The Evolution of Galaxies and Stars.

I. Aims of the theory.-A hydrodynamical scheme of evolution is proposed, confined to events after the time when the average density in the universe was comparable to the density inside a galaxy at our time. ii.Hydrodynamical conditions.-Gas in cosmic space is moving according to hydrodynamics, mostly in a turbulent and compressible manner. Dust is carried with the gas, probably by magnetic coupling. Star systems cannot be described- hydrodynamically and hence do not show turbulence and supersonic compressibility. III. The spectral law of incompressible turbulence.-The relative velocity of two points at a distance 1 is proportional to 11/3. This is deduced from the picture of a hierarchy of eddies. I V. Compressibility and interstellar cThnds.-A hierarchy of clouds is considered. V. General evolutional scheme for a gaseous body.-A gravitationally stable, turbulent cloud is first flattened into a rotating disk, which then is dissolved into a uniformly rotating central body and a part returning into cosmic space. The time scale of these changes is somewhat larger than the diameter of the cloud divided by the turbulent velocity. VI. The origin of galaxies.-They seem to have been formed by a competition between expansion and turbulence. VII. The evolution of galaxies and spiral structure.-Irreguiar nebulae must be young, spirals intermediate, elliptical nebulae genetically old. Spiral structure is the distortion of turbulent clouds by nonuniform rotation. A bar is more stable than a disk. A twoarmed spiral seems to be a distorted bar. VIII. The origin of the stars.-Three groups of stars are considered instead of Baade's two populations: (a) stars belonging to the galactic center dynamically; (b) old stars belonging to the disk; (c) "young" stars. Stars could be formed as long as there were no stars present, because stellar radiation inhibits the contraction of clouds to form new stars. IX. "Young stars."-They seem to be, more exactly, rejuvenated stars. The mechanism of the accretion of interstellar matter by a star is discussed hydrodynamically. K. Rotation, planetary systems, and double stars.-Stars must be formed rotating because of the turbulence of the original clouds. They lose their rotation probably by a combined magnetic-hydrodynamic process. Both formation and loss of rotation provide gaseous disks in which planets and double stars can be formed. Modifications of the author's theory on the origin of planets are discussed. XI. Giants and white dwarfs.-Giants seem to have highly condensed cores. Their model may be nonstationary, with a slowly expanding atmosphere. Planetary nebulae then would be special types of giants.