The first ideas for the symposium were generated in Brussels in the summer of 1986, dur ing exquisite lunches in between HST proposal consortium meetings. At the time it was expected that soon after the previous IAU symposium (No. 116) devoted to luminous hot massive stars, a bonanza of new exciting observational material would become available, together with significant advances on the theoretical front. Also it was felt that Wolf Rayet stars should feature predominantly, because that had not been the case since IAU Symposium No. 99 in 1981. Tradition requires that IAU symposia on hot massive stars take place in high lumi nosity beach resorts, and after Buenos Aires, Qualicum Beach, Cozumel and Porto Heli, Bali sounded like a reasonable place. Therefore we were only too pleased with the invitation of the Indonesian astronomical community to host the symposium in Sanur (Denpasar). The aim of the symposium was to bring together both observers and theoreticians active in the field of Wolf-Rayet stars and related objects, to present and discuss their recent results, in order to expose to what extent consensus exists as to the physical and chemical properties of Wolf-Rayet stars, their evolutionary status and their interrelations with other massive stars in galaxies."

The origin of stars is one of the principle mysteries of nature. During the last two decades advances in technology have enabled more progress to be made in the quest to understand stellar origins than at any other time in history. The study of star formation has developed into one of the most important branches of mod ern astrophysical research. A large body of observational data and a considerable literat ure now exist concerning this topic and a 1arge community of international astronomers and physicists devote their efforts attempting to decipher the secrets of stellar birth. Yet, the young astronomerjphysicist or more advanced researcher desiring to obtain a basic background in this area of research must sift through a very diverse and sometimes bewildering literature. A literature which includes research in many discip1ines and sub discip1ines of classical astrophysics from stel lar structure to the interstellar medium and encompasses the entire range of the electromagnetic spectrum from radio to gamma rays. Often, the reward of a suc cessfu1 foray through the current literature is the realization that the results can be obsolete and outdated as soon as the ink is dry in the journal or the conference proceeding in which they are published."

Highly ionized atoms in the general interstellar gas of the galactic disk were first detected through interstellar absorption line observations of 0 VI with the Copernicus satellite (Rogerson et al. 1973). Survey measurements by Jenkins (1978) of interstellar 0 VI absorption toward 72 stars demonstrated the general presence of 0 VI in the interstellar medium of the galactic disk. This researcIi. and parallel observational studies of the soft X-ra}' background (Williamson et al. 1974; McCammon et al. 1983; Marshall and ClarK 1984) provided direct evidence for the existence of hot low density gas in the mterstellar medium of the galactic disk. The extension of the aDsorption line studies to the distant gas of the galactic halo required the launch of the International Ultraviolet Explorer (IUE) satellite in 1978. The first measures of highly ionized gas m the galactic halo were obtained with the IDE when it was used to record high resolution spectra of bright stars in the Large Magellanic Cloud (Savage and de Boer 1979). Those early spectra revealed the presence of absorption by Si IV and C IV in the galactic nalo and have been followed by a number of surveys with IUE of nighly ionized gas in the galactic disk and halo (Savage and de Boer 1981; Pettini and West 1982; Savage and Massa 1987). The study of UV emission from highly ionized gas in the halo has progressed more slowly because of the intrinsic faintness of the emission.

A few years ago, a motivation for organizing one more IAU Symposium on star for mation in Grenoble, was the anticipated completion of the IRAM interferometer on the Plateau de Bures, close to Grenoble. This choice was also a sort of late celebration of the genius of Joseph Fourier, born in Grenoble, whose work is the very fondation of in terferometry. At the time when we finally announced the advent of this conference, the first reactions we got from the community were expressions of saturation and even reject, the Symposium being unfortunately scheduled almost simultaneously as two other major meetings on closely related topics, and sponsored by different organizations. A wave of disappointment then reached the organizers. Some of us were enthusiastic enough to help the others overcome their discouragement. Let them be thanked here. There was, indeed, a deeper motivation for organizing this conference. It was to trigger the meeting and communication of physicists and astrophysicists since many of the difficulties met now in understanding the physics of the interstellar medium and its evolution toward star formation are common to several, if not most, other fields of physics. They are assigned to one origin: complexity.

The galactic and extragalactic background radiation components are of major importance in astrophysical studies of a variety of problems related to galactic structure, interstellar matter, the distribution and evolution of galaxies and intergalactic matter, and cosmology. The back ground radiation is a unique source of information in cases where the source is of a truly diffuse nature, as with scattering and emission by interstellar dust and gas, or where the discrete emission sources are so faint (e. g. , very distant galaxies) that they can be observed only by their integrated emission. The galactic and extragalactic background must be discussed together because they are observationally entangled and their proper separation requires a knowledge of both components. The above points are equally valid for the entire electromag netic spectrum, from gamma rays to long radio waves. Thus, the study of the galactic and extragalactic background radiation is interdisciplinary in its character, and a multi wavelength approach would appear to be dictated by the nature of the topic. The proper separation of the galactic and extragalactic background components requires a knowledge of both. In addition, the foreground components, i. e. , zodiacal light, airglow, and atmospheric scattered light, must be treated carefully.

IAU Symposium 135 on Interstellar Dust was hosted and co-sponsored by NASA's Ames Research Center from July 26-30, 1988. The symposium was held at the lovely campus of Santa Clara University situated around the historic Santa Clara Mission in California. The meeting was made possible by generous grants from the Astron omy and Relativity Branch of the National Aeronautics and Space Administration and the Galactic Astronomy Program of the National Science Foundation. The International Astronomical Union provided travel grants to a few participants from countries with limited travel funds. We are particularly grateful for the support and services rendered by the dedicated staff at NASA's Ames Research Center and to the" SET Institute for professionally and expeditiously administering the U.S. grants. This symposium brought together 199 scientists representing 19 different coun tries. The wide range of interest and expertise of the participants - all in some way related to interstellar dust - is reflected in the great variety of topics that were discussed during the symposium ranging from UV, visible and IR observations of interstellar extinction to quantum-statistical calculations of the IR emission from highly vibrationally excited molecules. During the course of the meeting, 41 invited review papers and 140 contributed papers were presented. This book is a collection of the invited review papers. The contributed papers have been published in a companion volume, NASA CP-3036, available from National Technical Information Service, Springfield, Virginia 22161, USA."

The investigation of the Galactic nucleus and its surroundings is necessarily a modem endeavor, for traditional observations made at visual wavelengths have not even begun to penetrate the veil of -30 magnitudes of visual extinction that intercedes. On the other hand, infrared, and especially radio observers find a relatively unobstructed view of the central portion of the Galaxy, so the study of this arena has proceeded apace with the development of these branches of astronomy. Thus, it is no accident that the first IAU sponsored conference to be held on the Galactic center is timed to coincide with the initiation, or the immediate aftennath, of major technical developments at long wavelengths, including infrared array detectors, millimeter-wavelength aperture synthesis, and self-calibration and refmed deconvolution algorithms in aperture synthesis radio astronomy. The center of the Galaxy is also accessible to X and gamma-ray observers, and progress at high energies has been steady, especially as imaging capabilities are being realized at X-ray wavelengths. However, one might expect that the revolution in the high energy domain is still ahead of us, as instruments with larger collecting areas and improved spatial resolution are now being developed. The youth of this subject is evidenced by the relatively small number of meetings that have been devoted to it."

This book contains the proceedings of the sixth Eurographics Workshop on Vir tual Environments. The event took place from June 1 to June 2, 2000, in Am sterdam. We hope that readers will find these proceedings to be valuable, not only for virtual environment researchers, but also for practitioners developing or using virtual environment applications. We are glad to report that visibility of the workshop continues to expand and that virtual environment researchers and practitioners from allover the world are submitting papers. This year, 40 papers and case studies were submitted of which 20 were accepted. In addition, we are glad to see that the focus of the workshop is also expanding. We accepted 6 research papers on evaluation of virtual environments and there was a broad sampling of other topics. We would like to thank all those involved in organizing the symposium. In particular, thanks go to Mieke Brune who was in charge of the local organization. In addition, we want to thank the international program committee for their excellent, yet laborious, job in reviewing all submitted papers. The quality of the workshop is a reflection of the quality of the submitted papers and the quality of the reviewing process."

IAU Symposium No. 134 on Active Galactic Nuclei was hosted by the Lick Observatory, as part of the celebration of its centennial, for the Observatory went into operation as part of the University of California on June 1, 1888. Twenty years later, in 1908, Lick Observatory graduate student Edward A. Fath recognized the unusual emission-line character of the spectrum of the nucleus of the spiral "nebula" NGC 1068, an object now well-known as one of the nearest and brightest Seyfert galaxies and active galactic nuclei. Ten years after that, and seventy years before this Symposium, Lick Observatory faculty member Heber D. Curtis published his description of the "curious straight ray" in M 87, "apparently connected with the nucleus by a thin line of matter," which we now recognize as an example of one of the jets which are the subject of so much current AGN research. The symposium was held at Kresge College on the campus of the University of California, Santa Cruz, only a short walk through the redwood groves to the Lick Observatory offices. A total of 232 astronomers and astrophysicists from 24 countries attended and took part in the Symposium. About 200 more had applied to come, but could not be accepted in order to keep the meeting at a reasonable size. Most of the participants lived in the Kresge College apartments immediately adjacent to the Kresge Town Hall in which the oral sessions took place.

Ten years ago in August 1977 Malcom Longair and Jan Einasto organized IAU Symposium nO 79 on exactly the same exciting and most important topic i.e. the Large Scale Structure of the Universe. Many of us have the recollection of an outstanding meeting which fulfilled two goals (i) establish most of the foundation of a fast growing field (ii) set up a confrontation between the excellent observational and theoretical work performed in eastern and western countries. A decade after such a meeting Alex Szalay and I have felt the need to reassemble the cosmologists working actively on problems dealing with the Uni verse as a whole. Indeed a lot of progress has been achieved in the building of large surveys in the discovery of voids, sponges and filaments in the galaxy clus ter distribution, in refined numerical simulations, in experimental and theoretical particle physics (outcome of new particles (cold particles) and unification (GUT, supersymmetry) schemes), in research of quantum gravity and inflation scenarios etc ... A new confrontation between all the specialists working all throughout the world on such questions appeared to us to be most timely. This is why the location of Balatonfiired in Hungary to be accessible to anyone as Tallin in 1977 has been chosen."

The present book is a translation of the original German edition (published in 1982) with some minor corrections and improvements. The guide to sup plementary and advanced literature given in the Appendix, however, has been brought up to date. This book is addressed primarily to students taking astronomy as a prin cipal or subsidiary subject, and to scientists of related fields, but amateur as tronomers should also be able to profit from it. For most chapters an elementary knowledge of mathematics and physics will be sufficient, however, Chaps. 5 and 6 impose somewhat greater requirements. In addition the reader should already be acquainted with the basic concepts of stellar physics as treated in introduc tory books, including the spectral types, the system of stellar magnitudes and colours, absolute magnitudes and luminosities, the Herlzsprung-Russell dia gram and its interpretation. A modem textbook should use SI units. On the other hand, the use of the cgs system is still the prevailing custom in astrophysics - together with the special units of astronomy: length is quoted in parsecs [pc], mass in solar masses [M0] and time in years [a]. We have therefore compromised and employed both cgs and SI units in this book, whichever was the appropriate choice in each instance. A table for conversion of cgs units into SI units and vice versa is given in the Appendix.

In the centennial year, 1985-86, of Harlow Shapley's birth, the study of globular clusters was no less important to the development of astronomy than in 1915, when Shapley first noted their concentration on the sky. By 1917 Shapley had used the properties of the system of globular clusters to complete the Copernican revolution and locate the solar system, and its Earth-bound observers, far from the center of the Galaxy and the globular cluster distribution. Seven decades later, in the year of these proceedings, globular cluster research and the study of the system of globular clusters in our own and distant galaxies is undergoing a renaissance of activity. The introduction of new observational tools, particularly CCD imagers and digital spectrographs, as well as powerful theoretical methods have transformed the study of globular clusters into one of the main line areas of modern astrophysics. Thus it seemed particularly appropriate to one of us, when considering how the Harvard College Observatory might mark the Shapley centennial, to propose and plan for an IAU Symposium on Globular Cluster Systems in Galaxies. Planning for the Shapley Symposium, as it came to be called, was even more drawn out than the preparation of this volume. The Symposium was originally proposed to the IAU Secretariat in time for it to be held in August, 1985, so that it might occur in the centennial (calendar) year.

Every 5 years since 1967 a meeting has been held to discuss the subject of planetary nebulae and their central stars. Previous meetings have been held in Tatranska Lomnica (Czechoslovakia); Liege (Belgium); Ithaca, New York (U. S. A); and London (Great Britain). IAU Symposium 131 was sponsored by IAU Commision 34, on Interstellar Matter and co-sponsored by IAU Commisions 35 and 36 on Stellar Constitution and Theory of Stellar Atmospheres. The symposium was held at the Universidad Nacional Autonoma de Mexico in Mexico City, October 5-9, 1987. It took place in one of the old buildings of the University of Mexico in the downtown area. The inner patio of the building provided very pleasant surroundings for the poster sessions and for extensive discussions among the participants. The meeting was attended by 160 scientists from 22 countries. The Scientific Organizing Committee, under the chairmanship of J.B. Kaler, prepared a comprehensive scientific program based on a set of invited presentations. All contributed papers were presented in poster form. The Scientific Organizing Committee would like to thank the staff of the University of Illinois Department of Astronomy: Dr. Ron Allen for granting financial support; Carol Stickrod, Louise Browning, Deana Griffin and Sandie Osterbur for their help with the organization. IAU provided economic assistance to a group of young astronomers

Gaseous nebulae offer outstanding opportunities to atomic physicists, spectroscopists, plasma experts, and to observers and theoreticians alike for the study of attenuated ionized gases. These nebulae are often dusty, heated by radiation fields and by shocks. They are short-lived phenomena on the scale of a stellar lifetime, but their chemical compositions and internal kinematics may give important clues to advanced stages of stellar evolution. The material herein presented is based on lectures given at the University of Michigan, University of Queensland, University of California, Los Angeles, and in more abbreviated form at the Raman Institute, at the Scuola Internazionale di Trieste, and elsewhere. Much of it is derived origionally from the series "Physical Processes in Gaseous Nebulae" initiated at the Harvard College Observatory in the late 1930s. I have tried to emphasize the basic physics of the mechanisms involved and mention some of the uncertainties that underlie calculations of many basic parameters. Emphasis is placed on ionized plasmas with electron temperatures typically in the neighborhood of 10, OOOoK. Dust and other ingredients of the cold component of the interstellar medium are treated briefly from the point of view of their relation to hot plasmas of H II regions and planetaries. Chemical composition determinations for nebulae are discussed in some detail while the last section deals with interpretations of elemental abundances in the framework of stellar evolution and nucleogenesis. Gaseous nebulae offer some particularly engaging opportunities for studies of stellar evolution.

The idea for an international symposium on the interstellar medium was first discussed at the University of Wyoming during the summer of 1984. It was obvious that the outstanding natural beauty of the Teton mountain range in northwestern Wyoming must be matched by a meeting with the broadest appeal to the astronomical community. If the meeting was to produce a book, it must likewise be an important contribution to the astronomical literature. It was for these reasons that early in the discussions, it was decided that the University should host a "school." with the invited speakers presenting tutorials on a broad range of topics involving the interstellar medium. The symposium proceedings would then be a compilation of the written versions of these presentations. It has been nearly a decade since Lyman Spitzer published his classic text on the interstellar medium and we felt the need for a school and book that would focus on the recent developments in our understanding of the inter stellar medium. Thus, we view this two-volume set as an adjunct text to Spitzer's book."

The objective of this meeting was to bring together collea gues from different branches of observational astronomy and theoretical astrophysics to discuss and analyse the rapid progress in our knowledge and understanding of the matter surrounding stars, streaming off stellar surfaces, or fall ing onto stars. The meeting was sponsored by IAU Commis sions 36 (Theory of Stellar Atmospheres), 29 (Stellar Spec tra), and 34 (Interstellar Matter). There were two special reasons for organizing this meeting at Heidelberg in 1986: During this year the University of Heidelberg celebrated its 600th anniversary and the IAU symposium joined the many scientific events accompanying this celebration. Secondly, the year 1986 also marked the conclusion of a special co operative research project ("Sonderforschungsbereich") in astrophysics at Heidelberg, a major part of which had been devoted to the physics of circumstellar matter. The main topics discussed at this meeting were: (1) circumstellar matter, bipolar flows, and jets from young stars and protostars; (2) circumstellar envelopes of evolved stars; (3) stellar coronae; (4) stellar winds from hot and cool stars; (5) dust formation and circumstellar chemistry. Many exciting new results were presented in 21 invited or review papers, 26 contributed papers, and 127 poster papers. This symposium would not have been possible without the generous financial assistance of the International Astronomical Union, the German Science Foundation (DFG), and the State Government of Baden-Wurttemberg. The practi cal support of the University of Heidelberq and the Max Planck-Society was also very valuable."

IAU Symposium No. 121 was hosted by the Byurakan Astrophysical Observatory in Soviet Armenia, almost 30 years after Ambartsumian's pioneering ideas about galaxy activity were first published and almost exactly 20 years after the first Byurakan IAU symposium on nuclear activity (No. 29, "Non-stable Phenomena in Galaxies, May 1966). Although the proceedings of the first Byurakan symposium were not published in English, that conference provided a definitive impulse to the field, as Ambartsumian's ideas had done 10 years earlier. The Byurakan Astrophysical Observatory was thus a particularly appropriate setting for IAU 121. The symposium was also very timely since many new exciting results were presented which will surely revolutionize many of our present ideas about nuclear activity in galaxies and QSO's. The first results of the by now famous Markarian survey were presented by B.E. Markarian in the first Byurakan conference. Unfortunately, his untimely death prevented him from attending the second conference, but the influence of his fundamental work was certainly felt.

Proceedings of the 132nd Symposium of the International Astronomical Union, held in Paris, France, June 29-July 3, 1987

This is the first time that the International Astronomical Union has held a symposium on objects of totally unknown nature. In fact. M. Rees has pointed out that the mass of the individual particles that make up the dark matter is unknown to > 70 orders of magnitude. Since dark matter appears to make up ~ 90 % of the mass of the Universe. it presents us with one of the most fundamental problems in astrophysics. IAU Symposium 117 on Dark Matter in the Universe was held on June 24 - 28. 1985. Our hosts were Princeton University and the Institute for Advanced Study. which together form one of the most active centers of work on the dark matter problem. There were ~ 190 participants from 16 countries. These proceedings include the 31 review and invited papers. 72 of the 85 poster papers. and the two general discussions. The idea that the Universe might contain much more mass than we see in gas. stars and their remnants has been with us for over 50 years. In 1933. F. Zwicky pointed out that the Coma Cluster could be in equilibrium at the large observed velocity dispersion only if a great deal of unseen matter were present. However. in the absence of other evidence. the idea of "dark matter" was not widely pursued.

The idea to hold a Symposium on Star Forming Regions in Japan, be fore or after the New Delhi General Assembly of the IAU, originated in 1981. One of the main reasons was that the 45-m telescope was expected to be inoperationby 1982. In addition it was considered that Japan had been very active in the theory of star formation as well as in mm and IR observations of star forming regions. At the Patras General Assembly of the IAU the issue was brought up for discussion at meetings of Commis sions 34 (Interstellar Matter) and 40 (Radioastronomy), the former agreed to sponsor the Symposium, while the latter agreed to co-sponsore it. The IAU Executive Committee accepted the proposal to hold the Symposium on September 1983. The Symposium was held in Tokyo on 11-15 November 1985, just before the New Delhi General Assembly. It was attended by 250 scientists from 18 countries, and 212 papers were presented. The Scientific Organizing Committee elaborated a superb program of invited and review presentations. The Conference consisted of oral and poster sessions. The oral sessions included review papers, invited papers and a few of the contributed papers; after each presentation a discussion was held. The poster ses sions included most of the contributed papers and were very well attended. We consider that the scientific goals of the meeting were amply achieved and we express our thanks to all the participants and specially to the members of the SOC for making them possible."

Proceedings of the 116th Symposium of the International Astronomical Union, held at Porto Heli, Greece, May 26-31, 1985

IAU Symposium 127 was held in Princeton on May 28-31, 1986, at the Institute for Advanced Study. There were 150 participants from 19 countries. This was the first IAU Symposium devoted exclusively to elliptical galaxies. The last decade has been a period of exceptionally rapid progress regard ing our understanding of elliptical galaxies, driven on the observational side by a wealth of new photometric and spectroscopic data, and on the theoretical side by the recognition that ellipticals are slowly rotating triaxial systems, instead of rotationally flattened bodies. The 30 invited speakers reviewed all aspects of this progress, its consequences, and the major outstanding problems. Nearly 80 poster contributions were displayed for the duration of the meeting. These contained many of the most recent developments. The first part of these Proceedings contains the written versions of the invited reviews and the summary, in the order of their presentation. The accompanying discussions are based on the written versions of questions and answers as handed in by the participants. The second part of this book is devoted to the poster contribu tions. They have been grouped in a thematic order, proceeding from morphology to theory. Indices of objects, subjects, and names are provided. Unfortunately, none of the invited participants from the USSR were able to attend the Symposium. The Proceedings include the written versions of the planned contributions by Popov and Polyachenko."

The chemical processes of formation and destruction of molecules have been found to be widespread in astronomical scenario. It occurs in comets, planetary atmospheres including ionospheres, early solar system, stellar atmospheres, circumstellar shells, interstellar clouds, and possibly even in the early universe. Physical conditions are naturally different in the different situations, but there is unity in the underlying physics and chemistry in this diversity. Therefore, a need was felt to gather together astronomers, physicists and chemists in these and allied interdisciplinary fields to enrich each other with their experiences. This International Astronomical Union Symposium No.120 on Astrochemistry, the proceedings of which is this, has partially fulfilled this need. This symposium, held at the beautiful and scenic sea resort of Goa, India, between December 3-7, 1985, under the sponsorship of the IAU Commission No.34 and co-sponsorship of IAU Commissions 14, 15, 29 and 36, was attended by 116 registered participants with 19 accompanied guests from 19 countries. The scientific programme comprised of 34 review talks, 15 invited talks and 47 contributed poster papers, clima xed by a grand finale by Professor A. Dalgarno with an overview of th whole symposium. The Symposium covered almost all aspects of astro chemistry from the early universe to comets and meteorites, except iono spheres and planetary atmospheres. Basic atomic and molecular physics, experimental and observational results, and theoretical calculations and modelling, all were stressed."