Royaume-Uni - États-Unis/Allemagne
Prix Balzan 1994 pour l'astrophysique (évolution des étoiles)
Discours de remerciement – Rome, 16.11.1994 (anglais)
Members of the Balzan Foundation, Ladies and Gentlemen,
I am greatly honoured to be speaking on behalf of a long line of investigators of the problem of the evolution of the stars. Astronomers are concerned in their studies with two very different farms of activity. On the one band there is the expansion of the Universe, which is a dissipative, degenerative process. On the other hand there is the activity of the Stars. They supply light and beat, x-rays, y-rays even, and they drive high speed winds of particles through space. Stars are also responsible far the origin of the chemical elements. The carbon of living material, the oxygen that we breath, the metals on which modem industry is founded, the calcium in our bones, were ali produced by physical processes inside stars.
Stars are among the most reliable of time keepers, whether of short periods of a fraction of a second or of long periods of billions of years. This wide range arises from the enormous variety of the stars, from the swiftly rotating pulsars to the slowly evolving red dwarf stars. Shakespeare began a well-known speech with the words: « …one man in bis time plays many parts… », and in a similar way one star in its time plays many parts. An initial mass of material usually begins its life in a placid way, as fortunately far us our Sun is a placid star at the moment. Then, typically, there is a marked increase in energy output, accompanied by an increasingly large size of the outer envelope of the star. Sometimes the increase of the outer envelope is accompanied by repeated in-and-out oscillations that are used by astronomers far determining the distance scale of the large features of the Universe. Thereafter, and with increasing relative speed, stars incur internal explosions. They begin to eject material out into space. They shrink from their distended form, developing many kinds of violent activity, some becoming sources of x-rays, some undergoing the extreme violence of a supernova explosion, some going on to become placid again as white dwarfs, others become pulsars that are the sources of cosmic rays. The determination of all these processes in terms of known physics allied to mathematical calculation is the branch of astronomy called the evolution of stars. The underlying causes of it all are the nuclear transformations that take place within the interiors of the stars.
The processes I have described, albeit briefly, were not discovered all in a moment. The concept that it should be possible to understand the physics of the interior of stars as a matter of firm science began to take root already in the second half of the nineteenth century. Names that come to mind are van Helmholtz in Germany and Lord Kelvin in Britain. In the early part of the twentieth century we have Emden in Germany and Karl Schwarzschild, the father of my colleague Martin, also in Germany. Then Eddington in Britain, with others appearing in ever-increasing numbers – Hertzsprung and Russell, Chandrasekhar and Bethe.
This is all in the pre-WW II period. Bethe’s work immediately proceeding the war look the first long step towards what was to prove the key development of the post-war years. That is to say, the association together of the nuclear processes with what is actually seen in the sky. This was to prove an intricate and intriguing story in which many players emerged upon the stage. Of those who influenced me personally, I would like to pay especial tribute to the observer Walter Baade and to my colleague of to-day, Professor Martin Schwarzschild.
And so I pass finally to a younger generation than my own, unfortunately too numerous to be mentioned individually. Thanks to them, what began as a trickle a hundred years ago, what developed in my time into a healthy but still moderate river, has now become a mighty flood of discovery. In which exquisite details have become understood thai even flights of imagination would have failed to conceive of only a generation ago. In which indeed the evolution of the stars has become one of the best understood parts of modem physical science, a demonstration indeed that firm knowledge in science does not have to be acquired exclusively in the laboratory. Although without the painstaking work of many nuclear physicists done in the laboratory, little of what has been achieved would have been possible. It has been a development in which laboratory nuclear physics, observation by telescope, and mathematical calculation have been triumphantly joined together.
Members of the Balzan Foundation,
Ladies and Gentlemen,
The award of a recognition as significant as the Balzan Prize is obviously a cause for immense joy – but also for worrying contemplation. This applies particularly in my case: I was spoiled by fate ali through my scientific life. I was born into astrophysics. My father was the German astrophysicist Karl Schwarzschild, and my uncle was the Swiss astrophysicist Robert Emden – who now share two neighboring craters on the back side of the moon. My father died when I was four years old. My early science education was looked after by friends of my father’s, among them Cari Runge (of Runge-Kutta farne) and Ludwig Prandtl (pioneer of turbulence). My studies at the Gtittingen Observatory were guided by Hans Kienle and Otto Heckmann, both teachers as stem as stimulating. Finally, my education was polished in Oslo by the penetrating theoretician Svein Rosseland and in Harvard by Harlow Shapley, who moved the center of the universe 25,000 light years away from us.
With such an extremely advantaged start anything useful I may have done cannot be credited to me in the usual degree. And I have made big mistakes. The biggest was perhaps my choice of the theory of stellar structure and evolution for my main research topic exactly at the time when this field was deadly stuck. The cause of this impasse was that the physicists had not yet developed nuclear theory far enough to provide the quantitative data for the nuclear processes which provide the energy sources inside stars. Without these data stellar evolution could be speculated about, but one could not make a consistent theory for it.
Prior to this impasse the theory of stellar structure – not evolution – was carried through its first phase by Emden who analyzed hydrostatic equilibrium inside stars, by Eddington who showed how transport by radiation carries a star’s energy from its hot interior to its surface, and by Chandrasekhar who brilliantly developed the theory of degenerate stars, such as the white dwarfs. I was well tooled up in the state of the art at that time, largely by patient lessons from Chandrasekhar who became my final teacher at that time. But the nuclear physics was stili missing!
Then, in nineteen thirty-eight, two papers appeared, one by von Weizsacker and the other by Bethe, which isolated the specific nuclear processes operating in stars and gave the first quantitative estimates for their rates. Suddenly we all could start the real work on stellar evolution – though many of us were occupied otherwise until the war ended in 1945. The following years contained an entirely exhilarating period in the theory of stellar evolution. Forme personally a high point occured when Fred Hoyle came to Princeton for two successive spring semesters and he and I had a marvellous spell of collaborative work.
Thus, a kind fate did not punish me for my mistake of going into the stellar interior at the wrong time. I will not cover here other mistakes of mine. But I will brag here about two choices I made – all by myself – correctly. The first one was the choice of my wife. And the second one was my choice of accepting a position at Princeton University, for the express purpose of working in association with Lyman Spitzer. It was he who stimulated much of my work, who arranged my observing periods on Mount Wilson, who inveigled me into flying balloons with telescopes hanging from them – and that before Sputnik! – and who invited Fred Hoyle as a visiting professor to Princeton, which led to our happy collaboration. These two correct choices have compensated, so it seems to me, for a fair number of my mistakes.
But stellar evolution theory did not stop with Fred and me. New generations of young and daring theoreticians have opened entirely new areas in this field and are continuing in this work. One such area concems fast-rotating stars with their still mysterious magnetic fields, presenting problems which require nove! techniques to solve partial differential equations, in contrast to the simple ordinary differential equations relevant for the unperturbed spherical stars we used to consider exclusively.
Another modem area relates to the violent supernova explosions which occur during the final evolution phases of some stars, a phenomenon leading to the formation of heavy elements, a puzzle which Fred Hoyle look a leading role in solving. Still another modem area concerns close double stars which disturb each other so much that matter leaves the more extended companion and falls onto the surface of the more compact companion, thus causing X-ray emission. Last but not least, much experimental and theoretical effort is presently being directed towards the neutrinos that continuously stream out of the sun. Precise new models for the solar interior suggest that the observed rate of the solar neutrino flux does not agree with standard neutrino theory and may require attributing a small mass to the neutrino, which had been thought massless.
Clearly the theory of stellar structure and evolution is presently in an exciting epoch in which major problems should be solved. The same was gloriously true at the time Fred Hoyle and I were active in this field. To have been able to participate in such research makes fora full and rich life. To find one’s work earnestly applauded is cause for happiness indeed, and for deep gratitude.