Ambartsumian’ s view on astronomy of XX century

Ambartsumian on gas nebulae

Here are papers on this topic that enter in «A Life in Astrophysics. Selected papers of V.A. Ambartsumian», Edited by Rouben Ambartsumian, Allerton Press, 1988:  

«The exitation of metastable states in gaseous nebulae.»

«On the radiative equilibrium of planetary nebula.»

The physics of gas nebulae

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

A huge series of Ambartsumian’ s work is dedicated to the problems of the physics of the gaseous nebulae. Their numerous examples like the planetary and diffusive nebulae may be observed in our Galaxy.

The luminescence of the gaseous nebulae is induced by the neighboring hot stars (the temperature at the surface = 20000 degrees and more). Absorbing the ultraviolet radiation of these stars, invisible from Earth, the gaseous nebulae radiate their energy in the visible part of the spectrum. Ambartsumian was the first to suggest the mathematical explanation of the physical processes in the gaseous nebulae occurring during the processing (fluorescence) of the ultraviolet radiation of the hot stars. For this purpose he developed a method to study the radiation transfer in the gas nebulae, based on the separate examination of the radiations energy in the continued spectrum and in the lines. This new method, named the method of the field division, led to the creation of the theory of the radiative equilibrium of the planetary nebulae. The ideas of this theory are the fundaments of the modern theory of the gaseous nebulae.

Ambartsumian explained the colossal role of the ultraviolet radiation in the gaseous nebulae and the influence of the light pssure in the immobile gaseous envelopes of the stars. He proved that the planetary nebulae of regular shape with a hot star in the center psently are quickly expanding and will dissipate in the near future. The fact of the expansion of the planetary nebulae led to the important conclusion that these nebulae are very young formations originated as a result of an emission of the matter from the stars in the center. Later, the theory of the planetary nebulae expansion was developed.

In the spectra of the planetary nebulae two very bright lines are noticed that were never observed in the spectra of the terrestrial light sources. For a long time the psence of these lines in the spectra of the planetary nebulae caused a scientific mystery. It was supposed that there is an unknown chemical element «nebulium» that radiates the mentioned lines. In 1927 an American astronomer Ira Bowen explained the nature of the «nebulium» lines. In the gaseous nebulae exists the condition of the extremely low density of the matter and of the radiation (conditions practically unattainable on the Earth). Bowen showed that in these conditions there accumulate the double ionized, i.e. deprived of the two external electrons, atoms in the so-called meta-stable states. The probability of the transition from these states (levels) on the lower levels is insignificantly small. That is why such transitions on the Earth «are forbidden» ( the mentioned spectral lines are named forbidden as the nuclear trans-passes required for their originationoccur very rarely under the Earth conditionsas if they are «forbidden») and the corresponding spectral lines are not being observed. These types of transitions take place very often in the gaseous nebulae due to the big accumulation of atoms in the meta-stable states. The transitions of the double ionized atoms from meta-stable levels to normal lead to the radiation of the nebulium lines.

The explanation given by Bowen was only qualitative. It was still required to develop the general quantitative theory of the formation of the similar «forbidden» lines in the spectra of cosmic objects. The first step in this direction was made by a Norwegian scientist Rosseland.

However his theory was good only for particular cases and not for the «forbidden» lines in spectra of the gaseous nebulae.

Ambartsumian created the general excitation theory of the meta-stable atoms and the theory of the «forbidden» lines formation in the spectra of the gaseous nebulae. His theories have wide application in the astronomy. This theory allowed him to pdict the existence of the «forbidden» line of helium in the spectra of the non-stationary stars of Wolf-Rayet.

Ambartsumian developed special methods to determine the temperatures of the planetary nebulae nuclei and of the stars with gaseous envelopes. The gaseous envelopes are formed for example during the explosions of Novae and Supernovae stars as a result of an outburst of gaseous matter or as a result a continuous outflow of gaseous matter of the non- stationary stars. The fundaments of the theory of the atoms ionization and excitation were put forward by Amabartsumian.

Ambartsumian together with Kozirev suggested methods for determination the temperatures of planetary nebulae nuclei and for the stars with gaseous envelopes. This method allowed to discover that during its outburst a Nova star throws out a mass equal to the 10-5 mass of the Sun. Supernova star throws out a mass equal to even more — a mass of the Sun. Similar estimate of the gaseous envelope masses reveal the evolution rates of those stars.

In conclusion the only method of the gaseous nebulae mass definition based on their luminosity belongs to Amabartsumian (the luminosity is a measure unit of the radiation capacity of a cosmic object).
Two papers on this topic, published on the website, enter in «A Life in Astrophysics» (Allerton Press, 1998), and were originally published in:

«The excitation of the metastable states in the gaseous nebulae,» Циркуляры ГАО[Pulkovo Obs. Circ], no. 6, pp. 10-17, 1933.

«On the radiative equilibrium of a planetary nebula,» ИзвестияГАО[Bulletin De L’ Observatoire Central a Poulkovo], vol.XIII,no.114,pp. 1-27, 1933.