The interstellar gas shows wide variations in density and in velocity. Common words to describe the existence of these variations are cloudiness for the density and turbulence for the velocity field. These terms have initially been introduced as vague descriptions and were not based on mathematical analysis or dynamical theory. The word cloud as used by astronomers did not always mean a region of large density with relatively sharp boundaries; nor did the word turbulence necessarily imply a velocity field with well-defined statistical regularities and energy dissipation by means of viscosity.

This paper reports briefly on the provisional results of the 21-cm galactic work at the Radiophysics Laboratory, Sydney. Observations were made with a 36-ft, transit-type radio-telescope whose beam width at half-power is 1°.5 and a 4-channel receiver whose band width, 40 kcps, is equivalent to 8 km/sec.

The large scale structure of the galaxy has been studied by Kerr, Hindman, and Carpenter. They derive a general picture of the spiral structure in the southern hemisphere extending the Leiden results (see Fig. 1) and, in addition, a “relief map” of the galaxy showing the location of the densest parts of the hydrogen layer with respect to a central plane (see Fig. 2). Analysis of the spiral structure is for the present limited to regions beyond 3 kpc from the center. The Sydney and Leiden data agree well where they overlap, but location of spiral arms is sensitive to the choice of rotational model. Four or five spiral arms, or sections of arms, are indicated in the longitude interval l= 170° to l= 320°. The arms are clearly trailing in the rotation.

The large-scale distribution of gas and dust in external galaxies seems to be closely correlated with the spiral arms and other main structural patterns. This was shown, in particular, by Baade and Mayall at the 1949 Symposium. A great many structural patterns exist among external galaxies; it is likely that these will prove to be important indicators of the dynamics and possibly of the evolution of cosmic gas clouds in stellar systems. This was pointed out at the 1953 Symposium by Hubble, whose original classification system was described in the report by Blaauw which includes also some indications given by Sandage of the program of revision and illustration of this system.

Cygnus-x is an extended radio source centered at δ=40° and R.A. = 20h25m (l=48°, b=0°). Its emission in the continuous spectrum has been studied by Davies using high resolution surveys at the following frequencies: 92, 158, and 1425 Mcps, 242, 400, 600, and 900 Mcps. The background emission at the position of the source was removed by a smooth interpolation in galactic longitude between the two sides of the source. The resultant flux density did not change significantly with frequency and had the value 4.5 X 10−23 wm2 (cps)−1. Moreover, the brightness temperature Ts of the center of the source varied inversely as the square of the frequency v; Ts·v2 had the value 3.5X1019 °K sec−2.

In 1952 it was shown by one of us that the sources of galactic nonthermal radio emission form a nearly spherical system concentrating toward the plane and the center of the galaxy. It was pointed out independently by the other author that the field between the clouds must be sufficiently strong to retain the cosmic rays in the galaxy. The density of the kinetic energy of the gas between the clouds could be taken as equal to that of the magnetic energy. Thus the velocity dispersion of the rarefied gas in the space between the clouds should be large and form a spherical, not a flat, subsystem. The spherical distribution of the radio emission supports this suggestion. The wide H and K absorption lines appearing in the spectra of early supergiants are also an argument in favor of the reality of fast movements of the rarefied gas. However, L. Spitzer points out that at least a part of these lines belong to the stars. Spectrograms taken by G. Münch show that some of the wide lines consist of a few faint narrow lines. These phenomena may be explained by the density fluctuations of rarefied gas (kv ∝ n2), but some other interpretations are also possible. This phenomenon supports the hypothesis that the more rarefied gas possesses a higher velocity dispersion. The existence of wide H and K lines is not the principal argument of this theory. It will be shown below that the gas of the halo is too rarefied to give observable lines.

Because of the extreme irregularity in distribution of interstellar matter, some idealized model must be adopted in discussing the interstellar medium. Three simple models have been considered. The original one, long-since discarded, was a uniform distribution. This has been largely replaced by the random cloud picture in which space is visualized as filled with isolated clouds, 5–10 per kpc, about 10 pc in diameter, the array having an exponential velocity distribution with a mean of about 8 km/sec. Lastly, there is a turbulent or a random density fluctuation model.

Although the random cloud hypothesis has received most attenuation from astronomers, it has been well recognized that it is an over simplified model and should be used with some caution. This point of view has been clearly stated by many workers in this field, as the following statements by Oort and by Minkowski, respectively, demonstrate.

The characteristic features of interstellar gas clouds—existence of large density fluctuations, their connection with cosmic dust, stretching along the magnetic fields, and so on—may be described by a shock wave theory in interstellar space.

The author has developed a theory of stationary shock waves accompanied by losses of energy by means of radiation. Choosing two surfaces on both sides of the front, so that the regions of energy radiation should lie between them, we can write an equation for the mass flow and for the impulse conservation between these surfaces, and two equations which determine the stationary temperature of the gas in the field of interstellar radiation. The solution of this system of equations permits one to determine the general changes of thermodynamic and other parameters for the transition of gas through the shock wave with regions of radiative cooling. If changes of the degree of ionization take place, and a magnetic field is present, some terms should necessarily be added to the corresponding equations.

The boundary between the interstellar gas cloud and the intercloud medium must represent the shock wave accompanied by losses of energy by means of radiation, because such ruptures may probably be supposed as the sole explanation of stability of the great density changes (a hundred times and more) often observed in the interstellar space.

In this paper we give some results of the theory of shock waves accompanied by radiative losses of energy.

Almost ten years have passed since the idea of stellar associations as nonstable stellar systems was formulated. The complex of observational data obtained during this time indicates that stars contained in the associations are young objects of some million years of age. We would like to stress here that this concerns both the O and T associations. It is also known that those O associations which could be sufficiently investigated in this respect, contain, as a rule, T Tauri type stars and are consequently T associations as well. There are, on the other hand, T associations which do not contain hot giants. But apparently the mechanisms of stellar formations must be similar in O and T associations. This means that any theory of stellar origin for a given type of association must permit variations, which will provide an explanation of the origin of stars in associations of other type.

Two hypotheses on the origin of stellar associations have been thus far discussed. One of them, suggested by the author at the initial stage of the idea about associations, supposes that each association has originated as a result of an expansion from a body or a system, the volume of which was initially very small. The dimensions of the latter was in any case less than one parsec. According to this point of view, these initial bodies (protostars) have either not been observed up to the present, or have not yet been identified with any known object. This point of view does not give any indication about a concrete mechanism of stellar origin, postponing its explanation to the time, when the earliest stages of the expansion of the association may be studied in detail.

The discovery of globules by Bok and Reilly leads to various speculation concerning their cosmogonical meaning as protostellar bodies in our galaxy.

It is well known that the most favorable conditions for the discovery and study of globules are reached when they are connected with diffuse galactic nebulae (Dufay). Observations show that in this case globules possess various and generally irregular shapes. It is quite probable that these globules are connected with large dark clouds, genetically linked with bright nebulae. The number of globules in a nebula depends essentially on the peculiarity of its structure. Nevertheless the total area of the sky covered by the nebulae is relatively small and their physical conditions depending on nearby hot stars are peculiar. Consequently it is hardly possible to suppose that the properties of globules present in nebulae are the same as in the other regions of the galaxy. It is important to collect more data concerning the spatial distribution of globules, their density, sizes etc. The stellar clouds of the Milky Way give some possibility for further study in this respect. In the case of a sufficiently dense stellar field the projecting globules can be discovered as some fluctuations in the stellar population as observed on the photograph.

When investigating sufficiently dense star fields, one often finds some short chains of stars oriented with a certain regularity and in a few cases connected with dark filaments of the galactic nebulae. It is necessary to be very cautious in the interpretation of this phenomenon, because even completely accidental clustering of some objects can sometimes lead to interesting deviations from homogeneity.

The polarization of the stars in open clusters, explained on the basis of the Davis-Greenstein theory, gives some information on the microstructure of the galactic magnetic field.

The polarization is most conveniently described by the parameters Q, U, proportional to the Stokes parameters and defined by where p is the amount of polarization, θ is the position angle of the electric vector, and θ̄ is the mean value of θ for the region under consideration.

This paper reviews the many, by now familiar, processes that have at various times been attributed to the interstellar motions, in an effort to piece together some over-all picture of the grosser dynamical relations which might be expected to apply to the motions of the material and electromagnetic fields between the stars. We are interested in numerical values for the material densities, velocity fields, magnetic fields, etc., and we should also like to know to what extent the observed properties of the different fields are dependent upon each other.

Analyses of aerodynamic dissipation in ordinary un-ionized gases are all based upon the Navier-Stokes equations. These equations relate the rate of dissipation to the local gradients in velocity and temperature through the viscosity and heat conduction coefficients. Although it is true that in many flow situations the magnitude of the total dissipation in the gas does not depend on the magnitude of the viscosity coefficient, this coefficient does determine the minimum scale of variations observed in the gas and the form of the Navier-Stokes equations determines the type of phenomena which are observed on a small scale. In order to discuss dissipation in an ionized gas in the presence of a magnetic field, it is therefore necessary to re-examine the derivation of the basic flow equations. This paper attempts to do this for a case of a completely ionized gas and demonstrates that the basic microscopic dissipation mechanism is appreciably different. For example, it is shown that the minimum length in which the properties of the flow field can change noticeably is appreciably less than one mean free path.

Before entering the subject of magnetohydrodynamic dissipation we comment briefly on the question of the supply of kinetic energy to the interstellar gas. This topic has been considered in the 1953 Symposium by Schlüter and myself, and also by Oort. As had been pointed out already by Spitzer in Paris, 1949, the visible HII regions, owing to their excess pressure as compared with the HI regions and the dilute HII regions, must be assumed to expand with a velocity of the order of 10 or 20 km/sec. By this expansion part of the radiation energy of the star is converted into kinetic energy. It was estimated (p. 153 of the proceedings) that a typical HII region around a BO star feeds 1035 ergs/sec to the instellar gas, and that the number of these regions is such, that each region has to provide kinetic energy on the average to 1036−1037 g of interstellar material. Thus a value of 10−2−10−1 erg g−1 sec−1 was found (which corresponds to 10−26−10−25 erg cm−3 sec−1, assuming 10−24 g/cm3 for the mean density of the interstellar material in the disk).

Section II contains a summary of relevant collision theory methods. Section III is concerned with heavy particle collisions: excitation of H 1s hyperfine structure (hfs) states by H atom impact; excitation of H2 rotation by H and H2 impact; H 2s→2p transitions produced by proton impact; charge exchange reactions; excitation of atomic levels by proton impact. Section IV deals with inelastic collisions between atoms and electrons. Use of the Born and distorted wave approximations is discussed. Calculations of cross sections for excitation of forbidden lines in pq configurations are reviewed and new results presented for O+2, N+, C+, and Si+, both variational and semiempirical methods being used. In Sec. V, concerned with atomic photoionization, new results are given for photoionization from 2pq configurations.

In order to study recombination processes in gaseous nebulae, one requires the coefficients for radiative recombination to many excited states of neutral atoms and positive ions. These may be calculated as functions of electron temperature once the corresponding photoionization cross sections are known. For photoionization from valence-electron states a general formula has been obtained using a method which is an extension of that used by Bates and Damgaard for bound-bound transition probabilities.

We shall not discuss here in detail the problem of the temperature equilibrium of interstellar matter. Some improvement of our knowledge has been effected by Seaton's results concerning the collisional cross sections of the ions C+, Si+, Fe+, which are important for the temperature equilibrium of the HI regions, and by his results concerning the excitation of the forbidden lines in gaseous nebulae.