The circle of Ambartsumian’s scientific interests is extraordinary wide: physics of gaseous nebulae, dynamics and statistical mechanics of stellar systems, theory of light scattering, nature and structure of the interstellar matter, theory of stellar associations, evolution of stars and stellar systems, physics of non-stable stars and stellar statistics, theory of superdense baryon stars, non-stationary phenomena in galaxies, etc.
The scientist, together with his students, brought important and essential contribution in each of these branches of science, very often results of basic value, which considerably expanded and deepened our understanding on space formations and the phenomena taking place in them. Ambartsumian’s scientific contribution is not limited only to basic researches in various areas of astronomy. The new important results received by him mainly during the Leningrad period of his scientific activity, also concern the theoretical physics, the theory of differential equations and to other fields of science. In the last decades he brought considerable contribution to philosophy and to methodology of natural science.
“A characteristic streak of V.A. Ambartsumian’s scientific creativity, – according to academician V.V. Sobolev, – is the concentrated work within several years over one any problem without distraction of attention to other questions. When the problem “clears up” and the bases of the theory appear, V.A. Ambartsumian, leaving the completion and the theoretical development to his followers, passes to a new problem”.
Due to the popular character and the limited volume of the present book, we will not stop on Ambartsumian’s all researches, and we will try to give a short review only of the most important results of his scientific researches on the exact sciences, adhering to a chronological order.
Physics of gaseous nebulae
A big series of Ambartsumian’s works is devoted to the questions of physics of gaseous nebulae, numerous representatives of which, in the form of planetary and diffuse nebulae, are observed in our Galaxy.
The luminescence of gaseous nebulae is induced by neighboring hot stars (the temperature at the surface is about 20,000 degrees or above). And, absorbing the ultraviolet radiation of these stars invisible from the Earth, the gaseous nebulae radiate their energy in the visible part of the spectrum. Ambartsumian first offered a mathematical treatment of the physical processes proceeding in gaseous nebulae at processing (fluorescence) of ultraviolet radiation of hot stars. With this aim he developed a method of study of radiation transfer in gaseous nebulae, which is based on separate consideration of the energy of radiation in the continuous spectrum and in lines. This new method, which was named the method of field division, allowed creating the theory of radiative equilibrium of planetary nebulae. The ideas developed in this theory are the basis of the modern theory of gaseous nebulae.
Ambartsumian revealed the huge role of the ultraviolet radiation in gaseous nebulae and influences of light pressure in motionless gaseous envelopes of stars. He proved that the planetary nebulae, having regular shape, with central very hot star, expand nowadays and should very quickly dissipate. From the fact of expansion of the planetary nebulae, an important conclusion was obtained that these nebulae are very young formations, which have originated due to emission of matter from the central stars. Then the theory of expansion of planetary nebulae under the influence of light pressure in the gravitational field of the nucleus, the central star, was developed.
In spectra of planetary nebulae, two very bright lines are evident, which were never observed in spectra of terrestrial light sources. For long time, the presence of these lines in the spectra of planetary nebulae caused a scientific mystery. For its explanation it was supposed, that in the planetary nebulae there is an unknown on the Earth new chemical element, “nebulium”, which radiates the specified lines. In 1927, the American astronomer Ira Bowen managed to explain the nature of the “nebulium” lines. He showed that in conditions of extremely low density of matter and radiation existing in gaseous -nebulae (these conditions are practically unattainable on the Earth), there is an accumulation of twice ionized, i. e. deprived of two external electrons, atoms of oxygen, in so-called meta-stable states. The probability of transition from these states (levels) on lower levels is insignificantly small. Therefore such transitions on the Earth, as it is said, “are forbidden”, and spectral lines corresponding to them are not observed. Such transitions due to the big accumulation of atoms in meta-stable states are made very often in gaseous nebulae. Transitions of atoms of twice ionized oxygen from meta-stable levels to normal ones just lead to radiation of the nebulium lines. Bowen’s this explanation was only qualitative, and it was required to develop the general quantitative theory of formation of the similar “forbidden” lines in the spectra of cosmic objects. The first step in this direction was made by the Norwegian scientist Rosseland. However the theory developed by him was private and inapplicable to the most important cases of radiation of the “forbidden” lines in spectra of gaseous nebulae. Ambartsumian created the general theory of excitation of atoms, which are in meta-stable conditions, and formation of the “forbidden” lines, which found wide applications in astrophysics. In particular, on the basis of this theory, he predicted the existence of the “forbidden” line of helium in the spectra of Wolf-Rayet type non-stable stars that was later really revealed.
Ambartsumian developed special methods for determination of temperatures of the planetary nebulae nuclei and the stars surrounded with gaseous envelopes. Such gaseous envelopes are formed, for example, during the explosions of Novae and Supernovae stars, as a result of outburst of gaseous matter by them at that time or as a result of the continuous outflow of gaseous matter of some non-stable stars, for example, of Wolf-Rayet type stars. Bases of the theory of excitation and ionization of atoms in gaseous envelopes of small sizes were put by Ambartsumian.
Ambartsumian together with N.A. Kozyrev proposed methods of determination of masses of gaseous envelopes of stars. In the result of application of these methods it was shown in particular that during its outburst a Nova star throws out a mass equal to hundred-thousandth mass of the Sun, and the Supernova star, much more, a mass equal to the mass of the Sun. Similar estimations of the masses of gaseous envelopes thrown out by stars have great importance for revealing of the evolution rates of these stars.
At last, the only method of definition of masses of the gaseous nebulae by their luminosity belongs to Ambartsumian.
Dynamics and statistical mechanics of stellar systems
Ambartsumian’s researches devoted to questions of dynamics of stellar systems have basic value. The ideas put forward in them played an important role in revealing of the nature of the star formation process in the Galaxy.
The essence of the scientist’s new ideas concerning stellar dynamics is reduced to the following. In a stellar system each star during its motion is submitted to influence of forces of two sorts: 1) joint attraction force of all other stars of the system (regular force) and 2) the perturbation force arising in the result of close passages of stars (irregular force).
The time interval, during which the influence of irregular forces in the given stellar system is equal to the influence of regular forces, is called the relaxation time of the system. For our stellar system, the Galaxy, the relaxation time by Ambartsumian’s estimates makes about ten million billion years. It means that the influence of irregular forces is insignificantly small in the Galaxy due to the extremely rare close passages of stars. Therefore in many problems of star dynamics, the Galaxy can be considered as a system, in which stars move under the influence of only regular forces.
However, in real stellar systems, it is not always possible to neglect the influence of the irregular forces. Irregular forces can play an essential role in some of them (multiple stars, stellar clusters). On the other hand, stars in stellar systems interact according to Newton’s universal gravitation law. Because of the specified two features (rare close passages and gravitational interaction) of the real stellar systems, many common methods of statistical physics are not directly applicable to them.
Ambartsumian developed bases of the new physical statistics considering these features of real stellar systems, the so-called statistical mechanics of stellar systems.
The scientist received results of vital importance by application of original methods of statistical mechanics of stellar systems to double stars and star clusters, among which it is necessary to note the estimations of ages of stellar systems, in particular the estimation of the age of modern state of the Galaxy.
Let's consider this question in more details.
During their motions inside a star cluster, the stars forming systems often come nearer or go farther from each other. This leads to redistribution of velocities of stars of the cluster. As a result, some stars of the cluster get velocities sufficient for overcoming the gravitational field of the system, and leave from it. This process, periodically repeating, causes gradual disintegration of the cluster. By the way, the dwarf stars having small masses are thrown out from the star cluster first of all. Calculations show that the time necessary for half-decay of the galactic star clusters at their observed density of stars does not exceed ten billion years. The observational data about the presence of dwarf stars and the total number of stars in star clusters testify that many clusters of the Galaxy have not yet managed to break up in half. This important observational fact gave to Ambartsumian a basis to conclude that the duration of modern state of the Galaxy, that is its age, does not exceed ten billion years.
This result was confirmed by statistical researches of double stars.
A single star at a close passage by a double star causes changes of elements of its orbit. The accidental character of close passages with the time leads to equilibrium distribution of elements of orbits of double stars. The time required for the establishment of such equilibrium distribution in the Galaxy is equal to about ten billion years. However, the observations of double stars show that the equilibrium distribution of elements of their orbits in the Galaxy is not established yet, which may be considered as a confirmation of the abovementioned estimate of the age of the Galaxy.
At last, there are processes of formation, as well as of disintegration of double stars at close passages of stars. With some time, a state of equilibrium (dissociative equilibrium) should be established between these two opposite processes, when for a certain time interval the number of breaking up pairs of double stars is equal, on the average, to the number of the formed pairs. In case of wide pairs, about ten billion years are necessary for an establishment of dissociative equilibrium in the Galaxy. As it was shown by Ambartsumian, the fraction of wide pairs in respect of the single stars, expected at dissociative equilibrium, is a few tens of millions times less than observable fraction in the Galaxy. It means that the dissociative equilibrium between processes of formation and disintegration of wide pairs is not yet established in the Galaxy. This observational fact also indicates in favour of the abovementioned estimate of the age of the Galaxy.
Ambartsumian’s researches rejected the understanding unconditionally dominating in science based on the work of known English scientist James Jeans according to the statistics of double stars that the age of the Galaxy is defined by so-called “long time scale”, about ten thousand billion years. It was shown that the “long time scale” was a result of wrong interpretation of observational data about the elements of orbits of double stars. Actually, for the age of the Galaxy, these data indicate the “short time scale”, about ten billion years that is thousand times shorter than “long scale”. The estimation of the age of the Galaxy, given by Ambartsumian, received a general recognition.
The general theory of derivation of the distribution of spatial velocities of stars by means of observable distribution of their radial velocities developed by Ambartsumian has a big scientific importance.
The result obtained by the scientist on the basis of studying of RR Lyrae type variable stars, showing short-term periodic variations of light, has basic significance for the problem of origin and evolution of stars. He showed that the time intervals required for significant changes in spatial distribution or in distribution of spatial velocities of the stars of certain type, for many times exceed the duration of life of these stars. From this result directly follows that observable distributions of the specified values during the life of stars of the given type practically do not change. Therefore the stars representing various stages of evolution of given type should have similar distributions both in space and in velocities (should have identical spatial-kinematic characteristics).
Principle of invariance and light scattering theory
Ambartsumian’s surprising ability to find the simplest solutions of the most complicated physical problems was especially brilliantly shown at the creation by him of new light scattering theory in the opaque medium.
The problem of multiple light scattering has a long history. Many scientists were engaged in this problem, including very great ones. In their researches, the problem of scattering of light was usually led to an integral equation of very complicated form, the solution of which turns out only in the approximate and very long form.
For the solution of the problem of light scattering, Ambartsumian formulated following new and very fruitful principle, principle of invariance: reflective capacity of the medium consisting of plain-parallel layers and infinitely large optical thickness, should not change if a flat layer of final optical thickness having the same optical properties is added to it from its border side.
By application of this exclusively simple principle Ambartsumian brought the problem of light scattering in opaque medium to a system of equations of very simple type: so-called functional equations. Thus by means of the principle of invariance he managed to obtain the exact solution of the problem on multiple light scattering.
It became possible thanks to the fact that by application of the principle of invariance for obtaining a relation between falling and reflected beams of light on border of the medium, it is enough to know only the property of the scattering medium while at classical statement of the problem of light scattering, the knowledge of all changes occurring to the light beam in all points of the medium is required. These equations carry now Ambartsumian’s name.
The invariance principle was exclusively a powerful tool at the solution of various problems connected with the studying of atmospheres of planets, stars, and the Sun. The solution of part of them was obtained by Ambartsumian himself.
The principle of invariance became an initial point for the solution of questions connected with multiple scattering of electromagnetic radiation in general. This principle found numerous applications not only in astrophysics, but also in many various areas of theoretical and experimental physics, geophysics, radio physics, and even in diagnostics of illnesses.
Later, after a long-term break, Ambartsumian again returned to the problem of light scattering and made essential addition to applications of the principle of invariance. He found a way, which gave the possibility to use this principle in the nonlinear theory of light scattering. Nonlinear problems of scattering arise, when not only the scattering medium influences the light, but also the light itself makes appreciable impact on the medium in sense of changes of its optical properties. From the new results received by Ambartsumian, considerable interest represents the theoretical prediction of the phenomenon of the enlightenment of the medium under influence of radiation falling on it.
In conclusion of this section, we will notice that in the work devoted to the research of the integral equation of radiative equilibrium in stellar atmospheres the scientist opened some interesting features of this equation, which do not meet in the mathematical physics and have not only scientific, but also practical importance for the solution of such equations.
Nature of the interstellar matter and the theory of fluctuations
After the discovery of the phenomenon of light absorption in interstellar space of the Galaxy there was a necessity of study of properties of the interstellar absorbing matter. The light absorption of stars and nebulae by the interstellar matter makes considerable changes in their brightness, deforms their distances and, hence, distributions in space. It strongly complicates the research of the structure of the Galaxy.
Ambartsumian brought important contribution to studying of interstellar matter, to definition of its structure and optical properties. He showed that in the Galaxy it is impossible to explain light absorption by presence of gaseous matter in the interstellar medium and the interstellar dust matter should be considered as the reason of this phenomenon.
The scientist (jointly with his student Sh.G. Gordeladze) in a widely known investigation revealed the nature of observable connection of light dust nebulae in the Galaxy with stars illuminating them. It was shown by a simple and witty method that this connection is in most cases casual. In other words, only those dust nebulae are observed as light ones, nearby of which and in which there accidentally appear stars of sufficient high luminosity. It meant that the dust nebulae, in the neighborhood of which there are no stars of high luminosity, are not brightened and should be dark. Hence, it was necessary to admit, that light and dark dust nebulae are formations of identical nature. Calculations showed that in the Galaxy, stars of high luminosity brighten only an insignificant fraction (1/2000) of all dust nebulae. In other words, the number of not illuminated, dark dust nebulae should be in 2000 times more, than the number of light dust nebulae in our stellar system. From the fact of such abundance of dark dust nebulae essentially important conclusion that light absorption in interstellar space of the Galaxy is caused not by the continuous dust medium, but basically by individual dark dust nebulae, absorbing clouds, was obtained. Thereby it was established that the interstellar absorbing medium consists of individual absorbing clouds, i. e. it has a clumpy structure.
The absorbing clouds of big sizes having a large absorption capacity are directly observed in the form of dark clouds and can be investigated by means of the light absorption of the stars caused by them and located behind them. However the observation, and hence the investigation of the small clouds having insignificant absorbing capacity, is practically excluded, while they make the overwhelming majority of all absorbing clouds.
A powerful tool for research of the set of interstellar absorbing clouds of small sizes was the fluctuation theory developed by Ambartsumian. Absorbing clouds in interstellar space are concentrated in narrow enough layer around the plane of symmetry of the Galaxy. Because of the light absorption caused by them there are certain deviations in observable distribution of brightness of the Milky Way in the sky, and also the numbers of extragalactic nebulae, in comparison with uniform distribution.
In other words, while in the absence of interstellar absorption, for example, the brightness of the Milky Way in the neighboring areas of the sky should vary a little (it should change smoothly), in fact the presence of the interstellar absorbing clouds leads to the fact that at transition from one area to the next in the sky, abrupt changes of this brightness are observed. The character and size of observable deviations are completely defined by absorbing properties of the interstellar clouds and their number on the way of the light ray. Study of observable deviations by means of the fluctuation theory allowed defining important characteristics (average absorbing capacity, average sizes, etc.) of the interstellar absorbing clouds. It is necessary to add, that in the theory of fluctuations, Ambartsumian took into account the fluctuations in observable distributions of discussed values in comparison with uniform distribution, which are a consequence of existence of physical groups of stars and galaxies, and also presence of dispersion of light in interstellar space.
Now conclusions about the clumpy structure of the interstellar absorbing medium of the Galaxy and about the nature and properties of absorbing clouds have firmly entered into science.
It is interesting to notice that by consideration of random deviations of the observable values corresponding to two neighboring directions in the Galaxy from their average value, Ambartsumian faced the mathematical problem requiring some generalization of the law of distribution of Poisson’s random variables for the case when random variables are not completely independent from each other.
Stellar associations and evolution of stars
At all stages of scientific activity Ambartsumian paid a big attention to questions of the origin and evolution of stars and stellar systems.
In the researches devoted to studies of planetary nebulae, non-stable stars and, at last, the statistical mechanics of stellar systems, the scientist has found first signs of the changes taking place in the state of stars and stellar systems. The subsequent researches in this direction in 1947 led Ambartsumian to discovery of stellar systems of new type, stellar associations, centres of star-formation in the Galaxy.
The observable tendency of crowding in the sky of hot giant and supergiant stars (stars of O and B spectral types) and the dwarf stars showing irregular variations of brightness with emission lines in spectra (Ò Tauri type variable stars) were the initial point for the discovery of stellar associations. Investigation of their spatial distribution has shown that the formed by them groupings occupy limited volumes in the sky, which means that they are physical systems. These systems have received the name of stellar associations.
Stellar associations, with characteristic stellar population consisting of these stars of identical physical characteristics directly are not observed, unlike the earlier known stellar systems, star clusters, which due to the big stellar density are evident in the stellar sky photos. The average density of the stars in stellar associations is less than in the general stellar field of the Galaxy, and they are lost against the field of stars. However stellar associations are distinguished by their high partial density of stars of the abovementioned physical types.
According to the characteristic stellar population in the Galaxy two types of stellar associations are known: associations of hot stars (O-associations) and associations of T Tauri type stars (T-associations); all nearest O-associations comprise T Tauri type stars, i. e. are also T-associations (O + T-associations), while there is a big number of only T-associations.
An analysis of the forces active in stellar associations resulted Ambartsumian in the following basic result: stellar associations are dynamically extremely non-stable systems of stars due to what they expand at present and should break up inevitably during an order of tens millions years. This fact that modern stellar associations had not yet broken up, proves that their age is less than this time, tens millions years.
On the other hand, according to the “short time scale”“, the age of the Galaxy is one thousand times more. Hence, the stellar associations in the Galaxy are young formations. At the same time, multiple stellar systems, in particular stellar associations, could not be formed of earlier existing stars at their close passages. We already saw, that formation in such a way even the set of the double stars in the Galaxy is excluded. It is necessary to consider therefore that the stars forming associations are connected with each other since the time of their origin, i. e. also are young.
A number of other observational data indicate that the stellar associations and stars compounding these systems are young. We will note some of them. From the surface layers of many stars, which are a part of stellar associations (Wolf-Rayet and Ð Cygni type stars, stars with emission lines in spectra), there is a continuous and enough intensive outflow of gas matter, which cannot proceed long, no more than tens millions years. This fact shows that the specified stars really are in a stage of formation and had no time to reach an equilibrium state yet. In favour of the young age speaks also the abundance of dynamically extremely non-stable multiple stars in stellar associations (Trapezium type stars and stellar chains). The age of these multiple stars, by Ambartsumian’s calculations, does not exceed several millions years.
Ambartsumian’s idea about multiple systems of Trapezium type was a new word in stellar dynamics and in astronomy in general. These systems consist exclusively of very young stars and are dynamically extremely unstable. Therefore they very quickly break up, much faster than their host stellar associations.
Thus, on the basis of observational data of the diversified character, Ambartsumian showed that the stellar associations (and the stars entering into their structure) were born rather recently. For the first time in the history of science, it was established that the star-formation process in the Galaxy, begun a few billions of years ago, also proceeds in the modern stage of its evolution.
This conclusion had basic importance and completely rejected the understanding dominated before in the science that all stars in the Galaxy were formed simultaneously, a few billions years ago.
From the observational fact of the abundance of dynamically unstable, multiple stars and stellar chains in stellar associations, another fundamental result was received: the stars making physical system, have a common origin, stars are born in groups.
This new understanding on common origin of components making multiple stars has great significance also for the problem of origin of the Solar system. The point is that there are no bases to suppose that the process of formation of planetary systems, in particular our Solar system, essentially differs from the process of formation of multiple stars.
During the time passed after the discovery of stellar associations, in the world observatories numerous data have been obtained completely confirming essentially new conclusions about the physical nature of the stellar associations, in particular about their dynamical instability (expansion and the subsequent disintegration), about the process of proceeding at present star-formation and about the group origin of stars in the Galaxy.
Discovery and study of the stellar associations, these centres of star-formation in the Galaxy where stars are formed by groups, have played a decisive role in basic change of our understanding about the process of formation of stars and stellar systems. They stimulated a rapid stream of studies in this area of science.
Stellar associations became a powerful weapon in hands of researchers in studying of laws of the origin and evolution of stars and stellar systems.
Besides, the investigation of the stellar associations led to the new observational approach to the problem of origin and evolution of stars and stellar systems, to a new understanding about the nature of proto-stellar matter, about the energy sources of stars, etc.
New hypothesis on protostars. Theoretical study of possible superdense configurations of matter
Following from understanding on the dynamical instability of stellar associations, the phenomena of their expansion and the subsequent disintegration, the observational data on the nature and the structure of these recently originated stellar systems gave Ambartsumian a basis to put forward a new hypothesis on proto-stellar matter.
According to this new hypothesis on protostars, the origin and evolution of stars and stellar systems proceeds not by transition of the matter from diffuse (scattered) states to denser ones, as the classical hypothesis on condensation of diffuse matter in stars assumes, but on the contrary, the evolution of space matter corresponds to its transitions from denser states to less dense ones. In other words, in the new hypothesis an initial state of the matter is the superdense state and consequently it is possible to name this hypothesis a hypothesis of superdense protostars.
After initial superdense and dense state of cosmic matter in the course of evolution less dense states follow corresponding to observable forms of the existence of cosmic matter (stars, nebulae, planets, etc.).
For a long time it was considered that the densest cosmic formations were the white dwarf stars, the average density of matter of which makes up several tons to several tens of tons in cubic centimeter. Such high density is possible to explain if to admit that the matter of the white dwarfs consists of separate nuclei and free electrons located very close to each other. Meanwhile in terrestrial states nuclei and electrons are usually a part of atoms, where the distance between electrons and nuclei, hence the sizes of atoms, are for many times more than the sizes of nuclei and electrons. For this reason the number of nuclei and electrons, making atoms in a volume unit, i. e. the matter density, is essentially small on the Earth.
However a theoretical research showed that denser forms of existence of cosmic matter, which should consist mostly of neutrons, neutron stars, are basically possible. It is supposed that such neutron stars are the pulsars discovered in 1968, the sources of radio emission having rather fast and strictly periodic variability.
In connection with the hypothesis of superdense protostars Ambartsumian (together with G.S. Sahakian) considered the basic possibility of existence of even denser forms of matter in the nature. It was shown that in cases when the density of the gas consisting of elementary particles is much more than the density of neutron stars (about billion tons in cubic centimeter), in this gas the birth of hyperons, heavier elementary particles, should begin. At the further increase of density of gas, the number of arising hyperons gradually exceeds the total number of neutrons and protons in gas. The conclusion of the theory that extremely unstable in terrestrial conditions hyperons (average duration of their life on the Earth is equal to one ten-billionth part of the second) at such density of gas become steady is important. According to this new theory on the equilibrium configurations of superdense matter, beginning with certain value of mass, the superdense star should consist basically from hyperons.
In these studies, a rather important conclusion is also obtained proving that the equilibrium superdense configurations of matter possess huge stocks of internal energy necessary for explanation of the phenomena of physical and dynamic instability, which are observed as the investigation of stellar associations showed in recently arisen young stars and stellar systems.
However from the point of view of Ambartsumian’s hypothesis on superdense protostars, in these researches on theoretical studies of possible superdense forms of existence of cosmic matter, the most important result having basic value is the establishment of possibility of existence of superdense matter having density equal and more than the density of atomic nuclei.
Physics of young stars and sources of stellar energy
Among Ambartsumian’s researches on study of the nature and radiation of young stars, which are a part of stellar associations, of a big interest are the researches devoted to unusual excessive radiation observed at T Tauri type stars and related objects, so-called continuous emission. This interest is caused first of all by obtained in them new and essentially important conclusions about the energy resources of stars. They are rather interesting also due to the fact that they contain some indirect evidences on existence of a significant quantity of superdense matter in interiors of young stars that can be considered in favour of the hypothesis of superdense protostars. From the considered point of view, a special attention T Tauri type stars and related objects deserve, making up the characteristic population of stellar associations or young clusters. Ambartsumian established that irregular changes of brightness of these stars are caused not by changes of their temperature or sizes, but are caused by the sources of additional energy appearing from time to time in the surface layers of a star.
According to the understanding accepted now in science, energy source of stars are thermonuclear reactions taking place in their central regions. During these reactions occurring at temperatures of the medium of tens millions degrees, helium nuclei (alpha particles) are formed from hydrogen nuclei (protons), an alpha particle from four protons with a release of enormous quantity of energy. The energy released as a result of thermonuclear reactions in the form of radiation leaves then from the surface layers (photosphere) of a star.
Observed in the surface layers of T Tauri type stars and related with them flare stars cases of direct and sometimes rather short-term (of an order of a minute) release of huge quantities of energy is impossible to explain by thermonuclear reactions. The point is that the temperature of the medium in surface layers of stars is insufficient for occurring of thermonuclear reactions. For the explanation of this puzzle, Ambartsumian admitted that energy released in specified cases is taken out from the inside layers of the star together with a matter-carrier of this energy. As such abrupt changes of brightness (power of radiation) are observed only at T Tauri type stars and related with them stars, i. e. at very young stars, it is necessary to consider, apparently, that it is connected with the presence of certain quantities of matter in a star, which are in proto-stellar state. Hence, there is a rather plausible assumption that the presence of the continuous emission of unusual nature in the general radiation of the mentioned non-stable stars is a direct consequence of an exit of the proto-stellar matter to the surface layers of a star and disintegration, transition of matter from proto-stellar states to stellar ones.
Observations show that processes of direct release of the internal energy in surface layers of stars are accompanied by birth of new atomic nuclei sometimes rather unstable ones. The abundance of unstable nuclei in atmospheres of some non-stationary stars, in particular, testifies to it. Serious observational evidences were obtained in favour of the unusual nature of continuous emission in Byurakan and abroad (Mexico, USA, and Germany).
The problem of the continuous emission is now still far from its full solution. However, there is no doubt, that investigation of the phenomenon of direct release of the interstellar energy proceeding in physical conditions, unattainable on the Earth, has exclusively big scientific importance not only for the problem of sources of stellar energy, but also for the nuclear physics in general.
Physics and evolution of flare stars
The properties of radiation of flare stars, first of all the appearance of continuous emission in radiation of these stars during the flares of their brightness, in 1953 gave Ambartsumian a basis to conclude that the flare stars by their physical nature are related to Ò Tauri type stars. The discovery of flare stars in stellar systems, associations, and young star clusters by the Mexican astronomer Guillermo Haro was a weighty argument in favour of this conclusion and indicated on evolutionary importance of the flare stars.
In 1968, Ambartsumian managed to prove that flare stars really represent one of the earliest stages of evolution of the dwarf stars. He developed an original statistical method for estimation of the full number of flare stars in a physical system on the basis of observational data on flare stars already known in this system. By application of this method, the scientist established that in relatively young Pleiades star cluster (having an age of about 70 million years), there should be at least a few hundreds flare stars. Further on, using the known total mass of the cluster, as well as the mass of the bright, non-flare stars of the cluster, he determined the mass of other stars of the cluster. It appeared that this latter mass practically coincides with the estimation of the mass of all possible flare stars obtained by the scientist. In this way it was shown that all stars of low luminosity of the cluster should be flare ones.
Taking into account that these stars make a physical system, a cluster, their joint formation does not raise doubts. Hence, it is necessary to consider that a property to show flares is a typical feature of stars in this stage of evolution, and the stage itself is a regular stage in the life of the dwarf stars. Thus, it was established that the stage of flare star when the star has a property to show from time to time flares, is an evolutionary stage, one of the earliest in evolution of stars through which all dwarf stars pass.
This conclusion, important for studying of evolution of stars, initiated regular and systematic photographic observations of flare stars in stellar associations and clusters. The observations were mainly carried out in Asiago (Italy), Budapest, Byurakan, and Tonantzintla (Mexico) observatories, led to its full confirmation and gave valuable data on this early stage of evolution of stars.
On the basis of these observations, Ambartsumian considered the problem on genetic relation between the two early stages of evolution, Ò Tauri type stars and flare stars, and showed that the stage of a flare star follows a stage of Ò Tauri type, beginning even before the termination of the last one. During this period of life of a star, the stages of Ò Tauri type and of a flare star are mutually overlapped, and Ò Tauri type stars along with continuous and irregular changes of brightness also show flare type changes.
Among Ambartsumian’s studies devoted to flare stars in systems, work on derivation of the function of distribution of average frequencies of flares in the given system on the basis of observation of stellar flares in it is distinguished by its originality. In this work the problem of definition of the abovementioned function is reduced to the solution of an inverse problem by means of the chronology of discovery of flare stars (the first flares) and chronology of confirmation of their flare nature (observation of the second flares). The new method was applied by the scientist to the sample of flare stars in the Pleiades cluster. The obtained function of the distribution of average frequencies of flares satisfactorily represents the observations of this most studied system of flare stars and once again confirms their abundance in the cluster.
It is necessary to note also that, based on the hypothesis of protostars and understanding about the transfer of bundles of proto-stellar matter to the surface layers of young stars, the carrier of energy from the stellar interiors, Ambartsumian predicted the existence of “fast” and “slow” flares having different properties, which were later discovered, and gave an explanation to the surprising phenomenon of the Fuor.
Stellar statistics
A number of Ambartsumian’s statistical studies are devoted to definition of the shape of the Galaxy, distribution of the stars and the interstellar matter in it.
For the first time by him (together with G.A. Shain) it was shown that the real number of white dwarfs in the Galaxy should be very large. The observable number of white dwarfs is limited to their low luminosity due to what they are observed only in the Solar vicinity. They also proposed a method for detection of relatively far white dwarfs.
On the basis of study of the surface brightness of O-associations, Ambartsumian showed that they are a typical feature of the structure of the external regions of our Galaxy. This feature is common for all spiral galaxies with the developed spiral arms. Hence, the abundance of O-associations in the Galaxy indicates that it is a spiral with well developed spiral arms.
In another research of the scientist, it was shown that when observing from the outside, the Solar vicinity would appear on the edge of an observable part of the Galaxy. The parts farther from its centre could not be observed because of their low surface brightness. This result has a great value for measuring of the real sizes of other galaxies.
Calculations of stars of different brightness gave a basis to Ambartsumian to conclude that in a Galaxy the stars are on the average concentrated around the plane of symmetry of the system more strongly than the interstellar gas-dust matter.
A special interest is represented by the results of the statistical analysis of the question on existence of real Trapezium type multiple stars and stellar chains. It was established that multiple stars observed in stellar associations and stellar chains of giant and supergiant hot stars mostly are really young, dynamically unstable physical systems of stars while the multiple systems observed in similar configurations and consisting of colder stars not belonging to stellar associations are almost without an exception false Trapeziums formed as a result of accidental favorable projection of stars on the celestial sphere.
Ambartsumian’s many other investigations also concern the stellar statistics, which are considered in other sections.
Galaxies and their systems. Activity of galactic nuclei
A logic continuation of researches of non-stable phenomena in stars and their systems were researches of non-stationary phenomena in galaxies and their systems, the phenomena of much more powerful scales and more unusual by the nature.
For these researches an assumption was initial, which completely was justified that in the world of galaxies in the instability phenomena the displays of unknown states of the matter connected with the process of formation of new structural components should be more powerful and long than those, which are in the world of stars.
It was shown that the most typical feature of the spatial distribution of galaxies is their tendency to appear in physical groups; multiple galaxies, clusters of galaxies, etc. There appeared, for example, that the relative number of multiple systems among galaxies is more than in case of stars.
Researches by Ambartsumian revealed a remarkable feature of distribution of galaxies: systems of galaxies in most cases are dynamically unstable, breaking up. Such observational facts testify to it as the abundance among the multiple galaxies of Trapezium type systems and very big internal motions in some systems of galaxies. There are serious bases to admit, that some multiple galaxies and clusters of galaxies break up nowadays because a part of the galaxies making up these systems have spatial velocities sufficient to overcome the forces of gravitation and leaving from corresponding systems of galaxies.
These observational facts have formed the basis for Ambartsumian’s important conclusion that in the world of galaxies at present there are phenomena of dynamic -instability of large scales connected with the formation of new galaxies. In other words, in the world of galaxies processes of the origin and evolution of new systems proceed at present. Weighty evidences in favour of this conclusion were received on the basis of studies of manifestations of the physical instability in many galaxies.
As a push for researches on studies of various displays of physical instability of galaxies was the discovery by the American astronomers Walter Baade and Rudolf Minkovski of radio galaxies, the galaxies having unusually powerful radio emission, of an order of their optical radiation.
Ambartsumian on the basis of the deep analysis of all existing data about radio galaxies showed that this phenomenon is caused not by external reasons (collision of galaxies) as the authors of the discovery of radio galaxies considered, but physical instability of the corresponding galaxies.
The theoretical study of the numerous observational evidences of various sorts of physical instability in galaxies led the scientist to a fundamental conclusion that in processes of origin and evolution of galaxies, the role of the central small in their sizes condensations, the nuclei of galaxies, is huge. He justified an essentially new understanding that all observational evidences of the instability of galaxies are a consequence of activity of the galactic nuclei. Further on he established that to various degrees of activity of nuclei of galaxies correspond various manifestations by the form and power in structure and radiation of galaxies.
A great scientific interest is represented first of all by those forms of display of activity of galactic nuclei, which are connected with the release of enormous quantities of energy. Radio bursts concern such forms of nuclear activity, the explosions accompanied by eruptions of big gas masses, emissions of powerful jets of matter and whole galaxies-companions, so-called compact galaxies. Forms of display of the most powerful energy release by the galactic nuclei are also strong radio emitting gas clouds observed around galaxies and unusually intensive ultraviolet radiation of galaxies.
The idea on the activity of galactic nuclei and dynamical instability of physical systems of galaxies, developed by Ambartsumian, allowed to understand the before inexplicable phenomena and predicting absolutely new phenomena. In particular, a big scientific interest represents an explanation of radio galaxies as a certain stage of evolution of galaxies.
For the development of the idea on activity of galactic nuclei, a great importance had the discovery in Byurakan of blue eruptions and companions of elliptical (blue giant) galaxies. It is difficult to explain the unusually blue colour of these formations even at an assumption that they consist entirely of hot (blue) stars. Therefore, the observed blue formations, emissions from nuclei of galaxies, apparently should be considered as -evidence of existence of still unknown states of matter in the galactic nuclei.
Ambartsumian’s theoretical consideration of observational data about the known displays of activity of nuclei of galaxies gave serious bases to admit that activity of nuclei is caused not by stars and not by diffuse matter containing in them. They could not explain, at least, such observed forms of nuclear activity, which are connected with allocation of enormous quantities of energy and eruptions of unusually big masses of matter. Hence, it is necessary to consider that in corresponding nuclei there are bodies of at present unknown nature, which contain very big stocks of matter and have huge energy. In other words, it is necessary to consider, that in galactic nuclei physical conditions of matter are extremely unusual and strongly differ from the conditions observed in other parts of the Universe. In particular, in some bodies containing in nuclei of galaxies, the matter density should be extremely high. Only in this case the nuclei can provide the continuous outflow of matter or emissions and eruptions of the big masses from the nuclei, phenomena revealed by observations in some galaxies. These reasons also have formed a basis for working out of new important understanding that the galactic nuclei are sources of huge quantities of matter and energy, which then give rise to formation around them galaxies or systems of galaxies and supply them with energy of the observed non-stationary motions. Ambartsumian showed that the results of studying of non-stationary systems of galaxies and various forms of display of activity of nuclei of separate galaxies represent huge scientific interest not only for discovery of the laws of the origin of stars and stellar systems of various scales, but also for detection and research of while unknown states of matter, including the proto-stellar ones. And the results received by the scientist in this area are in a full consent with understanding of the theory of stellar associations already mentioned earlier that matter development in the Galaxy has a certain orientation from denser states to less dense ones.
Observations of the last decades carried out by the world largest telescopes, completely confirm Ambartsumian’s conclusions on the unusual features of the galactic nuclei and their decisive role in origin and evolution of galaxies and their systems. Especially it is necessary to note thereupon the discovery of quasars, galaxies having nuclei of extremely high activity and the detection of the consequences of powerful explosions and eruptions from nuclei of some active galaxies.
For the problem of the origin and evolution of galaxies, works on discovery and study of galaxies with unusually strong ultraviolet radiation, galaxies with very active nuclei and so-called compact groups of compact galaxies, carried out under Ambartsumian’s supervision had also a great importance.