Widened, image-tube spectra at a dispersion of 120 Å mm-1 were obtained of the nuclei of 36 spiral (Shapley-Ames) galaxies. Spectral features in the region λλ 3800 - 4300 Å were used to assign a spectral index to each galaxy and to estimate the luminosity classes of the stars contributing the majority of the light at these wavelengths. Bulge magnitudes were either estimated or determined from published aperture photometry, and converted to absolute magnitudes, MN, assuming H = 100 km s-1 kpc-1. A correlation was found between the spectral index and MN such that the luminosities of galaxies can be estimated to an accuracy of ˜1 mag.

The nuclei of late type spirals have spectra which resemble those of A and F stars, and it has been assumed that this indicated that these nuclei contain younger stars than those of early type spirals and elliptical galaxies. However, inspection of the luminosity sensitive features in the spectra indicate a preponderance of giant stars, and since A giants are relatively rare, it is probable that these blue stars are metal poor horizontal branch stars which are spectroscopically similar in appearance. The spectral index variation is thus due to metal abundance rather than age.

Complete details are to be published in the Astrophysical Journal by McClure, Cowley, and Crampton (1979).

I want to discuss the origin of non-circular gas motions observed in the nuclei of normal spiral galaxies and the possibility that recurring violent activity in normal nuclei excites such motion. But first, let us review several basic aspects of the nearest normal galactic nucleus — the nucleus of our own Galaxy.

The rotation curve as observed in the 21-cm line of neutral hydrogen gives some indication of the form of the gravitational field in the central region of the Galaxy. Figure 1 is a smooth fit to the rotation curve in the inner few kiloparsecs (solid line) taken essentially from the data of Rougoor and Oort (1960) and Simonson and Mader (1973). This rotation curve, within 1 kpc of the centre, is completely accounted for by the mass distribution implied by the extended 2.2-μ emission (Becklin and Neugebauer 1968, Oort 1971). Moreover, there is little doubt that this centrally condensed mass distribution should be identified with the bulge or spheroidal component of the Galaxy, because the spatial distribution of the 2.2-μ intensity is practically identical to the distribution of visible starlight in the bulge of M31 (Sandage, Becklin, and Neugebauer 1969). The conclusion is that the bulge overwhelmingly dominates the gravitational field inside of 1 kpc.

Please let me for fifteen minutes completely ignore all gas in the cores of galaxies. Clearly the core gas exhibits fascinating, varied and consequential phenomena. Still it would seem to me a plausible assumption that while the gas might be decisively affected by the stars, this gas might not have major effects on the stars, at least in their dynamical behavior. Accordingly I shall here consider purely stellar systems.

Obviously in the core of a galaxy, for dynamical consideration we have to distinguish sharply between the nucleus of a galaxy and the larger components of the galaxy co-occupying the core. Following Baade’s usage the nucleus is a stellar system of at most a few parsecs in radius and of a density substantially higher than the central densities of the other components. That the nucleus of a galaxy, at least in its equilibrium state -though not necessarily in its origin- is a distinct, self-contained stellar system seems to me strongly indicated by the rotation velocity measurements in the nucleus of the Andromeda Nebula (Lallemand et al. 1960; Walker, 1974) as well as by the photometric profile determination of the same nucleus (Light et al. 1974). For a nucleus of 108 stars the relaxation time for stellar encounters is, according to Spitzer (1971), comparable to a Hubble time while stellar collisions are ignorable. On the other hand, for a nucleus containing 1010 stars, which may be the right order of magnitude for more pronounced nuclei, the relaxation time may be comfortably long enough so that encounters may be ignored, but the frequency of collisions is not ignorable. In both cases therefore it would seem that in the centermost portions of nuclei of galaxies either encounters or collisions play a decisive dynamical role but the same may not be true in the outer portions of these nuclei. For the larger subsystems occupying the core of a galaxy, obviously neither encounters nor collisions play a significant role. To stay within the limits of my knowledge, I shall limit myself here to encounterless and collisionless stellar systems, a marginal approximation for the nuclei proper.

For the purpose of this talk, I define all those galaxies as normal whose nuclear regions are not known to be dominated by a compact source. Thus I will exclude N galaxies, Seyfert galaxies and radio galaxies, but include galaxies like M81 and M82 which have weak compact nuclei. My review is organized in three parts, (i) tracers of star formation, (ii) star formation in the nuclear regions of observed galaxies, and (iii) the theoretical attempts to interpret the observational material.

Nuclei of galaxies are the phenomena intimately connected with the structure of galaxies, their origin and evolution. Spectral and photometrical characteristics and therefore the stellar population of the most nuclei of normal galaxies are like that of central regions or bulges (Walker, 1962, 1964; Vorontsov-Velyaminov, 1965, 1968). Processes of star formation in the nuclei must be considered together with the problem of formation of the central regions (bulges) or even of all galaxies as a whole.

The term “galactic nucleus” is usually applied to the central region (or center of symmetry) having the maximum stellar density (maximum surface brightness) and dimensions less than 100 pc (dimension of a globular cluster). The observational evidence of star formation at the present epoch is related to the nuclear regions within radii of 100-1000 pc or more. Information about the physical condition of the gas and about star formation just in the center of galaxies (≤ 1 pc) can be obtained only by theory. The red colour of the central star cluster of normal galaxies is evidence that star formation in this region is practically finished.

In 1972, when the request for observing programs for the International Ultraviolet Explorer (IUE) was first made by NASA, ESA and the United Kingdom, a number of scientists recognized that observations in the ultraviolet of the strong galactic X-ray sources could be a fruitful activity. The underlying nature of those objects was understood. Their binary nature had been discovered through the X-ray observations which, combined with extensive optical observations and the insight gained through the tie to traditional ideas of stellar evolution, led to a picture whose general features are still accepted. The strong X-ray sources were taken to be close binary systems in which one object was a compact star, either a neutron star or a black hole, and the other object a common star in a stage of its life when it was undergoing great mass loss, either as a stellar wind or through Roche-lobe overflow.

Antediluvian UV observations of planetary nebulae were made using rockets and the ANS, TD-1 and Soyuz-13 satellites: this work is reviewed by Gurzadyan (1978) and Pottasch (1978); see also Bohlin et al. (1978), Pottasch et al. (1978) and Lutz and Carnochan (1979).

With IUE came the deluge. To my knowledge, IUE observations have been made of 54 planetaries, and work continues by many groups, on making observations and reducing and interpreting the data. The present report describes some preliminary results.

Extensive mass loss is observed for hot subluminous stars, through P Cygni lines in the ultraviolet. This persists in some sub-dwarf 0 stars, but is generally not observed in white dwarfs. The ultraviolet provides determination of effective temperatures. Among nine sdO’s, the maximum temperature reported is definitely below 60, 000 K; an object at 100, 000 K would be distinguishable. The sdO’s show a wide variety of line strengths, notably in N V, C IV and Si TV, as well as He II. One halo sdB is reported as rich in peculiar elements; it shows anomalous N V for its temperature. The comparison of effective temperatures of white dwarfs observed from space and from the ground gives excellent agreement. The hottest white dwarfs are near 60, 000 K, although one (helium-rich) reaches 80, 000 K. Another helium-rich close binary probably has an accretion disk; it is the only white dwarf to show the expanding shell of N V, C IV, Si IV characteristic of some subdwarfs. Two magnetic white dwarfs have been observed; one has strong unidentifiable features and the smallest known radius.

A vast increase in the amount of information on cool stars has occurred during the past 3 years starting with the spectroscopy of the brightest objects from the Princeton experiment on Copernicus, the near ultraviolet balloon measurements from BUSS, low dispersion observations in the far ultraviolet from rocket instruments, and most recently, spectra from the International Ultraviolet Explorer satellite (IUE). IUE now gives access to a wide variety of cool stars and stellar systems enabling systematic studies to be made.

Drawing on the most recent material from IUE, we discuss the presence and structure of chromospheres and coronae in single stars of varying gravities, surface temperatures, and activity. Evidence of mass loss and the concurrent presence of a corona are also noted. Binary systems of late-type stars (the RS CVn and W UMa type) are briefly discussed since they display extremes of surface activity. A binary system such as VV Cep containing a late-type star and an early-type companion provides a unique probe of cool extended stellar atmospheres.

This discussion focuses on high dispersion spectroscopic observations of chemically peculiar (CP) stars of the upper main sequence, classical HgMn, Si and SrCrEu stars, obtained with the IUE. Two subjects will be emphasized - the confirmation of composition anomalies previously identified in ground-based spectra and evidence relating to the validity of the diffusion theory for the production of chemical pecularities.

The gallium anomaly occurs primarily in the hotter Mn stars (Cowley 1975). Model atmospheres analyses of Mn stars yield overabundances of Ga as large as 2×105 relative to the solar value (Heacox 1979). The identification of the Ga peculiarity and estimates of Ga abundances are based upon a few high excitation lines observed in the blue. Figure 1 illustrates the resonance line of GaII, λ1414.44, observed in six Mn stars. The feature is strong and broad in the hotter stars, к Cnc and μ Lep. It is present but weaker in 46 Dra (A+B). It is absent in the cooler stars г CrB, χ Lup and HR 4072. The sharp feature seen near λ1414.4 in the latter three stars is probably Mnll and is also seen in normal stars. The Gall λ1414.44 line varies in strength from star-to-star in the same manner as the blue Gall lines. The absence of the blue Gall lines in cool Mn stars cannot be an excitation effect, since the resonance line is also absent in these stars. Neither can the presence of the blue Gall lines in hotter Mn stars be due to non-LTE effects, because the resonance line is also abnormally strong in these stars. The IUE observations confirm the Ga peculiarity as a genuine abundance anomaly.

In addition to offering observing time on a regular schedule, the International Ultraviolet Explorer (IUE) satellite is also being used to observe “targets of opportunity.” Novae represent one of the most exciting targets of opportunity and also one of the most difficult because of their rapid time behavior. During the first year of operation of IUE we were extremely fortunate to have three bright novae outbursts occur: Nova Cygni 1978, WZ Sagittae, and U Scorpii.

Nova Cyg 1978, a fast nova, reached a maximum apparent visual magnitude of 6.2 on September 12, 1978 (Slovak and Vogt 1979). Figure 1 shows the relative flux of Nova Cyg 1978 in the long ultraviolet wavelength range (1900-3200Å) at several different times. One day after maximum (Sept. 13), the nova is still in its absorption line phase. At this point it looks like a supergiant F star with mostly Fell absorption lines (also see Cassatella et al. 1979). Later spectra show the emergence of emission lines, which is the characteristic behavior of novae in the visual. Magnesium II at 2800Å is the strongest emission line in this part of the spectrum for at least a month after outburst. The last spectrum (Nov. 1) shows mainly semi-forbidden lines of carbon, nitrogen, and oxygen. It should also be noted that the slope of the continuum flattens as the nova evolves.

Interstellar clouds are concentrations of cold (T ≲ 100 K) neutral gas (cf. Spitzer 1978) which are immersed within an intercloud medium. It is worthwhile to distinguish between diffuse clouds (roughly those with E[B-V] ≳ 0.5) and dark clouds (those with E[B-V] ≳ 0.5). This distinction is useful in the sense that diffuse clouds are relatively warm (T ∼ 100 K), they are composed mostly of atomic species except for hydrogen which can be appreciably molecular, and they are dynamically controlled by their interaction with the intercloud medium. Dark clouds are relatively cold (T ∼ 10 K), they contain a rich variety of molecules, and self-gravity is important in their evolution. Because the interstellar extinction is a rapid function of wavelength, most ultraviolet observations have been of diffuse clouds. The IUE satellite is sufficiently powerful that observations of some dark clouds are possible, and an important area of future research will be to delineate more quantitatively the similarities and differences between diffuse clouds and dark clouds.

With ultraviolet observations, considerable progress has been made in understanding the physical characteristics of clouds including determinations of their densities, temperatures, chemical compositions and dynamics (cf. Spitzer and Jenkins 1976). Because particular progress has been made on understanding the abundances within diffuse clouds and because of the limitations of space, we restrict this review to a discussion of abundances within diffuse clouds. These abundance measurements provide a set of fundamental astrophysical data.

Up-to-date models of the interstellar medium should account for the existence of at least three phases of the interstellar gas, a hot phase at ˜ 106 K (HICM), a warm phase at ˜ 104 K (WICM) and a cold phase at ≤ 102 K (clouds). Recent observations of interstellar absorption lines are used to derive information about the physical properties and the spatial distribution of these phases. Evidence based on observations of C+ absorption lines indicates that the photoelectric threshold of interstellar dust grains is about 6 eV, much lower than previously thought. If the photoelectric threshold is indeed of this magnitude the WICM can easily be heated to temperatures in the range 6000 to 8000 K by photons of the general interstellar radiation field with λ > 912 Å. The analysis of interstellar CH+ and 0 VI absorption lines suggests that these lines are formed in gas associated with clouds in regions of enhanced pressure that occupy about 10 percent of interstellar space (SN cavities). The CH+ lines are formed in compressed WICM shells around clouds and the 0 VI lines in conductive interfaces between the WICM and the HICM. The velocity characteristics of the observed lines make shock waves a less probable source of the absorbing gases.

The UV spectrum of galaxies provide us with the unique chance to detect those hot stellar components, which normally do not appear in the visible spectrum. Such components, interpreted as possibly due to horizontal-branch stars, have been found in spectra of early-type galaxies. All these spectra exhibit an UV excess shortward of 3000 A with respect to the energy distribution of a K-type star. UV excess is present in different amount also in spectra of late-type galaxies, but the data are fragmentary.

There is hope to define a standard UV spectrum of nearby elliptical galaxies to compare with that observed in the visible region in distant galaxies. This would allow the determination of the evolutionary effect.

The emission lines in elliptical galaxies are found much stronger than one would expect if the physical conditions are similar to those of HII regions.

Direct images of galaxies allow to trace the spiral structure defined by hot stars and to study the UV luminosity profile in the nuclear region.

The most important observations of Seyfert nuclei to be done in the ultraviolet are 1) the determination of the non-stellar continuum energy distribution and 2) the measurements of the intensities of the permitted lines. The main results obtained so far with IUE on these two points are as follows.

The Lyman/Balmer intensity ratio is smaller than predicted by the recombination theory. It cannot be forced into agreement with recombination theory by applying the reddening correction relevant to the narrow line region nor to the continuum. This Lyman/Balmer ratio is therefore caused by the particular physical conditions prevailing in the broad-line region. The smallness of the Lyman/Balmer ratio and the non-detection of the resonance lines of Fell neither in emission nor in absorption show that collisional processes play a dominant role in the excitation of the lines in the broad-line region and that the density in this region is at least 3 × 1010 cm-3.

The first thing to say is that IUE has been a great success and we should all congratulate the agencies in the U.S.A., Europe and the U.K. and the many scientists and engineers involved in the project on making it possible. In surveying what has been achieved so far, it is important to remember that we may well have IUE with us for a further 3 or even 5 years, meaning that it may be operating right up to the time of launch of Space Telescope. Undoubtedly, observations with IUE will pioneer many fields which will be explored in much more depth by the Space Telescope - there is no reason why IUE should not scoop some of the topics originally thought to be only the province of the Space Telescope.

It was always recognized that the study of extragalactic objects would be difficult with IUE. As Michael Penston expresses it, most astronomers don’t try to do extragalactic astronomy with an 18-inch telescope. Nonetheless, many observers have used their observing time to make long exposures on faint objects and there is no question about the significance of the science which has been accomplished. My own experience has been entirely with quasars and radio galaxies and, now that many of the obvious bright objects have been observed, we have settled into a pattern of only attempting one object per 8 hour shift, switching between the long and short wavelength spectrographs in mid-shift. We then repeat the observations in the following shift. Our experience is that this procedure is valuable in distinguishing whether faint features are real or not. Because of the long projected life of IUE, we do not regard this as an extravagant use of time and it is to be hoped that many more observers will adopt this point of view.

Recent observations from space have shown that the solar corona is spatially a very structured medium and temporally a very dynamic one. The consequent changes in the current theoretical ideas about coronal physics are reviewed. The role of the magnetic fields in shaping and heating the coronal structures is especially underlined.

I want to begin with the observation, which I will try to make clear in the following, that a solar flare comprises an incredibly complex set of phenomena. This is not only true with respect to what is seen and measured in spectacular examples, but also when one considers the constituent parts of simple, even idealized, cases. A series of different physical effects lead, as one illustration, to radiations from the flare-instability site and its surroundings which span the range from meter waves to gamma rays (Svestka 1976, Sturrock 1979).

To fit within the context of this discussion, I will concentrate on the high-temperature and quasi-thermal aspects of a flare, and on the basic physical mechanisms connected with the primary energization and dissipation processes. Thus, I will treat the reconnection of the magnetic field, the bulk acceleration of particles, the thermalization and the ultimate radiation of the energy. I will not treat the optical manifestations or, at the other extreme, the acceleration of very high energy particles.

In contrast to most astrophysical situations where information about hot plasmas can be obtained only from emission and absorption spectra, often without spatial resolution, plasmas in the solar system in many cases provide us with the opportunity to make direct, in situ measurements. Such plasmas, notably the solar wind and the plasmas contained in the magnetosphere of the earth, Jupiter, and other planets, must be heated by processes which are in many cases similar to those occurring in astrophysical plasmas and their behaviour should also be to some extent similar. It is therefore interesting and instructive to be able to compare our observations and understanding of these accessible solar system plasmas with those found elsewhere in the universe which are not so easy to observe in detail. This might provide us with some new ideas and perspectives on the nature and behaviour of hot plasmas in general and also an opportunity to test some of our ideas against “ground truth”.

Revolutionary changes in our awareness of the physical state of gases in interstellar space have been precipitated by new theoretical realizations and observational evidence. Mechanical energy released by supernovae and early-type stars is now thought to be an important source of heating and agitation for the interstellar medium (McCray and Snow 1979). The propagation and dissipation of this energy gives rise to a broadly distributed network of gas at low densities and high temperatures, revealed to us by its emission of soft x-rays (Cox 1977, Tanaka and Bleeker 1977) and the pervasiveness of highly ionized atoms (with 0 VI being the most conspicuous tracer, see York 1974, Jenkins and Meloy 1974, review by Jenkins 1977).