In Memoriam

In Memoriam V. A.AMBARTSUMIAN

(18 September 1908 - 12 August 1996)

Viktor Amazaspovich Ambartsumian was born on September 18, 1908 inTbilisi, the capital of Georgia. He graduatedin 1928 at Leningrad University, continued his studies as a post-graduate at Pulkovo Observatory (near Leningrad) inthe years 1928-1931, and next wasassociated with the University of Leningrad (now St.Petersburg), from 1934 as a professor. In subsequent years he devoted much ofhis time to the foundation and construction ofByurakan Observatory in Armenia of which he becamethe Director, and from 1947 he also was Professor of Astrophysics at the State University at Yerevan, the capital of Armenia. Theobservational programme of Byurakan Observatoryhas been strongly inspired by Ambartsumian's imaginative thinking.

Ambartsumian's scientific achievements are manifold. His earliest work,in theoretical astrophysics and in collaborationwith N. A. Kosirev, dealt mainly with solar physics:the solar atmosphere, sunspots and the theory of radiative equilibrium. He subsequently broadened his interest taking up problems of Wolf-Rayetstars and planetary nebulae, generalizing Zanstra's work on the determinationof the radiation field of the nebula and the temperature of the central star. Arelated result was his estimate, also made in collaboration with Kosirev, thatthe mass loss of an ordinary nova outburst is a minor fraction only of thestellar mass, which implies that it is a surface phenomenon only, not involving the whole star. A very impressiveextension of his work in theoreticalastrophysics is his demonstration of an invariant property of the law of diffuse reflection by a semi-infinite plane-parallelatmosphere. This preceded work in the same field by S. Chandrasekhar whoexpressed himself as follows on some ofthese topics in a 'Festschrift-paper' at the occasion of Ambartsumian's 80th birthday:

The formulation of the principles of invariance in the theory ofradiative transfer: a theoretical innovationthat is of the greatest significance. Many paperswere contributed to a symposium on this topic at Byurakan in the fall of 1982; and in my contribution to that symposium I narrated the influence of Academician Ambarisumian 's ideas on my own related work.

Ambartsumian's marvelously elegant formulation of the fluctuations in brightness in the Milky Way: 'in the limit of infinite optical depth,the probability distribution of the fluctuations in the brightness of the Milky Way is invariant to the location of theobserver'.

Ambartsumian's interest then broadened to include stellar evolution, theproblem of star formation, and the origin and evolutionof stellar systems. In early work on the statistics of double stars he had argued that these cannot have existedfor more than ten billion years, a time scalemuch shorter than was generally accepted at that time. Inhis work of the 1940s and later on star formation and the origin and evolutionof

small stellar systems, Ambartsumian's unorthodox approach drew muchattention.

In the years 1941^3, he postulated that certain groups containing starswith similar properties, drifting among the general stellar population, aredynamically unstable systems and must be of much more recent origin than thestellar population in general. He called them stellar associations anddistinguished two categories: the O-Associations characterized by membership ofthe massive O- and B-type stars, and the T-Associations containing the, lessmassive, T-Tauri stars. He pointed out the frequent occurrence of so-calledTrapezium-type systems in the O-Associations: compact groups of very massivestars whose lifetime cannot exceed a few million years at most and that musthave a common origin. This work has greatly contributed to the now generallyaccepted view that star formation has been a continuous - and still ongoing -process up to the present. As to the formation process itself, Ambartsumianwent even as far as postulating that stellar associations originate fromsuperdense primordial matter, a postulate he later extended to the formation ofgalaxies in general.

Ambartsumian earned world-wide recognition for his pioneering work. Hewas a Vice-President of the International Astronomical Union from 1948 to 1955and its President from 1961 to 1964 and he also served as President of theInternational Council of Scientific Unions. He received many honours, both frominside the USSR and internationally. Among the first were the Order of Leninand the Stalin Prize, both awarded soon after the end of World War П. In 1950 he became a Deputy to the Supreme Soviet andin 1961 a member of the Presidium of the Academy of Sciences of the USSR. In1960 he was awarded the gold medal of the Royal Astronomical Society, and inthe same year he received the Bruce medal of the Astronomical Society of thePacific. He was a foreign member of many Academies of Science.

Ambartsumian's term as a "Vice-President of the IAU coincided withthe years of the cold war between western powers and the Soviet Union. In thoseyears, the IAU went through a critical stage in its existence as a consequenceof the IAU Executive Committee's decision to postpone the General Assembly thathad been planned for 1951 in Leningrad. During the subsequent years, althoughvigorously contesting the EC's decision, Ambartsumian did not fail to continuehis support to the Union as the world-wide organization embracing astronomersfrom all countries. His election as President of the IAU in 1961 reflected boththe appreciation for his efforts in this respect and his outstanding scientificachievements.

Adrian Blaauw © Indian Academy of Sciences • Provided by the NASA Astrophysics Data System