Obituary for W.H.W. Baade J.H. Oort Originally published in Dutch in KNAW Yearbook, 1960-1961, Amsterdam, 281-284, 1960 WALTER BAADE (24 March 1893 – 25 June 1960). On 25 June, after a long-term illness in Gottingen,¨ one of the great astronomers of our time died, a man who paired imagination and vision with extreme care for the observations and the patience without which no experimental astronomer can find anything valuable. At the same time, however, Baade was impulsive. His mind was full of the innumerable things that he in his long life as an astronomer had observed and he understood as few others do the art of communicating his experience orally to others. Baade was born in 1893 in Schrottinghausen¨ in Westphalen. During his stud- ies in Gottingen¨ he was assistant for three years of Felix Klein, who had a great influence on him. From 1919 to 1931 he worked as an astronomer at the Hamburg Observatory in Bergedorf. With the relatively small ones there among the avail- able instruments he did wonderful work, which led to a job as an astronomer at the Mt Wilson Observatory in 1931 in Pasadena. He had spent almost all his life in Pasadena, first of all mainly working with the 100-inch reflector on Mt Wilson and later almost exclusively with the 200-inch Hale telescope on Mt Palomar. He used only one technique: direct photography. in which he focused in par- ticular on the photometry of faint stars. The reason for his great success and his numerous discoveries was on the one hand the great perfection to which he dev- beloped this techniques and, on the other hand, the almost infallible intuition with which he separated fertile and less fertile directions of research. His work was multifaceted. It included the composition of galaxies, the evo- lution of stars, the radio sources in the universe, novae and supernovae and many detailed investigations into other nebulae, stars and star clusters. 1 During his work on the galaxies he discovered that these exist from two types of stars, to which he gave the names ‘population I’ and ‘population II’. The objects of population I are interstellar clouds of gas and stars concentrated in the spiral arms of spiral galaxies and further in irregular galaxies of the type of the Mag- ellanic Clouds. Most so called ‘globular star clusters’ on the other hand consist almost entirely of stars of population II. Baade showed that stars of this popula- tion also occur in the spiral systems, but there in the form of a regular substratum. The essential difference between the populations is their age. While the popula- tion I stars are formed in the youngest stage of development of the galaxies, the population 11 stars are all very old. Some of them even have to be formed in the very first evolutionary stage of the galaxies, when they on just had split off from the expanding universe as separate units. The stars of the two populations also distinguish themselves by their composition: In the oldest stars of population 11 the fraction of elements heavier than helium, is very much (perhaps one hundred times) smaller than in the population 1. Baade’s work is dedicated to research on the distribution of the two population groups across the Galaxy and in several other galaxies in its immediate vicinity, like the Andromeda nebula. In partic- ular he also examined variable stars, because even among the faintest stars can be recognized relatively easily. At his instigation and under his guidance many investigations were also conducted on the brightnesses and temperatures of stars in star clusters of all possible ages. The combination of these investigations with the new insights in nuclear processes and in the internal construction of the stars, provided for the first time some insight into the evolution of individual stars. How- ever, Baade’s research in this area also has an important basis for the study of the evolution of galaxies. In 1951 Baade and Minkowski succeeded photographically to discover in the optical some of the brightest so-called radio sources. The very strongest radio source, Cassiopeia A, they identified as remnant of a 250 year old supernova, located within the borders of the Galaxy. The optical image of the all but one of the strongest source, Cygnus A, they interpreted as two colliding spiral galaxies at a distance of the order of a billion light years. Subsequent investigations, mostly by the same astronomers, have shown that the majority of radio sources originate from collisions of fast-moving gas masses, caused either by a supernova explosion in the Galaxy, or by interaction of the interstellar media of one or more close galaxies lying at a very great distance from the Galaxy. Independent of radio astronomy Baade already had preformed extensive and particularly interesting researches on that curious group of exploding stars that one indicates as supernovae. Among other things, he found that there are two 2 species of supernovae. The first belongs to the population I and must be very young, perhaps not yet fully formed, stars with huge masses, while the masses of the others, population II, supernovae are not much larger than those of the sun. Among the latter belongs the Crab Nebula, an object of which Baade has made extensive studies. His work on this area stretched out into some ‘ordinary’ novae (where in contrast to the supernovae only a very small fraction is expelled from the star) in particular Nova Persei 1901 and Nova Aquilae 1918. Baade has during the entire period of his stay in Pasadena had a lot of contact with Dutch astronomers. In 1953 he spent a few months at the Leiden Obser- vatory; in this time formed also with the young astronomers in Leiden a strong bond. It was also during this stay that, under Baade’s influence, the plan was born to create a European observatory in the southern hemisphere, a project that is now being realized. Some Leiden astronomers will now help to interpret a part of the extensive observational material that at his death still only was partially reduced, and make it ready for publication. J.H. OORT 3.