«The theory of fluctuations of the surface brightness in the Milky Way».

«Surface brightness in our Galaxy».

«On the patchy structure of the interstellar absorbing layer.»

The nature of the interstellar matter and the theory of fluctuations

From L.V.Mirzoyans book «V.A. Ambartsumian»

After the phenomenon of light absorption in interstellar space of the Galaxy was discovered, it was necessary to study the properties of the interstellar absorbing matter. The absorption of stars and nebulae light by the interstellar matter changes considerably the brightness, deforms their distances and the distribution in the space. This complicates the research of the structure of the Galaxy.

Ambartsumian’ s contribution to the study of the interstellar matter and the definition of its structure and of its optical properties is very important. Ambartsumian proved that in the Galaxy it is impossible to explain light absorption only by psence of the gaseous matter in the interstellar medium and that the interstellar dust matter has to be considered as the reason of this phenomenon.

Ambartsumian (with his student Sh.G. Gorgeladze) revealed the nature of the observable connection of the bright dust nebulae in the Galaxy and the stars that illuminate them. Ambartsumian suggested a very simple and efficient method and proved that this connection in the most of the cases is accidental and dust nebulae are observed as bright ones only if stars of sufficient luminosity are accidentally appearing in their close vicinity and inside of them. It means that if the dust nebulae do not have high luminosity stars in their neighborhood they are not brightened and should be dark. So, it was required to assume that light and dark dust nebulae are formations of identical nature.According to calculations in the Galaxy stars of high luminosity brighten only an insignificant fraction (1/2000) of the all dust nebulae. In other words the number of not illuminated dark dust nebulae should be 2000 times more than the number of the light nebulae in our stellar system. From the fact of such abundance of the dark nebulae Ambartsumian made an important conclusion that light absorption in the interstellar space of the Galaxy is caused not by a continuous dust environment but by the separate dark nebulae, absorbing clouds. It was established that the interstellar absorbing medium consists of individual absorbing clouds, i.e. it has a patchy structure.

The absorbing clouds of big sizes having a large absorption capacity may be directly observed in the form of the dark clouds and may be investigated by the means of the absorbtion of the light of stars caused by the clouds and located behind them. But the observation and the investigation of the small clouds with insignificant absorbing capacity (the majority of the all absorbing clouds) is practically not possible.

Ambartsumians fluctuation theory became a powerful tool for the research of the interstellar small size absorbing clouds. The absorbing clouds in the interstellar space are concentrated in a quite narrow layer around the symmetry plane of the Galaxy. As a result of the caused light absorption there are certain deviations in the observable distributions of the Milky Way brightness in the sky and in the numbers of the extragalactic nebulae compared to the uniform distribution.

In the absence of the interstellar absorption for example the brightness of the Milky Way in the neighboring areas should vary a little and should change smoothly, while in the psence of the interstellar absorbing clouds leads to abrupt changes when the transition from one sky area to the next is observed. The character and the size of the observable deviations are completely defined by the absorbing properties of the interstellar clouds and their number on the way of the light ray.

The study of the observable deviations by means of the fluctuation theory allows to define important characteristics of the interstellar absorbing clouds (the average absorbing capacity, the average sizes, etc). In the theory of fluctuations Ambartsumian took into account the fluctuations in the observable distributions of the mentioned values in comparison with uniform distribution, that are consequence of the existence of the physical groups of the stars and the galaxies and also of the psence of dispersion of light in the interstellar space. Presently the conclusions regarding the patchy structure of the interstellar absorbing medium of the Galaxy and the nature and the properties of the absorbing clouds are widely used in science.

It is remarkable that when examining the random deviations of the observable values corresponding to the two neighboring directions in the Galaxy from their average value, Ambartsumian solved a mathematical problem requiring certain generalization of the Poisson’ s Law of the distribution of the random variables for a case when random variables are not completely independent of each other.

From L.V. Mirzoyan’ s book»Victor Ambartsumian» («ВикторАмбарцумян»,Ереван, 1985)

The following three papers on this topic that enter in «A Life in Astrophysics» (Allerton Press, 1998), were originally published in:

’ ’ К теории флуктуации яркости в Млечном пути,» ДАН СССР [Dokl.Akad. Nauk SSSR], vol. 44, no. 6, pp. 244-247, 1944.

«О поверхностных яркостях в Галактике,» Астр.журн.[Sov. Astr.], vol. 23, pp. 257-268, 1946.

«On the patchy structure of the interstellar absorbing layer,» Trans. IAU, vol. 7, pp. 452-455, 1950.