydrogen atom from the first excited state to the ground state), and $\lambda_(abs)$ is directly measured. Thus, by identifying a set of emission (or absorption) lines and determining how redshifted they are, the redshift of the source can be measured.
In reality, identification is carried out along several lines at once, most characteristic of objects of one type or another (see Fig. 1.2). If absorption lines are found in the spectrum (gaps, as in the spectra in Fig. 1.2), this means that the object for which the red shift is determined is located between the radiation source (for example, a quasar) and the observer (Photons of very specific frequencies experience resonant absorption at atoms and ions (followed by isotropic re-emission), which leads to dips in the radiation intensity spectrum in the direction towards the observer). If emission lines (peaks in the spectrum) are detected in the spectrum, then the object itself is an emitter.
Rice. 1.2. Absorption lines in the spectra of distant galaxies. The top diagram shows the results of measurements of the differential energy flux from a distant (z = 2.0841) galaxy. The vertical lines indicate the location of atomic absorption lines, the identification of which made it possible to determine the redshift of the galaxy. In the spectra of closer galaxies, these lines are better distinguishable. A diagram with the spectra of such galaxies, already brought into the accompanying reference frame taking into account the redshift, is presented in the lower figure
For $z\ll 1$, Hubble's law is valid $$ z=H_0 r,\,\,\, z\ll 1, \,\,\,\,\,\,\,\,\,\,\, \,\,\,\, (1.4) $$ where $r$ is the distance to the source, and $H_0$ is modern meaning Hubble parameter. At large z, the dependence of distance on redshift becomes more complex, which will be discussed in detail.
Determining absolute distances to distant sources is a very difficult matter. One method is to measure the flux of photons from a distant object whose luminosity is known in advance. Such objects in astronomy are sometimes called standard candles .
Systematic errors in the determination of $H_0$ are not very well known and are apparently quite large. It is enough to note that the value of this constant, determined by Hubble himself in 1929, was 550 km/(s · Mpc). Modern methods of measuring the Hubble parameter give $$ H_0=73_(-3)^(+4)\frac(km)(c\cdot Mpc). \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, (1.5) $$
Let us clarify the meaning of the traditional unit of measurement of the Hubble parameter appearing in (1.5). A naive interpretation of Hubble's law (1.4) is that the redshift is due to the radial motion of galaxies from the Earth with velocities proportional to the distances to the galaxies, $$ v=H_0r,\,\,\, v\ll 1, \,\,\ ,\,\,\,\,\,\,\,\,\,\,\,\, (1.6) $$
Then the red shift (1.4) is interpreted as a longitudinal Doppler effect (at $v\ll c$, i.e. $v\ll 1$ in natural units, Doppler shift $z=v$). In this regard, the Hubble parameter $H_0$ is assigned the dimension [speed/distance]. We emphasize that the interpretation of the cosmological redshift in terms of the Doppler effect is not necessary, and in some cases is inadequate. It is most correct to use relation (1.4) in the form in which it is written. The quantity $H_0$ is traditionally parameterized as follows: $$ H_0=h\cdot 100\frac(km)(c\cdot Mpc), $$ where h is a dimensionless quantity of the order of unity (see (1.5)), $$ h= 0.73_(-0.03)^(+0.04) $$ We will use the value $h = 0.7$ in further estimates.
Rice. 1.3. Hubble diagram constructed from observations of distant Cepheids. The solid line shows Hubble's law with the parameter $H_0$ = 75 km/(s · Mpc) determined as a result of these observations. Dashed lines correspond to experimental errors in the value of the Hubble constant
To measure the Hubble parameter, Cepheids are traditionally used as standard candles - variable stars, whose variability is related in a known way to luminosity. This connection can be revealed by studying Cepheids in some compact star formations, for example, in the Magellanic Clouds. Since the distances to all Cepheids within one compact formation can be considered identical with a good degree of accuracy, the ratio of the observed brightnesses of such objects is exactly equal to the ratio of their luminosities. The period of Cepheid pulsations can range from a day to several tens of days, during which time the luminosity changes several times. As a result of observations, a dependence of luminosity on the pulsation period was constructed: the brighter the star, the longer the pulsation period.
Cepheids - giants and supergiants, so they can be observed far beyond the boundaries of the Galaxy. Having studied the spectrum of distant Cepheids, the redshift is found using formula (1.3), and by studying the time evolution, the period of luminosity pulsations is determined. Then using known dependence variability from luminosity, determine the absolute luminosity of the object and then calculate the distance to the object, after which the value of the Hubble parameter is obtained using formula (1.4). In Fig. Figure 1.3 shows the Hubble aperture obtained in this way - the dependence of the redshift on distance.
In addition to Cepheids, there are other bright objects that are used as standard candles, such as Type 1a supernovae.
1.2.3. Lifetime of the Universe and the size of its observable part
The Hubble parameter actually has a dimension of $$, so the modern Universe is characterized by a time scale of $$ H_0^(-1)=\frac 1h\cdot \frac(1)(100)\frac(km)(c\cdot Mpc)=\ frac 1h\cdot 3\cdot 10^(17)c=\frac 1h\cdot 10^(10)\approx 1.4\cdot 10^(10) yr. $$ and cosmological distance scale $$ H_0^(-1)=\frac 1h\cdot 3000 Mpc \approx 4.3\cdot 10^3 Mpc. $$
Roughly speaking, the size of the Universe will double in about 10 billion years; galaxies located at a distance of about 3000 Mpc from us are moving away from us at speeds comparable to the speed of light. We will see that the time $H_0^(-1)$ coincides in order of magnitude with the age of the Universe, and the distance $H_0^(-1)$ coincides with the size of the visible part of the Universe. We will refine our ideas about the age of the Universe and the size of its visible part in the future. Here we note that a straight-line extrapolation of the evolution of the Universe into the past (according to the equations of classical general theory relativity) leads to the idea of the moment of the Big Bang, from which classical cosmological evolution began; then the lifetime of the Universe is the time that has passed since the Big Bang, and the size of the visible part (the size of the horizon) is the distance that signals traveling at the speed of light have traveled since the Big Bang. Moreover, the size of the entire Universe significantly exceeds the size of the horizon; in the classical general theory of relativity, the spatial size of the Universe can be infinite.
Regardless of cosmological data, there are observational lower bounds on the age of the Universe $t_0$. Various independent methods lead to close limits at the level of $t_0\gtrsim 14$ billion years $=1.4\cdot 10^(10)$.
One method by which the latter constraint is obtained is by measuring the luminosity distribution of white dwarfs. White dwarfs, compact stars of high density with masses roughly equal to the mass of the Sun, gradually dim as a result of cooling through radiation. White dwarfs of various luminosities are found in the Galaxy, but starting from a certain low luminosity, the number of white dwarfs drops sharply, and this drop is not related to the sensitivity of the observation equipment. The explanation is that even the oldest white dwarfs have not yet cooled enough to become so dim. The cooling time can be determined by studying the energy balance as the star cools. This cooling time—the age of the oldest white dwarfs—is a lower limit on the lifetime of the Galaxy, and therefore the entire Universe.
Among other methods, we note the study of the prevalence of radioactive elements in earth's crust and in meteorite composition, comparing the evolutionary curve of main sequence stars on a Hertzsprung-Russell diagram (luminosity-temperature or brightness-color) with the abundance of the oldest stars in metal-depleted globular clusters of stars ( Globular clusters are intragalactic structures with a diameter of about 30 pc, including hundreds of thousands and even millions of stars. The term "metals" in astrophysics refers to all elements heavier than helium.), studying the state of relaxation processes in star clusters, measuring the abundance of hot gas in galaxy clusters.
1.2.4. Spatial flatness
The homogeneity and isotropy of the Universe does not mean, generally speaking, that at a fixed moment in time three-dimensional space is a 3-plane (three-dimensional Euclidean space), i.e., that the Universe has zero spatial curvature. Along with the 3-plane, the 3-sphere (positive spatial curvature) and the 3-hyperboloid (negative curvature) are homogeneous and isotropic. The fundamental result of observations recent years was the establishment of the fact that the spatial curvature of the Universe, if different from zero, is small. We will repeatedly return to this statement, both in order to formulate it at a quantitative level and in order to outline what data indicate the spatial flatness of the Universe. Here it is enough to say that this result was obtained from measurements of the anisotropy of the cosmic microwave background radiation and, at a qualitative level, boils down to the fact that the radius of spatial curvature of the Universe is noticeably larger than the size of its observable part, i.e. noticeably more than $H_0^(-1)$.
We also note that the data on the anisotropy of the cosmic microwave background radiation are consistent with the assumption of a trivial spatial topology. Thus, in the case of a compact three-dimensional manifold with a characteristic size of the order of the Hubble size on celestial sphere circles would be observed with a similar picture of the anisotropy of the cosmic microwave background radiation - the intersection of the sphere of the last scattering of photons remaining after recombination (formation of hydrogen atoms) with the images of this sphere resulting from the action of the motion group of the manifold. If space had, for example, the topology of a torus, then a pair of such circles in diametrically opposite directions would be observed on the celestial sphere. CMB radiation does not exhibit such properties.
1.2.5. "Warm" Universe
The modern Universe is filled with a gas of non-interacting photons - relict radiation predicted by the Big Bang theory and discovered experimentally in 1964. The density of the number of relict photons is approximately 400 per cubic centimeter. The energy distribution of photons has a thermal Planck spectrum (Fig. 1.4), characterized by temperature $$ T_0=2.725 \pm 0.001 K \,\,\,\,\,\,\,\,\,\,\,\,\ ,\,\, (1.7) $$ (according to the analysis). The temperature of photons coming from different directions on the celestial sphere is the same at a level of approximately $10^(-4)$; this is another evidence of the homogeneity and isotropy of the Universe.
Rice. 1.4. Measurements of the spectrum of cosmic microwave background radiation. The data was compiled in . The dotted curve shows the Planck spectrum (black body spectrum). Recent analysis gives the temperature value (1.7), and not T = 2.726 K, as in the figure
Rice. 1.5. WMAP data: angular anisotropy of the cosmic microwave background radiation, i.e., the dependence of the temperature of photons on the direction of their arrival. The average photon temperature and dipole component (1.8) are subtracted; the temperature variations depicted are at the level of $\delta T \sim 100\mu K$ $\delta T/T_0\sim 10^(-4)-10^(-5)$
At the same time, it has been experimentally established that this temperature still depends on the direction on the celestial sphere. The angular anisotropy of the temperature of relict photons is currently well measured (see Fig. 1.5) and, roughly speaking, is on the order of $\delta T/T_0\sim 10^(-4)-10^(-5)$. The fact that the spectrum is Planckian in all directions is controlled by taking measurements at different frequencies.
We will repeatedly return to the anisotropy (and polarization) of the cosmic microwave background radiation, since, on the one hand, it carries the most valuable information about the early and modern Universe, and on the other hand, its measurement is possible with high accuracy.
Let us note that the presence of cosmic microwave background radiation allows us to introduce a selected reference system in the Universe: this is the reference system in which the gas of relict photons is at rest. solar system moves relative to the cosmic microwave background radiation in the direction of the constellation Hydra. The speed of this movement determines the magnitude of the dipole component of anisotropy $$ \delta T_(dipol)=3.346 mK \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, ( 1.8) $$
The modern Universe is transparent to relict photons ( In reality, "transparency" different parts Universes are different. For example, hot gas ($T\sim 10$ keV) in galaxy clusters scatters relict photons, which thereby acquire additional energy. This process leads to “heating” of relict photons - the Zeldovich-Sunyaev effect. The magnitude of this effect is small, but quite noticeable when modern methods observations.): today their mean free path is large compared to the size of the horizon $H_0^(-1)$. This was not always the case: in the early Universe, photons interacted intensely with matter.
Since the temperature of the cosmic microwave background radiation $T$ depends on the direction $\vec(n)$ on the celestial sphere, to study this dependence it is convenient to use the expansion in spherical functions (harmonics) $Y_(lm)(\textbf(n))$ that form a complete set of basis functions on the sphere. By temperature fluctuation $\delta T$ in the direction $\vec(n)$ we mean the difference $$ \delta T(\textbf(n))\equiv T(\textbf(n)) -T_0-\delta T_(dipol) =\sum_(l,m)a_(l,m)Y_(l,m)(\textbf(n)), $$ where for the coefficients $a_(l,m)$ the relation $a^*_(l ,m)=(-1)^m a_(l,-m)$, which is a necessary consequence of the reality of temperature. Angular momenta $l$ correspond to fluctuations with a typical angular scale $\pi /l$. Existing observations make it possible to study various angular scales, from the largest to scales less than 0.1° ($l\sim 1000$, see Fig. 1.6).
Rice. 1.6. Results of measurements of the angular anisotropy of the cosmic microwave background radiation by various experiments. The theoretical curve was obtained within the framework of the $\Lambda$CDM model.
Observational data are consistent with the fact that temperature fluctuations $\delta T(\textbf(n))$ represent a random Gaussian field, i.e. the coefficients $a_(l,m)$ are statistically independent for different $l$ and $m$, $$ \langle a_(l,m) a_(l",m")^*\rangle = C_(lm)\cdot \delta_(ll")\delta_(mm"), \,\,\,\,\,\,\,\,\,\,\,\,\,\,\, (1.9) $$ where under angle brackets imply averaging over an ensemble of universes similar to ours. The coefficients $C_(lm)$ in an isotropic Universe do not depend on m, $C_(lm)=C_(l)$, and determine the correlation between temperature fluctuations in different directions: $$ \langle \delta T(\textbf(n) _1)\delta T(\textbf(n)_2) \rangle = \sum_l \frac(2l+1)(4\pi)C_lP_l(\cos\theta), $$ where $P_l$ are Legendre polynomials depending only from the angle $\theta$ between the vectors $\textbf(n)_1$ and $\textbf(n)_2$. In particular, for the mean square fluctuation we obtain: $$ \langle \delta T^2\rangle = \sum_l \frac(2l+1)(4\pi)C_l\approx \int \frac(l(l+1))( 2\pi)C_ld\ln l. $$
Thus, the value $\frac(l(l+1))(2\pi)C_l$ characterizes the total contribution of angular momenta of the same order. The results of measuring this particular value are shown in Fig. 1.6.
It is important to note that measuring the angular anisotropy of the CMB gives not just one experimentally measured number, but a whole set of data, i.e., $C_l$ values for different $l$. This set is determined by a number of parameters of the early and modern Universe, so its measurement provides a lot of cosmological information.
Among objects of increasingly fainter brilliance, the number of stars increases rapidly. Thus, G. brighter than 12th magnitude is known to be approx. 250, 15th - already approx. 50 thousand, and the number of geographies that can be photographed by a 6-meter telescope at the limit of its capabilities is many billions. This indicates means. remoteness of most cities.
Extragalactic astronomy studies the sizes of stellar systems, their masses, structure, optical, infrared, x-ray properties. and radio emissions. The study of the spatial distribution of geology reveals the large-scale structure of the Universe (we can say that the observable part of the Universe is the world of geology). In the study of the spatial distribution of gases and the paths of their evolution of extragalactic. astronomy merges with cosmology - the science of the Universe as a whole.
One of the most important in extragalactic. in astronomy there remains the problem of determining the distance to G. Due to the fact that in the nearest G. found, as well as brightest stars constant brightness (supergiants), it was possible to establish the distances to these planets. To even more distant planets, in which it is impossible to distinguish even supergiant stars, the distances are estimated in other ways (see).
In 1912, Amer. astronomer V. Slifer discovered a remarkable property of G.: in the spectra of distant G. all the spectrum. the lines turned out to be shifted to the long-wave (red) end in comparison with the same lines in the spectra of sources stationary relative to the observer (the so-called lines). In 1929, Amer. astronomer E. Hubble, comparing the distances to Earth and their red shifts, discovered that the latter grow on average in direct proportion to the distances (see). This law gave into the hands of astronomers effective method determining distances to Earth based on their red shift. The redshifts of thousands and hundreds of Gs have been measured.
Determining the distances to gases and their position in the sky made it possible to establish that there are single and double gases, groups of gases, large clusters of them, and even clouds of clusters (superclusters). Wed. the distances between cities in groups and clusters are several. hundreds of pcs; this is approximately 10-20 times the size of the largest G. Avg. the distances between groups of gases, single gases, and multiple systems are 1-2 Mpc, the distances between clusters are tens of Mpc. Thus, gases fill space with a higher relative density than intragalactic stars. space (the distances between stars are on average 20 million times greater than their diameters).
Based on the radiation power, G. can be divided into several. luminosity classes. The widest range of luminosities is observed in ellipticals. G., in the central regions of certain clusters of G. the so-called. cD galaxies, which are record-breaking in luminosity (absolute magnitude - 24 m, luminosity ~10 45 erg/s) and mass (). And in our Local Group of G., an elliptical was found. G. low luminosity (absolute values from -14 to -6 m, i.e. luminosity ~10 41 -10 38 erg/s) and mass (10 8 -10 5). In spiral G. the interval is abs. stellar magnitudes range from -22 to -14 m, luminosities - from 10 44 to 10 41 erg/s, mass range 10 12 -10 8. Incorrect G. in abs. weaker magnitudes - 18 m, their luminosity is 10 43 erg/s, mass .
The formation of young stars is still underway in the central region of the Galaxy. Gas that has no rotational momentum falls toward the center of the Galaxy. 2nd generation spherical stars are born here. subsystems that make up the core of the Galaxy. But there are no favorable conditions for the formation of supergiant stars in the core, since the gas disintegrates into small clumps. In the same in rare cases when the gas transmits torque environment and is compressed into a massive body - with a mass of hundreds and thousands of solar masses, this process does not end happily: compression of the gas does not lead to the formation of a stable star, it can and does occur. The collapse is accompanied by the ejection of part of the matter from the galactic region. kernels (see).
The more massive a spiral gas is, the stronger gravity compresses the spiral arms; therefore, massive gases have thinner arms, more stars and less gas (more stars are formed). For example, in the giant nebula M81, thin spiral arms are visible, while in the nebula M33, which is a medium-sized spiral, the arms are much wider.
Depending on the type, spiral stars also have different rates of star formation. The highest speed is for the Sc type (approx. 5 per year), the lowest for Sa (approx. 1 per year). The high rate of star formation in the former is also apparently associated with the supply of gas from galactic stars. crown
Elliptical star systems evolutionary path should be simpler. The substance in them from the very beginning did not have significant torque and magnetism. field. Therefore, compression during the evolution process did not lead such systems to noticeable rotation and magnetic enhancement. fields. All the gas in these systems from the very beginning turned into spherical stars. subsystems. During the subsequent evolution, the stars ejected gas, which sank to the center of the system and went to the formation of stars of a new generation of the same spherical. subsystems. The rate of star formation in an elliptical. G. should be equal to the rate of gas inflow from evolved stars, mainly supernovae, since the outflow of matter from stars into the elliptical. G. insignificant. Annual loss of gas from stars in an elliptical. G. is calculated to be ~0.1 per galaxy with a mass of 10 11 . It also follows from the calculations that the central parts are elliptical. Due to the presence of young stars, the G. should be bluer than the peripheral regions of the G. However, this is not observed. The point is what it means. part of the resulting gas into the elliptical. The gas is blown out by the hot wind that occurs during supernova explosions, and in gas clusters it is also blown out by fairly dense hot intergalactic air. gas, recently discovered by its X-ray. radiation.
Comparing the number of stars of different generations large number of the same type, it is possible to establish possible paths of their evolution. In older stars, there is a depletion of interstellar gas reserves and, as a result, a decrease in the rate of formation and the total number of stars of new generations. But they contain many super-dense stars of small sizes, representing one of the last stages of the evolution of stars. This is the aging of planets. It should be noted that at the beginning of their evolution, planets apparently had a higher luminosity, since they contained more massive young stars. It is possible, in principle, to identify evolutionary changes in the luminosity of a planet by comparing the luminosities of nearby and very distant planets, from which light travels for many billions of years.
Extragalactic astronomy has not yet given a definite answer to questions related to the emergence of gas clusters, in particular, why in spherical. clusters are dominated by elliptical ones. and lens-shaped systems. Apparently, spherical clouds were formed from relatively small clouds of gas that had no rotational momentum. clusters with a predominance of elliptical and lens-shaped systems, which also have low torque. And from large clouds of gas, which had a significant rotational moment, gas clusters arose, similar to the Virgo Supercluster. Here there were more options for the distribution of torque among individual gas clumps from which gases were formed, and therefore spiral systems are more common in such clusters.
The evolution of gas in clusters and groups has a number of features. Calculations have shown that during collisions of gases, their extended gas coronas should be “stripped off” and scattered throughout the entire volume of the group or cluster. This intergalactic the gas was detected by high-temperature X-ray. radiation coming from clusters of gases. In addition, massive members of clusters, moving among the others, create “dynamic friction”: with their gravity they drag neighboring gases, but in turn experience braking. Apparently, this is how the Magellanic Stream was formed in the Local Group of Geographies. Sometimes massive Geographies located in the center of a cluster not only “rip off” the gas coronas of Geographies passing through them, but also capture “visitor” stars. It is assumed, in particular, that cD galaxies with massive halos formed them in such a “cannibal” way.
According to existing calculations, in 3 billion years our Galaxy will also become a “cannibal”: it will absorb the Large Magellanic Cloud approaching it.
The uniform distribution of matter on the scale of the Metagalaxy determines the sameness of matter and space in all parts of the Metagalaxy (homogeneity) and their sameness in all directions (isotropy). These important properties of the Metagalaxy are, apparently, characteristic of modern times. states of the Metagalaxy, however, in the past, at the very beginning of expansion, anisotropy and heterogeneity of matter and space could exist. The search for traces of anisotropy and inhomogeneity of the Metagalaxy in the past is a complex and urgent problem of extragalactic astronomy, which astronomers are only just approaching.