Sect. 13. 203 and the geometric elements for the large orbit are re-computed. From the analysis it cannot be decided whether the third body is close to the visual primary or secondary component; this can only be decided by measuring the orbital motion on an astrometric background of other stars (Chap. IV). In accurate long-term problems the perspective change in the apparent orbit due to motion of the center of mass in the line of sight, must be taken into account. In the future this feature of orbital analysis will undoubtedly become more and more important. An example of the required reduction is the orbital analysis of the 61 Cygni system by A. FLETCHER!. 1890 90° 1960 0° Fig. 11. Relative orbit of the visual binary Kruger 60. visual observations. 0 photographic observations. IV. Analysis of orbit referred to external reference system. 13. Introduction. In the preceding Chap. III we dealt with the analysis of the relative orbit of the two components of a binary star. We now extend the analysis to the measurements and orbits of the individual components referred to a system of, presumably distant, reference stars. Both micrometer measure ments and, particularly, meridian circle positions have been successfully employed in the past; the current observing technique is primarily long-focus photographic astrometry. The problems studied include those of mass-ratio and parallax, and of perturbations. Long-focus technique and analysis are in the first place suitable for the so-called resolved binaries, whose components are clearly separated.

Inhalt
Les classifications spectrales des étoiles normales.- I. La théorie du type spectral.- II. Les premières classifications d'apres I'aspect des spectres.- III. Comparaison de classifications spectrales.- IV. La variation du type spectral avec T? et P?.- V. Les classifications d'Upsal et de Stockholm.- VI. Les classifications à deux dimensions dérivées de Harvard.- VII. La classification des spectres très peu dispersés.- VIII. Classification par le spectre continu des étoiles.- IX. La classification des étoiles par photométrie.- X. Classification des étoiles à raies anormalement fortes ou faibles.- XI. Conclusion.- Bibliographie générale.- Stars with Peculiar Spectra.- General references.- Les bandes moléculaires dans les spectres stellaires.- I. Bandes moléculaires dans le spectre solaire.- II. Utilisation des bandes moléculaires pour la classification spectrale; description générale des bandes stellaires.- III. Absorption continue due aux molécules.- IV. Déterminations de températures basées sur les bandes moléculaires.- V. Interprétation du comportement des bandes moleculaires dans les étoiles froides.- Conclusions.- Bibliographie.- Die Spektren der planetarischen Nebel.- I. Allgemeine Eigenschaften der planetarischen Nebel.- II. Die Spektren der planetarischen Nebel.- Literatur.- The Spectra of the White Dwarfs.- I. Introduction.- II. Relevant observational data.- III. The spectroscopic results.- IV. Description of typical spectra.- V. Theoretical interpretations.- General bibliography.- Visual Binaries.- I. Observational.- II. Theoretical.- III. Analysis of relative orbit.- IV. Analysis of orbit referred to external reference system.- V. Masses and luminosities of visual binaries.- VI. Special topics.- General references.- TheEclipsing Binaries.- 1. Introduction.- 2. Definitions.- 3. Formulae used in computing the elements.- 4. The absolute dimensions of eclipsing variables.- 5. Eclipsing variables and astrophysical problems.- 6. Population of eclipsing variables in the galaxy.- 7. Eclipsing binaries in other galaxies.- 8. Conclusion.- Spectroscopic Binaries.- 1. Introduction.- 2. The determination of orbital elements.- 3. Apsidal motion.- 4. The determination of stellar masses.- 5. Rotation of binary stars.- 6. Peculiar line profiles.- 7. The luminosities of binary stars from spectroscopic studies.- 8. Departures from the mass-luminosity relation in close binary stars.- 9. Statistical studies of spectroscopic binaries.- 10. Gaseous motions and other effects.- 11. The nature of emission lines in close binaries.- 12. The peculiar system UX Monocerotis.- 13. Recent developments regarding the evolution and origin of binary stars.- 14. Concluding remarks.- General references.- Théorie générale des atmosphères stellaires.- Bibliographie générale.- Theorie der planetarischen Nebel.- I. Die mathematische Formulierung der wichtigsten Elementarprozesse in den Nebelhüllen.- II. Zur Theorie des Strahlungsumsatzes in den Nebelhüllen.- III. Theorie der Ionisationsschichtung.- IV. Die Zanstra-Theorie (Temperaturen der Zentralsterne).- Zusammenfassende Darstellungen.- Sachverzeichnis (Deutsch-Englisch).- Subject Index (English-German).- Table des Matières (Français).