1984 Balzan Prize for Astrophysics
Acceptance Speech – Rome, 15.11.1984
Mr. President, Excellency,
members of the Balzan Foundation,
To have been elected a recipient of the magnificent prize awarded by your Foundation is a very great honour. The pleasure of receiving this is particularly enhanced because it comes from Italy, the country where Galileo made his great contribution to our insight in the vastness of the world we live in, and where in the present time astronomy has made such admirable progress, which has brought it to the forefront of astronomical research.
During the last twenty years strong ties have developed between astronomers in Italy and the Netherlands due to the fact that rather similar long arrays of radiotelescopes had been built near Bologna and in Westerbork in the North of the Netherlands. These instruments were of a unique new type. In the early days of our Westerbork telescope we have profited from the great experience of the Italian astronomers with their similar instrument. Since that time there has hardly been a month without Italian astronomers at the Leiden Observatory.
On the occasion of a high award one’s thoughts naturally turn to the sources of one’s scientific interest.
The main inspiration has undoubtedly come from Professor J.C. Kapteyn in Groningen, who was one of the earliest explorers of the Milky Way System. It has also been his manner of thinking, and the way in which he stressed the importance of « seeing through » things which has penetrated the style of my work and of all who bad studied with him.
During the 19th century astronomy had been mostly restricted to the study of the solar system and the motions of the planets and their satellites. Kapteyn tackled a radically new field. When I came to Groningen in 1917 his maps of our Galaxy were still very incomplete. As a student I was particularly intrigued by the thought whether it would ultimately become possible to study the orbits of the stars within the Galaxy in a similar way as the orbits of the planets within the solar system. The difficulties seemed overwhelming. For, while the planets revolve in periods of years, the times of revolution in the Galaxy must be hundreds of millions of years.
Ten years later, by a piece of particularly good luck, I discovered some peculiarities in the motions of distant stars which showed convincingly that these stars were ascribing nearly circular orbits around a distant centre, much the same way as planets move around the Sun. The centre was found to lie at a great distance, well outside the system of stars which Kapteyn and his collaborators had outlined. The discovery helped opening the way to a new insight into the dynamics and structure of the Galaxy.
The exploration of our Galaxy is greatly restricted by clouds of gas and dust particles between the stars, which entirely obstruct our view of its more distant parts. In fact, no more than a few per cent of the thin galactic disk was accessible to optical observations. Entirely new horizons were opened after the war through the advent of radio astronomy and the discovery by van de Hulst of the so-called 21-cm line emitted by hydrogen atoms. With their traditional interest in the Milky Way System Dutch astronomers did pioneering work on discovering new parts of the Galaxy with the aid of radiotelescopes.
Though unimaginably large in human context the Milky Way System is but a tiny island in the Universe. In recent years my work has carried me to study subsequent steps in the hierarchy of the Universe touching upon the largest structures discernible and their evolution in the course of time. I wonder whether this work would have the approval of my former professor, strongly interwoven as it is with speculative aspects. But on this point I derive comfort from a line by Browning: « But a man’s reach should exceed his grasp, or what is heaven for? »
Even more than in other sciences much of the research in astronomy is team work extending over the whole Earth. The world of astronomers is like a strongly bound family. I am grateful to be one of its grandfathers.
I have been happy as an astronomer. But the greatest happiness has come from my family and, more than anything, from my wife.