The development of astronomical knowledge has now reached a stage where a precise description of the details of star formation has become a major issue. The enormous degree of contraction that occurs in stellar condensation, a concentration sufficient to increase the density of material from, say, 10-22gr./cm.3 up to ultimate values of the order of I gm./cm.3, gives a sure indication of the important part played by gravitational forces. Apart from gravitation, gas pressure must be accorded a significant place in the early stages of the process. It has been suggested that radiation pressure should also be included among the forces that determine the onset of condensation.

La matière gazeuse diffuse se manifeste dans l'Univers sous les aspects les plus divers et ses rapports avec les autres formes de la matière, avec les étoiles et la poussière interstellaire, sont fort complexes. Bien peu de données sont connues dans ce domaine pour qu'on puisse se faire une conception du rôle de la matière diffuse dans la cosmogonie. Nous nous permettrons de nous limiter à un exposé de quelques faits ayant probablement rapport à la cosmogonie, faits que nous avons recueillis au cours des 2-3 dernières années à l'observatoire de Siméiz.

Les propriétés les plus importantes des étoiles sont définies par ses sources d'énergie. Les réactions nucléaires représentent semble-t-il à l'heure actuelle la seule source, capable de produire une quantite suffisante d'energie pendant toute l'existence de l'étoile. Ainsi, par exemple, notre Soleil, perdant en moyenne deux ergs par seconde et par gramme de sa masse, devrait perdre, rien que dans la période d'existence de notre Terre, sa radiation restant à peu pres au niveau actuel, 2.108 milliards d'ergs par gramme au minimum. Actuellement, on suppose que pour les étoiles de la partie superieure de la Serie principale la production d'énergie s'etablit par le cycle du carbone de Bethe-Weizsäcker.

I wish to say something today regarding the theoretical basis of the fission theory of binary stars. For many years it has been thought to be one of the established conclusions of astronomy that at least the close binaries have resulted from the fissional break-up of single massive stars, and theories of stellar evolution have regarded this as one of the settled issues that must be incorporated in a general theory accounting for all types of stars.

In discussing the basis of the fission theory it will not be my concern so much to consider actual stars, but only the theoretical stars as postulated in the dynamical formulation of the fission theory.

On the one hand the rate at which stellar evolution progresses will depend on the chemical composition of the star. On the other, as a by-product of the nuclear processes involved in the energy production, the chemical composition of the reacting material will change. Stellar models can be used to provide information as to the composition of the reacting zones, stellar spectroscopy will provide the composition of the atmospheres, and of those zones of the stellar envelope which have been well mixed with the atmosphere by convection and circulation currents.

Strömgren gave a brief analysis of three points dealt with in his article on ‘ Evolution of Stars’ (A.J 57, 65, 1952):

1. For main-sequence stars the extent of the convective regions changes with the mass. The more massive stars (A stars brighter than 2m-3m) have a convective core roughly corresponding to the Cowling model. The outer convection zone, first considered in connexion with problems of the solar interior by Biermann in 1938, is quite narrow. Passing from bright to faint main-sequence stars one finds that the extent of the convective core decreases, vanishing for faint stars. This is due to the fact that the energy-production mechanism changes from the carbon cycle to the proton-proton process while the relation between opacity and distance from the centre does not change very appreciably. At the same time the outer hydrogen convection zone increases in thickness and reaches appreciable depths.

L'abondance des naines blanches dans notre voisinage est très considérable. C'est ainsi que Parenago(1), procédant en 1946 à une révision des analyses antérieures du nombre de naines blanches par parsec cube en a trouvé 0·15, c'est-à-dire un nombre du même ordre de grandeur que le nombre communément admis d'étoiles par parsec cube dans notre voisinage. Le nombre obtenu par Parenago est en bon accord avec les dénombrements d'étoiles de Krieger dans le nuage de Scutum. Ce nombre est beaucoup plus éléve que les nombres antérieurement admis, compris entre 1 et 10 centièmes du nombre total d'étoiles par parsec cube dans notre voisinage.

An important step forward in astronomical optics, and one of an entirely novel kind, was made by Bernhard Schmidt in 1930 when he introduced the new type of optical system which bears his name. As you will remember, the Schmidt camera consists of a spherical mirror together with a nearly plane-parallel figured plate situated at the centre of curvature of the mirror, as shown in Fig. 1.

The Super-Schmidt Meteor Camera was designed by James G. Baker and manufactured by the Perkin-Elmer Corporation for the Bureau of Ordnance of the United States Navy, to be used by the Harvard Observatory. The camera, of fine optical components including a spherical mirror, has an aperture of 12·3 inches, a focal length of 8-0 inches and covers a field of 55° with a spherical focal surface of 7-3 inches diameter. The optical focal ratio is nominally 7/0-65 and effectively f/0·85.

The first complete telescope was installed in New Mexico during the summer of 1951. Since March 1952 two of them have been operating at two stations for the simultaneous photograph v of meteors.

Les phénomènes dont je vais parler sont de deux ordres différents. J'examinerai en premier lieu les effets qui résultent des différences de température qui existent dans l'air, à l'interieur d'un instrument et dans son voisinage immédiat, sur le trajet du faisceau lumineux: c'est là ce qu'on pourrait appeler la micrométéorologie de l'observation télescopique. Comme resultat de cette etude je decrirai brièvement les dispositions qui ont ete introduites dans l'installation du telescope de 193 cm. actuellement en cours de réalisation a l'Observatoire de Haute-Provence, comme une tentative pour ameliorer ces conditions micrométéorologiq ues.

L'interaction d'un photon avec la matière peut donner naissance a la liberation d'un electron, c'est l'effet photoelectrique exterieur; cest un phenomene physique des plus remarquables ou on assiste a la quantification de la lumiere. Ce qui nous interesse surtout en astronomie c est qu'il semble bien etabli que la probabilite d'emission d'un electron par un photon ne depend pas du nombre des photons recus par unite de temps; autrement dit, disposant d'un rayonnement lumineux aussi faible que Ton voudra, on pourra toujours en augmentant le temps d'exposition ou temps de pose, recueillir un nombre suffisant, arbitraire de photo-électrons.

One of the first problems investigated at the start of the 200-inch telescope project was that of a suitable site. Extensive tests of climatic and ‘seeing’ conditions at some dozens of sites in the semi-arid south-western part of the United States led to the selection of Palomar Mountain. This is a flat-topped mountain some 10 x 20 km. in extend located 150 km. south-east of Los Angeles. The altitude of the telescope is 1700 m. Temperature conditions are very stable with a diurnal range of about 6° C. and observing conditions during the past two years have proved to be at least as satisfactory as those at Mount Wilson.

L'astrométrie est la branche la plus ancienne de l'astronomie. Le catalogue stellaire de Hipparque a été élaboré il y a 2,000 ans; le catalogue d'Oulougbek, il y a 500 ans. L'astrométrie exacte compte deux siècles; il est convenu de rapporter sa naissance à l'époque des observations remarquablement précises de James Bradley à l'Observatoire de Greenwich. L'étape la plus importante du développement de l'astrométrie fut la création de l'Observatoire de Poulkovo, que B. Gould et S. Newcomb appelaient ‘la capitale astronomique du monde Après la publication des premiers catalogues absolus de Poulkovo, qui se distinguaient par la rigueur de leur construction et par leur haute précision, devinrent possibles des travaux d'astrométrie fondamentale, dont le développement est lié aux noms de S. Newcomb, A. Auwers et L. Boss. Leurs longues recherches continuées par leurs élèves et adeptes ont abouti de nos jours à l'achèvement de deux ceuvres grandioses: l'élaboration du General Catalogue de Boss (GC) et du Dritter Fundamentalkatalog des Berliner Jahrbuchs (FK 3). Ce dernier catalogue, compose sous la direction de A. Kopff, a ete adopte depuis 1940 en qualité de catalogue fondamental international.

Die Domäne der schwachen Sterne ist die Photographic Trotzdem ist es auch in Zukunft notwendig, direkte Beobachtungen von Sternen bis etwa 9·5 Gr visuell (besonders am Meridiankreis) zu beobachten. Hierzu bedarf es eines FK, der auch Sterne bis zu dieser Helligkeit entha.lt. Ein (FK)S ist in die Gesamtheit eines FK einzubauen.

Unter einem Fundamentalsystem oder Fundamentalkatalog im strengen Sinn sei die Gesamtheit von Ortern, Eigenbewegungen und der Prazessionskonstanten zu verstehen. Das System beruht auf absoluten (oder fundamentalen) Positionen beider Koordinaten; die Eigenbewegungen sind durch strenge Ausgleichung der Positionen über den ganzen Zeitraum der Beobachtungen hinweg zu erhalten. Es genügt nicht, etwa aus den Mittelwerten von zwei Gruppen von Positionen für verschiedene Zeiten die Eigen-bewegung zu ermitteln (vergl. Abschnitt 11).

In his introduction to the General Catalogue Boss reports on the outlook for positional astronomy at the end of the nineteenth century with these words:

Towards the end of the nineteenth century it became increasingly and discouragingly apparent that the outlook for positional astronomy was sorry indeed unless new developents were forthcoming. An immense amount of raw material had been produced, with no apparent market.

As you all know, the considerations which are expressed in these words led Boss, father and son, to the derivation of the famous Preliminary General Catalogue and later on the General Catalogue.

I.General remarks. In the report of Prof. Zverev the most important problems of the catalogue of faint stars have been exposed. I restrict my remarks to the organizational topics of meridian observations in connexion with the catalogue and to instrumental difficulties which arise in such a work.

There are three essential problems which are to be solved by meridian observations:

• (1) The absolute determination of co-ordinates of 931 fundamental stars;

• (2) The differential observations of the general catalogue of about 20,000 stars;

• (3) The comparison of the new fundamental system with the FK3 system.

In 1950 the Yale Observatory published in Vol. 22 of its Transactions the concluding volume of the photographic repetition of the AG catalogues between declinations +30°and — 30° During the past year the supplementary Vol. 23 has also appeared. An important part of the contents of this supplementary volume is the comparison with the General Catalogue, both as to position and proper motion, of all the zones within the limits +30° and — 30° This is therefore an appropriate time for an evaluation of the programme to date and an indication of its future possibilities. Before taking up this subject I should like to record that the project was initiated by the late Prof. Frank Schlesinger who was the Director of the Yale Observatory from 1920 until 1941. Much of the credit, for the work belongs to Dr Ida Barney, who has made one-half of all the measurements and supervised all of the calculations and the preparation of the printer's copy for the various volumes. She has been in full charge of the work since Prof. Schlesineer's retirement.

A programme of photographic astrometric work on faint stars was initiated at the Cape Observatory by Sir Harold Spencer Jones in 1930. Its aim was to do for the sky south of — 30° what the Yale Observatory in America and various observatories in Germany and Russia were doing for the sky north of — 30° Though somewhat interrupted by the war, the work has progressed steadily and it is anticipated that the whole project will be finished by 1960.

These may be classed under four headings:

• (a) Systematic motions at different points in space.

• (b) Distribution of ‘random’ motions.

• (c) Check on distances of faint stars as determined by spectrographic methods.

• (d) Distances and motions of special types of stars.

Investigations of proper motions of faint stars generally are concerned with either of two problems: the distribution of the peculiar motions of the stars, or the mean parallax of certain groups of stars. In both kinds of investigations the knowledge of absolute proper motions is of fundamental importance. The present paper deals with some of the actual problems.