of Volume 3.- 24 The Stars.- 24.1 The Positions of the Stars.- 24.2 Stellar Magnitudes and Colors.- 24.3 Trigonometric Determination of Distances to Nearby Stars.- 24.4 Absolute Magnitudes and Distance Moduli.- 24.5 Stellar Spectral Types.- 24.6 The Hertzsprung-Russell Diagram.- 24.7 Luminosity Classes.- 24.8 Two-Color Diagrams.- 24.9 Bolometric Magnitudes.- 24.10 Stellar Diameters.- 24.11 Stellar Temperatures.- 24.12 Stellar Masses.- 24.13 Energy Generation and Lifetime.- 24.14 The Chemical Composition of the Stars.- 24.15 The Evolution of Stars with Time.- 24.16 Subjects for Amateur Observations.- References.- 25 Variable Stars.- 25.1 Introduction.- 25.1.1 Initial Discoveries.- 25.2 Nomenclature and Classification.- 25.2.1 Nomenclature.- 25.2.2 Classification.- 25.3 Eclipsing Variables.- 25.3.1 The Structure of Close Binaries.- 25.3.2 Classification and Analysis of Light Curves.- 25.3.3 Detached Systems.- 25.3.4 Ellipsoidal Variables.- 25.3.5 RS CVn Systems.- 25.3.6 Semi-detached Systems.- 25.3.7 Contact Systems.- 25.4 Pulsating Variables.- 25.4.1 The Physics of Radial Pulsations.- 25.4.2 The Period-Luminosity Relation; Cepheids as Distance Indicators.- 25.4.3 Periods and Light Curves.- 25.4.4 Non-radially Pulsating Stars.- 25.4.5 R Coronae Borealis Stars.- 25.4.6 Pulsating Variables in the Galaxy.- 25.4.7 Semi-regular and Irregular Variables.- 25.5 Rotating Variables.- 25.5.1 Magnetic Variables (?2 Canum Venaticorum Stars).- 25.5.2 BY Draconis Stars (Spotted Stars).- 25.5.3 Radio Pulsars.- 25.6 Eruptive Variables.- 25.6.1 Supernovae.- 25.6.2 Cataclysmic Variables.- 25.6.3 X-ray Binaries.- 25.6.4 Symbiotic Stars.- 25.6.5 Flare Stars.- 25.7 Young Irregular Variables.- 25.7.1 Orion Variables and RW Aurigae Stars.- 25.7.2 T Tauri Stars.- 25.7.3 Variable Be Stars.- References.- 26 Binary Stars.- 26.1 General Overview.- 26.2 Features of Visual Double Stars.- 26.3 Micrometer and Visual Observations.- 26.4 Speckle and Photographic Observations.- 26.5 Orbital Elements and Ephemerides.- 26.6 Photometric Binaries.- References.- 27 The Milky Way Galaxy and the Objects Composing It.- 27.1 The Visual Appearance of the Milky Way.- 27.2 Interstellar Dust and Its Effects Upon Distance Determinations Within the Galaxy.- 27.3 The Galactic Objects: A Survey.- 27.4 Catalogues and Atlases of Galactic Nebulae.- 27.5 Open Clusters.- 27.5.1 Classification of Open Clusters.- 27.5.2 Color-Magnitude Diagrams and Two-Color Diagrams.- 27.5.3 Membership of Stars in Open Clusters.- 27.6 The Globular Clusters.- 27.6.1 Color-Magnitude Diagrams of Globular Clusters.- 27.6.2 Determination of Distances to Globular Clusters.- 27.7 H II Regions.- 27.7.1 The Exciting Stars in HII Regions.- 27.7.2 The Spectra of HII Regions.- 27.7.3 The Radio Continuum Radiation from HII Regions.- 27.7.4 The Radio Recombination Lines of HII Regions.- 27.7.5 The Infrared Radiation from HII Regions.- 27.7.6 Typical Structures in HII Regions.- 27.7.7 Physical Parameters of HII Regions.- 27.7.8 The Best-Known HII Regions.- 27.8 Reflection Nebulae.- 27.9 Galactic Cirrus or Hagen Clouds.- 27.10 Supernova Remnants.- 27.11 Planetary Nebulae.- 27.12 Molecular and Dark Clouds.- 27.12.1 The Spatial Distribution of Interstellar Extinction.- 27.12.2 The Distances and Total Extinction of Dark Clouds.- 27.12.3 Molecules in Dark Clouds.- 27.12.4 Molecular Clouds.- 27.12.5 Molecular Clouds and Star Formation.- 27.13 Herbig-Haro Objects, Bipolar and Cometary Nebulae.- 27.14 The Structure of the Milky Way System.- 27.14.1 The Galactic Center.- 27.14.2 Stellar Populations.- 27.14.3 Differential Galactic Rotation—Kinematic Distances.- 27.14.4 The Spiral Arms of the Milky Way System.- 27.14.5 The Surface Brightness of the Milky Way.- 27.14.6 The Immediate Solar Neighborhood.- 27.15 Hints for the Observer.- 27.15.1 Requirements of the Site and Sky.- 27.15.2 Visual Observations.- 27.15.3 Photographic Observations.- References.- 28 Extragalactic Objects.- 28.1 Introduction.- 28.2 Catalogues and Photomaterials.- 28.3 Classification of Galaxies.- 28.3.1 Interacting and Peculiar Galaxies.- 28.4 The Structure of Galaxies.- 28.4.1 General Properties of Galaxies.- 28.4.2 Formation of Structure in Galaxies.- 28.5 Determination of Distances.- 28.6 Active Galaxies and Quasars.- 28.7 The Universe.- 28.8 Amateur Techniques and Projects.- References.- Appendix B: Astronomical Data.- B.0 Abbreviations and symbols used in astronomy, mathematics, and physics.- B.4 Normal refraction (for standard atmosphere at 1013 mb and 0°C).- B.5 Semi-diurnal arc (including normal refraction).- B.6 Astronomical constants.- B.7 Beginning and end times of twilight at 20-day intervals for different latitudes.- B.8 Julian day numbers.- B.9 Conversion of days into decimal fractions of the Julian Year.- B.11 Solar data.- B.12 Position angle P of the solar axis and heliographic latitude Bo of the apparent center of disk at 5-day intervals.- B.13 Coordinates of lunar formations in the Mucke-Rükl system.- B.14 Technique and exposure data for lunar and planetary photography.- B.15 Planetary data.- B.16 Temperature and atmospheric data for the major planets and the satellite Titan.- B.17 Planetary satellite data.- B.18 The IAU Mars map 1958.- B.19 Data on the bright minor planets.- B.20 Table of change of the central meridians of Mars and Jupiter.- B.21 Atmospheric currents on Jupiter.- B.22 Periodic comets with periods under 200 years, observed in more than one perihelion passage.- B.23 Brightness of the zodiacal light.- B.24 The 88 constellations.- B.25 The 170 brightestxStars (to visual magnitude +3.0).- B.26 The nearest stars (within 6 pc).- B.27 Absolute magnitudes of stars in the MK system.- B.29 Abbreviations of some chemical elements.- B.30 Wavelengths of selected spectral lines in visible light.- B.31 Radial velocities for selected stars.- B.32 Double stars.- B.33 The Messier list of nebulae and clusters (1784).- B.34 Open clusters.- B.35 Globular clusters.- B.36 Planetary nebulae.- B.37 Diffuse galactic nebulae.- B.38 The strongest emission lines in HII regions, with intensities given for the Orion Nebula.- B.39 Selected abbreviations of organisations and periodicals.- B.40 Abbreviations of reference catalogues.- Supplemental Reading List for Vol. 3.