More than 20 years ago V. A. Ambartsumian proposed that much of the activity in galaxies was dominated and even generated by their nuclei. Subsequent observational work in radio, optical and x-ray frequencies has borne out his prophecy, and major interest has centered about the nature of the machine in the galactic nucleus. The major characteristic of this machine is that it releases energy rapidly and often spasmodically by processes which are not thermonuclear in origin.

The original studies which led to the conclusion that nuclei were all important were observations of the powerful radio sources and Seyfert galaxies, and evidence for the ejection of gas from galaxies of many types. The realization that the synchrotron mechanism was the dominant radiation mechanism and the later studies of Compton radiation were fundamental in leading to the conclusion that large fluxes of relativistic particles must be generated in galactic nuclei.

This joint discussion is dedicated to the memory of Sir Arthur Eddington who was born 100 years ago. He laid the foundation of much work on stellar evolution, which was summed up in his book The Internal Constitution of the Stars (ICS) which appeared in 1926. Although he did no substantial work on mass loss from stars, the possibility of mass loss is mentioned in several places in ICS and I shall mention these and other aspects of Eddington’s work which are relevant to our present interest.

In the last decade or so, significant progress has been made in developing the theory of the winds of early-type stars. This theory, as is by now well known, is built on the basic assumption that selective radiation pressure, acting in general through a multitude of lines, is responsible both for the expulsion of matter from these stars' atmospheres and for its subsequent acceleration to beyond escape velocity (Lucy and Solomon 1970; Castor, Abbott and Klein 1975; Weber 1981). In order to investigate the effects presumed to be dominant as well as to keep within reasonable bounds the effort required to compute complete models and work out their observational implications, contributors to this line of research have concentrated on a simplified version of the general problem. Thus, for the gas dynamic part of the problem, spherically symmetric flow has usually been assumed and time-dependent effects neglected while, for the radiative transfer part, the Sobolev approximation has usually been adopted and multiple scattering effects neglected. The validity of these assumptions and approximations, and indeed of the whole enterprise, is of course ultimately a matter of observational test.

Much of our knowledge of winds from late-type stars comes from the detection of ejected material, called circumstellar shells, as observed in absorption lines of low-excitation species such as Mg II, Ca II, Na I, and K I (cf. Reimers 1977). Observations of CS shells are difficult to translate into quantitative mass loss rates, a limitation which has not helped to test various mass loss mechanisms. The data clearly demonstrate one very important fact: shell velocities are very low. In fact, they are so much lower than surface escape velocitiesthat it was not clear that material is actually being lost until Deutsch (1956) detected the existence of the CS absorption shells ejected from α Her and α Sco in the spectra of distant companion stars. Today it is possible to demonstrate the expansion of shells out to several thousand stellar radii in K I scattering (Honeycutt et al. 1980).

Theoretical models for the interaction of stellar winds and the interstellar medium are described. Stellar wind energy sources are listed, and observations of expanding shells around stars and star clusters are reviewed. Giant expanding shells, or “superbubbles,” with radius > 100 pc, are formed by the combined action of stellar winds and supernovae from clusters of OB stars. Smaller expanding shells around Wolf-Rayet stars are probably formed by the interaction of the WR stellar wind with matter ejected in a previous red supergiant phase of stellar evolution. Stellar wind activity is evident in regions of star formation in dense molecular clouds.

The physics of mass loss from these objects is reviewed. Both winds of different types and nearly instantaneous ejection are involved. Supercritical winds driven by the pressure of radiation from an object above the Eddington limit, probably exist for novae after their outbursts. Observations outside the optical region and in particular in the satellite UV have been very important.

High speed winds from quasars interact with the gas and stars in a surrounding galaxy and produce a variety of effects. The winds sweep up the general interstellar matter into fast moving shells of cool gas which can produce lower velocity (≲, 0.01c) sharp absorption line systems. The impact of the wind on dense interstellar clouds may contribute to the narrow emission line region. Finally, supernovae or stellar winds near the QSO set up shock waves in the outflow. Shocked QSO wind material is responsible for the broad emission line material.

In this summary of the Joint Discussion on mass loss I discuss successively:

1. (1) stellar mass loss data and possible mechanisms

2. (2) mass loss for special types of stars and suggestions for future research in the stellar case

3. (3) interaction with the interstellar medium

4. (4) mass loss from quasars

With the advent of powerful instruments in various spectral domains many scientists became involved in the study of giant HII regions - i.e. regions as important as 30 Doradus in the Large Magellanic Cloud or NGC 604 in Messier 33 - and of HII complexes still more active such as isolated extragalactic HII regions, some galactic central regions, chains in large spirals and clumpy irregular galaxies.

In view of the importance of these investigations for the problems of formation and evolution of stars and of galaxies, we organized a Meeting at thelAU XVIIIth General Assembly on”Giant HII complexes outside our Galaxy”with the following program:

1. a- Observational results and their primary interpretations in the X-ray, UV, visible, IR and radio domains.

2. b- Confrontation with related investigations in our Galaxy and general interpretations.

Giant extragalactic HII regions are found in the disks of nearby spiral and irregular galaxies, in the nuclear regions of spiral and elliptical galaxies, and in a variety of peculiar and interacting systems. At radio wavelengths they may emit thermal continuum radiation from the ionized gas and/or nonthermal synchrotron radiation if high energy electrons and magnetic fields are present. In some instances line radiation from associated molecular and neutral hydrogen clouds may also be detected. Table 1 illustrates the sorts of objects in which radio HII regions are observed and indicates the range of radio parameters found. Columns 1 and 2 give the galaxy and specific HII regions within the galaxy. Column 3 is the adopted distance of the galaxy. Column 4 indicates whether the emission is thermal (T) or nonthermal (N-T); column 5 is the 20 cm luminosity, and column 6 the linear size of the radio emitting region. For thermal sources the electron density, derived from the luminosity and size, is listed in column 7. The final column gives references. Note that almost all of the observational information obtainable from radio continuum observations is contained in columns 4, 5, and 6. Very occasionally there may be additional data concerning polarization or variability.

Many galaxies emit significantly more infrared radiation than expected as directly observable photospheric emission. This emission is not restricted solely, or even predominantly, to the galactic nuclei. Rather, where detailed observations exist, they indicate that pronounced emission often originates throughout the disks and especially in the more extended central regions which are distinct from the nuclei. The process which invariably dominates the emission at wavelengths longer than a few microns is radiation from heated dust. At shorter infrared wavelengths, nebular continuum and line radiation, as well as the stellar photospheres, has been observed. In many cases, this emission has been unambiguously associated with HII complexes.

In this review, we focus first on infrared observations of relatively isolated giant HII complexes in the LMC and the spiral arms of M33 and Ml01. We then examine complexes which extend over several kiloparsecs near the galaxian centers. We will not consider the large body of data on extragalactic infrared emission which, although surely relevant to our discussion, is not straightforwardly attributable to HII complexes rather than unusual phenomena in the nuclei (see Rieke and Lebofsky 1979).

By definition giant H II complexes are luminous sources in the spectra of thermal photoionized interstellar gas. Therefore they are easily detected out to large distances. As a consequence the list of objects is vast and it is impossible to cover in detail all the results brought together by an increasing number of investigators. For more complete bibliography the reader is referred to the reference lists in the papers quoted. Let me line out here instead a number of aspects which have received considerable attention or which are still subject to major improvements.

UV observations of extragalactic H II regions were pioniered by the ANS Satellite which collected spectra of 30 Dor in the LMC (Koorneef 1977, 1978, Israel & Koorneef 1979), of NGC 604 in M 33 (Israel et al. 1982) and of the brightest regions in M 101 (Blitz et al. 1981). It was however the International Ultraviolet Explorer, IUE, which allowed to extend the UV spectroscopic observations to a larger number of giant H II complexes with a combination of spectral and spatial resolution and sensitivity not available before. It should be noted that IUE, together with the data on the extragalactic H II regions, has provided a wealth of UV spectra of all class of astrophysical objects. These archive data are easily accessible and being all processed and calibrated in the same way, represent a unique tool for the interpretation of a composite spectrum such as the one obtained from extragalactic H II regions. For this reason the present review mainly deals with new IUE data.

Observations of the recently discovered jet feature in the symbiotic variable R Aquarii (M7e+pec) were obtained with the International Ultraviolet Explorer (IUE). A comparison of low dispersion UV-spectra reveals important differences between the central ionized source and the jet feature. The UV-continuum flux Fλ between 1200-2000A that rises with decreasing wavelength in the jet feature, essentially is independent with wavelength in the central compact nebula. The prominent emission lines of Si III] λ1892A and Si II λ1817A, evident in the central star, are virtually absent in the jet. The carbon lines of C IV, C III] and C II also suggest the general excitation of the jet is comparatively lower than the central star. This is further indicated by enhanced S II λ1250, 1295A and C II λ1335A emission in the feature. We speculate that material that is ejected from the symbiotic system cools through free-free and nebular recombination emission.

Symbiotic stars are suspected to be binary systems of large dimensions in which one star is a cool primary giant (regular M-giant or Mira-type variable) and the secondary a hot subdwarf (Boyarchuk 1982) or even a main-sequence star with an accretion disk around it (Bath and Pringle 1982). Observations of symbiotic stars in the far ultraviolet with the “International Ultraviolet Explorer” (IUE) enable us to determine the properties of the system in the binary picture. A number of critical observations are needed to do this including an estimate of the photoionizing radiation and the temperature of the photoionizing source.

The IUE and Einstein satellites have changed the clean and simple contact binary (W UMa-type) surface to a more “dirty” one where spots, flux tubes, coronal loops, flares etc. disturb the stellar image. The situation is, of course, the same for many other active stars as well. Here we discuss some aspects of this increased activity which may be relevant for the contact binary evolutionary theory.

As part of a multi-faceted study of gas streams, disks, and accretion processes in Algol type binaries, R.S. Polidan and I have obtained high resolution IUE observations of seven systems. Perhaps one of the most interesting results from these observations was the discovery of strong, variable absorption lines of N V, C IV, and Si IV in the spectra of stars whose photospheres are much too cool to form these ions. N V (1239,1243) and C IV (1548,1551) have been observed in AU Mon (B5V, 11ḍ1), CX Dra (B3V, 6ḍ7), and U CrB (B4V, 3ḍ5) while Si IV (1394, 1403) was found in TX UMa (B8V, 3ḍ1). The systems in which these ions have been observed generally contain primaries whose dimensions are greater than the Lubow and Shu (1975) parameter ϖmin.

Radial velocity measurements of the N V, C IV, and Si IV features suggest that the line formation region is associated with the primary star and small in size. The strengths of the N V, C IV features appear to be variable on a time scale of several orbital periods and may be determined by the instantaneous rate of mass transfer. However, when the high ionization features are observed, their presence appears to be global and not restricted to the domain of gas stream impact.

Unresolved, spectrum or spectroscopic binaries containing F or G giant or supergiant primaries are being observed with the IUE satellite. In most cases the secondary is a B or early A star probably still on the main sequence. The temperature class of the secondary is determined accurately by UV spectral classification and/or analysis of the composite flux distribution. The flux analysis uses a large grid of intrinsic colors and yields a best trial-and-error fit for the spectral types of primary and secondary, their V magnitudes, and the interstellar reddening. From the temperature class of the secondary, its probable main-sequence luminosity leads to a luminosity estimate for the primary with an uncertainty of about one magnitude. Several single-lined spectroscopic binaries are also being observed at high dispersion (in collaboration with Ake, Fekel, Harvel, and Kondo), to get the differential velocity between components from which mass ratios can be determined and masses estimated for the supergiants.

Ultraviolet spectroscopic observations made with the Copernicus and IUE spacecraft have been analyzed. The observations represent a collaborative effort with G. J. Peters.

Observational evidence supporting the existence of a turbulent accretion layer has been discussed earlier by Peters. Ionization equilibrium calculations suggest the following physical conditions in the layer: T ~ 125000 K°, Ne ~ 109 cm-3; composition: Nitrogen rich, carbon poor. Observations also show the region to be turbulent and not co-rotating with the accreting star. We conclude that the region is formed by new material arriving from the secondary and that the accretion process is turbulent.

IUE has been used to study 11 high luminosity X-ray binaries, of which 3 are in the Magellanic Clouds. In the supergiant systems, X-ray ionisation bubbles have been found in most cases, leading to a greater understanding of the winds and accretion processes. Further studies of precessing objects such as LMC X-4 with IUE and ST are clearly of considerable interest, relating to X-ray heating and blanketing. Detailed studies of the Cyg X-l ionisation bubble may resolve the long standing puzzle of its orbit inclination and masses. UV continua have furnished valuable information on extinction, temperatures and luminosities, and the presence of non-stellar (i.e. disk) luminosity. Here too, more detailed studies are clearly indicated for the future. A unique object of interest is the LMC transient 0538-66 whose UV spectrum has quasarlike lines and luminosity which varies oppositely to the visible. This may be a case of supercritical accretion generating an optically thick shell (“disk”) about the pulsar.