The 'Ledoux Criterion' on the stability of the inner layers of stars
The "Ledoux criterion" on the stability of the inner layers of stars is still used today in stellar evolution models.
"In this paper the effect of a discontinuity in the mean molecular weight in stellar models is examined": in one sentence of the preamble of one of his most famous articles (1), published in 1947 in The Astrophysical Journal, Paul Ledoux summarizes a whole part of his research.
Before reaching this point, however, his life was as eventful and unstable as that of the stars he studied. As with all researchers of his generation - but for him more than for others - Ledoux's working conditions were indeed dependent on the events that shook the world at the time. But in spite of all these vicissitudes, Paul Ledoux remained faithful to one line of research, present from his first publication and which would not cease to assert itself over the following years: to study the stability of stars and thus their evolution over time. Where did this passion come from? "Eric Gosset and Jean-Pierre Swings, astrophysicists, respectively Maître de recherches FRS-FNRS and Honorary Professor, explain that reading Arthur Eddington's The Internal Constitution of stars during his studies of physics at the University of Liège was a determining factor. The book was published in 1926, a time when the understanding of what stars really are was very sketchy. Where does their fabulous energy come from? For Kelvin and others, it is from the collapse of the star on itself, but Eddington suggests on the contrary that nuclear reactions - particularly the fusion of hydrogen atoms to give helium - are at work in the center of stars. He is also the first to determine a relationship between mass and luminosity and to explain the pulsation of variable stars. Next to Eddington, whom Ledoux seems never to have met, two personalities emerged during the 1930s in the study of stars; and these two names were to be important for Paul Ledoux: Svein Rosseland (1894-1985) and Subrahmanyan Chandrasekhar (1910-1995).
The "Chandra" gesture
Svein Rosseland is a Norwegian astrophysicist who, a little older than Paul Ledoux, will guide him in his first research. Thanks to Professor Polydore Swings with whom he worked as a young graduate in physics, Paul Ledoux arrived at the Institute of Theoretical Astrophysics in Oslo in 1939 to study the structure of stars and their stability, Rosseland's specialty. The year, one guesses, is not favourable to fundamental research on stars: the important will soon take place on the battlefields. In spite of this, as we shall see, ledoux's stay in Oslo was beneficial. During the invasion of Norway, Paul ledoux started a rather incredible journey (see his biography ) which led him from Oslo to the United States, more precisely to the Yerkes Observatory of the University of Chicago. There he met a brilliant young Indian astrophysicist of the same generation as him, Subrahmanyan Chandrasekhar, future Nobel Prize winner in physics, with whom he worked and became friends. Paul Ledoux stayed at Yerkes from December 1940 to September 1941 before putting his knowledge at the service of the Allies on the meteorological front... meteorology. " As eventful as it was, this first period of Paul Ledoux's career was fruitful, explain Eric Gosset and Jean-Pierre Swings. The proof is an article published in 1941 (2). Ledoux starts from Rosseland's work and applies for the first time the theory of stability to stars." Paul Ledoux is thus the first to conceive the existence of a maximum mass of stable stars of the main sequence (3) and to calculate it! The verdict: a star with a mass greater than 100 solar masses cannot be stable. He did not stop there and despite the world conflict and his occupations in the Royal Air Force, he continued his research and published another article, very technical, on the same subject in 1945 (4). an article that has a history. "Chandrasekhar had been interested in Ledoux's first article as well as in his later work, which remained in the form of unpublished notes. He wanted to publish his own work, but he was ethically embarrassed because he should have referred to the unpublished work of his colleague. Chandra, as he was called, and had it published! Without co-signing it, an elegant gesture on the part of the future Nobel Prize winner, " an article whose signature is unique in the annals of the Astrophysical Journal (and probably of all scientific journals): P. Ledoux, F/L.R.A.F., Stanleyville, Belgian Congo!
Source: Subrahmanyan Chandrasekhar, Nobel Prize in Physics. He and Paul Ledoux worked together and became friends at the Yerkes Observatory at the beginning of the war. "Chandra" edited and published Ledoux's work while he was serving with the Allied Forces in Africa.
The Ledoux criterion
The 1947 paper already mentioned (1) and the following ones will lead to a theoretical formulation of stellar evolution. Back at the University of Liège, Paul Ledoux was one of the first to build stellar models based on a convective nucleus that became depleted as the hydrogen atoms disappeared. convection was one of his favorite subjects, " explain Eric Gosset and Jean-Pierre Swings. The progressive disappearance of hydrogen within a star leads to a discontinuity in what Ledoux calls the average molecular weight - even though there are no molecules in the strict sense - and he studied the effect of this discontinuity on the structure of the star. to explain it, he introduced the notion of semi-convection. Recall that in the core of an average star, the nuclear fusion energy is transmitted by radiation (this is the radiative zone of the star). Around this zone, there is the convective zone where the energy is transmitted by the movements of the matter according to a cycle: the hot matter rises, expands and eventually heats the surrounding matter, cools down, and then descends towards the base of the convective zone, and so on. This is the phenomenon of convection dear to Paul Ledoux. As early as 1906, the German astrophysicist Karl Schwarzschild (1873-1916) had established a criterion based on the temperature gradient to determine if a zone of the star is stable or not (no movement of matter or movements, especially radiax, of it). To explain the shape of the molecular weight discontinuity resulting from the transformation of hydrogen into helium at the edge of the convective zone, he postulated the existence of a semi-convective zone. he also defined a stability criterion for the convective zone that took into account the variation of the average molecular weight, thus considerably refining the Schwarzschild criterion. "This criterion - now called the Ledoux criterion - is still used to develop models of stars. Thanks to this, we can determine where the convective zones are located in a star."
Paul ledoux (right) during an astrophysics symposium in Cointe.
The precursor of asteroseismology
Among the major publications of Paul Ledoux, one cannot ignore the two articles (5) that he published in the Handbuch der Physik of 1958. The first one, written in collaboration with the Dutch astronomer Theodore Walraven (1916-2008) deals with variable stars. The second, of which he is the only signatory, deals with stellar stability. "These two articles, and particularly the second one, have become the bible of all those who have started to do theoretical work on stellar stability and evolution. One can even say that the second one is considered as the founding article of asteroseismology (6). a reference, in particular for those who were (and are) part of the so-called liège school of theoretical astrophysics that Paul ledoux initiated.
Scientific references
(1) Stellar models with convection and with discontinuity of the mean molecular weight, P. Ledoux, The Astrophysical journal, vol. 105, 1947.
(2) On the vibriational stability of gaseous stars, P. Ledoux, The Astrophysical journal, vol. 94, 1941.
(3) To make it short and simple, the main sequence stars are those whose energy comes from the fusion of hydrogen into helium. This is the case for the majority of stars. This is because this phase of a star's life is by far the longest and most stable.
(4) On the radial pulsation of gaseous stars, P. ledoux, The Astrophysical Journal, vol. 102, 1945.
(5) Variable Stars, Ledoux, Paul; Walraven, Theodoor and Stellar Stability, ledoux, Paul. Handbuch der Physik, volume 51, 1958.
(6) This discipline studies the seismic movements in stars (their oscillations, pulsations). These movements are either self-sustained or due to violent convection movements that occur in stars. The study of these seismic movements allows to determine the internal structure of stars. It is therefore understandable that Ledoux's work on the stability of stars. It is therefore understandable that Ledoux's work on the stability of stars, and more particularly on the phenomena at work in the convection zone, made the Liege astrophysicist a precursor of modern asteroseismology.
Documentary sources
A. Noels, In Memoriam : Paul Ledoux, Ciel et Terre, Vol 105, 1989.
Robert Halleux, Geert Vanpaemel, Jan Vandersmissen en Andrée Despy-Meyer (éds.), Histoire des sciences en Belgique, 1815-2000, bruxelles : Dexia/La Renaissance du livre, 2001.
Paul Ledoux (1914-1988)
Paul Ledoux was born in Forrières, in the Ardennes. He completed his secondary education at the Athénée de Marche-en-Famenne and then studied physics at the University of Liège. Professor Polydore Swings noticed this brilliant student with a bright future. In 1939, he sent him to Oslo to study with Svein Rosseland, with whom he really discovered theoretical astrophysics. But the Germans soon invaded Norway and Paul Ledoux took refuge first in Sweden and then tried to reach the United States, which he did only at the end of 1940, after a journey that took him to Russia and then to Japan (and having been suspected of espionage in these two countries!). He finally settled down at the Yerkes Observatory, where he found Pol Swings, who had taken refuge there, and befriended Subrahmanyan Chandrasekhar, the future Nobel Prize winner in physics. After receiving a scholarship from the Belgian American Educational Foundation from December 1940 to September 1941, he asked to join the Belgian armed forces in Canada. Leaving his family in the United States, he joined the meteorological service integrated into the Royal Air Force (RAF), first in London (where he calculated trajectories for bombing planes), then in Nigeria and the Congo. He ended the war as a weather officer (captain).
Demobilized in 1946, he returned to Liège to present his doctoral thesis and then escaped to Yerkes where he was finally able to find his family after almost 5 years of absence. His daughter, born just before his departure, is, it seems, quite surprised to discover a father who speaks French! The year 1947 and the return to Liege did not mean the end of Paul Ledoux's adventures. There was no room for him at the university and he had to accept a job in the weather service of the Régie des Voies Aériennes. The European services were trying to fill a gap discovered during the war: in order to make forecasts more reliable, one could not be satisfied with data collected on the American continent and on the European continent, one had to collect data on the Atlantic! Paul Ledoux was therefore sent back to the United States with an astonishing mission: to negotiate the purchase of a boat equipped to take wind measurements at altitude! He also gave courses to forecasters, which he combined with his duties as a physics assistant at the University of Liege. After having presented his thesis in 1949, he was finally appointed in 1950 as head of works at the University of Liège, an institution he would never leave.
Appointed professor, he gathered around him and his research a large number of young people who would soon constitute what would be called the Liège school of theoretical astrophysics. Paul Ledoux received the Francqui Prize in 1964 and the Eddington Medal of the Royal Astronomical Society in 1972. And if he was above all a remarkable theoretician, Paul Ledoux, often at the instigation of Pol Swings, did not disdain the practical side of research. He was thus president of the Program Committee of the European Southern Observatory - ESO - in the 1970s and president of the ESO Council in the early 1980s.
Consult the list of Paul Ledoux's scientific publications on ORBi
Eric Gosset
Although Eric Gosset has always been attracted to physics, his initial interest was in nuclear and electronics. His passion for astrophysics came later: it wasn't until his year of rhetoric and his two years as a candidate in the physical sciences, that he attended numerous conferences, in particular those of the Société Astronomique de Liège, and went on nightly trips with a friend to observe the constellations that it became obvious. After graduating in physics from the University of Liège in 1978, he became for a year the student assistant and then the assistant of the astrophysicist Léo Houziaux, before obtaining the status, in the group of J.P. Swings, of researcher at the F.R.F.C. (Fonds de la Recherche Fondamentale Collective - Fundamental Research Collective) in January 1980.
After spending several years working on the development of an instrument to acquire star spectra in the very near infrared, a field that was still little explored at the time (a project whose funding was stopped), Eric Gosset reoriented his research towards the nuclei of distant and very energetic galaxies known as QUASARs (QUAsi-Stellar Objects), which became his doctoral subject. In December 1987, he defended his thesis on the spatial distribution of quasars, with an accompanying thesis on the variability of Wolf-Rayet stars.
It was during a 3-year post-doctoral stay in Munich, at the ESO headquarters, that Eric Gosset seriously entered the universe of massive stars and Wolf-Rayets, a universe he has not left since. Back at the University of Liege, he obtained a position as a qualified researcher at the FNRS in 1993. He passed his agrégation in 2007, and became a senior researcher at the FNRS in 2008. He teaches a course on time series analysis in astronomy.
Consult the list of scientific publications of Eric Gosset on ORBi.
Jean-Pierre SWINGS
Jean-Pierre Swings is Honorary Professor (space astrophysics) at the University of Liège, where he obtained his degrees (engineer-physicist in space techniques, Doctor of Science and Agrégé de l'Enseignement Supérieur). Between the last two, he spent three years as a post-doc at the Joint Institute for Laboratory Astrophysics (Boulder, Colo.) and at the Hale Observatories (Pasadena, California... his birthplace).
His interests include solar physics, line emission and excess infrared objects, extragalactic astrophysics, space research, (very) large telescopes and their instrumentation, and the solar system and its exploration, especially Mars. He gradually evolved from astrophysical observation to "astropolitics" as Secretary General of the International Astronomical Union (IAU), member of numerous committees of the European Space Agency (ESA) and member of the Council of the European Southern Observatory (ESO), where he was strongly involved in the committees related to the development of the Very Large Telescope and the choice of its site. He was one of the four founders of the European Astronomical Society (EAS). Jean-Pierre Swings was also Chairman of the European Space Sciences Committee (ESSC) of the European Science Foundation (ESF) from May 2007 to November 2014 and a member of the Space Advisory Group (SAG) of the European Commission (7th Framework Programme).
Consult the list of scientific publications of Jean-Pierre Swings on ORBi
