15 AMBARTSUMIAN 



diffusion in a turbid medium, which is of great importance in 

 many questions of geophysics, physics, and astrophysics, and 

 for which he received the State Prize in 1946. This traditionally 

 well-known problem in science has generally been reduced to 

 an integral equation for which the solution was found in a very 

 cumbersome fashion by means of consecutive approximations. 

 Ambartsumian applied an entirely new method to the solution of 

 this problem: reducing it to simple functional equations, he ob- 

 tained an exact solution to it. These equations have become 

 known as "Ambartsumian' s Functional Equations." Also in the 

 forties, he completed a cycle of studies dealing with the problem 

 of the structure of the Galaxy which had been partially carried 

 out during his stay in Leningrad. The structure of the Galaxy 

 (the basic problem of modern Astronomy) became a more com- 

 plex question in the 1930' s with the discovery of dark, light- 

 absorbing matter in the interstellar space. In studying (along 

 with Sh. G. Gordeladze) the distribution of hot stars and of dif- 

 fuse nebulae, Ambartsumian revealed the patchy structure of 

 the dark matter, and drew the conclusion that interstellar ab- 

 sorption is conditioned by the total mass of dark clouds, in the 

 form of separate, obscure nebulae, not a continuous medium, 

 as had been previously believed. On the basis of the patchy 

 structure of the dark matter, he elaborated a mathematical 

 theory of the fluctuations in the distribution of the stars, of the 

 brightness of the Milky Way, and of the extragalactic nebulae 

 which was subsequently developed in the work of Ambartsumi- 

 an' s pupils and by a number of foreign scientists (such as 

 Chandrasekar, Munch). His work also dealt with the relation- 

 ship between the luminosity of interstellar matter in space and 

 the neighboring stars, a method to calculate the mass ejected 

 by Nova (the order of magnitude of only one part in a thousand 

 of the mass of the sun), a theory on radiation equilibrium in 

 planetary nebulae, and a theory for determining the space ve- 

 locity distribution of stars from their radial velocities. 



A new development of Ambartsumian is concerned with the 

 origins and development of celestial bodies. An analysis and 

 synthesis of observation material accumulated enabled him in 

 1947 to discover the existence in the Galaxy of a new type of 

 stellar systems which he designated as stellar associations. 

 These he found to be subject to break-up through the dropping 

 out of individual stars, and to be of comparatively recent origin 

 (State Prize, 1950). He established the continuous process of 

 star formation at the present stage in the development of the 

 Galaxy. This was a refutation of the concept held of the 



