Double and multiple stars are the rule in the stellar population, and single stars the minority, as the abundance of binary systems in the space surrounding the sun shows beyond doubt. Numerous stellar features, and methods of their exploration, ensue specifically from the one but widespread property, the binary nature. Stellar masses are basic quantities for the theory of stellar structure and evolution, and they are ob- tained from binary-star orbits where they depend on the cube of observed parameters; this fact illustrates the significance of orbits as well as the accuracy requirements. Useful in dating stellar history is the knowledge that components of a system, different though they may appear, are of the same origin and age. Between star formation and the genesis of binaries a direct connection can be traced. The later stages of stellar life branch into a great variety as mutual influence between the components of a close binary pair develops. Transfer and exchange of mass and the presence of angular momentum in the orbit give rise to special tracks of evolution, not found for single stars, and to peculiar spectral groups.
This is not a new story but it has a new ending: The patterns of evolution involving mass transfer appear to lead ultimately to single objects.
1. Classes of Double Stars.- 2. When Double-Star Research Began.- 3. 19th-Century Achievements.- 4. Three Quarters of This Century.- 5. Catalogues and Data References.- 6. Selective Discovery and Abundance of Binaries.- 7. Optical and Temporary Double Stars.- 8. Relative Positions in Visual Double Stars.- 9. Micrometric Observation.- 10. Interferometric Observation.- 11. Photographic Positions.- 12. Magnitudes of Double-Star Components.- 13. The Orbital Elements of a Visual Binary.- 14. Ephemeris Formulae.- 15. Determination of Visual Binary Orbits.- 16. The Thiele-van den Bos Method.- 17. The Methods by Danjon and Rabe.- 18. The Edge-on Orbit.- 19. Correcting an Orbit.- 20. Formulae for Differential Corrections.- 21. The Parabolic Orbit.- 22. Absolute Orbits and Mass Ratios.- 23. Some Results from Visual Binary Orbits.- 24. Stellar Masses.- 25. The Mass-Luminosity Relationship (MLR).- 26. Dynamical Parallaxes.- 27. Multiple Systems.- 28. Unresolved Systems.- 29. Some Systems of Special Interest.- 30. Radial Velocities.- 31. Spectroscopic Double Stars.- 32. The Elements of Spectroscopic Orbits.- 33. The Differential and the Sterne Formulae.- 34. The Combined Spectroscopic-Visual Orbit.- 35. Distortions of Radial-Velocity Curves.- 36. Photometric Double Stars and Light Curves.- 37. Classes of Eclipsing Binaries and the Roche Lobe.- 38. Limb Darkening.- 39. Concepts of Photometric Orbit Determination.- 40. The Spherical Model.- 41. Rectification and Related Precepts.- 42. Eccentric Orbits.- 43. Mass Determinations from the MLR.- 44. On the Origin of Binary Stars.- 45. Mass Transfer After the Main-Sequence Phase.- 46. Binaries of the Type W UMa.- 47. Gas Streams and Hot Spots.- 48. Apsidal Rotation.- 49. Systemic Mass Loss.- 50. Atmospheric Eclipses.- 51. The Largest Stellar Masses.- 52. Binary Components of Wolf-Rayet Type.- 53. More Systems of Special Interest.- 54. Peculiar Prototypes: ? Per and ? Lyr.- 55. Cataclysmic Binary Components.- 56. X-Ray Binaries.- References.- Index of Stars.