This discussion actually took place after the Reports by Lüst and by Pottasch. However, the remarks pertaining to Pottasch's Report have been combined with the discussion following Boyarchuk's Report, since the latter two Reports appeared to have more in common than the first two. I have condensed several contributions, notably a very long remark by Gordon.

In this summary we shall attempt to evaluate the mass loss from several kinds of high luminosity stars, especially planetary nebulae, OB supergiants and M giants and supergiants. The purpose is to give an observational basis for the discussion of the mechanism of mass loss and of the consequences of stellar mass loss for the interstellar medium and for stellar evolution. For reasons which will presently be discussed, we are now certain that mass loss is occurring in all the objects mentioned, and probably to a similar extent in all high luminosity stars as well. The precise values of the mass loss rate are uncertain at present; for some objects the uncertainty will be large (two orders of magnitude) and have important influence on the consequences of the mass loss. Therefore we shall discuss in some detail how the different loss rates quoted in the literature have been obtained and what assumptions have been made (see also the Report by Boyarchuk, p. 281). On the basis of this discussion we will indicate the most probable loss rates and their consequences, always remembering the possible influence of the uncertainties.

The previous reports by Lüst and by Pottasch concerned quasi-stationary mass loss. The loss is called quasi-stationary because the basic characteristics of a star, its temperature and its radius, do not change significantly during the time that a particle needs to flow from the stellar surface out to a point where the stellar influence is negligible. However there are many stars which show significant changes of temperature and radius within a short time. Very often these changes are accompanied by instantaneous ejection of a great amount of material. For example, during the outburst of a nova the ejected mass can reach 10−4M⊙. Such effects are very appreciable in the so-called ‘eruptive variables’, in which the brightness changes spasmodically. It is very likely that the rate and the mechanism of mass loss of eruptive variables are different from that of normal stars.

This Discussion contains the remarks made during the discussion after Boyarchuk's Report on Tuesday, September 16, and about half of the remarks made in the discussion after the Reports on Sunday, September 14. The other half of the latter discussion is presented in Chapter 18. At the end of the Session on Tuesday Thomas asked Busemann to comment on the interrelation between aerodynamics and interstellar gas dynamics. Busemann's response has been transferred to the Final Discussion (Chapter 27). Some general comments by Goldsworthy have been moved to Chapter 5, Section 6.

The problem of the interstellar grains is in many ways still unsettled. The theories of the formation, growth, and destruction of the grains have undergone a number of modifications because of the very significant increase in our understanding of the dynamical processes in our Galaxy. I will emphasize here how the density wave theory of the origin of the spiral structure may lead to a new broad concept in dust formation and its generic relationship with star formation. The basic physical processes are not changed but rather the framework in which we consider them.

The observation of molecules in the interstellar medium offers an excellent oppor- tunity to specify the physical conditions present there. This must be done through the combined use of theory and observation. In the last few years the advancement in this area has accelerated spectacularly. McNally (1968) reviewed the subject and Spitzer (1968) included molecules in his more general review; these are both excellent references and the reader is referred to them for the development and state of the subject in 1966.

There are two levels of interest in the abundance of molecular hydrogen in the interstellar gas. (i) Even a low abundance can enhance the cooling rate at a few hundred K. (ii) Densities comparable with or larger than those of atomic hydrogen could affect the dynamics in the galactic disk, if widespread, or at least affect star formation, if restricted to isolated large clouds.

The material of this Discussion has been rearranged in five sections: 1. Grain Orientation; 2. The Diffuse Galactic Light; 3. The Extinction Curve in the Far UV; 4. Formation and Destruction of Grains; 5. The OH Molecule. Except for rearranging, very few changes have been made.

I have been invited to report on globules and OH emission sources. However, I will extend the topic to protostars and other condensations in Hii regions, of which globules and OH emission sources are only special, although rather conspicuous, objects.

The Discussion has been rearranged into five sections: 1. Density Concentrations, Cocoon Stars, and OH Sources; 2. The Carbon Recombination Line; 3. The He/H Abundance; 4. Various Shorter Comments; 5. A New Mechanism for Star Formation. A part of the Discussion which related to Spiegel's Report has been transferred to Chapter 13. A short exchange on Hi shells around Hii regions has been deleted and added to Chapter 5. A short part on the dust cloud in Orion at the end of Section 4 has been transferred from Tuesday, September 16.

In the present form the Final Discussion is divided into three sections: 1. Discussion on the Overall Energy Flow in the Interstellar Medium; 2. Summaries and Suggestions for Future Research (more or less prepared talks); 3. Summaries and Suggestions for Future Research (more or less free discussion). The discussion in Section 1 actually took place on Saturday, September 13, but it belongs in the Final Discussion. The (invited) summary papers, presented in Section 2, were given by Parker, van Woerden, and Kaplan. Section 3 starts with the remarks by Busemann on the interrelation between interstellar gas dynamics and aerodynamics, as viewed by an aerodynamicist; these remarks were made on Tuesday, September 16. The rest of Section 3 is taken up by the actual discussion held on the final day of the Symposium.