The space between stars is filled with rarefied gas, dust, magnetic fields and cosmic rays.
interstellar gas. Its total mass is quite large - a few percent of the total mass of all the stars in our Galaxy. The average density of the gas is about 10 -21 kg/m 3 . With such a density, 1-2 cm 3 of interstellar space contains only one atom of gas.
The chemical composition of interstellar gas is about the same as that of stars: most hydrogen, then helium, and very little of everything else. chemical elements.
Interstellar gas is transparent. Therefore, he himself is not visible in any telescopes, with the exception of those cases when it is near hot stars. Ultra-violet rays, as opposed to beams visible light, are absorbed by the gas and give it their energy. Due to this, hot stars heat the surrounding gas with their ultraviolet radiation to a temperature of about 10,000 K. The heated gas begins to emit light itself, and we observe it as a bright gaseous nebula (see Nebulae).
The colder, "invisible" gas is observed by radio astronomical methods (see Radio astronomy). Hydrogen atoms in a rarefied medium emit radio waves at a wavelength of about 21 cm. Therefore, streams of radio waves propagate continuously from regions of interstellar gas. By receiving and analyzing this radiation, scientists will learn about the density, temperature and movement of interstellar gas in outer space.
It turned out that it is distributed unevenly in space. There are gas clouds ranging in size from one to several hundred light years and with a low temperature - from tens to hundreds of degrees Kelvin. The space between clouds is filled with hotter and rarefied intercloud gas.
Away from hot stars, the gas is heated mainly by X-rays and cosmic rays, which continuously penetrate interstellar space in all directions. It can also be heated to high temperatures by supersonic compression waves - shock waves propagating at great speed in gas. They are formed in the explosions of supernovae and in the collisions of rapidly moving masses of gas.
The higher the density of a gas, or the more massive the gas cloud, the more energy is required to heat it. Therefore, in dense clouds, the temperature of the interstellar gas is very low: there are clouds with temperatures from a few to several tens of degrees Kelvin. In such areas, hydrogen and other chemical elements are combined into molecules. At the same time, radio emission at a wavelength of 21 cm weakens, because hydrogen from atomic (H) becomes molecular (H 2). But on the other hand, lines of radio emission of various molecules appear at wavelengths from several millimeters to several tens of centimeters. These lines are observed and can be used to judge physical condition gases in cold clouds, which are often referred to as molecular clouds or molecular gas complexes.
By radio observations in the emission lines of molecules in our Galaxy, it was discovered big number giant molecular clouds with a mass of at least 100 thousand solar masses. The total amount of gas contained in them is comparable to the amount of atomic hydrogen in the Galaxy. Areas with the most high density molecular gas form in the Galaxy a wide ring around the center with a radius of 5-7 kpc.
Using the lines of radio emission in the interstellar medium, astronomers managed to detect several dozen types of molecules: from simple diatomic molecules CH, CO, CN up such as a molecule formic acid, ethyl or methyl alcohol, and more complex polyatomic molecules. But the most common molecules are still hydrogen molecules H 2.
The density and temperature of molecular clouds are such that the gas in them tends to compress and condense under the influence of its own gravity. This process appears to lead to the formation of stars. Indeed, cold molecular clouds often coexist with young stars.
Due to the transformation of interstellar gas into stars, its reserves in the Galaxy are gradually depleted. But the gas partially returns from the stars to the interstellar medium. This occurs during the outbursts of new and supernovae, during the outflow of matter from the surface of stars, and during the formation of planetary nebulae by stars.
In our Galaxy, as in most others, the gas is concentrated towards the plane of the stellar disk, forming a layer about 100 ps thick. Towards the edge of the Galaxy, the thickness of this layer gradually increases. The gas reaches its highest density in the core of the Galaxy and at a distance of 5÷7 kpc from it.
At a great distance from the disk of the Galaxy, space is filled with very hot (more than a million degrees) and extremely rarefied gas, but its total mass is small compared to the mass of interstellar gas near the plane of the Galaxy.
Interstellar dust. The interstellar gas contains dust as a small admixture to it (about 1% by mass). The presence of dust is noticeable primarily by the absorption and reflection of starlight. Due to the absorption of light by dust, we can hardly see in the direction of Milky Way those stars that are located further than 3-4 thousand light years from us. Light attenuation is especially strong in the blue (short wavelength) region of the spectrum. This is why distant stars appear reddened. Particularly opaque due to the high density of dust are dense gas and dust clouds - globules.
Individual dust particles are very small size- a few ten-thousandths of a millimeter. They may be composed of carbon, silicon, and various frozen gases. The nuclei or cores of dust grains are most likely formed in the atmospheres of cold giant stars. From there, they are “blown out” by the pressure of star light into interstellar space, where molecules of hydrogen, water, methane, ammonia and other gases “freeze” on them.
Interstellar magnetic field. The interstellar medium is permeated with a weak magnetic field. It is about 100,000 times weaker magnetic field Earth. But the interstellar field covers gigantic volumes of outer space, and therefore its total energy is very large.
The interstellar magnetic field has practically no effect on stars or planets, but it actively interacts with charged particles moving in interstellar space - cosmic rays. By acting on fast electrons, the magnetic field "makes" them emit radio waves. The magnetic field orients elongated interstellar dust grains in a certain way, and the light of distant stars passing through the interstellar dust acquires a new property - it becomes polarized.
The magnetic field has a very great influence on the movement of interstellar gas. It can, for example, slow down the rotation of gas clouds, prevent strong gas compression, or thus direct the movement of gas clouds to force them to gather into huge gas and dust complexes.
Cosmic rays are described in detail in the corresponding article.
All four components of the interstellar medium are closely related to each other. Their interaction is complex and not yet entirely clear. When studying the interstellar medium, astrophysicists rely both on direct observations and on such theoretical branches of physics as plasma physics, atomic physics and magnetic gas dynamics.
Gas nebulae. The most famous gaseous nebula is in the constellation Orion (229), over 6 ps long, visible on a moonless night even with the naked eye. No less beautiful are the Omega, Lagoon and Trifid nebulae in the constellation Sagittarius, North America and the Pelican in Cygnus, the nebulae in the Pleiades, near the star h Carina, Rosette in the constellation Monoceros, and many others. In total there are about 400 such objects. Naturally, their total number in the Galaxy is much larger, but we do not see them because of the strong interstellar absorption of light. The spectra of gaseous nebulae contain bright emission lines, which proves the gaseous nature of their glow. The brightest nebulae also show a weak continuous spectrum. As a rule, the hydrogen lines Ha and Hb and the famous nebular lines with wavelengths of 5007 and 4950 Å, which appear during forbidden transitions of doubly ionized oxygen O III, stand out most strongly. Before these lines could be identified, it was assumed that they were emitted by the hypothetical element nebulium. Also intense are two close forbidden lines of singly ionized oxygen O II with wavelengths of about 3727 Å, lines of nitrogen, and a number of other elements. Inside a gaseous nebula or in its immediate vicinity, one can almost always find a hot star. spectral type O or B0, which is the cause of the glow of the entire nebula. These hot stars have very powerful ultraviolet radiation, which ionizes and causes the surrounding gas to glow, just as is the case in planetary nebulae (see § 152). The energy of the ultraviolet quantum of the star absorbed by the atom of the nebula is mostly used to ionize the atom. The rest of the energy is spent on giving speed to a free electron, i.e., it is ultimately converted into heat. In an ionized gas, reverse recombination processes should also occur with the return of an electron to bound state. However, most often this is implemented through intermediate energy levels, so that instead of the initially absorbed hard ultraviolet photon, the atoms of the nebula emit several less energetic visible rays (this process is called fluorescence). Thus, in the nebula, there is a kind of “crushing” of ultraviolet quanta of the star and their processing into radiation corresponding to the spectral lines visible spectrum. Radiation in the lines of hydrogen, ionized oxygen, and nitrogen, which leads to gas cooling, balances the heat input through ionization. As a result, the temperature of the nebula is established at a certain level of the order, which can be verified by the thermal radio emission of the gas. The number of quanta emitted in any spectral line is ultimately proportional to the number of recombinations, ie, the number of collisions of electrons with ions. In a strongly ionized gas, the concentration of both is the same, i.e., since, according to (7.18), the frequency of collisions of one particle is proportional to n, total number collisions of all ions with electrons per unit volume is proportional to the product of nine, i.e. Therefore, the total number of quanta emitted by the nebula, or its brightness in the sky, is proportional to the summed along the line of sight. For a homogeneous nebula of length L, this gives. The product is called the measure of emission and is the most important characteristic gaseous nebula: its value is easily obtained from direct observations of the brightness of the nebula. At the same time, the emission measure is related to the main physical parameter nebulae - the density of the gas. Thus, by measuring the measure of emission from gaseous nebulae, one can estimate the particle concentration ne, which turns out to be on the order of 102–103 cm–3 and even more for the brightest of them. As can be seen, the concentration of particles in gaseous nebulae is millions of times less than in the solar corona, and billions of times less than the best modern vacuum pumps can provide. The unusually strong rarefaction of the gas explains the appearance of forbidden lines in its spectrum, which are comparable in intensity with the allowed ones. In an ordinary gas, excited atoms do not have time to radiate a forbidden line, because much earlier than this happens, they will collide with other particles (primarily electrons) and give them their excitation energy without emitting a quantum. In gaseous nebulae at a temperature of 104 ёK, the average thermal velocity of electrons reaches 500 km/sec, and the time between collisions, calculated by formula (7.17) at a concentration ne = 102 cm −3, turns out to be 2 × 106 sec, i.e., a little less than a month, which is millions of times longer than the "lifetime" of an atom in an excited state for most of the forbidden transitions. Zones H I and H II. As we have just seen, hot stars ionize gas at large distances around them. Since this is mainly hydrogen, it is mainly Lyman quanta with a wavelength shorter than 912 Å that ionize it. But in in large numbers they can only be given by stars of spectral classes O and B0, in which effective temperatures Teff ³ 3×104 ёK and the emission maximum is located in the ultraviolet part of the spectrum. Calculations show that these stars are capable of ionizing gas with a concentration of 1 atom in 1 cm3 to distances of several tens of parsecs. Ionized gas is transparent to ultraviolet radiation, neutral, on the contrary, greedily absorbs it. As a result, the region of ionization surrounding the hot star (in homogeneous environment it's a ball!) has a very sharp boundary, beyond which the gas remains neutral. Thus, the gas in the interstellar medium can be either completely ionized or neutral. The first regions are called H II zones, the second - H I zones. There are relatively few hot stars, and therefore gaseous nebulae make up an insignificant fraction (about 5%) of the entire interstellar medium. The heating of the H I regions occurs due to the ionizing effect of cosmic rays, X-ray quanta, and the total photon radiation stars. In this case, carbon atoms are ionized first. The radiation of ionized carbon is the main mechanism of gas cooling in the H I zones. As a result, an equilibrium should be established between the energy loss and its gain, which occurs at temperature conditions, carried out depending on the density value. The first of them, when the temperature is set at several hundred degrees, is realized in one-time dust clouds, where the density is relatively high, the second - in the space between them, in which the rarefied gas is heated to several thousand degrees. Areas with intermediate values densities turn out to be unstable and the initially homogeneous gas must inevitably separate into two phases - relatively dense clouds and a very rarefied medium surrounding them. Thus, thermal instability is the main reason "ragged" and cloudy structure of the interstellar medium. Interstellar absorption lines. The existence of cold gas in the space between stars was proved at the very beginning of the 20th century. the German astronomer Hartmann, who studied the spectra of binary stars, in which the spectral lines, as noted in § 157, must experience periodic shifts. Hartmann discovered in the spectra of some stars (especially distant and hot) stationary (ie, not changing their wavelength) H and K lines of ionized calcium. In addition to the fact that their wavelengths did not change, like all other lines, they differed even in their smaller width. At the same time, the H and K lines are completely absent in sufficiently hot stars. All this suggests that stationary lines do not arise in the atmosphere of a star, but are due to the absorption of gas in the space between stars. Subsequently, interstellar absorption lines of other atoms were discovered: neutral calcium, sodium, potassium, iron, titanium, as well as some molecular compounds. However, the most complete spectroscopic study of cold interstellar gas became possible due to extra-atmospheric observations of interstellar absorption lines in the far ultraviolet part of the spectrum, where the resonance lines of the most important chemical elements are concentrated, in which, obviously, the "cold" gas should absorb most of all. In particular, resonance lines of hydrogen (La), carbon, nitrogen, oxygen, magnesium, silicon and other atoms were observed. The most reliable data on the chemical composition can be obtained from the intensities of the resonance lines. It turned out that the composition of the interstellar gas is generally close to the standard chemical composition of stars, although some heavy elements are contained in it in smaller quantities. A study of interstellar absorption lines with a large dispersion makes it possible to notice that most often they break up into several individual narrow components with different Doppler shifts corresponding on average to radial velocities of ±10 km/sec. This means that in the H I zones the gas is concentrated in separate clouds, the size and location of which exactly correspond to the dust clouds discussed at the end of the previous paragraph. The only difference is that the mass of gas is on average 100 times more. Consequently, gas and dust in the interstellar medium are concentrated in the same places, although their relative density can vary greatly from one region to another. Along with individual clouds consisting of ionized or neutral gas, the Galaxy is observed in the Galaxy, which are much larger in size, mass and density of cold interstellar matter, called gas-dust complexes. The closest to us is the well-known complex in Orion, which includes, along with many remarkable objects, the famous Orion Nebula. In such regions, which are distinguished by a complex and highly inhomogeneous structure, the process of star formation, which is extremely important for cosmogony, takes place. Monochromatic radiation of neutral hydrogen. Interstellar absorption lines to some extent give only indirect way elucidate the properties of H I regions. In any case, this can only be done in the direction of hot stars. The most complete picture of the distribution of neutral hydrogen in the Galaxy can be drawn up only on the basis of hydrogen's own emission. Fortunately, such a possibility exists in radio astronomy due to the existence of a spectral line of neutral hydrogen radiation at a wavelength of 21 cm. Total of hydrogen atoms emitting the 21 cm line is so large that the layer lying in the plane of the Galaxy turns out to be substantially opaque to the 21 cm radio emission for only 1 kpc. Therefore, if all the neutral hydrogen in the Galaxy were stationary, we could not observe it beyond a distance of about 3% of the size of the Galaxy. In reality this takes place, fortunately, only in the directions to the center and anticenter of the Galaxy, in which, as we saw in § 167, there are no relative motions along the line of sight. However, in all other directions, due to galactic rotation, there is a difference in the radial velocities of various objects that increases with distance. Therefore, we can assume that each region of the Galaxy, characterized by certain value radial velocity, due to the Doppler shift, it radiates, as it were, “its own” line with a wavelength not of 21 cm, but slightly more or less, depending on the direction of the radial velocity. Closer volumes of gas have a different mixing and therefore do not interfere with observations of more distant regions. The profile of each such line gives an idea of the gas density at a distance corresponding to a given value of the differential rotation effect of the Galaxy. Figure 230 shows the distribution of neutral hydrogen in the Galaxy obtained in this way. It can be seen from the figure that neutral hydrogen is distributed unevenly in the Galaxy. There are increases in density at certain distances from the center, which, apparently, are elements of the spiral structure of the Galaxy, confirmed by the distribution of hot stars and diffuse nebulae. Based on the polarization of light found in distant stars, there is reason to believe that the lines of force of the main part of the magnetic field are directed along the spiral arms. Galaxies, which will be discussed later in connection with cosmic rays. The influence of this field can explain the fact that most of both bright and dark nebulae are elongated along spiral branches, the very appearance of which must be somehow connected with the magnetic field. interstellar molecules. Some interstellar absorption lines have been identified with the spectra of molecules. However, in the optical range they are represented only by CH, CH+, and CN compounds. Significantly new stage in the study of the interstellar medium began in 1963, when in the wavelength range of 18 cm it was possible to register hydroxyl absorption radio lines predicted as early as 1953. In the early 1970s, they were discovered in the radio emission spectrum of the interstellar medium. lines of several dozen more molecules, and in 1973, a resonance line of the interstellar H2 molecule with a wavelength of 1092 Å was photographed on a special satellite "Copernicus". It turned out that molecular hydrogen makes up a very significant fraction of the interstellar medium. Based on the molecular spectra, a detailed analysis of the conditions in "cold" H I clouds was carried out, the processes determining their thermal equilibrium were refined, and data on the two thermal regimes given above were obtained. A detailed study of the spectra of interstellar molecular compounds CH, CH+, CN, H2, CO, OH, CS, SiO, SO and others made it possible to reveal the existence of a new element in the structure of the interstellar medium - molecular clouds, in which. a significant part of the interstellar matter is concentrated. The temperature of the gas in such clouds can range from 5 to 50 eK, and the concentration of molecules can reach several thousand molecules per 1 cm −3, and sometimes much more. Space masers. In the radio spectrum of some gas-dust clouds, instead of absorption lines of hydroxyl, quite unexpectedly, ... emission lines were found. This radiation has a number of important features. First of all, the relative intensity of all four radio lines of hydroxyl radiation turned out to be anomalous, i.e., not corresponding to the gas temperature, and the radiation in them was very strongly polarized (sometimes up to 100%). The lines themselves are extremely narrow. This means that they cannot be emitted by ordinary atoms undergoing thermal motion. On the other hand, it turned out that the sources of hydroxyl emission are so small (tens of astronomical units!), that in order to obtain the radiation flux observed from them, it is necessary to attribute to them a monstrous brightness - such as that of a body heated to a temperature of 1014−1015 ёK! It is clear that there can be no question of any thermal mechanism for the emergence of such powers. Shortly after the discovery of the emission, OH was discovered new type exceptionally bright "ultra-compact" sources emitting a radio line of water vapor with a wavelength of 1.35 cm. The conclusion about the extraordinary compactness of OH emission sources is obtained directly from observations of their angular dimensions. Modern methods radio astronomy makes it possible to determine the angular dimensions of point sources with a resolution thousands of times better than optical telescopes. For this, synchronously operating antennas (interferometer) are used, located in various parts the globe(intercontinental interferometers). With their help, it was found that the angular dimensions of many compact sources are less than 3×10−4 arc seconds! An important feature radiation from compact sources is its variability, which is especially strong in the case of H2O emission. In a few weeks and even days, the profile of the lines completely changes. Sometimes significant variations occur in 5 minutes, which is possible only if the source sizes do not exceed the distance that light travels during this time (otherwise the fluctuations will be statistically compensated). Thus, the sizes of regions emitting H2O lines can be on the order of 1 AU! As observations show, in the same region with dimensions of several tenths of a parsec there can be many sources, some of which emit only OH lines, and some only emit H2O lines. The only radiation mechanism known so far in physics capable of producing enormous power within an exceptionally narrow range of the spectrum is coherent (i.e., identical in phase and direction) radiation quantum generators, which are usually called lasers in the optical range, and masers in the radio range. The compact sources of OH and H2O emission are most likely giant natural cosmic masers. There is every reason to believe that cosmic masers are associated with regions where the process of star formation is taking place literally before our very eyes. They are most often found in H II zones, where young massive and very hot stars of spectral classes O and B have already arisen. In many cases, they coincide with very compact, dust-rich, and therefore very opaque special zones H II, which are detected only due to their thermal radio emission. The dimensions of these zones are about 0.1 ps, and the density of matter is hundreds of times greater than in ordinary interstellar clouds. The reason for their ionization is obviously an unobservable hot star surrounded by a dense opaque cloud. Sometimes these objects are observed as point sources of infrared radiation. They must certainly be exclusively young formations with an age of the order of tens of thousands of years. Behind more time the dense gas-dust medium surrounding the newly formed hot star must expand under the action of light pressure hot star, which will then become visible. Such stars, surrounded by an expanding dense shell, received the figurative name "cocoon stars". In these very specific, but nonetheless natural conditions, the maser effect is apparently realized.
is the substance observed in the space between stars.
It was only comparatively recently that it was possible to prove that stars do not exist in absolute emptiness and that outer space is not completely transparent. Nevertheless, such assumptions have been made for a long time. Back in the middle of the 19th century. Russian astronomer V. Struve tried (though without special success) scientific methods find indisputable evidence that space is not empty, and light from distant stars is absorbed in it.
The presence of an absorbing rarefied medium was convincingly shown less than a hundred years ago, in the first half of the 20th century, by comparing the observed properties of distant star clusters at different distances from us. This was done independently by the American astronomer Robert Trumpler (1896–1956) and the Soviet astronomer B.A. Vorontsov-Velyaminov (1904–1994). not completely transparent, especially in directions close to the direction of the Milky Way. The presence of dust meant that both the apparent brightness and the observed color of distant stars were distorted, and in order to know their true values, a rather complicated calculation of extinction was needed. Dust, thus, was perceived by astronomers as an unfortunate hindrance, interfering with the study of distant objects. But at the same time, interest arose in the study of dust as a physical medium - scientists began to find out how dust particles are created and destroyed, how dust reacts to radiation, and what role dust plays in the formation of stars.
With the development of radio astronomy in the second half of the 20th century. it became possible to study the interstellar medium by its radio emission. As a result of purposeful searches, radiation of neutral hydrogen atoms was discovered in interstellar space at a frequency of 1420 MHz (which corresponds to a wavelength of 21 cm). Radiation at this frequency (or, as they say, in the radio line) was predicted by the Dutch astronomer Hendrik van de Hulst in 1944 on the basis of quantum mechanics, and it was discovered in 1951 after calculating its expected intensity by the Soviet astrophysicist I.S. Shklovsky. Shklovsky also pointed out the possibility of observing the radiation of various molecules in the radio range, which, in fact, was discovered later. The mass of interstellar gas, consisting of neutral atoms and very cold molecular gas, turned out to be about a hundred times greater than the mass of rarefied dust. But the gas is completely transparent to visible light, so it could not be detected by the same methods that dust was discovered.
With the advent of X-ray telescopes installed on space observatories, another, the hottest component of the interstellar medium, was discovered - a very rarefied gas with a temperature of millions and tens of millions of degrees. It is impossible to “see” this gas either by optical observations or by observations in radio lines - the medium is too rarefied and completely ionized, but, nevertheless, it fills a significant fraction of the volume of our entire Galaxy.
The rapid development of astrophysics, which studies the interaction of matter and radiation in outer space, as well as the emergence of new observational possibilities, made it possible to study in detail the physical processes in the interstellar medium. Entire scientific areas arose - cosmic gas dynamics and cosmic electrodynamics, which study the properties of rarefied cosmic media. Astronomers have learned to determine the distances to gas clouds, measure the temperature, density and pressure of the gas, its chemical composition, and estimate the speed of movement of matter. In the second half of the 20th century a complex picture emerged spatial distribution interstellar medium and its interaction with stars. It turned out that the possibility of the birth of stars depends on the density and amount of interstellar gas and dust, and the stars (first of all, the most massive of them), in turn, change the properties of the surrounding interstellar medium - they heat it, support the constant movement of gas, replenish the medium with their substance change its chemical composition. The study of such complex system as "stars - interstellar medium" turned out to be a very difficult astrophysical problem, especially considering that the total mass of the interstellar medium in the Galaxy and its chemical composition slowly change under the influence of various factors. Therefore, we can say that the entire history of our stellar system, lasting billions of years, is reflected in the interstellar medium.
Kaplan S.A., Pikelner S.B. Physics of the interstellar medium. M., 1979
Shklovsky I.S. Stars: their birth, life and death. M., 1984
Spitzer L. The space between the stars. M., 1986
Bochkarev N.G. Fundamentals of physics of the interstellar medium. M., 1992
Surdin V.G. The birth of the stars. M., 1997
Kononovich E.V., Moroz V.I. General course of astronomy. M., 2001
To find " INTERSTELLAR MEDIUM" on the
Part One ASTRONOMIC ASPECT OF THE PROBLEM
3. Interstellar medium According to modern ideas, stars are formed by condensation of a very rarefied interstellar gas and dust medium. Therefore, before talking about the ways of the evolution of stars, we will have to dwell on the properties of the interstellar medium. This question also has independent meaning for the problem we are interested in. In particular, the issue of establishing various types links between civilizations located on different planetary systems, depends on the properties of the medium that fills the interstellar space separating these civilizations. Interstellar gas was discovered at the very beginning of this century due to absorption in the lines of ionized calcium, which it produces in the spectra of distant hot stars *. Since then, methods for studying interstellar gas have been continuously improved and reached high degree perfection. As a result of many years of extensive work done by astronomers, now the properties of the interstellar gas can be considered fairly well known: The density of the interstellar gaseous medium is negligible. On average, in regions of interstellar space located not far from the galactic plane, there is approximately 1 atom in 1 cm3. Recall that in the same volume of air there are 2.7x10 19 molecules. Even in the most perfect vacuum chambers the concentration of atoms is not less than 10 3 cm 3 . And yet the interstellar medium cannot be regarded as a vacuum! The fact is that, as is well known, a vacuum is a system in which the mean free path of atoms or molecules exceeds the characteristic dimensions of this system. However, in interstellar space average length The free path of atoms is hundreds of times less than the distance between stars. Therefore, we have the right to consider the interstellar gas as a continuous, compressible medium and apply the laws of gas dynamics to this medium. The chemical composition of interstellar gas is fairly well studied. He is similar to chemical composition outer layers of stars main sequence. Hydrogen and helium atoms predominate, there are relatively few metal atoms. The simplest molecular compounds (for example, CO, CN) are present in fairly noticeable amounts. It is possible that a significant part of the interstellar gas is in the form molecular hydrogen. The development of extra-atmospheric astronomy opened up the possibility of observing lines of molecular hydrogen in the far ultraviolet part of the spectrum. The physical properties of interstellar gas essentially depend on whether it is relatively close to hot stars or, on the contrary, sufficiently far away from them. The fact is that ultraviolet radiation hot stars, completely ionizes hydrogen at great distances. Thus, a class 05 star ionizes hydrogen around itself in a giant region with a radius of about 100 pc. The temperature of the interstellar gas in such regions (defined as a characteristic of random thermal motions of particles) reaches 10 thousand K. Under these conditions, the interstellar medium emits individual lines in the visible part of the spectrum, in particular the red hydrogen line. These regions of the interstellar medium are called "HII zones". However most of the interstellar medium is far enough away from hot stars. Hydrogen is not ionized there. The temperature of the gas is low, about 100 K or lower. It is here that there is a significant amount of hydrogen molecules. In addition to gas, the composition of the interstellar medium includes cosmic dust. The dimensions of such dust grains are 10 -4 - 10 -5 cm. They are the reason for the absorption of light in interstellar space, due to which we cannot observe objects located in the galactic plane at distances greater than 2-3 thousand pc. Fortunately, cosmic dust, like the interstellar gas associated with it, is highly concentrated towards the galactic plane. The thickness of the gas-dust layer is only about 250 pc. Therefore, radiation from space objects, directions at which make significant angles with the galactic plane, is absorbed insignificantly. Interstellar gas and dust are mixed. The ratio of the average densities of gas and dust in interstellar space is approximately 100:1. Observations show that the spatial density of the gas and dust interstellar medium varies very irregularly. This medium is characterized by a pronounced "ragged" distribution. It exists in the form of clouds (in which the density is 10 times higher than the average), separated by regions where the density is negligible. These gas and dust clouds are concentrated mainly in the spiral arms of the Galaxy and participate in the galactic rotation. Separate clouds have speeds of 6-8 km/s, as already mentioned. The densest of these clouds are observed as dark or light nebulae. A significant amount of information about the nature of interstellar gas has been obtained over the past three decades thanks to the very effective use of radio astronomical methods. Investigations of interstellar gas at a wave of 21 cm were especially fruitful. What kind of wave is this? Back in the forties, it was theoretically predicted that neutral atoms hydrogen in interstellar space should emit a spectral line with a wavelength of 21 cm. The fact is that the main, most "deep" quantum state The hydrogen atom consists of two very close levels. These levels differ in the orientations of the magnetic moments of the nucleus of the hydrogen atom (proton) and the electron rotating around it. If the moments are oriented in parallel, one level is obtained, if anti-parallel - another. The energy of one of these levels is somewhat greater than the other (by a value equal to twice the energy of interaction between the magnetic moments of an electron and a proton). According to the laws quantum physics, from time to time, transitions from the level more energy to a lower energy level. In this case, a quantum will be emitted with a frequency proportional to the energy difference between the levels. Since the latter is very small in our case, the radiation frequency will also be low. The corresponding wavelength will be equal to 21 cm. Calculations show that such transitions between the levels of a hydrogen atom occur extremely rarely: on average, one transition takes place for one atom in 11 million years! To feel the negligible probability of such processes, it is enough to say that when spectral lines are emitted in the optical range, transitions occur every hundred millionth of a second. And yet it turns out that this line, emitted by interstellar atoms, has a completely observable intensity. Since interstellar atoms have various speeds along the line of sight, then, due to the Doppler effect, the radiation in the 21 cm line will be "smeared" in a certain frequency band around 1420 MHz (this frequency corresponds to a wavelength of 21 cm). From the intensity distribution in this band (the so-called "line profile"), one can study all motions in which interstellar hydrogen atoms participate. In this way, it was possible to study the features of the galactic rotation of interstellar gas, the random movements of its individual clouds, and also its temperature. In addition, the number of hydrogen atoms in interstellar space is determined from these observations. Thus, we see that radio astronomical research at a wavelength of 21 cm is the most powerful method for studying the interstellar medium and the dynamics of the Galaxy. AT last years other galaxies, such as the Andromeda nebula, are studied by this method. As the size of radio telescopes increases, new opportunities will open up for studying more distant galaxies using the hydrogen radio line. At the end of 1963, another interstellar radio line belonging to OH hydroxyl molecules was discovered, with a wavelength of 18 cm. The existence of this line was theoretically predicted by the author of this book back in 1949. ) turned out to be very high ** . This confirms the above conclusion that in certain regions of interstellar space the gas is predominantly in the molecular state. In 1967, an H 2 O water radio line with a wavelength of 1.35 cm was discovered. Investigations of gaseous nebulae in the OH and H 2 O lines led to the discovery of cosmic masers (see the next chapter). Over the past 20 years, which have elapsed since the discovery of the OH interstellar radio link, many other radio links of interstellar origin have been discovered, belonging to various molecules. Full number already more than 50 molecules discovered in this way. Among them, especially great importance has a CO molecule whose radio line with a wavelength of 2.64 mm is observed in almost all regions of the interstellar medium. There are molecules whose radio lines are observed exclusively in dense, cold clouds of the interstellar medium. Quite unexpected was the discovery in such clouds of radio links of very complex polyatomic molecules, for example, CH 3 HCO, CH 3 CN, etc. This discovery, perhaps, is related to the problem of the origin of life in the Universe that concerns us. If discoveries continue at this rate, who knows if interstellar DNA and RNA molecules will be detected by our instruments? (see ch. 12). Very useful is the circumstance that the corresponding radio lines belonging to different isotopes of the same molecule have rather noticeably different wavelengths. This makes it possible to study the isotopic composition of the interstellar medium, which is of great importance for studying the problem of the evolution of matter in the Universe. In particular, the following isotopic combinations of carbon monoxide are observed separately: 12 C 16 O, 13 C 16 O, and 12 C 18 O. help of the so-called "recombination" radio lines, the existence of which was theoretically predicted even before their discovery by the Soviet astronomer N.S. Kardashev, who also dealt a lot with the problem of communication with extraterrestrial civilizations(see ch. 26). "Recombination" lines appear during transitions between very highly excited atoms (for example, between levels 108 and 107 of the hydrogen atom). Such "high" levels can exist in the interstellar medium only because of its extremely low density. Note, for example, that only the first 28 levels of the hydrogen atom can exist in the solar atmosphere; higher levels are destroyed by interaction with particles of the surrounding plasma. For a relatively long time, astronomers have received a number of indirect evidence of the presence of interstellar magnetic fields. These magnetic fields are associated with clouds of interstellar gas and move with them. The strength of such fields is about 10 -5 Oe, i.e., 100 thousand times less than the strength of the earth's magnetic field on the surface of our planet. General direction magnetic lines of force coincides with the direction of the branches of the spiral structure of the Galaxy. We can say that the spiral arms themselves are gigantic magnetic force tubes. At the end of 1962, the existence of interstellar magnetic fields was established by British radio astronomers through direct observations. For this purpose, very subtle polarization effects were studied in the 21 cm radio line observed in absorption in the spectrum of a powerful source of radio emission - crab nebula(for this source, see Chapter 5) *** . If the interstellar gas is in a magnetic field, one can expect the 21 cm line to split into several components that differ in polarization. Since the magnitude of the magnetic field is very small, this splitting will be completely negligible. In addition, the absorption line width of 21 cm is quite significant. The only thing to be expected in such a situation is small systematic polarization differences within the absorption line profile. Therefore, the confident detection of this subtle effect is a remarkable achievement. modern science. The measured value of the interstellar magnetic field turned out to be in full agreement with the theoretically expected value according to indirect data. To study interstellar magnetic fields, the radio astronomical method is also used, based on studying the rotation of the plane of polarization of radio emission from extragalactic sources **** as it passes through the "magnetized" interstellar medium ("Faraday phenomenon"). This method has already succeeded in obtaining a number of important data on the structure of interstellar magnetic fields. In recent years, pulsars have been used as sources of polarized radiation for measuring the interstellar magnetic field by this method (see Chap. 5). Interstellar magnetic fields play a decisive role in the formation of dense cold gas and dust clouds in the interstellar medium, from which stars condense (see Chap. 4). Interstellar magnetic fields are closely related to the primary cosmic rays that fill the interstellar space. These are particles (protons, nuclei of heavier elements, as well as electrons), whose energies exceed hundreds of millions of electron volts, reaching up to 10 20 -10 21 eV. They move along the lines of force of magnetic fields along helical trajectories. Electrons of primary cosmic rays, moving in interstellar magnetic fields, radiate radio waves. This radiation is observed by us as the radio emission of the Galaxy (the so-called "synchrotron radiation"). Thus, radio astronomy opened up the possibility of studying cosmic rays in the depths of the Galaxy and even far beyond its borders. For the first time, it put the problem of the origin of cosmic rays on a solid scientific foundation. the problem of the origin of life, until recently the question of primary cosmic rays was ignored. Meanwhile, the level of hard radiation that causes mutations is, in our opinion, a very significant evolutionary factor. There is every reason to believe that the course of evolution of life would be completely different, if the level of hard radiation (which is now largely due to primary cosmic rays) would be tens of times higher than the current value. important question: Does the level of cosmic radiation remain constant on any planet on which life develops? It's about about terms, calculated in many hundreds of millions of years. We will see in later chapters of this book how modern astrophysics and radio astronomy answer this question. The mass of interstellar gas in our Galaxy is close to a billion solar masses, which is slightly more than 1% of the total mass of the Galaxy, due mainly to stars. In others star systems ax, the relative abundance of interstellar gas varies within fairly wide limits. At elliptical galaxies it is very small, about 10 -4 and even less, while in irregular stellar systems (such as the Magellanic Clouds) the content of interstellar gas reaches 20 and even 50%. This circumstance is closely related to the question of the evolution of stellar systems, which will be discussed in Chap. 6.- * There are no intrinsic absorption lines of ionized calcium in such stars, since the temperatures of their surface layers are too high.
- ** The OH line consists of four components with close frequencies (1612, 1665, 1667 and 1720 MHz).
- *** The 21 cm absorption line, due to interstellar hydrogen, forms in the radio spectrum of any source in exactly the same way as interstellar calcium lines in the spectra of distant hot stars.
- **** Radio emission from megagalactic sources is linearly polarized, and the degree of polarization is usually on the order of several percent. The polarization of this radio emission is explained by its synchrotron nature (see below).
The nature of the interstellar medium has attracted the attention of astronomers and scientists for centuries. The term "interstellar medium" itself was first used by F. Bacon in the city of. "Oh, the Heaven between the stars, it has so much in common with the stars, revolving (around the Earth) just like any other star." The later natural philosopher Robert Boyle objected in 1674: "The interstellar region of the heavens, as some modern Epicureans believe, must be empty."
After the creation of modern electromagnetic theory, some physicists postulated that the invisible luminiferous ether is the medium for the transmission of light waves. They also believed that the ether filled the interstellar space. R. Patterson wrote in 1862: "This outflow is the basis of vibrations or oscillatory movements in the ether, which fills the interstellar space."
The use of deep photographic surveys of the night sky allowed E. Barnard to obtain the first image of a dark nebula, which stood out in silhouette against the background of the stars of the galaxy. However, the first discovery of cold diffuse matter was made by D. Hartmann in 1904 after the discovery of a stationary absorption spectrum in the emission spectrum of binary stars, observed in order to test the Doppler effect.
In his historical research spectrum of Delta Orion Hartmann studied the orbital motion of the companions of the Delta Orion system and the light coming from the star and realized that some of the light is absorbed on the way to Earth. Hartmann wrote that "the absorption line of calcium is very weak", and also that "it turned out to be somewhat surprising that the calcium lines at a wavelength of 393.4 nanometers do not move in the periodic divergence of the lines of the spectrum, which is present in spectroscopic binary stars". The stationary nature of these lines allowed Hartmann to suggest that the gas responsible for the absorption is not present in the atmosphere of Delta Orion, but, on the contrary, is located outside the star and is located between the star and the observer. This study was the beginning of the study of the interstellar medium.
After studies by Hartmann, Eger in 1919, while studying absorption lines at wavelengths of 589.0 and 589.6 nanometers in the systems of Delta Orion and Beta Scorpio, sodium was discovered in the interstellar medium.
Further research The "H" and "K" lines of calcium by Beals (1936) made it possible to detect double and asymmetric spectrum profiles of Epsilon and Zeta Orionis. These were the first comprehensive research interstellar medium in the constellation Orion. The asymmetry of the absorption line profiles was the result of the superposition of numerous absorption lines, each of which corresponded to atomic transitions(for example, the "K" line of calcium) and occurred in interstellar clouds, each of which had its own radial velocity. Since each cloud moves at different speeds in interstellar space, both towards the Earth and moving away from it, as a result of the Doppler effect, the absorption lines shifted either to the violet or to the red side, respectively. This study confirmed that matter is not evenly distributed throughout interstellar space.
Intensive studies of interstellar matter allowed W. Pickering in 1912 to state that “the interstellar absorbing medium, which, as Kaptein showed, absorbs only at certain wavelengths, may indicate the presence of gas and gaseous molecules which are expelled by the Sun and the stars."
Thorndike wrote in 1930: “It would be terrible to realize that there is an unbridgeable gulf between the stars and complete emptiness. Auroras are excited by charged particles emitted by our Sun. But if millions of other stars also emit charged particles, and this is an indisputable fact, then an absolute vacuum cannot exist in the galaxy at all.
Observational Manifestations
We list the main observational manifestations:
- The presence of luminous nebulae of ionized hydrogen around hot stars and reflective gas-dust nebulae in the vicinity of cooler stars.
- Weakening of stellar light (interstellar absorption) due to dust that is part of the interstellar medium. As well as the associated redness of the light; the presence of opaque nebulae.
- Polarization of light on dust grains oriented along the magnetic field of the Galaxy.
- Infrared radiation from interstellar dust
- Radio emission of neutral hydrogen in the radio range at a wavelength of 21 cm
- Soft x-rays hot rarefied gas.
- Synchrotron radiation of relativistic electrons in interstellar magnetic fields.
- Radiation from cosmic masers.
The structure of the ISM is extremely non-trivial and heterogeneous: giant molecular clouds, reflection nebula, protoplanetary nebula, planetary nebula, globule, etc. This leads to a wide range observational manifestations and processes occurring in the environment. The following table lists the properties of the main components of the disk environment:
| Phase | Temperature (TO) |
Concentration |
cloud mass () |
The size (PC) |
Share of occupied volume | Observation method |
|---|---|---|---|---|---|---|
| coronal gas | ≈5 | ~0.003 | - | - | ~0.5 | X-ray, absorption lines of metals in UV |
| Bright HII areas | ≈ | ~30 | ~300 | ~10 | ~ | Bright line Hα |
| Low density HII zones | ≈ | ~0.3 | - | - | ~0.1 | Hα line |
| Intercloud environment | ≈ | ~0.1 | - | - | ~0.4 | Lyα line |
| Warm HI regions | ~ | ~1 | - | - | ~0.01 | HI radiation at λ=21 cm |
| Maser condensations | ~ | ~ | ~ | Maser radiation | ||
| HI clouds | ≈80 | ~10 | ~100 | ~10 | ~0.01 | HI absorbances at λ=21 cm |
| Giant molecular clouds | ~20 | ~300 | ~3 | ~40 | ~3 | |
| molecular clouds | ≈10 | ~ | ~300 | ~1 | ~ | Absorption and emission lines of molecular hydrogen in the radio and infrared spectrum. |
| Globules | ≈10 | ~ | ~20 | ~0.3 | ~3 | Absorption in the optical range. |
Maser effect
Crab Nebula, green color- maser radiation
In 1965, very intense and narrow lines with λ = 18 cm were found in a number of radio emission spectra. Further studies showed that the lines belong to the OH molecule, and their unusual property is the result of maser radiation. In 1969, he discovers maser sources from a water molecule at λ=1.35 cm, later masers were discovered that work on other molecules as well. For maser emission, an inverse population of the levels is necessary (the number of atoms at the upper resonant level is greater than at the lower one). Then, passing through the substance, light with a resonant frequency of the wave is amplified, and not weakened (this is called the maser effect). To maintain an inverse population, a constant pumping of energy is necessary, so all space masers are divided into two types:
Physical features
Absence of local thermodynamic equilibrium (LTE)
In the interstellar medium, the concentration of atoms is small and the optical depths are small. This means that the radiation temperature is the radiation temperature of stars (~5000 K) and does not correspond to the temperature of the medium itself. In this case, the electron and ion temperatures of the plasma can differ greatly from each other, since the energy exchange upon collision occurs extremely rarely. Thus, there is no single temperature even in the local sense.
The distribution of the number of atoms and ions over the level populations is determined by the balance of recombination and ionization processes. LTE requires these processes to be in equilibrium so that the condition of detailed balance is satisfied, however, in the interstellar medium, direct and reverse elementary processes are of a different nature, and therefore a detailed balance cannot be established.
The solar wind is a stream of charged particles (mainly hydrogen and helium plasma) flowing out of the solar corona with increasing speed with great speed. The speed of the solar wind at the heliopause is approximately 450 km/s. This speed exceeds the speed of sound in the interstellar medium. And if we imagine the collision of the interstellar medium and the solar wind as a collision of two streams, then shock waves will arise during their interaction. And the medium itself can be divided into three regions: the region where there are only ISM particles, the region where only stellar wind particles and the region of their interaction.
And if the interstellar gas were completely ionized, as was originally assumed, then everything would be exactly as described above. But, as the first observations of the interplanetary medium in Ly-aplha have already shown, neutral particles interstellar medium penetrate the solar system. In other words, the Sun interacts with neutral and ionized gas in different ways.
Motion solar system in the Local Interstellar Cloud
Interaction with ionized gas
shock wave boundary
At first sunny wind slows down, becomes denser, warmer and turbulent. The moment of this transition is called border shock wave (termination shock) and is located at a distance of about 85-95 AU. e. from the Sun. (According to data from space stations Voyager 1 and Voyager 2, which crossed this border in December 2004 and August 2007.)
heliosphere and heliopause
About 40 more a.m. e. the solar wind collides with interstellar matter and finally stops. This boundary separating the interstellar medium from the matter of the solar system is called heliopause. In shape, it is similar to a bubble, elongated in opposite movement Sun side. The region of space bounded by the heliopause is called heliosphere.
