National administrative career

• October 1946-October 1950 : Research Associate, CNRS, Paris Institute of Astrophysics
• October 1950-October 1952 : Research Associate, CNRS, Paris Institute of Astrophysics
• October 1952-October 1955 : Lecturer, Université de Clermont-Ferrand (two lectures : " Physique du P.C.B. ", " Certificat d'Astronomie et d'Astrophysique "), except : October 1953-October 1954 : Fulbright Foundation Fellow
• Associate Fellow at the High Altitude Observatory, Boulder, Colorado (one year)
• October 1955-October 1962 : Assistant astronomer at Paris Observatory
• Head of the General Astrophysics Department (created for the occasion) located at the Meudon Observatory
• October 1982-summer 1964 : Full astronomer at Paris observatory, functions and location unchanged
• Summer1964-September 1988, elected in 1963 : Professor at the Collège de France, chair of Theoretical Astrophysics
• January 1962-July 1969 : Director, Nice Observatory
• October 1972-October 1979 : Director, Paris Institute of Astrophysics
• Since September 1988 : Honorary Professor at the Collège de France

Various national responsibilities

Official

• 1981-1985 : Chairman of COMIST (preparatory committee for the La Villette science and technology museum)
• 1985-1987 : Chairman, Comité national (interministériel) de la Recherche scientifique et technique
• 1974-1978 : Chairman of the Science Committee of the National Commission for UNESCO
• 1991-1997 : Vice-Chairman of the National Commission for UNESCO
• 1994-1995 : General Curator of the CNRS fiftieth anniversary exhibition
• Member of the editorial board of the journal Technê of the Musées de France research laboratory

Non-official positions

• 1973-1976 : President of the French Astronomical Society
• Member of the Scientific Council of the Société Astronomique de France
• Honorary Chairman of CLEA (Comité de Liaison Enseignants-Astronomes)
• Honorary Chairman of ALPF (Association des Planétariums de Langue Française)
• Honorary Chairman, Association d'Astronomie Champagne-Ardennes
• 1999-2001 : President of the Association Française pour l'Information Scientifique (French Association for Scientific Information)
• Various international responsibilities
• August 1961-August 1964 : Deputy Secretary-General of the International Astronomical Union (IAU)
• August 1964-August 1967 : Secretary-General of the International Astronomical Union (IAU)
  (in this capacity, IAU representative on the ICSU on several occasions)
• 1973-1979 : Chairman of IAU Commission 5 (Documentation, Bibliography, Data Centers, etc.)
• 1994 and 1997 : Chairman of the IAU Resolutions Committee
• 1989-1992 : Vice-President of the Academia Europaea

Honorary distinctions

• Commander of the Legion of Honor
• Grand officier dans l'ordre national du Mérite
• Commandeur dans l'ordre des Palmes académiques
• 1964-1977 : Corresponding member of the Académie des Sciences
• 1977 : Member of the Académie des Sciences
• 1961 : Corresponding Member of the Bureau des Longitudes
• 1977 : Foreign Associate of the Royal Academy of Belgium
• Member (and member of the Bureau) of the Académie internationale d'humanisme (Amherst, N. Y.)
• Member of various academies

Awards and medals

• Prix Forthuny (Académie des sciences)
• Manley-Bendall Prize (Bordeaux Academy)
• Prix Stroobant (Royal Academy of Belgium)
• Prix Jean Perrin (Société Française de Physique)
• Lodén Prize (Astronomical Society of Uppsala, Sweden)
• Prix des Trois-Physiciens
• Union Rationaliste Prize
• CNRS silver medal
• Jules Janssen gold medal (French Astronomical Society)
• Jules Janssen gold medal (Société française de photographie)
• ADION medal

At Rue d'Ulm (1945-1946), I attended lectures by Kastler, an old family friend. After the agrégation, he advised me to go into astrophysics, a field with a bright future, he said. At the same time, I met Evry Schatzman, who was spending his final year there before leaving for Copenhagen. I was going to start my thesis (in October 1946) with him. He said a simple thing to me : " In France, there is no theoretical astrophysics, except on very specific questions. Me, Evry, I'm interested in the internal structure of stars... Why don't you take an interest in stellar atmospheres ? Since I can't teach you anything about that, you'll have to study elsewhere. "
Evry went to Copenhagen and Princeton, I went to Utrecht and Copenhagen again. My teachers at the time, Minnaert, Strömgren and Schatzman, had a profound impact on my research.

The first problem Evry proposed I tackle was that of determining the abundances of light elements (hydrogen, helium) in stars. To do this, we used the standard equations of internal structure for some forty stars, and applied a method proposed by Schwarzschild for the Sun. This subject is at the confluence of the questions dealt with by Evry on the internal and global structure of the star, and those I dealt with later in the more localized framework of stellar atmospheric physics. It is also central to modern cosmology, one of whose key arguments is the measurement of H/D/He3/He4/Li7 ratios.

From the outset, therefore, it was this problem of elementary abundances that preoccupied me, and has not ceased to preoccupy me since ; it was by completely modifying the problematic that, in many ways, I tackled it then, and still do. It is in this area that my contribution has, I believe, largely changed the way in which the problem is posed and resolved. I myself have carried out very few abundance determinations ; but my students, and a great many foreign colleagues, have applied them ; and subsequently, everyone has exploited them, almost without knowing it... ! A whole chapter is devoted to them in A. Unsöld's seminal treatise Physik der Sternatmosphären, and another in John Jefferies' classic Stellar Atmopheres.

But let's go back a bit.

My first work concerned the structure of hot atmospheres, O and B stars, where the Lyman discontinuity in the hydrogen spectrum coincides roughly with the spectral region of the radiation flux maximum. As a result, the deviations from classical solutions, which predate my work, are the greatest for these stars. Classical models are said to be grey. In other words, the absorption coefficient is replaced by a constant mean value, which imposes a decoupling between opacity and temperature distribution, enabling easy solutions. Chandrasekhar, around 1945, gave a non-gray solution, assuming the deviation to be small... This was a big mistake ! The reality is very different from the grey case. In my thesis (defended in 1950, chapter 2), I presented the first rigorous solution to the non-gray case, and calculated the ratio of the surface temperature to the effective temperature of the star (To/Teff = 0.86 according to the gray theory, and in the range 0.55-0.60 for OB stars according to my calculations, since confirmed by more complete calculations made by Roger Cayrel, who had worked for his thesis with Schatzman and myself).

The case of the Sun is less serious : we're closer to the grey case. However, observation leads to contradictions. My own measurements of the variation of molecular lines (CN, CH, C2) on the solar disk seemed to suggest a lower temperature. Based on the idea that the layers of the solar atmosphere are non-spherical (the so-called roughness effect), I calculated a surface temperature of 4200° (instead of the previously adopted 4900°). I was the first to suggest such a low temperature and to justify it. Current models still accept this value, in the vicinity of the temperature minimum (in regions outside the surface, the temperature rises sharply, due to the dissipation of non-radiative, mechanical or magnetic energy). I have since extended the concept of roughness to other applications.

Calculating the overall intensity of spectral lines is an essential step between understanding the structure of atmospheres and determining their abundances. I therefore developed an exact theory, the first (and only !) on the market, applicable to all lines on all stars, using a new function, the Y saturation function (sometimes referred to as the Pecker function), which shapes the observed growth curves (abundance-intensity relationship). Many researchers have used this function to analyze individual stars. Claas (Belgium) and Aller, Goldberg, Muller (USA), for example, did so in the 1950s, until rapid computer calculation methods rendered this Y function useless.

Between the measurement of the intensity of a line (from a given level of a given atom) and the total abundance of that atom, there are the ionization rates of that atom and the excitation rates of the level in question. Until around 1950, these calculations were made within the framework of the classical hypothesis of local thermodynamic equilibrium (LTE), which assumes the validity of Boltzmann-Saha and Maxwell's laws and perfect coupling between atoms and the radiation field.

However, we know that this is not the case in gaseous nebulae (Menzel, 1930...) ; in the solar chromosphere, Thomas showed in 1947 that hydrogen behaves very differently from ETL conditions. In 1957, I was the first to propose extending this work to all regions of the atmosphere, and to show the importance of these differences, at least for all lines involving fairly low energy levels. With Thomas, Menzel and Athay, I calculated a chromospheric model that took this into account, using measurements from the Khartoum eclipse (1952). Above all, I developed methods for the empirical diagnosis of ETL deviations (physical data are lacking to calculate them, except for a few light atoms, H, He, or C...), and then launched my whole team on the systematic empirical determination of ETL deviations in stellar atmospheres, notably that of the Sun. A dozen researchers (R. Kandel, L. Vogel, Y. Cuny, S. Dumont, F. Praderie, J. Lefèvre, N. Gökdogan, M. Hotinli, A. Kiral, J. Lesh-Rountree) worked on this project and, in a dozen publications (1960s and 1970s), studied the lines of the best-known elements in the solar spectrum.

It's worth noting that this work is very similar to what was going on in physics at the time : the processes at work in stars are similar to the optical pumping of Kastler and Brossel ; they also involve other forms of pumping, etc. They all imply that the population of stars in the solar spectrum is very small. They all imply that the population of energy levels is no longer controlled solely by the parameters of temperature and electron density, but by the precise evaluation of all the processes of populating or depopulating, through collisions and interaction with the radiation field, all the energy levels of all the atoms... !

The important point, here too I believe, that my work (and that of my colleagues in the USA, with me, including Thomas in the forefront, and S. Pottasch) is that taking ETL deviations into account, after the grey case deviations, after the introduction of the non-sphericity of the layers of stellar atmospheres, does not just involve corrective terms. It involves an in-depth revision of the mechanisms by which energy (produced at the star's center) flows, as a symptom, an index of the immense reservoir of energy that is the star.

The stellar atmosphere, where the observed spectrum is formed - the only piece of information we have - is a transition medium between something (the interior, which behaves much like a black body) and the interstellar medium, which is completely out of equilibrium. None of the simplifying assumptions (ETL, ER-radiative equilibrium, EH-hydrostatic equilibrium, PP-spherical structure, or parallel plane) can decently apply to this transition medium. This is not a corrective term, but an essential, dominant phenomenon. The atmosphere is a region of transition. We can't treat it as if we knew its physics a priori. It seems to me that I have made an important contribution to the formulation of a coherent physics of stellar atmospheres, by rigorously treating situations that are not cas-gris, not ETL, not plane-parallel... and thus rejecting artificial hypotheses involving various decouplings.

It should also be noted that abundances depend on the geometric assumptions made about the model. Thus, the effect of roughness has a strong influence on the diagnosis of the various types of atmospheres that can be encountered. For the Sun, we are led (see above) to reduce its minimum temperature; for the solar chromosphere, to increase the optical thickness of the regions responsible for this or that X-UV emission line ; for supergiants, to significantly increase an order of magnitude (at least the abundances of the elements - work in progress, first note with J. Zimba, USA).

With these ideas in mind, I embarked on various special studies of stars (cepheids, with R. Canavaggia, RS Ophiuchi with S. Pottasch, Novae with oscillatory phase). In addition, with Goldberg, Jordan and Thomas, we edited the series of volumes (eight in all) of NASA-CNRS monographs (in English with a French abstract) on non-thermal phenomena in stellar atmospheres, a voluminous series which, with numerous authors, constitutes a sum of modern work on questions that we helped to open up.

To sum up, my publications on stellar atmospheres have helped to develop ideas and obtain new results on the atmospheres of stars, or on the atmosphere of the Sun, considered as a star, in the following fields (some of which, the least decisive in my view, have not been mentioned above) :

• Rigorous theory (and calculations, for O and B stars) of atmospheric models in the gray case;
• Complete calculation of the blanketing effect in solar-type stars (in my opinion, the best to date... but it's disputed ! I have serious criticisms to make of subsequent work !)
• Rigorous theory of spectral line formation (and saturation function)
• Rigorous method for diagnosing ETL deviations in heavy-element lines
• Establishment of the roughness effect; application of the existence of this effect to the determination of surface temperatures and elemental abundances
• Synthesis of a modern theory of stellar atmospheres
• Comprehensive theory of the polarization (by electron scattering) of light in the continuum and lines of stars and the sun

I have also published numerous articles in fields of Astronomy other than stellar physics, namely : the physics of the circumstellar medium (equilibrium of dust particles, charged or neutral) ; the evolution of galaxies (disk accretion, polar ejection, size distribution of non-ejected dust) ; the size of ionized hydrogen regions (with E. Schatzman) ; non-standard cosmology ; solar activity (discovery of the coronal hole) ; relations between solar activity and terrestrial phenomena, notably geomagnetism and rainfall.

Some of these works attracted considerable attention at the time. In particular, the discovery of coronal holes, based on their effect on geomagnetism, which Roberts and I had christened " cones of avoidance ", was not recognized until rather late, as our deductions predated direct observations by a considerable margin.

I should mention here the keen interest I took in the cosmological debate from the 1950s onwards, to which I devoted numerous publications. I have regularly expressed doubts about the standard model (known as the " big bang ") and suggested alternative, albeit partial, solutions. I still think we're a long way from a coherent solution to cosmological problems.