This volume contains the fifteenth tri-annual reports of the Presidents of the forty Commissions of the International Astronomical Union; it refers to the progress in our discipline during the three years 1970, 1971 and 1972. As compared to earlier volumes a gradual change in character is unmistakable. The ever increasing flow of publications, combined with the obvious necessity to keep the Reports at a reasonable size and price level has gradually forced the Commission Presidents to be more selective than before in drafting their Reports. I have certainly stimulated them into that direction - in order that Reports like these be valuable and lasting, it seems imperative that the individual contributions have the character of a critical overall review, where a fairly complete summary is given of the major develop ments and discoveries of the past three years, and in which the broad developments and new trends be clearly outlined, while at the same time essential problems for future research are identified. With respect to the latter item I have suggested the Commission Presidents to add to their reports a brief section on scientific priorities for future research in the field of their Commissions. In order to save space I have suggested to Commission Presidents that references to published papers are given on the basis of their number in the published issues of Astronomy and Astrophysics Abstracts. For instance, the indication (06. 078. 019) or (AAA 06. 078."

The primary inducement for organizing an international Conference on 'Image Processing Techniques in_Astronomy' was the fact that the recording microdensitometer VAMP ('Vol Automatische Micro Photometer') of the Utrecht Astronomical Institute was operative for a few years. The necessity of comparing the in strument and its performance with similar instruments nowadays available at many other institutes, was stimulating enough to organize a meeting on the above subject. It took place in Utrecht on March 25, 26 and 27, 1975. The Scientific Organizing Committee consisted of J. Borgman (Groningen), R.B. Dunn (Sacramento Peak), H. Elsasser (Heidelberg), L.D. de Feiter, T. de Groot, J.R.W. Heintze, C. de Jager, H. Nieuwenhuijzen (Utrecht) and W. Wiskott (Geneve). About 175 scientists from 14 countries participated in the meeting which appeared to be successful and offered a good opportunity of exchanging information and comparing experiences. The VAMP was bought with financial support of the Utrecht University and the Netherlands Foundation for Scientific Research (Z.W.O.). The conference was organized with financial support from The Netherlands Ministry of Science and Education, The European Southern Observatory, The Leids Kerkhoven-Bosscha Fonds, The Astronomical Institute of Utrecht, to which Institutes and Organisations we express our sincere gratitude. C. de Jager H. Nieuwenhuijzen editors PAR T WHAT INFORMATION DO WE NEED, FOR WHICH ASTRONOMICAL PROBLEM? ASTROMETRY K. Aa. Strand U. S. Naval Observatory Washington, D. C, INTRODUCTION Considerable progress has taken place in astrometry over the past two decades."

This volume contains the fifteenth tri-annual reports of the Presidents of the forty Commissions of the International Astronomical Union; it refers to the progress in our discipline during the three years 1970, 1971 and 1972. As compared to earlier volumes a gradual change in character is unmistakable. The ever increasing flow of publications, combined with the obvious necessity to keep the Reports at a reasonable size and price level has gradually forced the Commission Presidents to be more selective than before in drafting their Reports. I have certainly stimulated them into that direction - in order that Reports like these be valuable and lasting, it seems imperative that the individual contributions have the character of a critical overall review, where a fairly complete summary is given of the major develop ments and discoveries of the past three years, and in which the broad developments and new trends be clearly outlined, while at the same time essential problems for future research are identified. With respect to the latter item I have suggested the Commission Presidents to add to their reports a brief section on scientific priorities for future research in the field of their Commissions. In order to save space I have suggested to Commission Presidents that references to published papers are given on the basis of their number in the published issues of Astronomy and Astrophysics Abstracts. For instance, the indication (06. 078. 019) or (AAA 06. 078."

From 17 to 21 April 1967 a Study Week was held in the hotel 'De Bilderberg' near Arnhem, Holland, with the purpose to establish a new, and if possible, generally acceptable working model for the quiet parts of the solar photosphere and low chromosphere. The organizers of the conference hoped that even if this latter goal appeared too far to be reached, such a meeting would still be useful, if only for enumerating the crucial problems in solar photospheric research, and for defining future subjects of research. About twenty solar physicists from outside the Netherlands participated in the Study Week, while some others, though prevented from actively attending, sub mitted their comments before the meeting. The two above-mentioned goals were reached: a working model could be estab lished; yet it became clear that not everyone would agree about this model, and it became obvious too that future research is strongly needed, in particular in the field of line formation (coherence, or non-coherence; local thermal equilibrium), while also the motion field of the photosphere and chromosphere is insufficiently known, and its influence on the formation of spectral lines hardly understood."

This book was conceived to commemorate the continuing success of the guest observer program for the International Ultraviolet Explorer (IUE) satellite observatory. It is also hoped that this volume will serve as a useful tutorial for those pursuing research in related fields with future space observatories. As the IUE has been the product of the three-way collaboration between the U.S. National Aeronautics and Space Administration (NASA), European Space Agency (ESA) and the British Engineering and Research Council (SERC), so is this book the fruit of the collaboration of the American and European participants in the IUE. As such, it is a testimony to timely international cooperation and sharing of resources that open up new possibilities. The IUE spacecraft was launched on the 26th of January in 1978 into a geosynchronous orbit over the Atlantic Ocean. The scientific operations of the IUE are performed for 16 hours a day from Goddard Space Flight Center in Greenbelt, Maryland, U.S.A, and for 8 hours a day from ESA Villafranca Satellite Tracking Station near Madrid, Spain.

After the same pattern as the XIII th General Assembly of the International Astronom ical Union the present Volume of the Highlights in Astronomy contains the texts of the invited discourses given at the XIVth General Assembly held in Brighton, England, August 1970. It contains further the papers and discussion remarks presented at the six joint discussions, as well as the invited papers given at the special session on the Moon. In addition this Volume contains the papers given at the joint meeting of Commissions 24, 27, 30, 33 and 37 on RR Lyrae Stars. It goes without saying that the nearly hundred papers printed in this Volume represent only a minor part of all matter dealt with at the XIVth General Assembly of the Union; the many important discussions that took place in a few hundred commission meetings are not included. For short abstracts and reviews of these the reader is referred to Transactions of the International Astronomical Union XIVB. I wish to thank those who contributed to this Volume for the speed in submitting the manuscripts of their papers. This, together with the efficiency of the Publishers allowed for a rapid publication.

This volume contains the fifteenth tri-annual reports of the Presidents of the forty Commissions of the International Astronomical Union; it refers to the progress in our discipline during the three years 1970, 1971 and 1972. As compared to earlier volumes a gradual change in character is unmistakable. The ever increasing flow of publications, combined with the obvious necessity to keep the Reports at a reasonable size and price level has gradually forced the Commission Presidents to be more selective than before in drafting their Reports. I have certainly stimulated them into that direction - in order that Reports like these be valuable and lasting, it seems imperative that the individual contributions have the character of a critical overall review, where a fairly complete summary is given of the major develop ments and discoveries of the past three years, and in which the broad developments and new trends be clearly outlined, while at the same time essential problems for future research are identified. With respect to the latter item I have suggested the Commission Presidents to add to their reports a brief section on scientific priorities for future research in the field of their Commissions. In order to save space I have suggested to Commission Presidents that references to published papers are given on the basis of their number in the published issues of Astronomy and Astrophysics Abstracts. For instance, the indication (06. 078. 019) or (AAA 06. 078."

From 17 to 21 April 1967 a Study Week was held in the hotel 'De Bilderberg' near Arnhem, Holland, with the purpose to establish a new, and if possible, generally acceptable working model for the quiet parts of the solar photosphere and low chromosphere. The organizers of the conference hoped that even if this latter goal appeared too far to be reached, such a meeting would still be useful, if only for enumerating the crucial problems in solar photospheric research, and for defining future subjects of research. About twenty solar physicists from outside the Netherlands participated in the Study Week, while some others, though prevented from actively attending, sub mitted their comments before the meeting. The two above-mentioned goals were reached: a working model could be estab lished; yet it became clear that not everyone would agree about this model, and it became obvious too that future research is strongly needed, in particular in the field of line formation (coherence, or non-coherence; local thermal equilibrium), while also the motion field of the photosphere and chromosphere is insufficiently known, and its influence on the formation of spectral lines hardly understood."

The primary inducement for organizing an international Conference on 'Image Processing Techniques in_Astronomy' was the fact that the recording microdensitometer VAMP ('Vol Automatische Micro Photometer') of the Utrecht Astronomical Institute was operative for a few years. The necessity of comparing the in strument and its performance with similar instruments nowadays available at many other institutes, was stimulating enough to organize a meeting on the above subject. It took place in Utrecht on March 25, 26 and 27, 1975. The Scientific Organizing Committee consisted of J. Borgman (Groningen), R.B. Dunn (Sacramento Peak), H. Elsasser (Heidelberg), L.D. de Feiter, T. de Groot, J.R.W. Heintze, C. de Jager, H. Nieuwenhuijzen (Utrecht) and W. Wiskott (Geneve). About 175 scientists from 14 countries participated in the meeting which appeared to be successful and offered a good opportunity of exchanging information and comparing experiences. The VAMP was bought with financial support of the Utrecht University and the Netherlands Foundation for Scientific Research (Z.W.O.). The conference was organized with financial support from The Netherlands Ministry of Science and Education, The European Southern Observatory, The Leids Kerkhoven-Bosscha Fonds, The Astronomical Institute of Utrecht, to which Institutes and Organisations we express our sincere gratitude. C. de Jager H. Nieuwenhuijzen editors PAR T WHAT INFORMATION DO WE NEED, FOR WHICH ASTRONOMICAL PROBLEM? ASTROMETRY K. Aa. Strand U. S. Naval Observatory Washington, D. C, INTRODUCTION Considerable progress has taken place in astrometry over the past two decades."

In this volume we compare modem observations of solar flares with results from recent theoretical research and simulation studies on current-carrying loops and their interaction. These topics have undergone rapid developments in the course of recent years. Observational results by X-ray monitoring and imaging spacecraft in the seventies and by dedicated imaging instrumentation in the satellites Solar Max imum Mission and Hinotori, launched 1980 and 1981, have shown the importance of X-ray imaging for understanding the ignition processes of solar flares. Such observations, in tum, stimulated theoretical studies, centered around the flux-tube concept. The classical idea that flares originate by interaction of current-carrying loops was developed and proved to be promising. Concepts on reconnection and coalescence of flux tubes were developed, and their consequences studied. The Yohkoh spacecraft, launched 1991, showed the overwhelming importance of coro nal flux tubes and their many possible ways of interaction. Subsequent and parallel theoretical studies and simulations, differentiating between the topology of interact ing fluxtubes, demonstrated that the mutual positioning and the way of interaction are important for the subsequent processes of energy release in flares and the many associated phenomena such as the expUlsion of jets and the emission of X -ray and microwave radiation. The new developments now enable researchers to understand and classify flares in a physically significant way. Various processes of accelera tion are active in and after flares on greatly varying timescales; these can now be distinguished and explained.

No part of the Hertzsprung-Russell diagram shows a more pronounced diversity of stellar types than the upper part, which contains the most luminous stars. Can one visualize a larger difference than between a luminous, young and extremely hot Of star, and a cool, evolved pulsating giant of the Mira type, or an S-type supergiant, or - again at the other side of the diagram - the compact nucleus of a planetary nebula? But there is order and unity in this apparent disorder! Virtually all types of bright stars are evolutionally related, in one way or the other. Evolution links bright stars. In many cases the evolution is speeded up by, or at least intimately related to various signs of stellar instability. Bright stars lose mass, either continuously or in dramatic sudden events, they vibrate or pulsate - and with these tenuous, gigantic objects this often happens in a most bizarre fashion. Sometimes the evolution goes so fast that fundamental changes are observable in the time span of a human's life - several of such cases have now been identified.