Physical properties of interstellar gas. Interstellar gas and dust

interstellar gas

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 a few percent of the total mass of all the stars in our Galaxy. The average concentration of atoms in interstellar gas is less than 1 atom per cm³. Its main mass is contained near the plane of the Galaxy in a layer several hundred parsecs thick. The average density of the gas is about 10 −21 kg/m³. The chemical composition is about 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. Cold molecular clouds, rarefied intercloud gas, clouds of ionized hydrogen with a temperature of about 10 thousand K. (Orion Nebula), and vast areas of rarefied and very hot gas with a temperature of about a million K are observed. Ultra-violet rays, unlike rays of visible light, are absorbed by the gas and give it their energy. Due to this, hot stars with their ultraviolet radiation heat the surrounding gas 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. The colder, "invisible" gas is observed by radio astronomical methods. 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.

interstellar medium
Components	interstellar gas Interstellar dust Interstellar cloud Cosmic rays Magnetic field
Nebulae	Diffuse (light) nebula Dark nebula Emission nebula Reflection nebula Supernova remnant Planetary nebula Protoplanetary nebula
Regions of star formation	Molecular cloud Globule Region H II
Circumstellar formations	Accretion disk Protoplanetary disk Polar jet streams Herbig-Haro object
Radiation	Stellar wind CMB

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See what "Interstellar gas" is in other dictionaries:

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Gas dynamics is a branch of physics that studies the laws of gas motion. We often encounter questions of gas dynamics in everyday life- these are sound waves, and the flow around fast moving bodies, and shock waves, which are well known to everyone in the age of supersonic speeds. But the conditions of the interstellar medium significantly change the laws of gas motion.

Let's start with sound waves. As the reader probably knows, sound waves are a series of gas compressions and rarefications propagating through a medium. If you slightly compress the gas in a certain volume, and then give it the opportunity to return to its original state, then by inertia it will then expand a little, compress the layers of gas adjacent to this volume, and then compress itself again. Oscillations will arise, which will be transmitted to neighboring layers, and from them - even further. This is the propagation of sound waves. Their speed depends only on the temperature of the gas. The speed of sound waves in air at a temperature of 300 K is well known - 330 m / s, and with increasing temperature it increases proportionally ( T) 1/2 .

But such sound waves are adiabatic, i.e., it is assumed that the compression and rarefaction of gas in sound waves occurs without loss of heat. This is not the case in interstellar space. As the density increases, so does the radiation loss. Therefore, interstellar sound waves are by no means adiabatic. In the first approximation, they can also be considered isothermal, i.e., it can be assumed that during compression and expansion of the gas, the temperature in the wave does not change at all. Then the speed of sound waves will be somewhat less (in air - by 20%) and it can be calculated by the formula: withs = (RT/mu) 1/2 , where R is the universal gas constant, and mu is the molecular weight. It is curious that even Newton, who was the first to calculate the speed of a sound wave, assumed it to be isothermal, and therefore for a long time it was not clear why the speed of sound in air turned out to be higher than calculated. However, for interstellar sound waves, this formula, obtained by Newton, is quite applicable.

The next important phenomenon, which also changes its properties under interstellar conditions, is shock waves. To explain this, consider the case shown in Fig. 16. Let a gas flow into a long pipe closed at one end with a concentration p 1 and speed v. Hitting the wall, he must stop. A region of immobile gas is formed, which should increase all the time as more and more new portions of gas flow in. A boundary is formed between the gas at rest and the moving gas (dotted line in Fig. 16), which moves along the pipe towards the gas flow.

Let us denote the gas concentration beyond this boundary as n 2 . It turns out that if the speed v is very large (much greater than the speed of sound), then this boundary is sharp (shock wave), and the jump in concentration, i.e., the value n 2 / n 1, turns out to be limited (for example, in a monatomic gas p 2 / p 1<4, в двухатомном p 2 / p 1<6). It is explained simply. The kinetic energy of the incident gas not only compresses but also heats the stopped gas. Thus, in a stationary region, a large gas pressure arises, which prevents further compression.

But in interstellar space, this may not be the case. As soon as the gas is compressed, its radiation will sharply increase and the temperature will no longer rise. The gas pressure remains small, and it does not prevent further compression of the gas. As a result, very large concentration shocks can occur in interstellar shocks, better termed "compression shocks". jump size p 2 / p 1 can be determined by comparing the gas pressure in the compressed region (i.e., a value proportional to n 2 RT) with dynamic pressure of the incident gas flow proportional to p 1v 2 . Thus, we find that the density jump in the interstellar shock wave is characterized by the quantity n 2 /p 1 ~muv 2 / RT~ v 2 / c s 2 , where T- the usual temperature of interstellar gas (about 10 4 K in HII zones and much less, 10-20 K, in molecular clouds). The reader can easily see that even at low gas velocities (for example, at a speed of 7-8 km / s, the usual speed of interstellar clouds), one can obtain (when they collide with each other) shock waves of tens and even hundreds of times changing concentration .

Of course, the case shown in Fig. 16, there is an idealization - there are no pipes in interstellar space, but the general features of the movement there are just that.

One of the important cases of the dynamics of the interstellar medium is shown in Fig. 17 - the fall of interstellar gas under the action of its own gravity to the center of the cloud. This fall creates a compression region at the center of the cloud, surrounded by a spherical shock wave propagating from the center. Obviously, here too there can be a very strong compression of matter, but in a real object, i.e., this phenomenon is very possible during the formation of stars.

The third feature of interstellar gas dynamics is the essential role of magnetic fields. Let us consider this feature using an example familiar to the reader from a school physics course. If a conductor is moved through a magnetic field, an electric current is induced in it, which, in turn, creates a magnetic field. As a result of the interaction of these fields, a force arises that slows down the movement of the conductor (Lenz's rule). When the electrical resistance of a conductor is high, the induced currents and magnetic fields are weak and the conductors move easily in the magnetic field. But if the electrical resistance of the conductor is very small, then rather strong induced currents arise, and the resistance force to the movement of the conductor increases significantly - the conductor "gets stuck". It is known, for example, that it is generally impossible to push a superconductor into a region occupied by a magnetic field. (We remind you that if the conductor moves along the magnetic field, then no current arises in it at all and there is no resistance to such movement.)

Now back to interstellar gas. Here, as we know, there are many free electrons, and therefore the electrical conductivity of the interstellar gas is quite high (even better than the electrical conductivity of copper). Therefore, the movement of such a gas through the interstellar magnetic field can be quite similar to the movement of a good metal conductor in the same field. It must also be taken into account here that huge size interstellar clouds make the effect of their deceleration in a magnetic field very noticeable.

Thus, the interstellar magnetic field should slow down the movement of interstellar clouds across the direction of the field and not prevent their movement along the field. It can be expected that interstellar gas flows are directed predominantly along magnetic field lines. This conclusion is confirmed by observations: indeed, the gas most often moves parallel to the plane of the Galaxy, and the magnetic field also has approximately the same direction.

However, if the interstellar magnetic field is weak, so that it can no longer stop the movement of gas across the lines of force, then the gas begins to drag the magnetic field along with it. In other words, moving gas flows will, as it were, pull magnetic lines of force along with them, stretching and twisting them. In this case, the magnetic field lines are said to be "frozen" into the interstellar gas (or the interstellar gas is "glued" to the magnetic field lines).

From the definition of the concept of magnetic field lines, it is known that the magnetic field strength R (or magnetic induction AT) proportional to the number of lines of force passing through a unit area. When the movement of the gas stretches and "entangles" the magnetic lines of force, it thereby increases H(and B). We can say that here the kinetic energy of the gas is converted into magnetic energy. The growth of the magnetic field during gas motion stops when these energies are of the same order: pv 2 /2~ B 2 /8p(here p is the density of the gas; on the left is the density of kinetic energy, on the right is the density of magnetic energy). The enhancement of the magnetic field is especially noticeable in the density jumps mentioned above. An increase in density is accompanied, by virtue of the principle of "freezing" the field, by a proportional increase in the value AT.

The fourth feature of interstellar gas dynamics is the existence of ionization fronts - moving boundaries between HII zones and HI regions. They appear due to the fact that the gas pressure in the NI zones is usually much greater than the gas pressure in the HI regions. Indeed, considering interstellar thermodynamics, we have seen that in a two-component system consisting of clouds and the intercloud medium, the pressure value (more precisely, the product Fri) no more than 3 10 3 K/cm 3 . On the other hand, in the NI zone, where T\u003d 10 4 K, this value at the "standard" value of the concentration of protons and electrons (p~s m -3) more, and at high concentrations the difference is even more noticeable.

Thus, the HII zones should expand into the surrounding space. But with expansion, the density of the gas inside the zone decreases, the number of recombinations decreases, and as a result, a part of the “unused” ionizing quanta remains in this zone. They pass through the boundary of the initial mass of the HII zone and ionize new hydrogen atoms. Thus, the whole process consists not only of the expansion of the substance of the H II zone itself, but also of an even faster advance of the boundary between the regions of ionized and non-ionized hydrogen - the H II zone grows both in size and in size.

Such a movement of the boundary of the HII zone is called the movement of the ionization front, the movement speed of which can be compared with the speed of sound in the HI region. If the speed of the ionization front is greater than the speed of sound in the same gas, then we speak of a front R-type. Here, when passing through this front, the gas is ionized and condensed.

Conversely, if the front speed is less than the corresponding speed of sound, then on the ionization front (called the front D-type) concentration decreases. To ensure this reduction, the front D-type often “sends” a shock wave in front of it, which preliminarily “compresses” the gas in the HI region.

As soon as a new hot star forms in the HI region, it first creates a small HII zone, which rapidly expands as an ionization front. R— type. Then the speed of the expanded HII zone decreases, a shock wave is sent forward, followed at a close distance by an ionization front D-type.

Knowledge of the properties of interstellar gas dynamics is absolutely necessary for understanding the processes of condensation of stars from the interstellar medium - after all, this condensation is nothing but the movement of interstellar gas. And as we will see below, the features of interstellar gas dynamics manifest themselves in various aspects of the problem of star formation.

An important role in the dynamics of stellar processes, in stellar evolution is played by the interstellar medium, which is closely connected with stars: they are born in the interstellar medium, and when they “die”, they give it their substance. Thus, between the stars and the interstellar medium there is a circulation of matter: interstellar medium > stars > interstellar medium. In the course of such a circulation, the interstellar medium is enriched with chemical elements created in the interiors of stars. About 85% of all chemical elements heavier than helium arose at the dawn of our galaxy, about 15 billion years ago. OBE time, an intensive process of star formation took place, and the lifetime, evolution massive stars was relatively short. Only 10-13% of chemical elements (heavy helium) are less than 5 billion years old.

Although even with powerful optical telescopes we see in our galactic space only stars and a dark “chasm” separating them, in fact, interstellar galactic space is not an absolute void, it is filled with matter, substance and field.

The only question is what are the forms of this matter, in what state are the matter and the field here.

The interstellar medium consists of 90% interstellar gas, which is fairly evenly mixed with interstellar dust (about 1% of the mass of the interstellar medium), as well as cosmic rays, is permeated by interstellar magnetic fields, neutrino fluxes, gravitational and electromagnetic radiation. All components of the interstellar medium influence each other (cosmic rays and the electromagnetic field ionize and heat the interstellar gas, the magnetic field determines the movement of gas, etc.) The interstellar medium manifests itself in attenuation, scattering, polarization of light, absorption of light in individual lines of the spectrum, radio emission, infrared, x-ray and gamma radiation, through the optical glow of some nebulae, etc.

The main component of the interstellar medium is interstellar gas, which, like the substance of stars, consists mainly of hydrogen atoms (about 90% of all atoms) and helium (about 8%); 2% are represented by other chemical elements (mainly oxygen, carbon, nitrogen, sulfur, iron, etc.). The total mass of molecular gas in our Galaxy is approximately 4 billion solar masses, which is approximately 2% of the entire mass of matter in the Galaxy. Approximately 10 new stars are formed from this material every year!

Interstellar gas exists in both atomic and molecular states (the densest and coldest parts of molecular gas). At the same time, it is usually mixed with interstellar dust (which is the smallest solid refractory particles containing hydrogen, oxygen, nitrogen, silicates, iron), forming gas and dust formations, clouds. Of revolutionary importance for cosmochemistry was the discovery in gas and dust clouds of various organic compounds- hydrocarbons, alcohols, ethers, even amino acids and other compounds in which the molecules contain up to 18 carbon atoms. To date, more than 40 organic molecules have been discovered in the interstellar gas. Most often they are found in places of the highest concentration of gas and dust. Naturally, the assumption arises that organic molecules from interstellar gas and dust clouds could contribute to the emergence of the simplest forms of life on Earth. Gas and dust clouds are under the influence various forces(gravitational, electromagnetic, shock waves, turbulence, etc.), which either slow down or accelerate the inevitable process of their gravitational compression and gradual transformation into protostars.

In all likelihood, the first extraterrestrial objects that attracted the attention of man in ancient times were the Sun and the Moon. Contrary to the well-known joke that the moon more useful than the sun because it shines at night, and during the day it is already light, the paramount role of the Sun was noted by people in the primitive era, and this was reflected in the myths and legends of almost all peoples.

The question of what is the nature of the stars obviously arose much later. Having noticed wandering stars - planets, people, perhaps for the first time, made an attempt to analyze the relationship of various phenomena, although astrology that arose in this way replaced knowledge with superstitions. It is curious that astronomy, one of the most generalizing sciences of nature, made its first steps on the shaky ground of delusions, the echoes of which have survived even to this day.

The reason for these misconceptions is easy to understand if we consider that the first stage in the development of the science of the sky in the literal sense of the word was based on contemplation and abstract thinking, when there were practically no astronomical instruments. It is all the more striking that this stage ended brilliantly with the immortal creation of Copernicus - the first and most important revolution in astronomy. Prior to this, it seemed obvious that the observed, the visible coincides with the real, really existing, copies it. Copernicus proved for the first time that the real can radically and fundamentally differ from the visible.

The next equally decisive step was taken by the great Galileo, who managed to see what even such a subtle observer as Aristotle did not notice. It was Galileo who first realized that, contrary to the obvious, the process of moving a body does not at all mean the constant impact of another body on it. The principle of inertia discovered by Galileo then allowed Newton to formulate the laws of dynamics, which served as the foundation of modern physics.

If Galileo made his most ingenious discovery in the field of mechanics - and this later brought great benefits to astronomy - then the science of the sky is directly obliged to him for the beginning new era in its development - the era of telescopic observations.

The use of the gastronomy telescope, first of all, immeasurably increased the number of objects available for research. Even Giordano Bruno spoke about the countless worlds of suns. It turned out to be right: the stars are the most important objects in the Universe, almost all cosmic matter is concentrated in them. But stars are not just reservoirs for storing mass and energy. They are thermonuclear boilers, where the process of formation of atoms of heavy elements takes place, without which the most complex stages of the evolution of matter would not have been possible, which led to the emergence of flora, fauna, man and, finally, human civilization on Earth.

With the improvement of telescopes and methods for recording electromagnetic radiation, astronomers are able to penetrate into ever more remote corners of outer space. And this not only expands the geometric horizon of the world known to us: more distant objects also differ in age, so that the part of the Universe known to us, which is commonly called the Metagalaxy, contains rich information about the history of development, in other words, about the evolution of the Universe. Modern astronomy has been enriched by the doctrine of the development of the worlds, just as biology was enriched by the teachings of Darwin in its time. This is already a higher stage of transition - from the visible to the real, because by what is visible today, we learn the essence of phenomena in the distant past and can foresee the future!

Recently in astronomy there has been another important transition from the observed to the real. The observable itself has now turned out to be the property of many astronomers, armed with the most modern technology, which uses the slightest possibilities hidden in the recesses of physical laws and makes it possible to wrest from nature its secrets. But penetration into a reality unknown to us is not just an idea of ​​what revolves around what, and not even what is the cause of the movement or what certain bodies looked like in time immemorial, but something much more. This is the knowledge of the properties of space and time as a whole, on a scale that is not accessible to our direct perception and contemplation.

The space between the stars, with the exception of individual nebulae, looks empty. In fact, all interstellar space is filled with matter. Scientists came to this conclusion after the beginning of the 20th century. Swiss astronomer Robert Trumpler discovered the absorption (attenuation) of starlight on its way to an earthly observer. Moreover, the degree of its weakening depends on the color of the star. Light from blue stars is absorbed more intensely than from red stars. Thus, if a star radiates the same amount of energy in blue and red rays, then as a result of light absorption, the blue rays are attenuated more than the red ones, and from the Earth the star appears reddish.

The substance that absorbs light is not uniformly distributed in space, but has a ragged structure and is concentrated towards the Milky Way. Dark nebulae, such as the Coal Sack and Horsehead Nebulae, are sites of denser absorbing interstellar

substances. And it consists of the smallest particles - dust particles. The physical properties of dust grains have been studied quite well by now.

In addition to dust between stars, there is a large number of invisible cold gas. Its mass is almost a hundred times greater than the mass of dust. How did the existence of this gas become known? It turned out that hydrogen atoms emit radio waves with a wavelength of 21 cm. Most of the information about interstellar matter is obtained using radio telescopes. This is how clouds of atomic neutral hydrogen were discovered.

A typical cloud of atomic neutral hydrogen has a temperature of about 70 K (-200 °C) and a low density (several tens of atoms per cubic centimeter spaces). Although such a medium is considered a cloud, for an earthling it is a deep vacuum, a billion times rarer than the vacuum created, for example, in a TV kinescope. The sizes of hydrogen clouds are from 10 to 100 pc (for comparison: stars are on average 1 pc apart from each other).

Subsequently, even colder and denser clouds of molecular hydrogen were discovered, completely opaque to visible light. It is in them that most of cold interstellar gas and dust. These clouds are approximately the same in size as the regions of atomic hydrogen, but their density is hundreds and thousands of times higher. Therefore, large molecular clouds can contain a huge mass of matter, reaching hundreds of thousands and even millions of solar masses. Molecular clouds, which consist mainly of hydrogen, also contain many more complex molecules, including the simplest organic compounds. Some of the interstellar matter is heated to very high temperatures and "glows" in ultraviolet and X-rays. In the X-ray range, it emits the hottest gas, which has a temperature of about a million degrees. This is coronal gas, named after the heated gas in the solar corona. Coronal gas has a very low density, about one atom per cubic decimeter of space.

Hot rarefied gas is formed as a result of powerful explosions- supernova explosions. A shock wave propagates from the explosion site in the interstellar gas and heats the gas up to high temperature at which it becomes an X-ray source. Coronal gas is also found in the space between galaxies.

So, the main component of the interstellar medium is a gas consisting of atoms and molecules. It is mixed with dust, containing about 1% of the mass of interstellar matter, and penetrated by fast flows of elementary particles - cosmic rays - and electromagnetic radiation, which can also be considered components of the interstellar medium. In addition, the interstellar medium turned out to be slightly magnetized.

Magnetic fields are associated with clouds of interstellar gas and move with them. These fields are about 100 thousand weaker than the Earth's magnetic field. Interstellar magnetic fields contribute to the formation of the densest and coldest clouds of gas, from which stars condense. Cosmic ray particles also react to the interstellar magnetic field: they move along its lines of force along spiral trajectories, as if winding on them. In this case, the electrons that make up cosmic rays emit radio waves. This so-called synchrotron radiation is born in interstellar space and is reliably observed in the radio range.

GAS NEBLES

Observations with telescopes made it possible to detect a large number of faintly luminous spots in the sky - bright nebulae. The systematic study of nebulae began in the 18th century. William Herschel. He divided them into white greenish ones. The vast majority of white nebulae are formed by many stars - these are star clusters and galaxies, and some turned out to be connected by interstellar dust that reflects the light of nearby stars - these are reflection nebulae. As a rule, a bright star is visible in the center of such a nebula. But the greenish nebulae are nothing more than the glow of interstellar gas.

The brightest gas nebula in the sky is the Great Nebula of Orion. It is visible through binoculars, and with good vision it can also be seen with the naked eye - just below the three stars located in one line that form Orion's Belt. The distance to this nebula is about 1000 light years.

What makes interstellar gas glow? After all, the air we are accustomed to is transparent and does not emit light. The blue sky overhead glows with the light of the sun scattered on the molecules of the air. The night sky becomes dark. However, sometimes you can still see the glow of the air, for example, during a thunderstorm, when lightning occurs under the influence of an electric discharge. In the northern latitudes and in Antarctica, auroras are often observed - multi-colored stripes and flashes in the sky. In both cases, the air emits light not by itself, but by the action of a stream of fast particles. The flow of electrons generates a flash of lightning, and the entry into the Earth's atmosphere of energetic particles from the radiation belts that exist in the near-Earth outer space, auroras.

In a similar way, radiation occurs in neon and other gas lamps: a stream of electrons bombards the atoms of the gas and causes them to glow. Depending on which gas is in the lamp, its pressure and the electrical voltage applied to the lamp, the color of the emitted light changes.

In the interstellar gas, processes also occur that lead to the emission of light, but they are not always associated with the bombardment of the gas by fast particles.

It is possible to explain how the glow of interstellar gas occurs using the example of atomic hydrogen. The hydrogen atom consists of a nucleus (proton), which has a positive electric charge, and a negatively charged electron rotating around it. They are connected by electrical attraction. Having spent a certain energy, they can be separated. This separation leads to the ionization of the atom. But electrons and nuclei can reconnect with each other. Each time the particles combine, energy will be released. It is emitted in the form of a portion (quantum) of light of a certain color, corresponding to a given energy.

So, in order for a gas to radiate, it is necessary to ionize the atoms of which it consists. This can happen as a result of collisions with other atoms, but more often ionization occurs when gas atoms absorb ultraviolet radiation quanta, such as from a nearby star.

If a blue hot star flashes near a cloud of neutral hydrogen, then, provided that the cloud is sufficiently large and massive, almost all the ultraviolet quanta from the star will be absorbed by the atoms of the cloud. A region of ionized hydrogen is formed around the star. The released electrons form an electron gas with a temperature of about 10 thousand degrees. The reverse process of recombination, when a free electron is captured by a proton, is accompanied by re-emission of the released energy in the form of light quanta.

Light is emitted not only by hydrogen. As it was believed in the 19th century, the color of greenish nebulae is determined by the radiation of a certain "heavenly" chemical element, which was called nebulium (from Latin nebula - "nebula"). Later it turned out that in green glowing oxygen. Part of the energy of movement of electron gas particles is spent on the excitation of oxygen atoms, i.e. that is, to transfer an electron in an atom to an orbit farther from the nucleus. When the electron returns to a stable orbit, the oxygen atom must emit a quantum of green light. Under terrestrial conditions, he does not have time to do this: the gas density is too high and frequent collisions "discharge" the excited atom. And in an extremely rarefied interstellar medium, a sufficiently long time passes from one collision to another for the electron to make this forbidden transition and the oxygen atom to send a quantum of green light into space. The radiation of nitrogen, sulfur and some other elements similarly arises.

Thus, the region of ionized gas around hot stars can be represented as a "machine" that converts the ultraviolet radiation of the star into very intense radiation, the spectrum of which contains lines of various chemical elements. And the color of gaseous nebulae, as it turned out later, is different: they are greenish, pink and other colors and shades - depending on the temperature, density and chemical composition of the gas.

Some stars in the final stages of evolution gradually shed their outer layers, which, slowly expanding, form luminous nebulae. When observed through telescopes, these nebulae resemble the disks of planets, which is why they are called planetary. In the center of some of them you can see small very hot stars. Expanding gaseous nebulae also occur at the end of the lives of some massive stars when they explode as supernovae; in this case, the stars are completely destroyed, scattering their matter into interstellar space. This substance is rich in heavy elements formed in nuclear reactions flowing inside the star, and further serves as material for new generation stars and planets.

What is happening at the center of our galaxy?

central region Milky Way attracted the attention of astronomers for many decades. From it to the Earth is only 25 thousand km. light years, while millions of light years separate us from the centers of other galaxies, so there is every reason to hope that it is the center of our Galaxy that will be studied in more detail. However, it was impossible to directly observe this region for a long time, since it is hidden by large dense clouds of gas and dust. Although the discoveries made in the observations of X-ray and gamma radiation are certainly important, the most extensive and valuable spectroscopic studies of the center of the Galaxy were carried out in the infrared and radio ranges, in which it was first observed. The radio emission of atomic hydrogen with a wavelength of 21 cm has been studied in some detail. Hydrogen is the most common element in the Universe, which compensates for the weakness of its radiation. In those regions of the Milky Way where interstellar gas clouds are not too dense and where ultraviolet radiation is not very intense, hydrogen is present mainly in the form of isolated electrically neutral atoms; it is the well-defined radio signals of atomic hydrogen that have been mapped in detail to establish the structure of our Galaxy.

At distances of more than 1000 light years from the center of the Galaxy, the radiation of atomic hydrogen provides reliable data on the rotation of the Galaxy and the structure of its spiral arms. From it one cannot obtain much information about the conditions near the center of the Galaxy, since there hydrogen is predominantly combined into molecules or ionized (split into a proton and an electron).

Powerful clouds of molecular hydrogen hide the center of the Galaxy and the most distant objects located in the plane of the Galaxy. However, microwave and infrared telescopes make it possible to observe both these clouds and what is behind them in the galactic center. In addition to molecular hydrogen, clouds contain many stable carbon monoxide (CO) molecules, for which the largest characteristic wavelength of radiation is 3 mm. This radiation passes through the earth's atmosphere and can be registered by ground-based receivers; especially carbon monoxide in dark dust clouds, so it plays useful role to determine their size and density. By measuring the Doppler shift (the change in the frequency and wavelength of the signal caused by the movement of the source forward or backward relative to the observer), one can also determine the speed of the clouds.

Usually dark clouds are quite cold - with a temperature of about 15 K (-260 ° C), so carbon monoxide in them is in low energy states and emits at relatively low frequencies - in the millimeter range. Part of the matter near the center of the Galaxy is clearly warmer. Using the Kuiper Astronomical Observatory, researchers at the University of California, Berkeley have detected more energetic emission of carbon monoxide in the far infrared, indicating a gas temperature of about 400 K, which is roughly the boiling point of water. This gas is heated under the influence of ultraviolet radiation coming from the center of the Galaxy and, possibly, shock waves that arise when clouds moving around the center collide.

In other places around the center, carbon monoxide is somewhat colder and most of its radiation falls on longer wavelengths - about 1 mm. But even here the temperature of the gas is several hundred kelvins, i.e. close to the temperature at the surface of the Earth and much higher than inside most interstellar clouds. "Other molecules that have been studied in detail include hydrogen cyanide (HCN), hydroxyl (OH), carbon monosulfide (CS), and ammonia (NH^). A high-resolution HCN emission map was obtained with a radio interferometer at the University of California. The map indicates the existence of a clump-splitting, inhomogeneous disk of warm molecular clouds enclosing a "cavity" about 10 light years wide at the center of the Galaxy Because the disk is tilted relative to Earth's line of sight, this circular cavity appears elliptical (see figure below).

Carbon and oxygen atoms, some of which are ionized by ultraviolet light, are mixed in a disk with molecular gas. Maps of infrared and radio emissions corresponding to the emission lines of ions, atoms, and various molecules show that the gas disk rotates around the center of the Galaxy at a speed of about 110 km/s, and also that this gas is warm and collected in separate clumps. Measurements have also found some clouds whose movements do not at all correspond to this general circulation pattern; perhaps this substance fell here from some distance. Ultraviolet radiation from the central region "hit" the outer edge of the cloud disk, creating an almost continuous ring of ionized matter. Ionized streamers and gas clumps are also present in the central cavity.

Some fairly common ionized elements, including neon missing one electron, argon missing two electrons, and sulfur missing three electrons, have bright emission lines near 10 microns, in that part of the infrared spectrum for which the Earth's atmosphere is transparent. It was also found that of all the elements near the center, singly charged ionized neon predominates, while the three-charged sulfur ion is practically absent there. To take three electrons from a sulfur atom, you need to expend much more energy than to take one electron from a neon atom; the observed composition of matter indicates that in the central region the ultraviolet radiation flux is large, but its energy is not very high. From this it follows that this radiation is apparently created by hot stars with a temperature of 30 to 35 thousand degrees Celsius. Kelvin, and there are no stars with a temperature much higher than indicated.

The spectroscopic analysis of the ion radiation also gave detailed information about the velocities of rarefied matter inside

cavity with a diameter of 10 St. years surrounding the center. In some parts of the velocity cavity

close to the speed of rotation of the ring of molecular gas - about 110 km/s. Part of the clouds inside this area moves much faster - at about 250 km / s, and some have speeds up to 400 km / s.

In the very center, an ionized substance was found moving at speeds up to 1000 km/s. This substance is associated with an interesting set of objects near the center of the cavity, known as IRS16, which was discovered by Böcklin and Negebauer during their search for sources of shortwave infrared radiation. Most of the very small sources they found are probably single massive stars, but IRS 16 (the 16th infrared source on their list) is something else: subsequent measurements revealed five bright unusual components in it. This whole central region, both the warm gaseous disk and the inner cavity, appears to be the stage where some violent action has recently taken place. A ring or disk of gas rotating around the center of the Galaxy should gradually turn into a homogeneous structure as a result of collisions between fast and slowly moving clumps of matter. Doppler shift measurements show that the difference between the velocities of individual bunches in a ring of molecular gas reaches tens of kilometers per second. These bunches should collide, and their distribution should be smoothed out on time scales of the order of 100 thousand years, i.e., in one or two revolutions around the center. Hence it follows that during this time interval the gas was strongly perturbed, possibly as a result of the release of energy from the center or the fall of matter from a certain distance from the outside, the collisions between the bunches must still be strong enough for shock waves to arise in the gas. The validity of these conclusions can be verified by searching for "traces" of such waves.

Shock waves can be identified from the spectral lines of hot highly excited molecules. Such molecules were discovered during observations from the Kuiper Astronomical Observatory; these include hydroxyl radicals - electrically charged fragments of water molecules that have been torn apart with force. Short-wave infrared radiation of hot hydrogen molecules has also been registered; it indicates that in some places the temperature of the clouds of molecular gas reaches 2000 K - just such a temperature can be created by shock waves. What is the source of dense molecular dust clouds near the center? The substance contains heavy elements; this indicates that it was formed in the interior of stars, where as a result elements such as carbon, oxygen and nitrogen. Old stars expand and emit great amount matter, and in some cases explode as supernovae. In any case, heavy elements are ejected into interstellar space. The matter of the clouds located near the center of the Galaxy was, apparently, more thoroughly "processed" inside the stars than the matter located farther from the center, since near the center there are especially many of some rare isotopes that are formed only inside the stars.

Not all of this matter was created by pre-existing stars in close proximity from the center. Perhaps some of the clouds were attracted from outside. Under the influence of friction and magnetic fields, the substance gradually contracts towards the center, so it should accumulate in this area.

Gas in the Large Magellanic Cloud.

Luminous gaseous nebulae are some of the most beautiful and impressive objects in the universe. The 30 Doradus nebula is the brightest and largest of the gaseous nebulae of three dozen local group galaxies, including our Galaxy. It has an irregular shape and huge size. While the Great Nebula in the constellation Orion is visible to the naked eye as a blurry star. Nebula 30 Doradus occupies an area in the sky comparable to the disk of the sun or full moon, despite the fact that it is over 100 times farther from us than the Orion Nebula. Its diameter is about 1000 light-years, and the Orion Nebula is only three light-years. The gas in the nebula is largely ionized: most of the atoms have lost at least one electron. It turns out that the 30 Doradus Nebula contains 1500 times more ionized gas than the Orion Nebula. Ionization of the gas occurs under the action of ultraviolet radiation emitted by massive hot young stars located in the nebula.

The twentieth century has given rise to amazing science and technology, they allow human thought penetrate into the depths of the Universe, truly beyond known world. Our horizons and horizons of the visible world have expanded so much that the human mind, trying to cast off the shackles of earthly prejudices, is barely able to master it. Scientists working in various fields of science, trying to explain the mysterious objects discovered in our time with the help of physical laws, are convinced that the amazing Universe in which we live is largely unknown to us. If any information about the Universe becomes available , then often even the most daring mind is not prepared to perceive it in the form in which nature presents it. Struck by the unusualness of the newly discovered celestial objects, it should be remembered that in the entire history of mankind, no science has achieved such phenomenal rapid development, as a science about these unique objects. And all this is literally recent decades. Satisfying the inexhaustible thirst for knowledge inherent in man, astrophysicists tirelessly study the nature of these celestial objects that challenge the human mind.

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2. I. Levitt "Beyond the Known World" (1978)

3. John S. Mathis "An object of unusually high luminosity in the Large Magellanic Cloud" (In the world of science. October 1984)

4. Charles G. Townes, Reinhard Genzel "What's going on at the center of our Galaxy?" (In the world of science. June 1990)

5.Avanta plus. Astronomy.

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