Gas and dust complexes. Interstellar medium. Interstellar medium and star formation Interstellar gas and dust
Interstellar gas and dust.
The interstellar medium is the matter and fields that fill interstellar space inside galaxies. Composition: interstellar gas, dust (1% of gas mass), interstellar magnetic fields, cosmic rays, and dark matter. The entire interstellar medium is penetrated by magnetic fields, cosmic rays and electromagnetic radiation.
Interstellar gas is the main component of the interstellar medium. Interstellar gas is transparent. The total mass of interstellar gas in the Galaxy exceeds 10 billion solar masses or several percent of the total mass of all the stars in our Galaxy. The average concentration of interstellar gas atoms is less than 1 atom per cm³. Its bulk is contained near the plane of the Galaxy in a layer several hundred parsecs thick. The average gas density is about 10 −21 kg/m³. Chemical composition approximately the same as that of most stars: it consists of hydrogen and helium (90% and 10% by the number of atoms, respectively) with a small admixture of heavier elements (O, C, N, Ne, S, etc.).
Depending on temperature and density, interstellar gas is in molecular, atomic or ionized states.
The main data on interstellar gas were obtained by radio-astronomical methods after the radio emission of neutral atomic hydrogen at a wavelength of 21 cm was discovered in 1951. It turned out that atomic hydrogen having a temperature of 100 K forms a layer 200-300 pc thick in the galactic disk at a distance of 15- 20 kpc from its center. By receiving and analyzing this radiation, scientists learn about the density, temperature and movement of interstellar gas in space.
About half of the interstellar gas is contained in giant molecular clouds with an average mass of 10^5 solar masses and a diameter of about 40 pc. Due to the low temperature (about 10 K) and increased density (up to 10^3 particles per 1 cm^3), hydrogen and other elements in these clouds are combined into molecules.
There are about 4000 such molecular clouds in the Galaxy.
Regions of ionized hydrogen with a temperature of 8000-10000 K manifest themselves in the optical range as light diffuse nebulae.
Ultraviolet rays, unlike visible light rays, are absorbed by gas and give it their energy. Thanks to this, hot stars heat the surrounding gas with their ultraviolet radiation to a temperature of approximately 10,000 K. The heated gas begins to emit light itself, and we observe it as a light gas nebula.
It is these nebulae that are indicators of places of star formation currently occurring.
Thus, in the Great Orion Nebula, protostars surrounded by protoplanetary disks were discovered using the Hubble Space Telescope.
The Great Orion Nebula is the brightest gas nebula. It is visible through binoculars or a small telescope
A special type of nebula is planetary nebulae, which appear as faintly luminous disks or rings resembling the disks of planets. They were discovered in 1783 by W. Herschel, and now there are more than 1200 of them. In the center of such a nebula there is the remnant of a dead red giant - a hot white dwarf or neutron star. Under the influence of internal gas pressure, the planetary nebula expands at a speed of approximately 20-40 km/s, while the gas density decreases.
(Planetary Hourglass Nebula picture)
Interstellar dust is solid microscopic particles, along with interstellar gas, filling the space between stars. It is currently believed that dust grains have a refractory core surrounded by organic matter or ice shell. The chemical composition of the core is determined by the atmosphere of which stars they condensed in. For example, in the case of carbon stars, they will consist of graphite and silicon carbide.
The typical size of interstellar dust particles is from 0.01 to 0.2 microns, the total mass of dust is about 1% of the total mass of gas. Starlight heats interstellar dust to several tens of Kelvin, making interstellar dust a source of long-wave infrared radiation.
Because of the dust, the densest gas formations - molecular clouds - are almost opaque and appear in the sky as dark areas, almost devoid of stars. Such formations are called dark diffuse nebulae. (picture)
Dust also affects chemical processes occurring in the interstellar medium: dust granules contain heavy elements that are used as catalysts in various chemical processes. Dust granules also participate in the formation of hydrogen molecules, which increases the rate of star formation in metal-poor clouds.
Tools for studying interstellar dust
- Distance learning.
- Research of micrometeorites for the presence of inclusions of interstellar dust.
- Study of ocean sediments for the presence of cosmic dust particles.
- Study of cosmic dust particles present at high altitudes in the Earth's atmosphere.
- Launching spacecraft to collect, study and deliver interstellar dust particles to Earth.
Interesting
- Over the course of a year, over 3 million tons of cosmic dust fall onto the earth's surface, as well as from 350 thousand to 10 million tons of meteorites - stone or metal bodies that fly into the atmosphere from outer space.
- Over the last 500 years alone, the mass of our planet has increased by a billion tons due to cosmic matter, which is only 1.7·10 -16% of the Earth's mass. However, it apparently influences the annual and daily motion of our planet.
The idea that our Galaxy does not contain the entire stellar world and that there are other star systems similar to it was first expressed by scientists and philosophers in the mid-18th century. (E. Swedenborg in Sweden, I. Kant in Germany, T. Wright in England). In the sky, other star systems appear as distant giant clusters of stars. It was natural to assume that such “external” galaxies are light, nebulous spots of low brightness, discovered by astronomers in the sky when sufficiently large telescopes became available to them. English astronomer W. Herschel at the end of the 18th century. was able with the help of what he built large telescope was the first to “decompose” some of these nebulae into individual stars. Subsequently it turned out that they are star clusters that belong to our Galaxy. Other nebulae (including the large Andromeda Nebula) were not resolved into stars, and it was unknown whether they belonged to our Galaxy or lay beyond it. Later, at the end of the 19th century, it became clear that the nature of the observed light spots is not the same at all; some of them, indeed, may be distant star clusters, while others have a spectrum characteristic of gas, and not of stars, and, therefore, are clouds of heated interstellar gas.
In the middle of the 19th century. the presence of a spiral structure in some nebulae was first discovered (Lord Ross, UK). But their stellar nature remained unproven for a long time.
Photography came to the rescue. At the beginning of the 20th century. American astronomer J. Ritchie, using a new telescope with a diameter of 1.5 m at the Mount Wilson Observatory, for the first time, using long exposures, managed to obtain photographs of several nebulous spots (including the Andromeda and Triangulum nebulae) of such high quality that they could be seen Images large number very faint stars. But since no one could say what types these stars belonged to, Ritchie’s discovery did not solve the question of distance, and therefore the nature of the objects under study. This problem was finally solved in 1924, when the American astronomer E. Hubble, conducting observations on a new instrument - a 2.5-meter reflector, discovered stars of a familiar type in the Andromeda and Triangulum nebulae - Cepheids ( cm. STARS).
Astronomers were already able to determine the distance to these variable stars from their characteristic “period-luminosity” relationship. And although it later turned out that the distances obtained by Hubble were more than half the actual distances, his estimates convincingly showed that the observed star systems were far beyond the boundaries of our Galaxy. From that time on, it became possible to talk about the birth of a new branch of science - extragalactic astronomy.
Only three galaxies are visible to the naked eye - the Andromeda nebula in the northern hemisphere and the Large and Small Magellanic Clouds in the southern hemisphere. The Magellanic clouds are the closest galaxies to us: the distance to them is approx. 150 thousand St. years.
The space between galaxies is transparent, which makes it possible to observe very distant objects. Modern large telescopes have the potential to observe more than a billion distant galaxies, however, most of them are barely visible and are visible only as tiny specks a few arcseconds in size, often difficult to distinguish in appearance from the faint stars of our Galaxy. Therefore, modern ideas about galaxies are based on the study of several tens of thousands of relatively close objects that can be studied in more detail.
The first catalog containing information about the position of more than a hundred nebulous spots in the sky was compiled by the French astronomer who specialized in searching for comets, Charles Messier in the 18th century. Most of the spots he recorded later turned out to be galaxies, the rest - light gas nebulae and star clusters of our Galaxy. Messier objects are still designated by their catalog numbers (for example, the Andromeda nebula is designated M31). One of the more extensive catalogs, the numbers of which often denote galaxies, is the New General Catalog (NGC), the foundations of which were laid by the English astronomers William Herschel and his son John Herschel. Together with its addition (Index Catalogues, or IC), the NGC catalog contains the coordinates of more than 13 thousand objects.
The work on compiling more detailed galaxy catalogs has been significantly expanded by several publications Abstract catalog of bright galaxies J. de Vaucouleurs and colleagues. More extensive, but less informative catalogs, based on viewing photographic plates of the Sky Survey, obtained on the 1.2-meter Schmidt camera of the Palomar Observatory, were published even earlier by F. Zwicky in the USA (Zwicky Catalog), P. Nilsson in Sweden (UGC catalog ) and B.A. Vorontsov-Velyaminov in the USSR (Morphological catalog of galaxies). They contain coordinates, magnitudes, angular sizes and some other parameters for several tens of thousands of galaxies up to approximately the 15th magnitude. Later, a similar survey was carried out for the southern sky - using photographs taken using the wide-angle Schmidt cameras of the European Southern Observatory in Chile and Australia. Over time, numerous more specialized atlases and catalogs of galaxies with certain properties appeared, including those compiled from observations in the radio, X-ray or infrared spectral ranges.
The same galaxy under different numbers can be included in different catalogs. With the exception of a small number of objects, galaxies do not have proper names. Each corresponds to a digital designation, which, as a rule, is preceded by an abbreviation (the name shortened to several letters) of the corresponding catalog. Designations of galaxies from various catalogs, along with extensive information about their observed properties, can be found, for example, in the NASA database on extragalactic objects on the website.
GENERAL PROPERTIES OF GALAXIES
Galaxies are complex systems in composition and structure. The smallest of them are comparable in number of stars to large star clusters in our Galaxy, but they are significantly larger in size: the diameter of even the smallest galaxies is several thousand light years. years. The sizes of giant galaxies are hundreds of times larger.
Galaxies do not have sharp boundaries; their brightness gradually decreases with distance from the center outward, so the concept of size is not strictly defined. The apparent size of galaxies depends on the telescope's ability to highlight their low-brightness outer regions against the glow of the night sky, which is never completely black. The peripheral parts of galaxies “drown” in its weak light. Modern technology makes it possible to record regions of galaxies with a brightness of less than 1% of the brightness of the night sky. To objectively estimate the size of galaxies, a certain level of surface brightness, or, as they say, a certain isophote (this is the name of the line along which the surface brightness has a constant value) is conventionally taken as their boundary. Often, magnitude 25 per square arcsecond in the photographic region of the spectrum is taken as such a threshold brightness value. The corresponding brightness is tens of times lower than the brightness of the night sky, which is not “illuminated” by anything. The brightness of the central regions of galaxies can be several hundred times higher than the threshold value.
The luminosity of galaxies (i.e., the total radiation power) varies within even greater limits than their size - from several million solar luminosities (L c) for the smallest galaxies to several hundred billion L c for giant galaxies. This value roughly corresponds to the total number of stars in the galaxy, or its total mass. The luminosity of galaxies of this type like our Galaxy is several tens of billions of solar luminosities. However, for the same galaxy it can vary greatly depending on the spectral range in which the observation is made. Therefore, observations at different wavelength intervals play a very important role in the study of galaxies. The appearance of galaxies changes unrecognizably when moving from one spectral range to another - from radio waves to gamma rays. This is due to the fact that the main contribution to the emission of galaxies at different wavelengths comes from objects of different natures.
| Spectral range | Objects that make the main contribution to the galaxy's radiation | Note |
| Gamma | Active nuclei of some galaxies. Sources producing single short bursts of radiation, apparently associated with compact stars (neutron stars, black holes).. | Emission from galaxies in this range is rarely observed. It is registered only outside the atmosphere. |
| X-ray | Hot gas filling the galaxy. Active nuclei of some galaxies. Individual sources associated with close binary star systems with the flow of matter onto a compact star. | Radiation is received only outside the atmosphere. |
| UV | The hottest stars (in galaxies where star formation occurs, these are blue supergiants). Active nuclei of some galaxies. | The radiation is especially strong in galaxies with intense star formation. |
| Visible light region | Stars with different temperatures. Light gas nebulae. | Most galaxies emit most of their energy in this range. |
| Near infrared | The coolest stars (red supergiants, red giants, red dwarfs). | The luminosity of a galaxy in this range most accurately characterizes the total mass of the stars it contains. |
| Far infrared | Interstellar dust heated by stellar radiation. Active nuclei and near-nuclear regions of some galaxies. | The radiation is especially strong in galaxies with intense star formation. Registered only outside the atmosphere. |
| Radio | High-energy electrons studying in the interstellar magnetic field. Cold (atomic, molecular) interstellar gas emitting at certain frequencies. Active nuclei of some galaxies. | The radiation provides basic information about the cold interstellar gas of the galaxy and about magnetic fields in interstellar space. |
The masses of galaxies, as well as their luminosities, can also vary by several orders of magnitude - from values typical of large globular star clusters (millions of solar masses) to thousands of billions of solar masses in some elliptical galaxies.
Galaxies are primarily star systems; It is with the stars that their optical radiation is associated. Spatially, the stars form two main structural components galaxies, as if nested one within the other: a rapidly rotating stellar disk, the thickness of which is usually 1–2 thousand light years. years, and a slowly rotating spherical (or spheroidal) component, the brightness of which is concentrated not towards the plane of the disk, but towards the center of the galaxy. The inner, brightest part of the spherodal component is called the bulge, and the outer part of low brightness is the stellar halo . In the central part of massive galaxies, there is often a small and rapidly rotating circumnuclear disk about a thousand light years in size, which also consists of stars and gas. This structure of galaxies reflects the complex multi-stage nature of their formation. There are galaxies in which only one of two main components is observed: a disk or a spheroid.
In addition to stars of varying masses, chemical compositions, and ages, each galaxy contains a tenuous and slightly magnetized interstellar medium (gas and dust) riddled with high-energy particles (cosmic rays). The relative mass of the interstellar medium, as well as the power of radio emission, are also among the most important observable characteristics of galaxies. The total mass of interstellar matter varies greatly from one galaxy to another and usually ranges from a few tenths of a percent to 50% of the total mass of stars (in rare cases, the gas can even predominate in mass over the stars). The gas content in a galaxy is a very important characteristic, on which the activity of processes occurring in galaxies and, above all, the process of star formation largely depends.
MORPHOLOGICAL CLASSIFICATION AND STRUCTURE OF GALAXIES
The variety of observed forms of galaxies has caused astronomers to desire to combine similar objects and divide galaxies into a number of classes according to their appearance(by morphology). The most commonly used morphological classification of galaxies is based on the scheme proposed by E. Hubble in 1925 and developed by him in 1936. Galaxies are divided into several main classes: elliptical (E), spiral (S), lenticular (S0) and irregular (Irr).
E-galaxies look like elliptical or oval spots, not too elongated, the brightness inside of which gradually decreases with distance from the center. There is no noticeable disk in them, although precise photometric measurements in some cases make it possible to suspect its existence. Traces of dust or gas are also rarely found in them. According to the degree of oblateness, E galaxies are divided into several subclasses - from E0 (round) to E6 (elongated). The number after the letter “E” characterizes the apparent oblateness of the galaxy. It is approximately equal to the ratio 10·(a–b)/a , Where a And b – respectively, the major and minor axes of the ellipse describing the galaxy.
In spiral (S) galaxies, there is a central condensation of stars - a “bulge”, and an extended stellar disk, in which (unless it is turned “edge-on” towards the observer) spiral branches are observed. There are spiral galaxies without a bar and with a bar. In the latter case, in the central part of the galaxy, the stars form an elongated structure - a bar, beyond which spiral branches begin. Such galaxies are designated SB. In photographs taken in the visible part of the spectrum, bars are noticeable in at least a third of all S-galaxies. In infrared rays they can be detected in an even larger number of galaxies.
Spiral galaxies are also divided into subclasses: Sa, Sb, Sc, Sd, and for galaxies with a bar – SBa, SBb, SBc, SBd. Along the sequence from a to d, the brightness of the bulge decreases, and the spiral branches become increasingly clumpy, more “unfolded,” and less distinct in shape. Edge-on spiral galaxies do not have spiral arms visible, but the type of galaxy can be determined by the relative brightness of the bulge and disk.
Between types E and S there is a type of lenticular galaxies (S0). Like S galaxies, they have a stellar disk and a bulge, but they do not have spiral arms (although they may have a bar). It is believed that these are galaxies that in the distant past were spiral, but have now almost completely “lost” or used up interstellar gas, and with it the ability to form bright spiral branches.
Irr galaxies do not have an ordered structure; they do not have spiral branches, although they contain bright regions of various sizes (as a rule, these are regions of intense star formation). The bulge in these galaxies is very small or completely absent.
Several percent of the observed galaxies do not fit into the described classification scheme; they are called peculiar. Typically these are galaxies whose shape is distorted by strong interactions with neighboring galaxies, or which have an unusual structure - for example, a polar ring rotating in a plane perpendicular to the plane of the stellar disk.
Dwarf galaxies are a separate group - small in size, the luminosity of which is thousands of times less than that of galaxies such as ours or the Andromeda nebula. They are the most numerous class of galaxies, but their low luminosity makes them difficult to detect at great distances. The size of dwarfs usually does not exceed several kiloparsecs ( cm. PARSEC). Among them there are also elliptical dE, spiral dS (very rare), and irregular (dIrr). The letter d (from the English dwarf - dwarf) denotes membership in dwarf systems.
Two types of dwarfs were also discovered, which have virtually no analogues among high-luminosity galaxies. These are dwarf spheroidal systems (dSph) and dwarf blue compact galaxies (dBCG). The first are similar to globular star clusters, increased in volume thousands of times. Such galaxies hold the record for low surface brightness among dwarfs, which even in the inner region of galaxies is often significantly lower than the brightness of the dark night sky. Several dSph galaxies are satellites of our Galaxy. In contrast, dBCG galaxies have high surface brightness with a small linear size, and their blue color indicates intense star formation. These objects are especially rich in gas and young stars.
The difference between galaxies of different types is explained both by different formation conditions and by evolutionary changes that have occurred over the billions of years of their life.
ESTIMATION OF DISTANCES TO GALAXIES
Many characteristics of galaxies, such as luminosity, linear dimensions, mass of gas and stars, rotation period, cannot be estimated if the distance to them is not known. There is no universal method for determining distances to galaxies. Some methods are used for relatively close objects, others for very distant objects. The most diverse methods are used to estimate distances to relatively nearby galaxies, in which individual bright objects can be observed and studied. Such objects are usually stars with high luminosity: Cepheids, the brightest supergiants or giants (they are easy to distinguish by color), but other formations are also often used: star clusters ( cm. STARS), planetary nebulae ( cm. NEBULA), as well as new stars at maximum brightness. The characteristics of these objects are considered known, for example, by analogy with similar objects in our Galaxy. The most accurate method involves the use of Cepheids, since the luminosities of these stars can be obtained from a well-established period-luminosity relationship. To determine distances, photometric measurements of the apparent magnitudes (apparent brightness) of objects in certain galaxies are carried out. The resulting estimates are then compared with the luminosity of the selected objects (or their absolute magnitude); in this case, a correction for interstellar absorption of light is necessarily introduced. As a result, this allows us to estimate how far the galaxy is from us.
If m is the apparent magnitude of an object, corrected for interstellar extinction, and M is its known absolute magnitude, then the logarithm of the distance D to this object, expressed in megaparsecs, is determined by the formula:
log D = 0.2(m – M) – 5.
To convert the distance to millions of light years, its value in megaparsecs must be multiplied by 3.26.
The method of determining distances not from individual objects, but from estimating the parameters of small ripples (surface brightness fluctuations) in the visible image of galaxies, which is caused by stars that cannot be resolved individually, also turned out to be effective. But all these methods are quite crude and, when applied to individual galaxies, can produce a large error.
The brightest stars suitable for estimating distances, even with the help of the largest telescopes, are observed in galaxies no more than a few tens of millions of light years away (globular clusters are somewhat further away). The exception is supernovae; they can be captured at any distance from which galaxies are visible. They are also used to estimate distances, however, they flare up in galaxies rarely and in an unpredictable way. Therefore, other approaches have been developed for more distant galaxies. For example, it is assumed that the luminosity or linear size of galaxies of a certain type is known in advance (this is a very rough method). More accurate estimates are based on statistically established relationships that connect the luminosity of galaxies with any directly measured quantity characterizing the galaxy (rotation speed, width of spectral lines belonging to stars, or emission lines of interstellar gas in the radio range). But most often, the distance to distant galaxies is determined using the Hubble relationship “redshift of spectral lines - distance”. This method (redshift method) is based on measuring the shift of lines in the spectrum of the galaxy caused by the expansion of the Universe. The empirically discovered Hubble dependence received a reliable justification in the theory of the expanding Universe. However, to calibrate empirical dependencies, relatively close galaxies are still required, for which distances are found based on individual objects. Therefore, determining how many times one galaxy is farther than another can be much more accurate than estimating the distance to each of them. In general, the accuracy of distance estimation does not exceed 10–15%, and in some cases it is significantly lower.
COMPOSITION OF GALAXIES
Interstellar gas and dust.
The distribution of gas in a galaxy can be very different from the distribution of stars. Sometimes the gas can be traced to much greater distances from the center of the galaxy than the stars, clearly demonstrating that the galaxy can extend beyond its optical boundaries. The relative fraction of mass attributable to interstellar gas increases on average from E to Irr galaxies. For galaxies like ours, it is several percent, and E-galaxies contain less than 0.1% gas (although there are exceptions to this rule).
Interstellar gas consists mainly of hydrogen and helium with a small admixture of heavier elements. These heavy elements are formed in stars and, together with the gas lost by the stars, end up in interstellar space. Therefore, the content of heavy elements is important to know for studying the evolution of the galaxy.
In spiral galaxies, gas is concentrated towards the plane of the stellar disk, and inside the disk its density is greatest in the spiral branches, as well as in the central region of the galaxy. But gas is also observed in elliptical galaxies, where there are neither stellar disks nor spiral arms. In these galaxies, gas is a hot, rarefied medium that fills the entire volume of the star system. Due to its high temperature (hundreds of thousands of degrees Kelvin), it can be observed in X-rays.
The gas in S- and Irr-galaxies exists in three main states, or phases. Firstly, these are clouds of cold (less than 100 K) molecular gas. Such a gas does not emit light, but its presence allows radio observations to be detected because different molecules in a rarefied environment emit at specific, well-known wavelengths. It is in clouds of cold gas that stars are born. Secondly, it is an atomic, or neutral, gas that forms clouds and a more rarefied intercloud medium. Such gas also does not emit light. Atomic hydrogen was discovered by radio emission at a frequency of 1420 MHz (wavelength 21 cm). As a rule, the bulk of interstellar gas is in this state. Third, in visible light there are usually numerous bright regions formed by gas ionized by ultraviolet radiation from stars and heated to a temperature of about 10,000 K. These are regions of ionized gas. As a rule, the source of heating and ionization is young massive stars, so a large amount of ionized gas indicates intense star formation in the galaxy.
The gaseous environment of interstellar space also contains a finely dispersed solid component - interstellar dust. . She manifests herself in two ways. First, dust absorbs visible and ultraviolet light, causing an overall dimming and reddening of the galaxy. The most opaque (due to dust) areas of the galaxy are visible as dark areas against a light, bright background. There are especially many opaque regions near the plane of the stellar disk - this is where the cold interstellar medium is concentrated. Therefore, if you look at the disk of the galaxy edge-on, then a dust lane crossing the galaxy in diameter is usually clearly visible. Secondly, the dust itself radiates, releasing the accumulated light energy in the form of far infrared radiation (in the wavelength range 50–1000 microns). Therefore, the total energy of dust radiation is comparable to the energy of visible radiation coming to us from all the stars in the galaxy. The total mass of dust is relatively small: it is several hundred times less than the total mass of interstellar gas. There is especially little dust in E galaxies, where cold gas is also practically absent; and also in dwarf galaxies, where there may be a lot of gas, but the environment contains few heavy elements necessary for the formation of dust grains. Dust in galaxies is a product of the evolution of stars.
Stellar population and age of galaxies.
Stars differ from each other in mass, age and chemical composition. Each galaxy can contain stars with different characteristics: massive and low-mass, young and old. The percentage of long-formed (old) stars billions of years old and stars that can be called young (less than one hundred million years old) varies greatly from one galaxy to another. Although old stars are present in galaxies of all types, along the morphological sequence of galaxies - from E to Irr - the relative number of young stars increases on average.
In E galaxies, with rare exceptions, young stars are practically absent. The spectrum and color of galaxies of this type indicate that they mainly consist of stars that arose more than 10 billion years ago. The brightest stars in E-galaxies are red giants.
Spiral and irregular galaxies contain both old and young stars. The brightest of them are blue supergiants, whose age does not exceed several tens of millions of years.
The largest numbers of young stars are observed in some rare starburst galaxies. As a rule, they belong to the Irr or dBCG types, but they can also be S galaxies. Young massive stars give these systems their bluish color. An example of a spiral galaxy with a burst of star formation that is relatively close to us is NGC 253.
In addition to the age composition, the stellar population of galaxies (as well as the interstellar gas in them) can differ in their chemical composition, or more precisely, in the relative content of chemical elements heavier than helium. Because these elements are born in massive stars and then enter interstellar space and participate in the formation of new generations of stars, young stars have more heavy elements than older ones. Therefore, measuring the abundance of heavy elements in stars allows us to obtain information about the history of star formation in the galaxy. The least amount of heavy elements was found in dwarf galaxies. This is partly explained by the fact that such elements have not yet had time to appear in them, and partly by the fact that part of the gas enriched with those formed in stars chemical elements, when ejected from stars, receives such high speeds that it is not held by the gravitational field of a low-mass galaxy and leaves it forever.
The age of galaxies is estimated by their stellar composition, which is determined by the spectrum (or color) of stellar radiation, while relying on the theory of stellar evolution, which indicates the characteristic age of stars of different spectral classes. However, the very concept of the age of galaxies is not clearly defined, since the process of galaxy formation can take 1–2 (and in some cases more) billion years. However, analysis of observations has shown that in the vast majority of cases, the oldest stars in galaxies of all types have a similar age, exceeding 10 billion years.
The era in which the mass formation of galaxies as star systems from an initially gaseous environment began is 10–13 billion years away from us. However, among dwarf galaxies there are systems whose age is apparently significantly younger. Some very rare dwarf galaxies appear to be experiencing the first burst of intense star formation in their history only in our epoch. They contain a lot of interstellar gas (atomic hydrogen) and young stars, and no noticeable traces of old stars (red giants). At the same time, their stars and interstellar gas contain very few heavy elements that simply have not yet had time to arise. But more often than not, a large number of young stars does not indicate the youth of the system, but rather the fact that, for one reason or another, another burst of star formation has occurred in the galaxy.
Star formation in galaxies.
Stars and gas are the main components of galaxies, closely related friend with a friend. In cold clouds of gas, stars are born, and at a certain stage of evolution, the latter return part of the matter to the interstellar medium. At the same time, massive stars heat and ionize the gas with their radiation. The exchange of matter between stars and the interstellar medium is unbalanced: since stars lose only part of their mass, star formation leads to a slow decrease in the supply of gas in the galaxy. Therefore, in most galaxies, gas accounts for only a few percent of the matter contained in stars, i.e. Most of the gas has already been used up.
Galaxies with intense star formation are different a large number observed young high-luminosity stars (blue supergiants) with more blue and large numbers of regions of ionized gas, the spectrum of these stars contains bright emission lines. The presence of young massive stars makes such galaxies especially bright in the ultraviolet and far infrared regions of the spectrum, leading to the appearance of many regions of ionized gas. Frequent supernova explosions increase the radio emission power of the galaxy. Based on these characteristics, the intensity of star formation in galaxies is estimated.
On average, star formation rates (per unit galaxy mass or luminosity) decrease along the Hubble sequence of types from Irr to E, although there are exceptions to this rule. In E-galaxies, young stars are either absent altogether, or their faint traces are visible only in the very center of the galaxy. In S- and Irr-galaxies, on average, from several million to several tens of millions of solar masses of matter turns into stars every million years. Moreover, as a rule, the more gas in a galaxy, the higher the rate of star formation in it.
Star formation in galaxies almost always occurs in their disks, where the interstellar medium is most concentrated. The main feature of star formation in galactic disks is its focal nature. Gas and young stars tend to cluster in discrete regions of the disk several hundred light years across. Small galaxies may contain two or three large centers of star formation, and in giant galaxies hundreds of star formation regions of various sizes are scattered throughout the disk, concentrating towards the spiral arms where the gas density is highest. Most of the observed differences between galaxies are directly or indirectly related to their star formation - as in modern era, and in the past.
The rate of star formation and the location of regions where stars are born in a galaxy depend on many factors that can accelerate or, conversely, slow down the process of transformation of gas into stars. Identifying these factors and their role in the evolution of galaxies is an important and far from solved problem.
KINEMATICS OF GALAXIES
Rotation of galaxies.
Individual stars, star clusters and gas clouds move continuously in the galaxy, with each object describing a rather complex open path around the galaxy's center of mass. But it is impossible to directly measure the movement of stars or gas clouds. Determination of the speed of movement of various objects is based on the Doppler effect, and is made by measuring the shift of lines in their spectra. For stars, these are absorption lines; for clouds of ionized gas, these are emission lines in the optical spectrum. For clouds of cold gas that do not emit light, radio emission lines of hydrogen (wavelength 21 cm) or molecular compounds, primarily CO molecules, are used; Most of these radio links lie in the centimeter and millimeter bands. Of course, measurements provide only the magnitude of the velocity projection onto the line of sight, and restoration of the full velocity vector requires certain assumptions about the nature of the objects’ motion.
Estimating the velocities of gas and stars in galaxies has one peculiarity: the objects whose velocities are determined are usually not visible individually, so the measurements provide some average velocities at a given location in the galaxy. Moreover, each star or cloud of gas can have a speed noticeably different from the average. Therefore, they often talk not about the speed of individual objects, but about the speed of gas or stars of a given type in a certain region of the galaxy.
The speeds of movement of gas and stars range from several tens of kilometers per second in dwarf galaxies to 200–300 km/s (in rare cases, up to 400 km/s) in giant spiral galaxies.
All galaxies rotate, but not like solids: The orbital period of objects increases with increasing distance to the center of rotation (center of mass) of the galaxy. In this case, a set of stars and interstellar gas can have different rotation rates even at the same distance from the center. The nature of rotation of galaxies of different types is also not the same.
Elliptical galaxies.
The velocities of stars in them are greater, the more massive the galaxy, but the velocities of neighboring stars, as a rule, have different directions, so that the average value of the velocity in each local volume of the galaxy turns out to be small. Therefore, even at high speeds of movement of stars, the rotation of the galaxy as a whole is quite slow - several tens of kilometers per second. It is curious that the degree of compression of the galaxy, contrary to expectations, turned out to be unrelated to the speed of its rotation: a slowly rotating galaxy can be either spherical or oblate.
Spiral galaxies.
Different components of galaxies have different rotation rates. The stellar bulge and stellar halo rotate the slowest: their rotation speeds are almost as low as those of E-galaxies. Stars and gas in the galactic disk rotate faster because the speeds of all the objects in the disk are more ordered: they move predominantly in one direction. The velocities of gas clouds and young stars are most ordered. Their orbits in the galactic disk are close to circular, so the speeds of these objects are often called circular rotation velocities, or circular velocities.
The graph of the change in gas velocity with distance from the center of the galaxy is called the rotation curve of the galaxy. The characteristic appearance of galaxy rotation curves is shown in Fig. 15 Spiral branches can cause noticeable deviations of rotation velocities from the circular velocity, but the amplitude of these deviations is usually small compared to the circular velocity and, as a rule, does not exceed 20–30 km/s. More significant deviations from the circular velocity are observed in interacting galaxies, as well as in local star formation regions, where the impact of massive stars on gas causes heating and expansion of the interstellar medium.
Irregular galaxies.
These are slowly rotating systems. As in the disks of S-galaxies, the rotation velocities of gas and stars in them are close to circular. Unlike E galaxies, the low rotation speed in Irr galaxies is a consequence of their low mass.
Galactic masses and the dark halo problem.
In the middle of the 20th century. It was discovered that in large clusters of galaxies the average speeds of movement of individual members of the cluster are too high for them to be able to hold each other in the cluster by their gravitational attraction. But because clusters include old star systems, they cannot be short-lived entities. It followed that most of the mass must be in the unobservable medium, the radiation of which is almost or completely absent. It was discovered quite independently that a similar problem occurs for individual galaxies.
The principle of determining the masses of galaxies is quite simple. If the objects that make up the galaxy did not attract each other, then their movement at the observed speeds would lead to the destruction of the galaxy within a few hundred million years. But gravitational forces prevent parts of the galaxy from flying away. Therefore, by measuring the speed of movement of gas or stars, you can find out how matter is distributed in the galaxy and what its mass is. Let the speed of circular rotation in the disk of the galaxy at a distance R from the center is equal V. Then the mass M galaxy contained within R, to a first approximation is equal to M(R) = V 2 R/G, Where G– gravitational constant. This approach allows one to estimate its mass from the known rotation curve of a galaxy and find out how it is distributed in the galaxy.
In the 1970s, it was discovered that the shape of the rotation curves of many spiral galaxies at large distances from the center differs significantly from what was expected. Rotation speeds in the inner region of the galaxy increase with distance R from the center, but, as a rule, starting from a certain distance, almost do not change with R, remaining high even at the periphery of the disk. If the galaxy consisted only of ordinary (observable!) stars and gas, then the rotation speed in the outer regions of the galaxy should decrease with increasing R, similar to how the speed of revolution of planets around the Sun decreases with increasing size of their orbits. Faster rotation means higher mass of matter contained within a given radius. It follows that the mass of matter in the outer regions of galaxies should be higher than expected. This is how the problem of hidden or dark mass in galaxies arose. If in the inner region of galaxies the relative fraction of dark mass is small, then the further from the center, the greater it is. From indirect data it follows that the bulk of the dark mass is contained not in the disk, but in the spheroidal component of the galaxies. Therefore, they usually talk about the dark halo of galaxies.
In different spiral and irregular galaxies, the proportion of mass accounted for by dark matter is different. In most cases, within the optical boundaries of spiral galaxies, the mass of invisible matter is comparable to the total mass of “visible” matter: stars and gas. Dark matter continues the galaxy where no starlight is visible. But there are also known galaxies where the dark mass predominates over the visible mass at all distances from the center.
Independently, the conclusion was reached about the existence of dark mass in elliptical galaxies - based on observations x-ray radiation hot gas. Its temperature is tens of millions of degrees, and a galaxy consisting of ordinary stars would not be able to contain such gas for any long time.
The nature of dark mass in galaxies is still not entirely clear. Some of it can be associated with low-mass stars or bodies intermediate in mass between stars and planets. Their radiation is undetectably weak, and the search for such bodies poses a serious scientific problem. Low-mass bodies can be detected only by their gravitational effect on light rays from distant stars that accidentally find themselves in a straight line with any of these “dark” objects: the deflection of light rays in the object’s gravitational field leads to a short-term brightening of the star (gravitational microlensing effect) .
Another direction in the search for hidden mass is associated with an attempt to discover new elementary particles, responsible for this dark mass. Such particles must have a non-zero rest mass and interact weakly with ordinary matter, making them difficult to detect. total weight There must be a very large number of such particles; they must fill the entire galaxy, freely passing not only through the interstellar medium, but also through planets and stars. It is expected that the speeds of these particles in galaxies are approximately the same as the speeds of stars. Particles with the required properties have not yet been discovered by laboratory physics methods, but their existence is predicted within the framework of physical theories of elementary particles. Whether they can constitute the bulk of galaxies must be clarified by further research.
The nature of spiral branches.
Most of the observed high-luminosity galaxies are spiral. Their spiral branches are structural formations in the rotating gas-stellar disks of galaxies. In the vast majority of cases, the rotation of galaxies occurs in such a direction that the outer ends of the spirals “lag behind” in their movement (the spirals seem to twist). Although this form of spirals is characteristic of structures that arise in a wide variety of rotating media, the nature of spirals in galaxies remained unclear for a long time. The problem lies primarily in explaining their longevity. As already noted, the disks of galaxies do not rotate like solid bodies: their angular velocity decreases with distance from the center. This type of rotation should stretch, “smear” any structural pattern of the disk, so that it will not last even several revolutions of the galaxy. However, spiral arms are observed in most disk galaxies, despite their great age.
From an observational point of view, spiral arms in galaxies are regions of higher brightness, and this is mainly due to the concentration of young stars and clouds of ionized gas, which also owe their origin to young massive stars. Spiral branches seem to synchronize star formation in the galactic disk, stimulating the appearance of dense clouds of gas and young stars along the branches. The mechanism of such synchronization is the compression of the interstellar medium in spirals. In the branches there is indeed an increased density of all components of the interstellar medium - gas, dust, magnetic field, cosmic rays.
It turned out to be much more difficult to detect an increase in the density of the old population of the stellar disk in the spiral arms, which makes up its bulk. Only observations in the near-infrared range made it possible to verify that the spiral pattern affects not only gas and young stars, but, as a rule, all components of the disk. An increase in the density of the disk in the region of the spiral branches disturbs its gravitational field. This leads to the fact that stars and gas clouds in the disk, in their movement under the influence of “excess” forces of attraction of the spirals, experience systematic deviations from circular rotation, either increasing or decreasing their speeds, and this happens in such a way that the spiral pattern is not blurred during the rotation of galaxies, but is self-sustaining. This coordinated process is described mathematically as a density wave propagating across the disk. This means that the spiral pattern is not “glued” to the disk, but moves with its own angular velocity, which remains the same at any distance from the center of the galaxy, and therefore the spiral branch cannot quickly “spin and smear.” In this case, the inner regions of the disk rotate faster than the spiral pattern, and the outer regions rotate more slowly. The radius at which these two rotational speeds compare is called the corotation radius. Its position in the galaxy is determined from an analysis of the velocities of stars or gas measured for a large number of local regions of the disk.
Each star can cross the spiral arms several times during one revolution around the center of the galaxy. For stars, such intersections occur without a trace, but interstellar gas, being a continuous medium, reacts to spiral wave a sharp increase in density, which ultimately leads to increased star formation. In the absence of gas, the bright spiral arms of galaxies would not be able to form.
Identifying the mechanisms of excitation and maintenance of density wave oscillations in the disks of galaxies is a separate rather complex problem. A major role in these processes can be played by stellar bars existing in the central regions of SB galaxies, as well as satellites and neighboring galaxies that disturb the motion of stars and gas in the galactic disk with their gravitational field. The wave theory of spirals made it possible to explain the regular spiral patterns observed in galaxies. The validity of the wave concepts is confirmed by an analysis of the velocities of gas and stars in the disks. But in real galaxies the situation is usually much more complicated. Almost never the spiral pattern is mathematically correct; the spiral structure is often broken up into separate light spots; spirals sometimes partially or entirely consist of short arc segments that do not join together (in this case they are called flocculent spirals). This reflects both the complex nature of the process of star formation spreading across the disk and the simultaneous existence of waves with different frequencies and amplitudes in the disk.
GALAXY NUCLEUS
The central region of a galaxy, called its core, is the densest part of the star system. In the image of the galaxy, the core stands out due to its high brightness. Nuclei can be seen in all types of galaxies except irregular galaxies and most dwarf galaxies. In addition to the stars, interstellar gas and numerous regions of young stars are often concentrated within about a thousand light-years of the galactic center, forming a rotating circumnuclear disk.
Most amazing property nuclei, which cannot be explained by the presence of ordinary stars and gas in the core alone, is their activity, which is pronounced in a few percent of high-luminosity galaxies. In active nuclei, non-stationary processes associated with the release of large amounts of energy are observed. In some cases, the power of energy release in the core exceeds 10 37 W, which is comparable to or exceeds the total radiation power of all the stars in the galaxy combined, although usually it is still 1–2 orders of magnitude lower.
The form of energy release in nuclei, as well as the observed signs of activity, can be different. This is the rapid movement of gas at speeds of thousands of km/s, powerful non-thermal radiation of a non-stellar nature in various regions of the spectrum - from X-ray to radio, the formation of directed plasma jets (jets), emissions of high-energy elementary particles responsible for the powerful radio emission of the galaxy. Common feature active galactic nuclei is the variability of radiation over a wide range of time intervals: from several days or even hours to several years.
Galaxies with active nuclei are usually divided into several types. A distinction is made between Seyfert galaxies, radio galaxies, quasars and lacertids. The manifestation of nuclear activity in each of these types of galaxies has its own observable features. However, in all cases, the source of powerful core energy is tiny compared to the size of the galaxy (substantially less than a light year). The “core” of such a source is presumably a supermassive black hole, onto which the initially rarefied medium located in its vicinity falls, accelerating as it falls to near-light speeds (such a medium can be the interstellar gas of the circumnuclear disk or gas that was part of stars torn apart by the gravitational field black hole). This assumption is confirmed by the discovery in the cores of large galaxies of all types of massive objects (apparently black holes) that do not have noticeable radiation, but create a very strong gravitational field. Their masses range from several million to several billion solar masses. Theoretically, the kinetic energy of falling matter imparted to it by the gravitational field of a black hole can be tens of times greater than the energy that any thermonuclear reactions in this matter can produce. From this point of view, the activity of the nucleus is associated with various mechanisms for converting the energy of falling matter into other forms. In this case, the galactic core can be in an active or quiet state, depending on the presence of flows of matter into the black hole.
The core of our Galaxy, like our neighboring Andromeda Nebula, is in a relatively calm state, despite the fact that in the very center of these galaxies the existence of objects that apparently are massive black holes has been discovered. The closest spiral galaxy with an active nucleus to us is the Seyfert galaxy NGC 1068, located at a distance of about 50 million light years. years in the constellation Cetus. The nearest peculiar elliptical galaxy with an active nucleus is the radio galaxy NGC 5128 in the constellation Centaurus. The distance to it is several times smaller.
GALAXY SYSTEMS
Groups of galaxies.
Galaxies are often grouped into pairs, triplets, and more complex groups. Single, or, as they are not entirely correctly called, “isolated” galaxies, are rare. Thus, our Galaxy is surrounded by a system of small satellites, the largest of which are the Large and Small Magellanic Clouds. The Andromeda Nebula also has satellites. All these objects, in turn, are part of the Local Group of galaxies with a diameter of about 5 million light years, which contains several dozen galaxies (mostly dwarf ones), with our galaxy and the Andromeda Nebula being the brightest and most massive members of this group. More than a dozen similar groups have been discovered within 30 million light years of the Local Group.
| Table 2. MAIN GALAXIES OF THE LOCAL GROUP | |||||||||
| Visible | Absolute | ||||||||
| Galaxy | Type | Dist. 1 | Vel. 2 | Diam. 3 | Luminosity 4 | Diam. 5 | Mass 6 | M/L 7 | |
| Milky Way | Sbc | – | – | – | 14,5? | 80? | 200? | 14? | |
| BMO | Sm | 0,15 | 0,6 | 12° | 2,75 | 31 | 15 | 5,5 | |
| MMO | Smp | 0,18 | 2,8 | 4° | 0,52 | 13 | 3 | 5,8 | |
| M 31 | Sb | 2,10 | 4,4 | 3° | 22,9 | 110 | 400 | 17 | |
| M 32 | E2 | 2,10 | 9,1 | 4ў | 0,21 | 2 | 1? | 5? | |
| M 33 | Sc | 2,20 | 6,3 | 1° | 3,63 | 38 | 20 | 5,5 | |
| Sculptor | E | 0,35 | 9,2? | 45ў | 0,004 | 5 | – | – | |
| Bake | E | 0,75 | 9,0 | 50ў | 0,019 | 11 | 0,1? | 5 | |
| NGC 205 | E | 2,10 | 8,8 | 11ў | 0,27 | 6 | – | – | |
| NGC 6822 | Im | 1,80 | 9,3 | 20? | 0,11? | 7 | – | – | |
| IC 1613 | Im | 2,10 | 9,9 | 20ў | 0,076 | 10 | – | – | |
| 1 Distance in millions of light years. 2 Apparent magnitude in blue light. 3 Apparent angular diameter in degrees or minutes of arc. 4 Absolute luminosity in billions of solar units. 5 Linear diameter in thousands of light years. 6 Mass in billions of solar units. 7 Ratio of mass to luminosity in solar units. |
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The masses of pairs, groups and triplets of galaxies are estimated from the difference in radial velocities of their members, assuming that the gravitational field of the system should be sufficient to hold all galaxies together. The mass found in this way is usually greater than the total mass of all visible members of the group. This discrepancy is called the “hidden mass problem” in galaxy systems. This problem is related to the problem of hidden mass in individual galaxies and their clusters.
Clusters of galaxies.
Galaxy systems containing hundreds or thousands of individual members are called galaxy clusters. The closest of them is located in the constellation Virgo at a distance of more than 40 million light years. Its apparent diameter is about 12° (corresponding to a linear diameter of 8 million light years), and the brightest galaxies in the cluster are visible as objects of the 9th - 10th magnitude. Elliptical and lenticular galaxies in it are concentrated towards the center, and the proportion of spiral and irregular galaxies increases towards the periphery. Richer clusters are observed even further away, such as the giant cluster in the constellation Coma Berenices, located about 300 million light years away. Usually this cluster is simply called Coma (read - Coma, from Coma Berenices - Veronica's Hair). It contains more than 10 thousand galaxies, half of which are concentrated in a central region with a diameter of 1.5°, which corresponds to 8 million light years. 23
In rich Coma clusters, galaxies are highly concentrated towards the center, similar to the stars in elliptical galaxies. In the central part of the cluster, predominantly elliptical and lenticular galaxies are observed. The total mass of giant clusters reaches 10 14 solar masses. This mass is only partially contained in galaxies. A significant part of the cluster’s matter is hot intergalactic gas: Despite the very low density of the gas (the concentration of atoms is only 100–1000 atoms per cubic meter), its glow in many clusters is reliably recorded by X-ray space telescopes. But, as in many groups of galaxies and individual galaxies, the bulk of the mass of clusters comes not from stars and gas, but from the so-called “dark mass”, the radiation of which is undetectable.
Not only galaxies, but also clusters of galaxies are distributed non-uniformly in space. Vast areas are known where the frequency of occurrence of galaxies and galaxy clusters is 5–10 times higher than the average. Sometimes such compactions are called superclusters, however, they cannot be considered as clusters of more high level. Unlike ordinary galaxy clusters, they are not gravitationally connected systems and are in a state of cosmological expansion. This kind of compaction includes, for example, the elongated region of the Shapley Supercluster in the constellation Centaurus. The distance to it is about 650 million light years, and its extent exceeds 60 million light years. The concentration of groups and small clusters at a distance of several tens of millions of years around the Virgo cluster is often called the Local Supercluster.
Statistical analysis of the distribution of a large number of distant clusters shows that their totality forms a kind of cellular structure in space with a characteristic cell size of 400–500 million light years. Toward the boundaries of the cells, the concentration of galaxies and their clusters increases and becomes several times higher than the average, but inside the cells, vast spaces can be practically devoid of high-luminosity galaxies. Such a structure was formed at the early, pre-galactic stage of the expansion of the Universe under the influence of gravitational forces of matter that had not yet had time to break up into individual protogalaxies.
Interacting galaxies.
In pairs, groups or clusters of galaxies, close encounters or even collisions of individual galaxies quite often occur. In this case, as a rule, gravitational forces between approaching galaxies lead to a distortion of their shapes, the appearance of a common luminous “fog” of individual stars that have left the galaxies, and the appearance of bridges or long tails consisting of gas and stars directedly ejected from the galaxies. Systems of such galaxies are called interacting (the term was introduced by B.A. Vorontsov-Velyaminov, who was the first to systematically study these objects). Computer simulations have shown that most of the shapes of interacting galaxies can be naturally explained by their gravitational influence on each other. By selecting the magnitude and direction of the relative velocities of galaxies, their masses and the distance between them, it is possible to simulate the observed features of interacting galaxies, including the development of tails and bars as a result of the convergence of two systems. With each approach of galaxies in groups or pairs, they lose part of the energy of their orbital motion and must come closer to each other with each subsequent meeting. The final stage of such a process will inevitably be the mutual penetration of galaxies and their merging into one system, but this may take many billions of years.
The effects of interaction are not limited to the distortion of shapes or the appearance of long emissions of matter. In particular, they can greatly affect the nature of the movement of interstellar gas in the disks of galaxies, cause the appearance of large-scale shock waves, lead to a sharp increase in the rate of star formation in one or both galaxies, to the redistribution of gas in them, and even to a surge in nuclear activity. Particularly strong effects occur when galaxies interpenetrate or a small satellite falls inside a giant galaxy. In the latter case, as calculations show, the satellite should move in a spiral towards the galactic core, quickly collapsing in the process. In particular, the presence of gas and dust disks in some elliptical galaxies (including the radio galaxy NGC 5128 mentioned above) is apparently associated with the destruction of gas-rich satellites that were once captured by the galaxy.
When a sufficiently massive satellite is absorbed or two galaxies of comparable mass merge, the internal structure and even the morphological type of galaxies can change. The merger of galaxies and the absorption of small satellites by them is an important feature of the evolution of galaxies of all types. In our Galaxy there are also traces of the destruction of the star systems it captured, and one of the dwarf satellites, which relatively recently penetrated into the Galaxy and has not yet had time to collapse, is observed near the plane of the Galaxy on the other side of its center, in the constellation Sagittarius.
Anatoly Zasov
Literature:
Baade V. Origin and evolution of stars and galaxies. M.: Mir, 1966
Hoyle F. Galaxies, nuclei, quasars. M.: Mir, 1968
Origin and evolution of galaxies and stars. – Ed. S.B. Pikelner. M.: Nauka, 1976
Vorontsov-Velyaminov B.A. Extragalactic astronomy. M.: Nauka, 1978
Mitton S. Galaxy exploration. M.: Mir, 1980
Agekyan T.A. Stars, galaxies, Metagalaxy. M.: Nauka, 1981
Tayler R.J. Galaxies: structure and evolution. M.: Mir, 1981
Marochnik L.S., Suchkov A.A. Galaxy. M.: Nauka, 1984
Gurevich L.E., Chernin A.D. Origin of galaxies and stars. Science, 1987
Suchkov A.A. Galaxies familiar and mysterious. M.: Nauka, 1988
Hodge P. Galaxies. M.: Nauka, 1992
Zasov A.V. Physics of galaxies. M.: Publishing house of Moscow State University, 1993
Surdin V.G. The Birth of Stars. M.: URSS, 2001
Efremov Yu.N. Into the depths of the Universe. M.: URSS, 2003
"The Origin of Galaxies and Stars" - The Visible Universe. Formation of superclusters of galaxies. The retreat of galaxies. Milky Way. Critical density of the universe. Hadron era. Density of the Universe. Solar system. Extension. Astronomical structures. The expansion of the Universe arose as a result big bang. Density. Nucleosynthesis in the early universe.
“Properties of galaxies” - Types of spiral galaxies. Ultracompact dwarf galaxies. Irregular galaxies. Spiral galaxies. Gravity-bound system. Small Magellanic Cloud. Andromeda's nebula. Seyfert galaxies. Age of galaxies. Elliptical galaxies. Composition of spiral galaxies. Large Magellanic Cloud.
“Galaxies and Stars” - Black Hole. Age of the Metagalaxy. Northern direction. Andromeda's nebula. Types of galaxies. The energy of a thermonuclear reaction. Electrons. Stages of the existence of stars. Transformations. Galaxies are not evenly distributed. Substance. Stages of star formation. Gas cycle. Basic concepts. Galaxies and stars.
“Types of galaxies” - Galaxies. Spatial arrangement of galaxies. Clusters of galaxies. Irregular galaxies. Quasars and quasags. Distance to the galaxy. Hubble tuning fork classification. Elliptical galaxies. Spiral galaxies. Linearity. Protogalactic clouds. Barred spiral galaxies. Hubble's law.
“Galaxies and Nebulae” - A galaxy is a system of stars, interstellar gas, dust and dark matter. . Large and Small Magellanic Clouds. Cat's Eye Nebula. Andromeda nebula as seen from Earth. Ring Nebula. Andromeda's nebula. Galaxy Sombrero. Horsehead Nebula. Telescope image from space. By the early 1990s, there were no more than 30 galaxies.
"Types of Galaxy" - Virgo A Galaxy with jet. Irregular galaxy NGC1313. Radio galaxy NGC5128 (Centaurus A). Galaxy M64 (Eye). Galaxy M101. Spiral galaxy NGC2997. Quasar 3C273. Spiral galaxy M31 is a member of the Local Group along with the Milky Way. Intersecting spiral galaxy NGC 1365. Interacting Wheel galaxy.
There are 12 presentations in total
Slide 2
GALAXY A galaxy is a large system of stars, interstellar gas, dust and dark matter, bound by gravitational forces. Typically, galaxies contain from 10 million to several trillion stars orbiting a common center of gravity. In addition to individual stars and the rarefied interstellar medium, most galaxies contain many multiple star systems, star clusters and various nebulae. As a rule, the diameter of galaxies ranges from several thousand to several hundred thousand light years, and the distances between them are calculated in millions of light years.
Slide 3
There are countless stars in the sky. However, only about 2.5 thousand can be observed with the naked eye in clear weather in each hemisphere. Stars are distributed unevenly in the Universe, forming galaxies consisting of various numbers stars: from tens of thousands to hundreds of billions. There are an innumerable number of galaxies throughout the Universe. The stars are so far from us that even in the most powerful telescope they are visible as points. The closest star to the Sun, Proxima Centauri, is 4.25 light years away, and the closest galaxy, the Sagittarius Dwarf Galaxy, is 80 thousand light years away. Stars
Slide 4
Interstellar gas is a rarefied gaseous medium that fills all the space between stars. Interstellar gas is transparent. The total mass of interstellar gas in the Galaxy exceeds 10 billion solar masses or several percent of the total mass of all the stars in our Galaxy. The chemical composition is approximately the same as that of most stars: it consists of hydrogen and helium (90% and 10% by number of atoms, respectively) with a small admixture of heavier elements. Depending on temperature and density, interstellar gas is in molecular, atomic or ionized states. Interstellar Gas
Slide 5
Interstellar dust is an admixture of solid microscopic particles in interstellar gas. The total mass of interstellar dust is about 1% of the gas mass. The size of interstellar dust particles is from 0.01 to 0.02 microns. The dust grains probably have a refractory core (graphite, silicate or metal) surrounded by organic matter or an icy shell. Recent studies indicate that dust particles are generally non-spherical in shape. Dust affects the optical emission of stars, leading to absorption, reddening and polarization of star light. Interstellar Dust
Slide 6
The general name for a set of astronomical objects that are inaccessible to direct observation by modern means of astronomy (that is, not emitting electromagnetic radiation of sufficient intensity for observation), but observable indirectly by the gravitational effects exerted on the observed objects. The general problem of hidden mass consists of two problems: astrophysical, that is, the contradiction of the observed mass of gravitationally bound objects and their systems, such as galaxies and their clusters, with their observed parameters determined by gravitational effects; cosmological - contradictions between the observed cosmological parameters and the average density of the Universe obtained from astrophysical data. Dark matter
Slide 7
The Sun, the central body of the Solar System, is a hot ball of gas. It is 750 times more massive than all other bodies in the Solar System combined. That is why everything in the solar system can be approximately considered to revolve around the sun. The Sun outweighs the Earth by 330,000 times. The solar diameter could accommodate a chain of 109 planets like ours. The Sun is the closest star to Earth; it is the only star whose visible disk is visible to the naked eye. All other stars, light years away from us, even when viewed through powerful telescopes, do not reveal any details of their surfaces. Light from the Sun reaches us in 8 and a third minutes. According to one hypothesis, it was together with the Sun that our planetary system, the Earth, and then life on it was formed. Sun
Slide 8
A parallel world is a reality that somehow exists simultaneously with ours, but independently of it. This autonomous reality can have various sizes: from a small geographical area to an entire universe. In a parallel world, events happen in their own way; it can differ from our world both in individual details and radically, in almost everything. The physical laws of a parallel world are not necessarily similar to the laws of our world; in particular, it is sometimes allowed to exist in parallel worlds phenomena such as magic. A parallel world
Slide 9
The great Cosmonaut Yuri Alekseevich Gagarin was born on March 9, 1934 in the village of Klushino, Gzhatsky district, Western region of the RSFSR, not far from the city of Gzhatsk (later renamed the city of Gagarin) Gagarinsky district, Smolensk region. On April 12, 1961, for the first time in the world, it launched from the Baikonur Cosmodrome spaceship"Vostok", on board with pilot-cosmonaut Yuri Alekseevich Gagarin. For this feat, he was awarded the title of Hero of the Soviet Union, and starting from April 12, 1962, the day of Gagarin's flight into space was declared a holiday - Cosmonautics Day. Yuri Alekseevich Gagarin FIRST COSMONAUT OF THE PLANET
Slide 10
Comets are small celestial bodies with a nebulous appearance, usually revolving around the Sun in elongated orbits. When approaching the Sun, comets form a coma and sometimes a tail of gas and dust. The nucleus is the solid part of the comet, which is relatively small in size. A coma forms around the nucleus of an active comet (as it approaches the Sun). Comet nuclei consist of ice with the addition of cosmic dust and frozen volatile compounds: carbon monoxide and dioxide, methane, ammonia. Comets Ulyanovsk 2009
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Composition of the interstellar medium
The main component of the ISM is hydrogen (~ 70% of the total mass), which is present there in various forms: neutral atomic
hydrogen, molecular hydrogen (H2), ionized hydrogen.
About 28% of the mass is helium and ~2% is the share of other elements.
In addition to gas, the ISM contains solid particles (dust). The ratio of dust mass to gas mass is ~0.01.
Two-phase model of the interstellar medium
In the simplest two-phase model, in a certain pressure range, the neutral ISM breaks up into two stable phases (being in pressure equilibrium): a dense cold phase (“clouds”), T ~ 100 K,
n ~ 10 cm-3, and rarefied hot (“intercloud medium”), T ~ 104 K, n ~ 0.1 cm-3.
Main components of the MZS
Phase
Coronal gas
Low Density HII Zones
Cross-cloud environment
Warm areas HI
Clouds HI
dark clouds
Areas HII
Giant molecular clouds
Maser
condensation
T(K) |
n(cm-3) |
M (Msun) |
L (pc) |
|
~ 5·105 |
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~104 |
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~104 |
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~103 |
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~103 |
~ 10-5 |
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~104 |
~ 3·10-9 |
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~104 |
~ 10-4 |
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~ 3·105 |
~ 3·10-4 |
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~ 1010 |
~ 10-5 |
Heating and cooling mechanisms
Basic heating mechanisms
Ultraviolet radiation from stars (photoionization).
Heating by shock waves.
Volumetric heating of gas by penetrating radiation and cosmic rays
Volumetric heating of gas by hard electromagnetic radiation (X-ray and gamma quanta).
Basic cooling mechanisms
Free-free(bremsstrahlung) radiation
Recombination radiation
Emission in spectral lines
Dust radiation
Electron impact ionization
Cosmic rays
The cosmic ray flux in the vicinity of the Solar System is ~ 1 particle/cm 2·s. Hence the average concentration of fast protons in the interstellar medium is ~ 10-10 –10-11 cm-3.
Cosmic rays contain the most protons (~ 90% by number of particles). Helium nuclei make up about 7% by number of particles. A feature of the CR is the relatively large abundance of lithium, beryllium, boron nuclei (~ 0.14%), while in the interstellar There are very few of them in the gas-dust environment (~ 10-6%).
The CR energy spectrum has a power-law character, although the spectrum index can vary in different regions. The average CR energy density is close to 10-12 erg/cm3.
Most likely, cosmic rays are accelerated during supernova explosions and (or) in pulsars.
Differential spectrum of cosmic rays in interplanetary space near the Earth's orbit: 1 - protons; 2 - particles of galactic cosmic rays; 3 - protons from solar flares.
Shown for comparison
spectra of protons and -particles
Origin of cosmic rays
Dependence of the gamma ray flux on galactic longitude l according to observational data (vertical lines) in comparison with the calculation results (solid curve) based on the hypothesis of supernova remnants as the main source of cosmic rays.
CL acceleration mechanisms
Fermi mechanism.
Interaction between a particle and interstellar clouds that move along with frozen magnetic fields
(magnetic bottle). Traffic jams approach at speed U<< V . За одно столкновение частица приобретает скорость 2U , число столкновений в единицу времени V /2L .
V dL |
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Statistical acceleration mechanism (during chaotic motion of a particle between clouds). During oncoming collisions with clouds, the energy of the particle increases, and during overtaking collisions, it decreases. The relative speed during oncoming collisions is higher, and therefore the number of such collisions is greater. The gas of heavy clouds is in equilibrium with the gas of particles. The direction of the process should lead to the establishment of equidistribution of energy between clouds and particles. The role of the magnetic field is reduced to reflecting particles from clouds.
