Astronomer, educator and administrator. Ph.D., University of Groningen (1932). Harvard University, R.W. Willson Fellow in Astronomy (1927-1929), Professor of Astronomy (1933-1957), Associate Director of the Harvard Observatory (1946-1952); Director, Mount Stromlo Observatory, Professor of Astronomy, Australian National University (1957-1966); University of Arizona, Head of the Department of Astronomy, professor and professor emeritus (1966-1983). Research in galactic structure and dynamics, stellar and interstellar matter.