Biography and publications

National Civil Service Career

• October 1946–October 1950: Research Fellow, CNRS, Paris Institute of Astrophysics
• October 1950–October 1952: Research Fellow, CNRS, Paris Institute of Astrophysics
• October 1952–October 1955: Lecturer, University of Clermont-Ferrand (two courses: “Physics of the P.C.B.,” “Certificate in Astronomy and Astrophysics”), except: October 1953–October 1954: Fulbright Scholar
• Associate Fellow at the High Altitude Observatory in Boulder, Colorado (one year)
• October 1955–October 1962: Assistant Astronomer at the Paris Observatory
• Head of the (newly created) General Astrophysics Department at the Meudon Observatory
• October 1982–Summer 1964: Tenured Astronomer at the Paris Observatory; duties and location unchanged
• Summer 1964–September 1988, elected in 1963: Professor at the Collège de France, Chair of Theoretical Astrophysics
• January 1962–July 1969: Director of the Nice Observatory
• October 1972–October 1979: Director of the Paris Institute of Astrophysics
• Since September 1988: Honorary Professor at the Collège de France

Various national responsibilities

Official

• 1981–1985: Chair of COMIST (Preparatory Committee for the Museum of Science and Technology at La Villette)
• 1985–1987: Chair of the National (Interministerial) Committee on Scientific and Technical Research
• 1974–1978: Chair of the Science Committee of the National Commission for UNESCO
• 1991–1997: Vice Chair of the National Commission for UNESCO
• 1994–1995: Commissioner-General of the CNRS 50th Anniversary Exhibition
• Member of the editorial board of the journal *Technê* published by the Research Laboratory of the Musées de France

Unofficial

• 1973–1976: President of the French Astronomical Society
• Member of the Scientific Council of the French Astronomical Society
• Honorary President of CLEA (Teachers-Astronomers Liaison Committee)
• Honorary President of the ALPF (Association of French-Language Planetariums)
• Honorary President of the Champagne-Ardennes Astronomy Association
• 1999–2001: President of the 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, he represented the IAU within the ICSU on several occasions)
• 1973–1979: Chair of Commission 5 (Documentation, Bibliography, Data Centers , etc. ) of the IAU
• 1994 and 1997: Chair of the IAU Resolutions Committee
• 1989–1992: Vice President of the Academia Europaea

Honors

• Commander of the Order of the Legion of Honor
• Grand Officer of the National Order of Merit
• Commander of the Order of Academic Palms
• 1964–1977: Corresponding Member of the Academy of Sciences
• 1977: Member of the Academy of Sciences
• 1961: Corresponding Member of the Bureau des Longitudes
• 1977: Foreign Associate of the Royal Academy of Belgium
• Member (and member of the Board) of the International Academy of Humanism (Amherst, N.Y.)
• Member of various academies

Awards and Medals

• Forthuny Prize (Academy of Sciences)
• Manley-Bendall Prize (Academy of Bordeaux)
• Stroobant Prize (Royal Academy of Belgium)
• Jean Perrin Prize (French Physical Society)
• Lodén Prize (Astronomical Society of Uppsala, Sweden)
• Three Physicists Prize
• Rationalist Union Prize
• CNRS Silver Medal
• Jules Janssen Gold Medal (Astronomical Society of France)
• Jules Janssen Gold Medal (French Society of Photography)
• ADION Medal

On Rue d’Ulm, I attended (1945–1946) the lectures of Kastler, an old friend of my family. After I passed the agrégation, he advised me to pursue astrophysics, a field with a promising future, as he put it. Around the same time, I met Evry Schatzman there; he was spending his final year there before leaving for Copenhagen. I was about to begin my dissertation (as early as October 1946) under his guidance. He then told me something simple: “There is no theoretical astrophysics in France, except on very specialized topics. As for 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 go study elsewhere.”
Evry went to Copenhagen and Princeton, while I went to Utrecht and then to Copenhagen again. My mentors at the time—Minnaert, Strömgren, and Schatzman—had a profound influence on my research.

The first problem Evry had suggested I tackle was determining the abundances of light elements (hydrogen, helium) in stars. To do this, for about forty stars, we used the standard equations of internal structure and applied a method proposed by Schwarzschild for the Sun. This topic lies at the intersection of the questions Evry addressed regarding the internal and overall structure of stars and those I later addressed within the more localized framework of stellar atmospheric physics. It is also central to modern cosmology, one of whose key arguments is the measurement of the H/D/He3/He4/Li7 ratios.

It was therefore this problem of elemental abundances that first occupied my mind, and has continued to do so ever since; by completely reframing the problem, I approached it from various angles at the time, and continue to do so today. It is in this field that my contribution has, I believe, significantly changed the way the problem is posed and solved. I myself have carried out very few abundance measurements; but my students, and a great many foreign colleagues, have applied them; and subsequently, everyone has made use of them, almost without realizing it…! An entire chapter is devoted to this in A. Unsöld’s seminal treatise, *Physik der Sternatmosphären*, and another in John Jefferies’ classic work, *Stellar Atmospheres *.

But let’s take a step back.

My early work focused on the structure of hot atmospheres—O- and B-type stars—where the Lyman discontinuity in the hydrogen spectrum roughly coincides with the spectral region of maximum radiation flux. As a result, the discrepancies from the classical solutions—which predated my work—are most significant for these stars. The classical models are known as “gray” models. That is, the absorption coefficient is replaced by a constant average value, which decouples opacity from the temperature distribution and allows for straightforward solutions. Around 1945, Chandrasekhar had given a non-gray solution, assuming the deviation was small… That was a major mistake! Reality is very far from the gray case. In my dissertation (defended in 1950, Chapter 2), I therefore presented the first method for a rigorous solution of 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 of 0.55–0.60 for OB stars according to my calculations, which have since been confirmed by more comprehensive calculations carried out by Roger Cayrel, who had worked with Schatzman and myself on his dissertation).

The case of the Sun is less severe: we are closer to the gray-body case. However, observations lead to contradictions. My own measurements of the variation in molecular lines (CN, CH, C2) across 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 4,200° (instead of the previously adopted 4,900°). I was the first to suggest such a low temperature and to justify it. Current models still accept this value, which is close to the temperature minimum (in regions beyond the surface, the temperature rises sharply due to non-radiative, mechanical, or magnetic energy dissipation). Since then, I have extended this concept of roughness to other applications.

Calculating the total intensity of spectral lines is an essential step between understanding the structure of atmospheres and determining abundances. I therefore developed an exact theory—the first (and still the only one!) of its kind—applicable to all spectral lines of all stars, which utilized a new function, the Y-saturation function (sometimes formerly called the Pecker function), which determines the shape of the observed growth curves (the abundance-intensity relationship). Many researchers used this function to analyze various stars. For example, Claas (Belgium) and Aller, Goldberg, and Muller (USA) in the 1950s, until rapid computer calculation methods rendered the use of this Y function obsolete.

Between the measurement of a line’s intensity (originating from a specific energy level of a specific atom) and the total abundance of that atom lie the ionization rates of the atom in question and the excitation rates of the energy level in question. Until about 1950, these calculations were performed within the framework of the classical hypothesis—that of local thermodynamic equilibrium (LTE)—which assumes the validity of the Boltzmann-Saha and Maxwell laws and posits a perfect coupling between the atoms and the Radiation field.

However, it was known that this was not the case in gaseous nebulae (Menzel, 1930...); in the solar chromosphere, Thomas showed in 1947 that hydrogen behaves very differently under conditions of LTE. In 1957, I was the first to propose extending this work to all regions of the atmosphere and to demonstrate the significance of these deviations, at least for all spectral lines involving relatively low energy levels. Together with Thomas, Menzel, and Athay, I calculated a chromospheric model that took this fact into account and utilized measurements from the Khartoum eclipse (1952). Above all, I developed methods for the empirical diagnosis of deviations from the ETL (physical data are lacking to calculate them, except for a few light atoms—H, He, or C...), and then I set my entire team to work on the systematic empirical determination of deviations from the ETL in stellar atmospheres, particularly that of the Sun. About ten 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 about ten publications (in the 1960s and 1970s), they studied the spectral lines of the best-known elements in the solar spectrum.

It should be noted that this work was very similar to what was taking place in physics at the time: the processes at work in stars are analogous to the optical pumping described by Kastler and Brossel; they also involve other forms of pumping, etc. They all imply that the population of energy levels is no longer governed solely by parameters such as temperature and electron density, but by the precise evaluation of all processes of population and depopulation—through collisions and interaction with the radiation field—of all energy levels of all atoms…!

The important point here, I believe, that my work (along with that of my colleagues in the U.S., including Thomas foremost among them, and S. Pottasch) has highlighted, is that taking into account the deviations from the ETL—after accounting for the gray-case deviations and introducing the non-sphericity of the stellar atmospheric layers—does not merely involve corrective terms. It involves a thorough reevaluation of the mechanisms by which energy (produced at the star’s core) flows out—as a symptom, an indication of the immense reservoir of energy that is the star.

The stellar atmosphere—the region where the observed spectrum is formed, the only piece of information we had—is a transitional medium between something (the interior, which behaves roughly 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 or parallel-plane structure) can reasonably be applied to this transitional medium. This is not a corrective term, but an essential, dominant phenomenon. The atmosphere is a transition region. It cannot be treated as if we knew its physics a priori. It seems to me that I have made a significant contribution to the formulation of a coherent physics of stellar atmospheres, through the rigorous treatment of situations outside the gray-case, ETL, and parallel-plane models… and thus by rejecting artificial assumptions involving various decouplings.

It should also be noted that abundances depend on the geometric assumptions made in the model. Thus, the roughness effect strongly influences the diagnosis of the various types of atmospheres that one may encounter. For the Sun, we are led (see above) to decrease its minimum temperature; for the solar chromosphere, to increase the optical thickness of the regions responsible for a given emission line in the X-UV range; for supergiants, to increase the element abundances very significantly by an order of magnitude (at least—work in progress, first note with J. Zimba, USA).

With these ideas in mind, I undertook various specific studies of stars (Cepheids, with R. Canavaggia; RS Ophiuchi, with S. Pottasch; and oscillating-phase novae). In addition, together 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 that, with contributions from numerous authors, constitutes a comprehensive compilation of modern research on topics we helped to pioneer.

To summarize, my publications on stellar atmospheres have helped develop new ideas and yield new results regarding the atmospheres of stars—or the atmosphere of the Sun, considered as a star—in the following areas (some of which, though less significant in my view, were not mentioned above):

• Rigorous theory (and calculations, for O- and B-type stars) of atmospheric models in the gray-case scenario;
• A comprehensive calculation of the greenhouse effect (blanketing effect) in solar-type stars (in my opinion, the best to date… but this is disputed! I have serious criticisms of subsequent work!);
• Rigorous theory of spectral line formation (and of the saturation function)
• A rigorous method for diagnosing deviations from the ETL in the spectral lines of heavy elements
• Establishment of the roughness effect; application of this effect to the determination of surface temperatures and elemental abundances
• Synthesis of a modern theory of stellar atmospheres
• A comprehensive theory of the polarization (via electron scattering) of light in the continuum and spectral 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, whether charged or neutral); the evolution of galaxies (disk accretion, polar outflow, size distribution of non-ejected dust); the size of regions of ionized hydrogen (with E. Schatzman); non-standard cosmology; solar activity (discovery of the coronal hole); and relationships between solar activity and terrestrial phenomena, particularly geomagnetism and rainfall.

Some of this work attracted attention at the time. In particular, the discovery of coronal holes based on their effect on geomagnetism—holes that Roberts and I had dubbed “avoidance cones”—was not recognized until quite late, as our deductions predated direct observations by a considerable margin.

I must mention here the keen interest I took, beginning in the 1950s, in the cosmological debate, to which I have devoted numerous publications. I have regularly expressed doubts about the standard model (known as the “Big Bang”) and suggested alternative—albeit partial—solutions. I continue to believe that we are far from a coherent solution to cosmological problems.