Byurakan, stellar associations and active galaxies
Born in Tiflis in 1908, Viktor Ambartsumian studied astronomy in Leningrad. In 1946 he founded Byurakan Observatory, where astronomers could test theoretical predictions against observations of the sky.
In 1947 Ambartsumian identified loose groups of young stars called stellar associations. Members of such a group gradually drift apart. Seeing them still together suggests that the group formed relatively recently. This was evidence that stars continue to form in our Galaxy.
Two stars close together in a photograph can lie at very different distances. Identifying an association means comparing positions, distances and physical properties. Among the groups Ambartsumian studied were those containing hot O and B stars or T Tauri variables. A massive hot star rapidly uses up its nuclear fuel. Its presence is therefore a clue to youth independent of the group’s loose structure.
Starlight also needs interpretation. Radiation passing through an atmosphere can scatter and change direction. Ambartsumian developed an invariance principle for radiative transfer, exploiting relations preserved when layers of a medium are combined. The problem connects a physical process inside a star with the signal that eventually reaches a telescope.
A separate line of work at Byurakan concerned active galactic nuclei. When intense radiation comes from a small central region, its energy source becomes a physical question. Ambartsumian insisted on studying systems in the course of change. The observed activity and his proposed explanations have different standing: some of his hypotheses are not accepted models in modern astrophysics.
He also held two distinct international offices: president of the International Astronomical Union in 1961–1964 and of the International Council of Scientific Unions in 1968–1972.
Imagine a group of stars gradually drifting apart. What would it look like millions of years later? How does finding such a group today give a clue to its age?