Ambartsumian’ s view on astronomy of XX century

«Stellar associations».

«Multiple systems of Trapezium type».

«Superassociations in distant galaxies».

Stellar associations and the evolution of the stars

From L.V.Mirzoyan’ s book «V.A. Ambartsumian»

The problems of the origin and the evolution of the stars and the stellar systems were always interesting to Ambartsumian.

The scientist found the first signs of the changes in the state of the stars and the stellar systems in his studies of the planetary nebulae, the non-stable stars and the statistical mechanics of the stellar systems.In 1947 Ambartsumian discovered a stellar systems of a new type — the stellar associations, the centers of the star-breeding in the Galaxy.

The initial point for the discovery of the stellar associations was the observed tendency of hot giant and super-giant stars (the stars of O and B spectral types) and the dwarf stars with irregular variations of brightness with emission lines in spectra (T Tauri type variable stars) to cluster in the sky. It turned out that these clusters occupy a bounded extent, i.e. they are physical systems. These systems are called stellar associations.

The stellarassociations are endowed by identical physical characteristics in contrast to the stellar systems known before. The stellar associations are not directly observed on the sky photos unlike the usual star clusters. The latters are evident on the sky photos due to the big stellar density. The average density of the stars in the stellar associations is less compared to the general stellar field of the Galaxy so they are lost in the star fields. However the stellar associations are distinguished by the high partial density of the stars of the mentioned physical types.

Two types of the stellar associations in the Galaxy are differentiated due to their characteristic stellar population: the associations of hot stars (O-associations) and the associations of T Tauri type stars (T-Associations). All the nearest O-associations comprise T Tauri type stars i.e. are also T-associations (O+T associations) while there is a big number of pure T-associations.

Ambartsumian analyzed the active forces in the stellar associations with a following principal result: the stellar associations are dynamically non-stable star systems that psently expand and should break up inevitably in an order of ten million years. The fact that the modern stellar associations had not broken up yet, proves that their age is less then tens million years.

This result was proven by observations of the Dutch scientist Adrien Blaau and the others. On the other hand, according to the short time scale the age of the Galaxy is more in thousand times. Hence, the stellar associations in the Galaxy are young formations. The multiple stellar systems in particular the stellar associations could not be formed on account of the close passages to the existing stars. We have seen that this type of formation even of totality of the double stars in the Galaxy is excluded. Therefore it is considered that the stars forming associations are linked to each other since the time of their origin, i.e. are also young.

The observation data indicates that the stellar associations and the stars of these systems are young. We will note some of them. From the surface layers of many stars, which are members of the stellar associations (Wolf-Rayet and P-Cygni type stars, and the stars with emission lines in spectra) there is a continuous and an intensive outflow of a gas matter, that proceeds for no more then tens millions years. This fact shows that the specified stars are in a formation stage and had not yet reach an equilibrium state. The abundance of the dynamically highly non-stable multiple stars in the stellar associations (Trapezium type stars and stellar chains) also indicates their young age. The age of these multiple stars according to Ambartsumian’ s calculations does not exceed several millions years.

Ambartsumian’ s idea of the multiple systems of Trapezium type was a pioneer discovery in the stellar dynamics and in astronomy in general. These systems consist of young stars only and have extremely unstable dynamics. Therefore they very quickly break up, much faster compared to their host stellar associations.

On the basis of the various observational data Ambartsumian showed that the stellar associations (and the stars entering into their structure) where born rather recently. Ambartsumian was first in the history of science to establish that the star-formation process in the Galaxy begun a few billion of years ago and is undergoing a modern stage of its evolution.

This conclusion had fundamental importance and completely rejected the dominating understanding 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 the stellar associations another principle result was obtained: the stars constituting a physical system are of a common origin, the stars are born in groups.

The new principle of common origin of the components forming the multiple stars has a great significance for the Solar systems origin question as well. There are no reasons to suppose that the processes of the planetary systems formation (in particular our Solar system) differs from that of the multiple stars.

After the discovery of the stellar associations the world’ s observatories obtained a rich data that completely confirmed the new conclusions on the physical nature of the stellar associations in particular the nature of their dynamical instability (expansion and the subsequent disintegration), the psently on going star-formation process and the group origin of the stars in the Galaxy.

The discovery and the study of the stellar associations, these star-formation centers in the Galaxy have a decisive role in the basic change of our understanding of the star and stellar system formation processes. These discoveries stimulated a rapid stream of studies in this area of science. The stellar associations became a powerful tool used by the researchers of laws of the origin and the evolution of stars and stellar systems.

The investigation of the stellar associations led to the new observational approach to the problem of the origin and the evolution of the stars and the stellar systems, to a new understanding of the nature of the proto-stellar matter and of the stars energy sources.

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

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

«Звёздные ассоциации,» Астр.журн.[Sov.Astr.], vol.26, pp. 3-9, 1949.

«Кратные системы типа трапеции,» Сообщения Бюраканской обсерватории [Communications of Byurakan Obs.], vol.15, pp.3-35, 1954.

.»Superassociations in distant galaxies,» The Galaxy and the Magellanic Clouds, IAU-URSI Symposium, no. 20, Canberra, March 18-28,1963, F. J. Kerr and A. W. Rodgers (eds.), Canberra, Austr. Acad. Sci., pp. 122-126, 1964.