Most important scientific results

Astronomy, Astrophysics and Cosmogony

Theory of Planetary Nebulae (PN)

For the first time the influence of the light pressure on the planetary nebulae dynamics was studied (1932). Each quantum of light owns a definite amount of energy and linear momentum. The bigger the frequency of the quantum (or shorter the corresponding wavelength), the bigger the momentum carried by them. As there are many hot stars in the center of planetary nebulae, which emit short wavelength photons, a question arises, how this emission may affect the circumstellar gas medium. The problem had a very important subtext. The matter was that the classical cosmogony claims that stars are being formed as a consequence of the condensation of gas and dust, and the joint existence of a star and a nebula could introduce a definite clarification in their evolutionary chain. And it was really obtained that owing to light pressure the planetary nebulae expand and should gradually dissipate in the space. It was also shown that the age of the planetary nebulae could not exceed 100000 years if no continuous outflow existed from the central star. Thus, for the first time the new evolutionary paradigm on the formation of objects from denser matter was formulated.

   

Zanstra’s method for determination of the planetary nebulae’s central star temperature was modified giving the probabilistic definition of short wave energetic photons transformation into less energetic ones (1932). It is known that the planetary nebulae radiate due to the transformation of short wave photons emitted by the hot stars located in their center. It means that the high energy photons emitted from the star, which are not visible to the eye due to their short wavelengths, are being absorbed in the nebula ionizing the gas atoms. Due to this, ions and electrons are being originated in the nebula. Later on electrons couple with ions and as a result numerous different energy photons form, including those visible to the eye. If a method is being developed allowing calculate the number of the transformed and hence visible photons, the number of those short wavelength photons that originated the visible ones also may be determined. Just due to this it becomes possible to calculate the surface temperature of the star. This definition leads to the determination of radiative equilibrium.

Novae and Supernovae

For the first time the amount of matter ejecting as a result of Novae and Supernovae explosions was estimated (together with N.A. Kozyrev, 1933). The phenomena of Novae and especially Supernovae belong to the largest stellar explosions. The first of these names has been formed historically. The matter was that in the sky from time-to-time people discovered stars that had never been observed before and it seemed as if they were newly born stars. Later on it was understood that they also existed before, however were not visible due to their faintness. They become visible at some moment due to a burst and increase of their brightness dozens of thousands or even million times. And in a few months they return to their previous state. The Supernova phenomenon is much more powerful compared to the ordinary Novae. If the Nova after the explosion returns to its normal state, the Supernovae after the explosion don’t return to their former state anymore. For the thrown-out matter as a consequence of the explosion Ambartsumian obtained the presently known values of 0.00001 and 1 solar masses correspondingly for the Novae and Supernovae phenomena.

   

Stellar Dynamics

For the first time the distribution of stellar spatial (3D) velocities was derived using only the coordinates and radial velocities. By classical method, the stellar spatial velocities are being determined by their radial and tangential velocities. However, the observations at different directions lead to a variety of errors. Taking into account that the spatial velocities are being given in three components, Ambartsumian applied a completely different method using three other variables for each star; the radial velocity and celestial coordinates. As a matter of fact, a necessity originated to find the required function that depended on three variables by means of a known function also depending on three other variables. This problem was reduced to the numerical inversion of the Radon transformation. Four decades later the same mathematical scheme was applied for the construction and exploitation of computer tomography. “It seems to me quite possible that Ambartsumian’s numerical methods might have made significant contributions to that part of medicine had they been applied in 1936”, – mentioned in 1985 Alan Cormack who won the Nobel Prize for creating the tomography.

Using the statistical studies of, so called, wide binaries it was shown for the first time that those did not obey the dissociative equilibrium conditions (1936-1937). The matter is that during the stellar approaches as physically coupled binaries may form so as the existing binaries may disrupt. So, if three physically uncoupled stars approach each other, an energy exchange may happen between them. One of the possible consequences of such exchange is that one of the three stars obtains large energy and leaves and the remaining two, losing some part of their energy, cannot anymore move away and form a binary. The inverse phenomenon may also take place, when a third star approaches a physically coupled binary, transfers part of its energy to them and decouples them from each other and three uncoupled stars form as a consequence. A dissociative equilibrium establishes in a definite time when these two inverse processes equal each other. Ambartsumian showed that such equilibrium has not yet been established and the number of binaries is many times larger than was expected in the case of the equilibrium state. The same studies allowed arriving at a conclusion that the components of binaries had been formed jointly. Moreover, the observed distribution put an upper limit for the Galaxy age, which agreed with “the short scale”.

   

The mechanism of star “evaporation” from the open star clusters was revealed (1938). The use of this phenomenon allowed him for the first time find the halftime of the clusters’ decay, and was applied to anticipate the gradual decrease of the number of low mass stars in clusters. A new stellar statistical method was developed to consider this problem. For this, it was taken into account that in the stellar system of any type mutually connected by gravitational forces an ongoing energy exchange was taking place between its components. As a consequence, a so-called equilibrium Maxwell distribution of energies is being established. On the other hand, in the case of equilibrium distribution some stars gain such velocities that allow them break away from the gravitational field of the system and irreversibly leave. As a result, the energy equilibrium is being broken and the energy exchange again establishes a new equilibrium state. This process, which is by its sense the same as the water evaporation, gradually depletes the star cluster. And by the way, the low-mass stars leave the cluster the first as in the case of the same energy low-mass stars gain larger velocities. These studies provided a theoretical basis for decreasing the accepted age of the Galaxy for thousand times, making it equal to 10 billion years and for introducing “the short scale” of the Galaxy age. Note that this estimate has not changed until nowadays, though with the development of science many new methods have been applied to calculate the age of the Galaxy. It is worth stating that by modern estimates the age of the Universe is 13.7 billion years.

Interstellar Medium

The patchy structure of the Milky Way’s absorbing dust component was revealed (together with Sh.G. Gordeladze, 1938). Our Galaxy has two components, one having disk-like flat form and spiral arms come out from its nucleus. The stars belonging to this component form in the sky the nebulous strip of the Milky Way. And the whole gas-dust matter of the Galaxy belongs just to this component. Even with the naked eye one can see that the stars in the Milky Way are not distributed equally; in some parts their number is rather small compared to the neighboring regions. This is simply because of the presence of the absorbing matter. Ambartsumian was the first to show that the distribution of the absorbing matter was rather inhomogeneous and, in the reality, it was formed of individual clouds. The sense of the name “patchy structure” is just the fact that it is consisted of these individual clouds. For the first time, the mean absorption of corresponding single clouds was estimated to be of the order of 0.2 magnitudes corresponding to some 1.2 times weakening of the light passing through it.

   

The theory of the fluctuations in brightness of the Milky Way was formulated (1944). In the simplest form it asserts that the probability distribution of fluctuations in the brightness of the Milky Way is invariant to the location of the observer. In the interstellar space the absorbing clouds are concentrated in a rather thin strip around the plane of symmetry of the Galaxy. Definite deviations of the observed distribution of the brightness of the Milky Way have formed in the sky due to the light absorption in them. For the same reason, deviations from the equal distribution of the number of other galaxies are also formed. In other words, for example if in the case of absence of interstellar absorption, the brightness of the Milky Way in the neighboring regions of the sky would not vary much, the presence of the absorbing clouds causes abrupt variations of this brightness. The nature and size of the observed deviations are completely determined by the properties and the number of the interstellar absorbing clouds. The investigation of the observed deviations by the theory of fluctuations allowed determine the properties of the absorbing clouds.

Radiative Transfer Theory

The Invariance principle was proposed to solve the radiative transfer problems (1941-1942). The principle in fact has a very simple physical sense and statement. For the first time it was established for the purpose of the formulation of the problem of the reflection of radiation from the semi-infinite medium. It is enough to imagine a radiation scattering and reflecting medium that has only one surface and fills half of the space. An example of such a medium is the ocean for the ordinary light. The radiation penetrates into the depth of the medium and may change its direction in the case of each scattering process and move in the opposite direction or continue moving in the same direction. The problem is to determine the number of the reflected radiation as a result of numerous scattering processes, which in general case depends on different variables, including the direction, wavelength, etc. Very simple physical reasoning that the reflection properties of the semi-infinite medium will not be changed if a very thin layer of the same physical properties is added to the medium boundary gave an excellent basis for creation of a new and strong research method. The mathematical equations describing the problem are much simpler and are being easily solved compared to the previously used so-called transfer equation. After many years Ambartsumian declared the Invariance Principle to be one of the most important tools he invented. This principle bears V. Ambartsumian’s name and the corresponding function was named V. Ambartsumian’s φ function.

Stellar Evolution

The importance of the stellar associations as dynamically unstable entities was revealed (1947-1949). The stellar associations, which are distinguished by a rather big partial density of similar stars, attracted Ambartsumian’s attention still in 1930s. A special interest caused the question how the similar stars had gathered in the same area. Investigating their structural properties, Ambartsumian showed that they had been formed together and that they could not be older than a few dozen million years. It meant that the stars belonging to the stellar association also could not be older than that. Taking into account that the prevailing part of the stars of the Galaxy are a few billion years old, a conclusion was made that these were newly born stars. Thus, it was shown for the first time that at present the star formation process also continues. It was the first case when it was shown that in a finite space volume there exist stars of different ages. The other important conclusion was that the star formation occurs in groups.

   

Ambartsumian showed that the continuous emission observed in the spectra of non-stable stars had nonthermal nature and put forward an idea about new possible sources of stellar energy, the hypothesis of the superdense protostellar matter (1954). Moreover, later on Ambartsumian concluded that the same source might be responsible for the phenomena taking place in the centres of galaxies, galactic nuclei. Observational evidences, particularly huge amounts of energy emitted from the central parts of galaxies, insist on the existence of powerful sources of energy. At present the most popular explanation for the AGN powerhouse, the Unified Scheme, involves accretion of gas onto a Super Massive Black Hole (SMBH). Thus, Ambartsumian’s suggestion that there existed a supermassive dense body in the center of galaxies is now well accepted and SMBH could be one of the possible models of such matter. Though Ambartsumian believed that the source of energy was inner, it is obvious that his idea on the activity of galaxies based on the energy sources hidden in the galactic nuclei is modified to fit known physical theories, which Ambartsumian himself did not find unambiguous.

Theoretical studies of the hypothetical superdense protostellar matter have been done in the frame of the modern knowledge of physics (together with G.S. Sahakyan, 1960-1961). The existence of stellar associations and the possibility of the formation of stellar groups in a small space volume already require the presence of matter of a new type that is by its density comparable to atomic nuclei. The activity of the galactic nuclei and the need of the presence of large masses in them even more increased the necessity of the theoretical justification of the superdense matter. These researches allowed later increase the Chandrasekhar limit of stellar masses. However, so far it has not been possible to theoretically prove the possibility of the existence of superdense matter concentrations having masses of galaxies or their nuclei. So far just this prevents the further dissemination of the point of view of the activity of galactic nuclei and the formation of daughter galaxies by decay and ejections from galaxies. Some researchers believe that this obstacle is in fact a consequence of the non-perfect knowledge of the laws of Nature.

   

Statistical studies of the flare stars revealed their evolutionary status (1968). In general, on all stars one can find various evidences of activity. Even on the Sun that is considered as a quiet star, giant explosions and bursts take place, though having essentially smaller energy compared to the total energy emitted by the Sun. However, there exists a whole class of dwarf stars, which show flares having emitted energy dozens and hundreds of times more than that of the star at quest state. These stars are called flare stars and no stellar inner structure model had predicted such a phenomenon. For this reason, for some time these stars were considered as “crazies” of the stellar family that do not obey the general regularities. However, their observational study and statistical investigation showed that the flare activity was a regular phase in the evolutionary path of low-mass and low-luminosity stars. It was proved that all the stars of the mentioned category inevitably possess flare activity in the early phases of their evolution. Later on (1978), on the basis of the chronology of discovery (first flares) and confirmation (second flares) of the flare stars, Ambartsumian by a solution of an inverse problem derived the distribution function of average frequencies of flares in the given stellar system.

Active Galactic Nuclei (AGN)

The hypothesis on the activity of galactic nuclei was proclaimed (1956). Previously it was believed that the nuclei of galaxies were their oldest parts that did not have any participation in the evolution of galaxies. Even there was an opinion that the nucleus was the “grave” of the dead matter. However, the observational facts showed evidence that there were ejection and outflow of matter from the nuclei of galaxies, including some cases when these processes were connected with expenditure of a rather large amount of energy. Moreover, the amount of the ejected matter sometimes can be enough to form a new smaller mass galaxy. This and other similar facts served as a basis for a formation of an unprecedented idea on the activity of galactic nuclei. The various forms of activity have been presented as different manifestations of the same phenomenon of activity. The evolutionary significance of the activity in the galactic nuclei was emphasized and further hypothesis was declared on the ejection of new galaxies from the active galactic nuclei.

   

Theoretical Physics

In 1930 he published a paper with Dmitri Ivanenko where the impossibility of the existence of free electrons in the atomic nuclei was proved. After the formation of the first understanding on the atomic structure, the explanation of the contents of the atomic nucleus was considered as the most important problem. It was known that the nuclei were charged positively and the simplest explanation was that they consisted of protons. However, as the experiments showed, the nuclear mass was bigger than the summarizing mass of the protons corresponding to their charge. To overcome this controversy the author of the atomic structure Ernst Rutherford believed that the nuclei were formed of as many protons as was needed to reach the nuclear mass and the excessive charge was neutralized by the corresponding number of electrons located in the nucleus. Ambartsumian and Ivanenko showed that only electrically uncharged elementary particles of approximately proton mass could exist together with protons in the nuclei. Two years later the English physicist James Chadwick discovered the neutron. Ambartsumian and Ivanenko also put forward an idea that not only the quanta of the electromagnetic field, photons, but also other particles (including particles having nonzero rest mass) may be born and disappear as a result of their interaction with other particles (this idea lays in the basis of modern physics of the elementary particles and quantum field theory).

Mathematics

For the first time the problem of finding the form of the differential equation corresponding to the known family of eigenvalues was solved. This problem is rather hard in its general statement. As an example, one can imagine such a problem. It is known that all atoms have discrete energy levels and spectral lines form due to transitions between them. A question arises; having the set of the spectral lines characterizing an atom, is it possible to find the equation with the line frequencies as its eigenvalues? The problem discussed by Ambartsumian belonged to the same family of problems but was incomparably simpler, though at that time also seemed unsolvable. It related to the frequencies of oscillations of a homogeneous string. It was shown that only the given string could have the given spectrum of frequencies. The paper devoted to the solution of this problem was published in the German Journal of Physics (Zeitschrift für Physik, 1929). It remained unnoticed for one and half dozen years. “If an astronomer publishes a paper on a mathematical problem in a physical magazine, he is not to be wondered that nobody has noticed it”, – was recalling he many years later. However, the paper was found, valued highly by mathematicians and it initiated a wide direction in mathematics – inverse problems.

Source: Areg Mickaelian, "Viktor Ambartsumian: Life and Activities", Yerevan, "Antares", 48 p., 2014