The main topic of long-term researches by G.A. Shajn is the nature of diffuse matter, its distribu-tion in the Galaxy and extragalactic systems, interaction with the interstellar medium and hot stars, the formation of emission and reflection nebulae and stars have become the main theme of researches by Shajn during some years. Based on the analysis of experimental data, mainly pho-tographic observations of nebulae in the Milky Way and extragalactic systems, he made conclu-sions and suggested well-founded hypotheses on a wide range of the considered problems, in-cluding those related to cosmogony. The structure of nebulae, their masses and sizes give a reason to the conclusion that most of them are formed not in the process of ejection of matter from the stars, but this is the objects, which are born and evolved, and quite often comprised in giant conglomerates of gas, dust and stars. The distribution of O-stars and nebulae in spiral branches points to their genetic relation and the fundamental role of the interstellar medium as a source of their formation. The structural features of nebulae are determined by the action of magnetohydrodynamic forces. Magnetic fields in a galaxy control the motion of diffuse gas-dust matter and ensure the maintenance of its spiral structure. These ideas continue being developed in modern directions of astrophysics.
An intensive study of interstellar matter, its properties and distribution in our Galaxy and extragalactic systems was began in the middle of the 20th century and posed a number of problems concerning the origin of stars and stellar systems, their stability and evolution (Ambartsumian 1933(Ambartsumian , 1949;;Whipple 1946;Bok & Reilly 1947;Sharpless 1954). The first ideas and theories proposed in a number of works were based on the results from scanty observational data (Weizs.acker 1951;Blaauw 1952;Sharpless & Oster-brock 1952;Oort 1954;Oort & Spitzer 1955).More extensive facts obtained by different methods had required for their verification and development.
Long-term investigations of nebulae in the Milky Way were carried out at Crimean Astrophysical Observatory (CrAO) for about 20 years according to the program developed by G.A. Shajn. Grigory Abramovich Shajn (1892Shajn ( -1956) is a famous astrophysicist, a member of the Academy of Sciences of the USSR and a number of astronomical societies. His scientific path began at Pulkovo Astronomical Observatory in 1921Observatory in , in 1925 he was sent to Simeiz Observatory, which under his leadership became the leading astrophysical observational base of the Soviet Union. Investigations of small planets and comets, the Sun, variable stars and galaxies were carried out there. Shajn was the initiator of the foundation of Crimean Astrophysical Observatory and its Director in 1944-1952. His bibliography contains about 150 works concerning different astronomical objects -meteors, comets, small planets, stars, nebulae, and galaxies. His results on the rotation of stars, the features of spectra of long-period variables and the abundance of carbon isotopes in cold stars, on the study of nebulae in the Milky Way and the interstellar medium made contribution to the physics of stars and galaxies and are determined the development of a number of directions in this field. Shajn’s biography has been presented in articles by Pikel’ner (1957), Struve (1958), Chuvaev (1995).His scientific heritage was generalized in reviews of different years (Pikel’ner 1957;Struve 1958;Pronik 1998Pronik , 2005Pronik , 2008;;Pronik & Sharipova 2003) and in proceedings of the memorial conference published in Izv. Krym. Astrofiz. Obs. (BCrAO) v. 90, 1995. In the late 1930s Shajn studied the distribution of matter in the Galaxy. He obtained an unexpected result which was named the “Shajn Paradox”. Observations have shown the absence of correlation between the reddening of stars and the brightness of the Milky Way. Stars with a small reddening were detected in dark areas, and in contrary, stars with a strong color excess were found in light areas (Shajn 1937). This fact found an explanation in the cloudy structure of the Milky Way, whose apparent brightness is associated with near clouds, at a distance of 100-150 pc from the Sun. Also Shajn & Dobronravin (1939) carried out the study of the continuous spectrum of the Milky Way.
In the years of the World War II the 1 m telescope at Simeiz Observatory was demounted and after the war the spectral investigations could not be continued. Shajn decided to choose a direction of research in which with small instruments it could be possible to obtain actual results relevant to the physics of interstellar medium and to cosmogony of stellar systems (Shajn & Gaze 1951). He saw the development of questions of cosmogony in the solution of problems relating to the physical nature of nebulae, the diversity of their physical characteristics, the distribution in the galaxy, and the interaction of interstellar matter and stars. Later, the important part of this program became a study of magnetic fields and their role in maintaining the structure of nebulae and spiral arms of galaxies.
The systematic survey of nebulae was started by G.A. Shajn in 1949, together with V.F. Gaze, and within two years they obtained images of nebulae in the Milky Way in the belt of latitude limited to ±10 (Shajn & Gaze 1951). At the longitude the observations covered the Milky Way from Sagittarius to Monoceros. The researches have been based on the experimental data obtained by Shajn together with colleagues in the Crimean Astrophysical Observatory (CrAO). Shajn used high-speed cameras (F:1.4) with a diameter of 450 mm and 640 mm for photography and narrow-band filters that extracted in a spectrum strong emission lines and narrow bands in the continuum. This method of observations and the appropriate choice of exposure made it possible to obtain unique images of nebulae with many details invisible in early images of already known nebulae and discover new nebulae.
The results obtained during the first two years Shajn presented in his report at the VIII General Assembly of IAU in 1952 in Rome (Shajn & Gaze 1954a). These studies were recognized important, and it was decided to create a new catalog of gas nebulae. For this aim the Sub-Commission of IAU was organized and V.F. Gaze active
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