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The mechanism of stochastic self-propagating star formation has previously been invoked to explain the origin of spiral arms in galaxies. In this paper we extend the application of this mechanism to account for the diversity of morphological types and the evolution of galaxies. The new property that arises from consideration of this mechanism is that the rate of star formation exhibits the critical behavior of a phase transition. This is a general property of the system and is not strongly dependent on the details of the star--interstellar gas interaction. Examination of the properties of this phase transition provides a general scenario for the evolution of galaxies and the origin of the various morphological types.

We have examined the properties of model galaxies produced by the stochastic, self-propagating, star-formation model and compared them with those observed in real galaxies. Observations have shown that a good correlation exists between the morphological type of a spiral galaxy and its rotational velocity, color, and gas fraction. We have shown that the stochastic, self-propagating, star-formation model directly correlates the morphological type with the rotation curve, the most important parameter being the maximum rotational velocity. The relative star-formation rates given by the model lead qualitatively to the observed variation of integrated colors and gas fraction as a function of morphological type, as well as to the observed decrease in the density of regions of recent star formation with radius. We show that this model is also capable of giving spiral arms in red plates and discuss the color profiles across spiral arms. Inclusion of the interaction of star formation and the interstellar gas shows that this model is also capable of inducing spiral ordering in the gas.

We present models for temperature and ionization structure of low, uniform-density (approximately 0.3 per cu cm) interstellar gas in a galactic disk which is exposed to soft X rays from supernova outbursts occurring randomly in space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ionized fraction, temperature, and their various observable moments. These time-dependent models predict a bimodal temperature distribution of the gas that agrees with various observations. Cold regions in the low-density gas may have the appearance of clouds in 21-cm absorption. The time-dependent model, in contrast to the steady-state model, predicts large fluctuations in ionization rate and the existence of cold (approximately 30 K), ionized (ionized fraction equal to about 0.1) regions.

A spectrometer was used on the satellite Copernicus to observe a chromospheric L alpha emission from the K0 giant beta Gem at 1218.4 A. This emission appears to be in the corona at temperatures near 260,000 deg K, since the ion it is identified with requires 77.4 eV to be produced.

The photodesorption of molecules and atoms from the surfaces of interstellar grains can be an important source of heating for the interstellar medium and the origin of instabilities which may separate grains and gas. For low densities, the force exerted on the grains is proportional to the gas density and independent of the radiation intensity; for high densities, it is proportional to the radiative flux and independent of the gas density. This force may act differently on grains of different sizes. The photoelectric effect may also be an efficient mechanism for the separation of gas and dust in diffuse clouds.

A heuristic treatment of diffusion in the solar chromosphere-corona transition region is developed. It is shown that diffusion becomes increasingly important with steeper temperature gradients, in active and quiet regions relative to coronal holes, and with increasing excitation potential. Numerical calculations are made for the resonance lines of He I and He II and show that diffusion can enhance these lines. Thus the helium lines may appear relatively weak in coronal holes due to a weakening of the enhancement mechanism. Most transition region lines will be less affected by diffusion than He I or He II.

We propose that dwarf elliptical galaxies are the bound remnants of more massive stellar systems, such as a parent galaxy which has lost more than half its mass in gas ejection during a burst of star formation, or an unbound tidal debris produced during the interaction of two giant galaxies. N-body calculations were performed to study the evolution of stellar systems with postive energy. The results indicate the existence of a power-law relation between the mass and the density sigma of the fragments over a large range of values of sigma, consistent with recent observational evidence. The calculations predict a mass-metallicity relation for the fragments. The amount of unbound material in Virgo Cluster that genetically connected with its dwarf elliptical galaxies is not detectable at present.