Science has spread from western Europe where it developed into recognisably-modern form in the seventeenth century, stimulated by Copernicus’s claim that the Earth is a planet. Copernicus however was an astronomer in the Greek tradition, whose task was to reproduce the planetary paths by geometrical constructions using uniform circular motions. Eudoxus’s attempt to do this with nests of concentric spheres had been superseded by the use of the more flexible techniques necessary to meet the observational standards of the Hellenistic era. Ptolemy’s Almagest synthesised the Greek achievement but its shortcomings led Copernicus to make the Earth a planet.

A plausible scenario for the early history of the sun can be constructed by combining the results of stellar astronomy with lunar and meteoritic chronologies. The meteorites apparently contain material exposed to two nucleosynthetic events, one about 108 yr and another a few 106 yr before solidification. Following II. Reeves, these are associated with supernovae occurring in star clusters in molecular clouds that formed during passage through successive galactic arm shocks. The Orion Trapezium Cluster may be a modern example; its density is such that encounters between members would have been close enough and frequent enough to have had major effects upon their circumstellar ‘solar nebulae,’ as would recurrent FU Ori-like eruptions of the stars themselves. The lunar bombardment continued for 7 × 108 yr following formation of our sun. If this represented disk cleanup, disks must persist for that long, and hence circumstellar activity may still be in progress around some young stars in the solar vicinity. The observed time decay of axial rotation and surface activity in solar-type stars can be extended backwards, and indicates that the ultraviolet radiation of the young sun would have had major photochemical consequences upon the primitive earth.

It is a qreat honour to pronounce this lecture before the General Assemhly of United astronomers of all the world.

In fact, the astronomers are really more united, than the representatives of manv other sciences. Perhaps this unity is connected with the noble spirit of inquiry into the immense distant worlds, characteristic for astronomers. And of all the problems and all the systems investigated the biagest one is surely the Universe as a whole i.e. the tonic of cosmoloay.

This branch of astronomy is the greatest challenge, one is always confronted with the danaer of being captured by prejudices.

In the last decades the situation chanaed. At least cosmology has transformed into a respectable science, which it was not say 50 or 60 years ago.

There remain deep unsolved questions beginning with the very existence and/or birth of the Universe.

But there are also in modern Cosmology definite successes, there are achievements in the understanding of the Universe. The successes are due to the work of many people, they are due to an international collective effort.

In a talk in the Odeon, an ancient theatre of Patrai, Greece and also in this high-light paper it seems me unappropriate to give precise citations and to divide priorities. Therefore no names will be called except a handful of great men gone of us - Einstein, Rubble, Friedmann, Jeans, Planck.

A review is given of knowledge on solar flares with particular emphasis on progress made during the international Solar Maximum Year (1979-1981).

The pre-flare structure is described by a flux tube model or a circuit model. The instability leading to a flare may occur by a disturbance of the field topology (mostly: field emergence) followed by field-line reconnection (circuit coupling). In the first, impulsive, phase of a flare this causes jets of energetic electrons, originating near the top of the flux tube, to bombard lower chromospheric regions: footpoint heating. In the second (‘gradual’ or ‘diffuse’) phase heated gas from the footpoints ascends convectively upward producing a large cloud of hot gas. Consequent shock wave phenomena cause moving fronts and associated waves in the high parts of the corona; these show up in coronagraphic or radio-observations. In some cases it happens that several hours after a large flare extended loop-like structures appear of fairly high temperature (> 6 MK), emitting a very faint X-ray flux. They are the basic structures of a more extended configuration, visible on metric radio waves, and pointing out from the area where the flare occurred. They may extend to distances of ≈ 106 km or more from the solar surface.

The most interesting result in solar luminosity studies in the past decade has been the detection of significant variations in the total irradiance by precision radiometers on the NIMBUS-7 and SMM spacecraft. A substantial fraction of the observed variation can be attributed to sunspot blocking. Thermal storage models indicate that the blocked flux can be stored in a slight increase of the thermal and potential energy of the convective zone. The thermal storage time is likely to far exceed one solar activity cycle, implying an 11-year modulation of the solar constant at a level of about 0.1%. Direct observations of the 11-year or longer variations are more difficult but there is some evidence for secular trends below about 0.4% amplitude over the 14-year period of modern sampling. Ongoing stellar photometric programs suggest that luminosity changes exceeding 1% may have been detected in young, chromospherically active stars.

H. S. Hudson (University of California, San Diego) reviewed observations of short-term solar irradiance variations from spacecraft, commenting principally on the precision measurements of the solar constant (S) made by the Active Cavity Radiometer (ACRIM) on the Solar Maximum Mission (SMM) spacecraft (Willson et al., 1981).

Almost 25 years ago Walter Baade told the Vatican conference about the meaning of stellar populations:

“We also understand now why the two stellar populations, either singly or combined, are such conspicuous features in most galaxies. They are age groups which represent two significant phases of the star formation in galaxies.”

There appeared a straight-forward picture with an old, metal-poor halo containing stars in elongated galactic orbits and a younger disk population, where the stars have near solar abundances and near circular orbits. The central bulge of the galaxy was considered part of the older system.

New observations have made the picture more complex and also more controversial, indicating that it still might not be fully understood. Stars in the bulge of our spiral galaxy have been shown to have rather high contents of heavy elements. The gas and perhaps also the stars of the disk show a metal content that is decreasing outwards in the disk. The abundances in globular clusters differ widely between individual clusters and even between individual stars of the same cluster. It has become clear that one must discuss separately the different heavy elements; that some abundances may be considered primordial and thus characteristic of the evolution of the stellar population, whereas other abundance differences may be caused by mixing into the stellar envelopes.

The discrepancy between the results of recent high-dispersion analyses and of studies at lower resolution for the classical metal-rich globular clusters is discussed and different explanations for this discrepancy are considered. It is concluded that further systematic investigations are motivated in order to establish the more or less interesting astrophysical explanations for it.

In this review I shall concentrate mainly on globular star clusters in our Galaxy since these are the objects for which most work has been done recently, both observationally and theoretically. However, I shall also discuss briefly the oldest open clusters and clusters in the Magellanic Clouds. Little can be said about more distant cluster systems, since the only observations available are of integrated colours or spectra and these seem to be rather unreliable indicators of age. It is perhaps worth pointing out that the title may be slightly misleading; the problem is not so much to determine the ages of clusters of known abundances, as to obtain the best simultaneous solution for both age and composition, since some of the most important abundances (notably helium and oxygen) are virtually unobservable in little-evolved low mass stars.

The abundance of the elements in the halo are mostly known from analysis of stellar atmospheres. Data from evolved stars and planetary nebulae have to be used with some caution. I will therefore rely primarily on unevolved cool dwarfs, using other information only as a complement.

I will designate these unevolved cool dwarfs indifferently by the words halo dwarfs or Population II dwarfs; such stars were formerly often called subdwarfs for historical reasons. Remaining in the field of vocabulary, I will call underdeficient an element which is deficient, but by a factor smaller than the general deficiency factor for the other elements. Finally, let us recall the classical notation:

Obviously such transcendental issues as the helium content and age of the oldest stars depend on whether we are correct in our belief that the answer to this question is “yes” I hardly need say that over the past 40 years compelling affirmative arguments have been developed. Thus, for example, the solar motion of the common subdwarfs can be shown (e.g., Carney 1979) to be essentially identical with that of globular clusters (Kinman 1959) (Table 1), and the Fe-peak metallicities of giants and RR Lyraes in the halo field have been shown to be the same as those in clusters [see, e.g., recent reviews by Kraft (1979) and Freeman and Norris (1981)]. It is hard to believe that we would be incorrect in identifying the main sequence of a globular cluster with the main sequence defined by the trigonometric parallaxes, magnitudes and colors of subdwarfs having the same [Fe/H]. It might seem, therefore, that raising such an issue at this late date is equivalent to discussing a non-existent problem.

Spiral galaxies are a composite of two dynamical population types: the spheroidal and disk populations. These can be studied in isolation in E and Irr galaxies, respectively. It is natural to expect that the combination in a spiral of a dense spheroidal population, having a deep central potential well, with a repository of interstellar gas and dust in the disk gives rise to special conditions not usually found in E or Irr galaxies. And in fact, we find spectacular concentrations of star forming regions in some spiral nuclei.

In this review, I will limit most of the discussion to later spiral types which are classified as “intermediate” in the Yerkes system (Morgan and Osterbrock 1969). Types earlier than Sbc usually have “k-nuclei,” where the spheroidal population dominates; excellent reviews are available for these types (van den Bergh 1975, Faber 1977).

Numerous studies have been attempted to determine the stellar content of the nuclei of galaxies. In the case of elliptical galaxies observations show that there is a change in spectral type from later to earlier types correlated with a variation from high to low luminosity (e.g. Faber 1977). This has been interpreted for the most part as being due to differences in metal abundances in an old stellar population, although some recent studies suggest a variation in stellar age may be important as well (e.g. O’Connell 1980, Heckman 1980). The nuclear bulges of spiral galaxies also display a change from late to early spectral type along a sequence from high to low luminosity bulges. In this case, however, the assumption has almost universally been made that these differences are due to differences in age of the stellar population. In particular, most stellar models for the nuclear bulges of spiral galaxies have used solar neighbourhood metal- rich stars, and assumed that the strong hydrogen lines and weak metal lines in late-type spiral nuclei are due to the predominance of a very young main-sequence population rather than old metal-poor stars.

New developments in understanding the stellar populations of elliptical nuclei are discussed. Variations in iron-peak spectral features have recently been detected, which are much smaller than changes in CN, Mg, and Na. A study is presently in progress to determine whether this pattern is consistent with lockstep enhancements in all heavy elements. Present evidence on the ultraviolet spectra of ellipticals now suggests that the source of the excess UV light is not young OB stars. A new correlation is also pointed out between UV excess and mean line strength. If real, this trend is further evidence against significant numbers of young OB stars in elliptical galaxies.

This review will take the linear view of the history of stellar systems. Thus the last billion years of a dwarf galaxy’s development receives no special attention. A considerable amount of information has recently come to light on the intermediate age populations of dwarf galaxies.

It was already realized by Hubble (1936) that galaxy morphology and stellar content were correlated. He pointed out that resolution into stars increases along the classification sequence Sa-Sb-Sc. Simultaneously the colours of spirals become bluer and their integrated spectral types become earlier as one proceeds from Sa to Sc. Baade (1944) speculated that the red stars in ellipticals and in the nuclear bulges of spirals were identical to those in globular clusters. He suggested that stars in galaxies belong to two distinct populations: young metal-rich stars of Population I which inhabit the disc and spiral arm regions of spirals, and old metal-poor stars of Population II which dominate the light of elliptical galaxies and the nuclear bulges of spirals. Subsequently Baade (1950) emphasized the strong correlation between the occurrence of gas and dust and the presence of young stars. As Baade put it so succinctly “No dust, no Population I”. Belief in a clear cut dichotomy between Population I and Population II was strengthened by the differences in their radial luminosity distributions. The surface brightness of Population I in spirals is well represented by an exponential disc, whereas the surface brightness of Population II stars in ellipticals and the bulges of spirals may be described by an r1/4 law (de Vaucouleurs 1959).

The basic characteristics of the stellar populations in galaxies are determined by the history of star formation and by the initial mass spectrum with which the stars are formed. Many attempts have been made to model the history of star formation in galaxies using various quasi-theoretical descriptions of star formation, but star formation remains a very poorly understood process and no theoretical understanding with any real predictive power has yet been attained. However, recent work suggests the importance for star formation of some mechanisms that have so far not been very extensively studied, and I shall mention some of these possible processes here, undeveloped though these ideas still are at present.

First, I shall review briefly the observational evidence concerning the history of star formation in galaxies of different Hubble type. Most of the information we have for galaxies other than our own comes from their colors, which can be compared with the predictions of models constructed with various assumed histories of star formation. Ever since the work of Tinsley (1968), it has been clear that most of the observed variation of color with Hubble type can be understood in terms of differing time scales for star formation, the reddest galaxies (E and SO) having formed the bulk of their stars at a relatively early stage, while the much bluer Sc galaxies have formed their stars much more gradually and at a rate that has remained nearly constant up to the present time. Recent studies of normal elliptical galaxies, reviewed by Faber in this volume, confirm that they have no detectable present star formation.