The credit for the first detection of magnetic fields on a star belongs to Hale who, in 1908, spectroscopically examined sunspots and determined their Zeeman splitting. Routine measurements of fields, however, did not come until the late 1940’s, with the development of the magnetograph by Babcock. This measurement technique relied on the fact that the split components of a magnetically sensitive line profile had opposite senses of circular polarization. Since the displacement of these components from the central wavelength was directly proportional to the strength of the magnetic field, it was found that the degree of circular polarization measured in the wings of an appropriate spectral line could be directly related to the magnetic flux present within the resolution element, provided the degree of magnetic splitting did not exceed the Doppler width of the line. The technique was applied to the Sun with highly successful results. It was possible to show that the magnetic fields were influential in nearly every form of solar activity. They structured the atmosphere and affected the energy transport, cooling some areas (sunspots) and heating others (e.g., plages and the corona). It was also found that the fields could act as resevoirs of energy. This energy can be explosively released, resulting in flare and mass ejection events.

Spectra of the 4.7 μm bands of carbon monoxide in absorption, and spectra of the cores of the 0.4 μm resonance lines of ionized calcium in emission, might be used to diagnose the presence of atmospheric inhomogeneities, caused by the action of surface magnetic fields, on stars as diverse as the Sun and the red giant Arcturus.

The general methodology to detect and study solar-like surface inhomogeneities on stars is presented. The most significant observations and typical results are discussed with particular emphasis on the inferred physical characteristics of starspots and plages and differential rotation, as compared to solar values. On account of the present observation and modelling limits, the need for complementary photometry and spectroscopy with space-born instruments is stressed, as well as the scientific relevance of the so-called solar-stellar connection studies in today’s Astrophysics.

I would like to present the results of the studies of three RS CVn-type eclipsing binaries that are conducted by our group at the Osmania University.

It is unfortunate that coronal magnetic fields cannot be easily measured, even in the case of the Sun. Except for a few measurements of magnetic fields in the transition region above sunspots, made using the conventional Zeeman effect, and except for the possibility of inferring the direction – not the intensity – of coronal magnetic fields using optical forbidden lines, direct measurements of coronal fields are virtually non-existent. The most promising method appears to be the use of the Hanle effect, i.e. the modification of polarization characteristics of spectral lines induced by magnetic fields. This method has been proposed for future space missions in solar physics, for instance for the European satellite SOHO, but its feasibility depends on the strength of the fields to be measured, which in any case must be higher than a few tens of Gauss.

Analysis of VLA and VLBI observations of non-thermal radio emission from close binary stellar systems can produce reliable estimates of magnetic field strength and geometry in the source emission region. Assuming a gyrosynchrotron emission model for the non-flare emission, one can model the magnetic field as a relatively small number of stable magnetic loops of overall size R ⋍ (1-3)×1012 cm. with an average field strength of 101 < B < 102 gauss. A correlation of mean fractional circular polarization with inclination angle along with an absence of any clear phase dependence, indicates that the loops are azimuthally symmetric with poles nearly orthogonal to the orbital plane, i.e. axisymmetric dipolar. The loops are probably the same as the loops derived from observations of x-ray emission of ‘hot’ (T ⋍ 107K) coronal gas.

Activities due to the magnetic effects in the stellar atmospheres are discussed with a special emphasis on the enhanced X-rays and radio activities in RS CVn binaries. Enhanced activities in these close binary systems are proposed to be due to an enhanced production of the twist of the subphotospheric magnetic flux tubes by the large differential rotation in radius, created between the inertially rotating core and more or less synchronously rotating envelope. The magnetic twist of the emerged flux tubes in the “active-longitude-belt” will be dynamically released into inter-star flux tubes together with the driven-out mass heated in the events of magnetic reconnections which are mediated by the magnetic fields of the companion star.

Coronal scaling laws and magnetic braking are discussed. The importance of future EUV and X-ray spectroscopy missions is emphasized.

At the last General Assembly of the IAU, in Patras, we had a Joint Discussion about the evolution in old stellar populations in galaxies. Then, as will be the case today, the great emphasis was placed on star formation and stellar evolution, on luminosity functions and mass functions. Today our theme does not include the word stellar. This is intentional. The young populations we are going to discuss will to a large extent concern the gaseous component in galaxies. The gas and the dust may from some points of view be considered old. However, when it takes part in star formation we discuss it as a part of the young population.

Several properties of massive star evolution are of great interest for the understanding of young populations in galaxies: -the genetic connections predicted by the models for the various types of massive stars allow us to understand their filiation; -in order to study the differences of the relative star frequencies in galaxies, we have to know which properties affect the lifetimes in the various evolutionary stages; -the composition of stellar winds is interesting to discuss the wind injections into the interstellar material, particularly the injections by Wolf-Rayet stars, and to discuss the influence of mass loss on nucleosynthesis and chemical yields. Here we shall briefly summarize some recent results on these various problems. For more details the reader may refer to general reviews (cf. Humphreys, 1984; Maeder, 1984a,b; Chiosi and Maeder, 1986).

A young cluster or association bears the imprint of the conditions at its birth for perhaps ten million years, after which the initial conditions are lost to either dilution in the galactic field or erasure by orbital mixing and stellar encounters. In its youngest years, however, the dynamical state of the system can provide valuable information concerning the structure and energetics of the parent gas, the star-formation efficiency and the star-formation process itself. This short review discusses recent theoretical and observational progress in the study of the very youngest of stellar systems.

This review discusses both the earlier and the most recent work on the IMF in young star clusters. It is argued that the study of the stellar content of young star clusters offers the best chance of developing a theory of star formation and of the IMF.

The mechanism of spiral structure in galaxies is a puzzle that is only partly understood. Galaxies do not revolve like solid bodies, so the spiral patterns cannot be entirely material in nature. Yet the observable tracers of spiral structure are unquestionably material: young stars, dust and gas. These objects must be organized by a collective, wave phenomenon, the spiral density wave in the disk star population. The density wave is a small (≈ 10%) local increase in the density of stars. The interstellar medium responds to the increased gravitational force by forming Giant Molecular Clouds (GMCs), concentrations of 105 M⊙ or more of interstellar matter in a region about 50 pc across. In a sense the GMCs are the spiral arms: in other galaxies, dust, gas and young star populations trace spiral structure; in the solar vicinity, these populations are seen to be associated with GMCs. This paper briefly reviews observational data supporting the hypothesis that spiral structure results from the agglomerative build-up of GMCs from smaller clouds, that this growth occurs preferentially in spiral arms, and that GMCs subsequently self-destruct because of the formation of massive stars.

Studies of molecular clouds in nearby galaxies require high angular resolution. Ten arcseconds corresponds to 0.5 kpc at the distance of M51. Typical gigant molecular clouds (GMC:s) have a size of 5-30 pc (Solomon et al. 1985). However complexes of GMC: s (Superclouds) can be several hundred parsecs (Elmegreen 1985; Rivolo et al. 1985). The higest angular resolution achived in CO(J=1-0) line observations of external galaxies is 7” (Lo et al 1984,1985). The resolution problem can be eased by observing M31 with a distance of only ⋍ 690 kpc (10” corresponds to 34 pc), which has been done by Combes et al. 1977a,b; Boulanger et al. 1984; Ryden and Stark 1985; Stark 1985; Blitz 1985; Ichikawa et al. 1985. In M31 the CO emission is strongly concetrated to the spiral arms with a arm interarm ratio of ≥ 25 (Ryden and Stark 1985; Stark 1985). The emission is caused by many small clouds unresolved with present resolution together with some larger clouds. Streaming is observed to occur across the arms. Extragalatic studies have the advantage of being more easy to interpret in terms of arm interarm contrast, noncircular motion, and galatic structure. They also make possible studies of the mass fraction of gas as a function of radius in different morphological types of galxies. Answers to questions like “Do any relation exist between galaxy type and molecular abundance?” are very important for our understanding of galatic evolution.

This paper concentrates on the molecular cloud component in its role as the site of current star formation – especially massive star formation. It is further restricted to the molecular component as traced by CO, the preeminent tracer of molecular gas in galaxies despite the problems discussed here. The basic topics addressed are (1) the variation of CO properties with galactic environment and type; (2) the problems involved in the inference of H2 distributions for individual galaxies from CO observations; and (3) the particular question of the presence of spiral structure in the CO component.

The three topics outlined in the title I have been given for this contribution are each far too complex to be thoroughly reviewed in the short time available. Instead of trying to duplicate the relevant (28, 34, 35) Commission Reports in the Transactions of the IAU (R.M. West 1985), I shall review only briefly current results on the stellar content of giant HII regions and then continue towards recent developments of our ideas about the mutual interrelation of galaxy wide star formation and spiral structure. The goal is to emphasize the simultaneous need for descriptions of details and unifying approaches.

A few of the interesting problems concerning star formation in irregular galaxies are reviewed.