Text no 1Radio Recombination Lines (RRLs), discovered in the USSR in 1964, have become a powerful research tool for astronomers. Available throughout the radio spectrum, these lines carry information regarding the density, temperature, turbulence and velocity of thermal plasmas. Their very existance shows the presence of thermal gas. They also can carry information regarding magnetic fields if Zeeman splitting were to be detected.Containing the proceedings of an IAU Colloquium celebrating the 25th anniversary of their detection, this volume tells us what has happened since. It contains the story of the detection of RRLs and reviews of many areas of physics of the interstellargas from which stars form, HII regions excited by newly formed stars, planetary nebulae involving dying stars, and the structure of our Milky Way and other galaxies reflecting the large-scale morphology of the star formation process. In addition there is an article describing modern laboratory studies of Rydberg atoms to probe the basic physics of atomic structure, and articles describing the theory of collisions and radiation upon Rydberg atoms leading to observate effects to be used as diagnostic tools in astromony.This book focuses on the 25 years of astronomical research with radio recombination lines (RRLs) since their discovery in 1965. It covers a wide range of topics: papers dealing with research into Rydberg atoms both in the laboratory and in the interstellar medium of our galaxy and others; papers on the interaction of radiation and atomic systems, as well as with the effects of inadiabatic collisions between these atoms and both ions and electrons. It deals with astronomical observations of atoms with `diameters' ranging from 0.08 to 50 mum a size factor of 625. It deals with RRLs in absorption, in emission and as true masers. And it deals with plasmas with temperatures ranging from 10 to greater than 104 kelvins, and with an even greater range of volume densities.Much new work is reported, including low frequency RRLs discovered in 1980 and the maser RRLs from the star MWC349, discovered in 1989. The advent of aperture synthesis telescopes and large single-element telescopes have made possible RRL studies with high angular resolution. The sum total of the work reported here will make the volume a platform from which to search new horizons in RRL research.

General Reviews.- Postulation, Detection, and Observations of Radio Recombination Lines.- Cavity Quantum Electrodynamics: Rydberg Atoms in Atomic Physics and Quantum Optics.- Theory of Radio Recombination Lines.- Review of the Populations of Highly-Excited States of Atoms in Low Density Plasmas.- The Broadening of Radio Recombination Lines by Ion Collisions: New Theoretical Results.- Radio Recombination Lines from HII Regions.- High Resolution Radio Recombination Line Observations.- Radio Recombination Lines from Compact HII Regions.- Radio Recombination Line Emission from Ultra-Compact HII Regions.- Radio Recombination Lines at Millimeter Wavelengths in HII Regions.- Maps of the 64? Radio Recombination Lines in Orion A.- Estimate of Electron Densities in HII Regions from Observations of Pairs of ?-Type Recombination Radio Lines.- Interferometric Observations of HII, CII, and H0 Regions in Orion B.- The Helium Abundance in the HII Region DR21.- RRLs from Planetary Nebulae and Stellar Envelopes.- Radio Recombination Lines from Stellar Envelopes: Planetary Nebulae.- Radio Recombination Lines from Compact Planetary Nebulae.- Radio Recombination Line Maser Emission in MWC349.- Recombination Emission from CII-Regions Around Be-Stars.- Low Frequency Recombination Lines.- Review of Decameter Wave Recombination Lines: Problems and Methods.- Low Frequency Radio Recombination Lines Towards Cas A.- Interferometric Observations of Carbon Recombination Lines Towards Cassiopeia A at 332 MHz.- Detection of Carbon Recombination Lines at Decameter Wavelengths in Some Galactic Objects.- RRLs from the Local Interstellar Medium.- Large-Scale Properties of the Galaxy.- Recombination Lines and Galactic Structure.- Study of the H166? Recombination Line in the Southern Milky Way.- RadioRecombination Line Imaging of Sgr A.- Limits of the Temperature and Filling Factor of the Warm Ionized Medium Towards the Galactic Centre.- Radio Recombination Lines from Extragalactic Objects.- VLA Observations of Recombination Lines from the Star Burst Galaxy NGC253.- Conclusion of the Colloquium.- Radio Recombination Lines at 25.- Indexes.- Author.- Citation.- Subject.