On 12 April Professor Adriaan Blaauw, emeritus professor of astronomy at Groningen and Leiden, celebrates his ninetieth birthday.

Adriaan Blaauw studied at Leiden. In 1938 he became assistent to Professor P.J. van Rhijn at the Kapteyn Laboratory in Groningen, where he obtained his doctorate in 1946. A year earlier he had already joined the Leiden Observatory as a senior scientist; in 1948 he was appointed Associate Professor. From 1953 till 1957 he was on the staff of the Yerkes and McDonald Observatories in the USA, from 1956 as Associate Director. In those years, Yerkes was at the top of American astronomy, together with Mount Wilson and Palomar, and Blaauw was enjoying excellent research facilities. Yet, when in 1957 he was asked to succeed Professor van Rhijn, he decided to return to the Kapteyn Laboratory - a big jump.

A big jump down indeed. Kapteyn had, in the years 1890-1920, brought world fame to Groningen with his research into the structure of the Milky Way, but the institute had only a very small staff, and it badly needed new research programs. Blaauw accepted the challenge, and the University gave strong support to his ambitious plans. He obtained a modern 60-cm reflector (the largest telescope in the Netherlands), and an ample travel budget for observing trips abroad; he could strongly expand the staff, modernize the institute's research program, and include astronomical instrumentation, radio astronomy, and interstellar physics. As Director of the Kapteyn Laboratory, Blaauw turned out to be a real democrat, who strongly involved his collaborators in his new plans and endeavours.

In addition to responsibilities at Groningen and in Dutch astronomy, Blaauw got a central role in preparations for a European Southern Observatory. In 1968 he became ESO's Scientific Director, and in 1970 Director-General. These heavy commitments forced him to reduce his Groningen position to a part-time professorship. But the Kapteyn Laboratory was back in the scientific mainstream; and in the seventies its involvement with the Westerbork Synthesis Telescope would turn it into a prominent centre of extragalactic research.

During Blaauw's years as Director-General ESO developed into a mature organisation; the 1-meter Schmidt camera began its surveys of the southern sky, and the 3.6-meter reflector (one of the largest telescopes in the southern hemisphere) started operation in 1976. In 1975 Blaauw returned to the Netherlands; he obtained a full-time professorship at Leiden, and retired in 1981.

Blaauw has done eminent research on the formation of stars and the properties of young, hot stars. He demonstrated the expansion of "OB-associations", groups of young stars, and he found that some hot, young stars move away from their birthplaces at very high speeds. This work culminated in 1961 in a now classical paper, which explained these "runaway stars" as the effect of supernova-explosion in binary star systems: the supernova loses most of its mass in a very short time, and its companion is released from its orbit with a speed exceeding 100 km/sec. Blaauw later followed this up with studies of the relationships between pulsars, runaway stars, supernovae and their birthplaces.

Together with Maarten Schmidt, Adriaan Blaauw edited a book on Galactic Structure, as Vol. 5 in the famous "Stars and Stellar Systems" Series. This excellent book, published in 1965, has remained a key reference for very many years.

Blaauw has done very important work in astronomical administration, both in The Netherlands and world-wide. We mention here only a few items. He was the first Chairman (1979-1984) of the ASTRON Foundation, which undertook the task of coordinating astronomical research in The Netherlands. As President of the International Astronomical Union (1976-1979) he succeeded - thanks to his diplomatic gifts - in bringing China back into the IAU, from which China had withdrawn after the admission of Taiwan in 1961. In the 1980s, he served as Chairman of the Hipparcos Satellite Program Selection Committee. He is a member of five foreign Academies of Science.

Professor Blaauw enjoys excellent health, and still takes an active part in the scientific life of the Kapteyn Institute. He has written books about the History of the IAU, and about ESO's Early History. He also has done research about the history of farming villages in the province of Drenthe, and has been Chairman of the Provincial Museum of Drenthe in Assen.