Physics of the Galaxy and Interstellar Matter

The present book is a translation of the original German edition (published in 1982) with some minor corrections and improvements. The guide to sup­ plementary and advanced literature given in the Appendix, however, has been brought up to date. This book is addressed primarily to students taking astronomy as a prin­ cipal or subsidiary subject, and to scientists of related fields, but amateur as­ tronomers should also be able to profit from it. For most chapters an elementary knowledge of mathematics and physics will be sufficient, however, Chaps. 5 and 6 impose somewhat greater requirements. In addition the reader should already be acquainted with the basic concepts of stellar physics as treated in introduc­ tory books, including the spectral types, the system of stellar magnitudes and colours, absolute magnitudes and luminosities, the Herlzsprung-Russell dia­ gram and its interpretation. A modem textbook should use SI units. On the other hand, the use of the cgs system is still the prevailing custom in astrophysics - together with the special units of astronomy: length is quoted in parsecs [pc], mass in solar masses [M0] and time in years [a]. We have therefore compromised and employed both cgs and SI units in this book, whichever was the appropriate choice in each instance. A table for conversion of cgs units into SI units and vice versa is given in the Appendix.

1. Introductory Survey.- 1.1 The Stellar System.- 1.2 Interstellar Matter.- 1.3 Dynamics and Evolution.- 2. Positions, Motions and Distances of the Stars — Concepts and Methods.- 2.1 Positions and Motions.- 2.1.1 Astronomical Coordinate Systems.- 2.1.2 Temporal Changes of the Star Coordinates.- 2.1.3 Space Velocities and Solar Motion.- 2.1.4 The Fundamental Astronomical Coordinate System.- 2.1.5 Extra-Galactically Based Reference System, Radioastrometry.- 2.2 Distances.- 2.2.1 Primary Methods.- 2.2.2 Secondary Methods.- 3. Structure and Kinematics of the Stellar System.- 3.1 Apparent Distribution of the Stars.- 3.1.1 General Star Counts, Integrated Starlight.- 3.1.2 Apparent Distribution of the Individual Star Types.- 3.1.3 Distribution of the Surface Brightness of the Milky Way in the Visible and in the Infrared.- 3.2 The Local Galactic Star Field.- 3.2.1 Methods of Stellar Statistics.- 3.2.2 Luminosity Function and Spatial Distribution of the Stars.- 3.2.3 Motions of the Stars in the Solar Vicinity.- 3.3 Large Scale Distribution of the Stars.- 3.3.1 OB Stars, Young Open Star Clusters and OB Associations.- 3.3.2 Globular Clusters, RR Lyrae Stars and Cepheids.- 3.3.3 Stars in the Galactic Halo.- 3.3.4 Surface Brightness of the Milky Way and Galactic Structure, Stellar Emission of the Central Region.- 3.4 Large Scale Motion of the Stars.- 3.4.1 Shear, Rotation and Dilatation of the Velocity Field.- 3.4.2 Rotation of the Galactic Disc.- 3.4.3 Objects of the Galactic Halo.- 3.5 General Summary, Stellar Populations.- 3.5.1 Sub-Systems.- 3.5.2 Stellar Populations and Evolution of the Galaxy.- 4. Interstellar Phenomena.- 4.1 The Generally Distributed Medium.- 4.1.1 Interstellar Extinction.- 4.1.2 Interstellar Polarisation.- 4.1.3 Reflection Nebulae and Diffuse Galactic Light.- 4.1.4 Interstellar Absorption Lines in Stellar Spectra.- 4.1.5 The 21 cm Radio Line of the Interstellar Hydrogen.- 4.1.6 Line Emission of Interstellar CO Molecules.- 4.1.7 Continuous Radio Emission.- 4.1.8 Faraday Rotation in the Interstellar Magnetic Field and Dispersion of the Radiation of Discrete Radio Sources.- 4.1.9 Interstellar Radiation Field, X- and Gamma Rays, Cosmic Particle Radiation.- 4.2 Interstellar Clouds.- 4.2.1 Diffuse Emission Nebulae: Optical Phenomena.- 4.2.2 Continuous Radio Emission from H II Regions.- 4.2.3 Radio Recombination Lines of H II Regions.- 4.2.4 Infrared Emission from H II Regions.- 4.2.5 Dark Clouds: Optical Data.- 4.2.6 Dark Clouds of High Extinction and IR Sources.- 4.2.7 Molecular Clouds.- 4.2.8 Maser Point Sources.- 4.2.9 Interstellar Clouds and Star Formation.- 5. Physics of the Interstellar Matter.- 5.1 Radiation in the Interstellar Gas.- 5.1.1 Radiation Transport.- 5.1.2 Line Emission and Absorption Under Interstellar Conditions.- 5.1.3 Continuous Emission and Absorption.- 5.2 State of the Interstellar Gas.- 5.2.1 Ionisation.- 5.2.2 Formation and Dissociation of Interstellar Molecules.- 5.2.3 Heat Balance and Kinetic Temperature of the Gas.- 5.2.4 State of Molecular Clouds and H II Regions.- 5.2.5 Elemental Abundances in the Interstellar Gas.- 5.3 The Interstellar Dust Grains.- 5.3.1 Optics of Small Solid Particles.- 5.3.2 Nature of the Interstellar Dust Grains.- 5.3.3 Interactions with the Radiation Field, the Gas and the Magnetic Field.- 5.4 Distribution and Motion of the Interstellar Matter.- 5.4.1 Local Distribution and Cloud Structure of the Dust.- 5.4.2 Density and Cloud Structure of the Neutral Hydrogen in the Solar Neighbourhood.- 5.4.3 The Neutral Hydrogen on the Large Scale.- 5.4.4 Large Scale Distribution of H II Regions and Molecular Clouds.- 5.4.5 Gaseous Galactic Halo.- 6. Dynamics of the Galaxy.- 6.1 Stellar Dynamics.- 6.1.1 Posing of the Problem, Fundamental Equations.- 6.1.2 Close Stellar Encounters, Relaxation.- 6.1.3 Stellar Dynamics Taking Account of Encounters.- 6.1.4 Collisionless Dynamics of the Galactic Stellar System.- 6.1.5 The Mass Distribution in the Galaxy.- 6.2 Gravitational Theory of the Spiral Structure.- 6.2.1 Statement of the Problem and General Review.- 6.2.2 Spiral-Form Density Waves.- 6.3 Dynamics of the Interstellar Gas.- 6.3.1 Posing of the Problem and Fundamentals.- 6.3.2 Dynamical Equilibrium of the Gas.- 6.3.3 Shock Fronts in the Interstellar Gas.- 6.3.4 Condensation of the Gas and Galactic Evolution.- A. Fundamental Formulae for the Transformation of Astronomical Coordinates.- B. Energy States and Transitions of Molecules.- C. Charts for the Graphical Determination of Approximate Galactic Coordinates.- D. Excitation and Ionisation in the Thermo dynamical Equilibrium.- Astronomical and Physical Constants.- Latin Names of Constellations.- References.- Supplementary and Advanced Literature.- Sources of Tables.- Sources of Figures.