« Professor Ledoux 's stature has grown continuously during the four decades I have known him. Certainly he must be counted among the very few great representatives of astrophysics during the past half century. » (S. Chandrasekhar)

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A biography written by Cécile Dohogne (ULiège Library)

Youth and studies

Paul Ledoux was born in Forrières, in the Province of Luxembourg, on August 8, 1914, into a modest family. The youngest of five children, he was quickly noticed for his intellectual abilities in elementary school.

He entered the University of Liège in 1933 and graduated in physical sciences in 1937. His thesis was on molecular dissociation in a stellar atmosphere.

It was while attending the Institute of Astrophysics that he met Marcel Migeotte, assistant to Pol Swings, and the latter. He then became interested in theoretical astrophysics in Oslo and Paul Ledoux, winner of the Government Scholarship Competition, obtained a grant to spend several months there. But as soon as he obtained his diploma, he was called up for military service.

He married Aline Michaix and, in 1939, the couple left for Norway. At the Institute of Theoretical Astrophysics in Oslo, Paul Ledoux began his research on the structure and stability of stars.

Second World War

But the war interrupted his work: Norway was invaded by Germany. The Ledoux decided to return to Belgium. While in Sweden, they learned of the invasion of Belgium. They stayed in Sweden for some time, where Paul Ledoux was housed at the Saltjöbaden Observatory.

Then, thanks to Pol Swings, he obtained a Graduate Felow scholarship from the Belgian American Educational Foundation, which allowed him to work at the Yerkes Observatory of the University of Chicago under the direction of the future Nobel Prize winner Subrahmanyan Chandrasekhar. On November 14, 1940, the Ledoux family left Sweden for Moscow and reached the United States via the USSR, Siberia and Japan.

In Chicago, Paul Ledoux presented Chandrasekhar with the results obtained in Oslo. Ledoux then put him in contact with another researcher whose research results converged with his own. Without ever meeting, they corresponded in order to write an article in collaboration which will be published in the Astrophysical Journal, " Radial Pulsations of Stars ".

But Paul Ledoux interrupted his research again. Indeed, the Baron de Dorlodot Forces assembles the Belgian Armed Forces in Canada. Ledoux joined him and was then mobilized. He is then sent to England where he joins the Belgian meteorological service which is formed within the Royal Air Force. He was then sent on a mission to Africa and ended the war in Stanleyville as a meteorological officer.

Forced to abandon his research, Paul Ledoux nevertheless remained in contact with Chandrasekhar and Pol Swings. While stationed in Stanleyville, he wrote to Chandrasekhar about his work on the use of the virial theorem in its differential form to study the radial pulsations of a star. Wanting, on the basis of these results, to publish his own research, Chandrasekhar rewrote Ledoux's article and had it published in the Astrophysical Journal: " On the Radial Pulsation of Gaseous Stars ".

Thus, as soon as he was demobilized and returned to Belgium, he was able to present his doctoral thesis which he defended in 1946. He then returned to the United States to join his wife and daughter, Jacqueline.


The Ledoux criterion

Back at the Yerkes Observatory, he conducts research on the effects on stellar stability of changes in chemical composition due to nuclear reactions taking place in the core of stars. He shows the establishment of a transition zone of variable mean molecular weight at the interface between the convective core and the stellar envelope. The mathematical expression that relates to this is called "Ledoux criterion" and generalizes the Schwarzschild criterion. It will appear later that this transition zone (called semi-convective) plays a role in the evolution of massive stars. These results will constitute an important part of the thesis of aggregation.

Career at the University of Liege


In 1947, he returned to Belgium and was appointed assistant to Pol Swings at the Institute of Astrophysics in Liege. He was also offered a position as assistant meteorological advisor at the Régie des Voies Aériennes. On the other hand, he shared his knowledge with the students of the school of forecasters and organized advanced courses for university level meteorologists.

At the same time, he prepared his dissertation, "Contribution to the study of the internal structure of stars and their stability", which he successfully defended in 1949. In addition to a detailed discussion of the semi-convective zones, he added a chapter on the dynamical stability of stars. He also discussed the problem of non-radial oscillations.

He then left his functions at the meteorological service of the Régie des Voies Aériennes to devote himself to his academic career. He was appointed head of works at the 1950, then associate professor the following year. In 1956, he was in charge of the theoretical astrophysics and analytical mechanics courses for the mathematical and physical sciences degree, and was appointed ordinary professor in 1959.

In 1951, he published an article in the Astrophysical Journal entitled " The Nonradial Oscillations of gaseous Stars and the Problem of Beta Canis Majoris ", while he was working at the Princeton University Observatory. This contributed to his recognition as a master in the fields of structure and stability of stars. It was therefore natural that the editor of the Handbuch der Physik would turn to Paul Ledoux a few years later to write the chapters on the stability of stars (" Variable Stars " in collaboration with Th. Walraven and " Stellar Stability ") that would become a classic on the subject.

He will be honored with many prizes in Belgium and abroad: the Franqui Prize in 1964, the Eddington Medal of the Royal Astronomical Society in 1972 and the Jansen Medal of the Institut de France in 1976.

He founded at the Cointe Observatory a real school of theoretical astrophysics, his reputation attracting many young researchers, including foreign researchers of international reputation. He will also be called to teach in many foreign institutions.

He will also be very active in different organizations in Belgium such as the Royal Belgian Astronomical Society, the Scientific Council of the Royal Observatory of Belgium or the Belgian National Committee of Astronomy. He was also active in international organizations such as the Observation Program Committee of the European Southern Observatory (ESO).

Shortly before his death, in Liege, on October 6, 1988, he was still president of ESO and remained active in the learned societies to which he belonged.

Awards and honors

  • Winner of the Government Travelling Scholarship Competition, 1937
  • A. De Potter Prize, 1949
  • Prize of the Friends of the University of Liege, 1949
  • E. Mailly Prize, 1952
  • A. Wetrems Prize, 1954
  • Prize of the Decennial Competition of Applied Mathematics, (7th period, 1953-1962)
  • Francqui Prize in 1964
  • Eddington Medal of the Royal Astronomical Society, 1972
  • Jules Jansen Medal of the Institut de France, 1976
  • Correspondent of the Class of Sciences of the Royal Academy of Belgium 1959 (director in 1973)
  • President of the commission 35 on the Internal Constitution of Stars of the International Astronomical Union, from 1964 to 1967,
  • Member of the Royal Academy of Sciences, Letters and Fine Arts of Belgium, 1966 (president for the year 1973)
  • Holder of the Francqui Chair at the Université Libre de Bruxelles, 1967-1968
  • Doctor Honoris Causa of the Université Libre de Bruxelles, 1969
  • President of the Belgian National Astronomy Committee, from 1972 to 1974
  • President of the Royal Belgian Astronomical Society
  • President of the Observation Programs Committee of the European Southern Observatory (ESO), from 1972 to 1974 (Belgian scientific representative on the Council from 1975 onwards - President again from 1981 to 1984)
  • Member of the committee of the "Advanced Institutes" organized by NATO, from 1972 to 1976
  • Associate of the Royal Astronomical Society, 1974
  • Member of the American Association for the Advancement of Science, 1980
  • Member of the European Academy of Sciences, Arts and Letters, 1980
  • Visiting Fellow at Princeton and Boulder (Colorado)
  • Visiting Professor at the University of California at Berkeley, at Boulder (Colorado), at the University of Washington and at Columbia University (New York).
  • Admitted to emeritus in 1984
  • Member of the Scientific Council of the Royal Observatory of Belgium, where he was appointed Astronomer-Correspondent in 1984.
  • Foreign Associate of the Academy of Sciences of Paris, 1984
  • Doctor Honoris Causa of the Katholieke Universiteit Leuven, 1985

Sources

Gough, D O (2002). « Reminiscences on Two Papers by Paul Ledoux ». In : ASP Conference Proceedings, vol. 259, p. 12.

Houziaux Léo (1995). « Ledoux, Paul ». In : Nouvelle Biographie nationale de l’Académie Royale de Belgique, t. 5, p. 225.

Houziaux Léo (1995). « Paul Ledoux ». In : Annuaire de l’Académie Royale de Belgique, 1995, p. 3.

Noels A & Hoge E (1989). « In memoriam : Paul Ledoux (1914-1988) ». In : Ciel et Terre, vol. 105, pp. 41-42

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