Published in "The Interstellar Medium in Galaxies", eds. Harley A. Thronson, Jr. and J. Michale Shull, 1990

For a postscript version of the article, click here.

Abstract. Giant H II regions can be observed out to large distances. They trace recent star formation and, through the analysis of their chemical composition, previous star formation activity. Systematic effects such as metallicity gradients across the disks of spirals and the correlation of metallicity with total galaxy mass offer important constraints on galaxy evolution. The helium abundance in regions where the gas has experienced little stellar nuclear processing provides an important test of cosmological theories. Some of the current uncertainties in interpreting nebular spectra are likely to be resolved by the next generation of ground-based, airborne, and spaceborne telescopes.

Table of Contents

• INTRODUCTION
• H II Regions in the Context of the ISM
• Environments and Systematics
• Internal Structure
• Ionizing Clusters
• METHODS OF ABUNDANCE DETERMINATION
• The "Conventional" (Direct) Method
• Nebular Models
• "Bright-Line" Methods
• ABUNDANCE GRADIENTS IN GALACTIC DISKS
• The Phenomenon
• Trends with Galaxy Type
• Elements other than Oxygen
• Comparison with the Galactic Abundance Gradient
• Comparison with Chemical Evolution Models
• ABUNDANCES IN METAL-POOR DWARF GALAXIES
• Nomenclature
• The "I Zw 18 Problem" and the Mass-Metallicity Relation
• Dwarf Galaxies as Tests of the Simple Model
• Dwarf Galaxies and the Extragalactic H II Region Sequence
• Elements other than Oxygen
• HELIUM ABUNDANCES AND THE PRIMORDIAL COMPONENT
• Cosmological Motivation
• Demands on the Observations
• Current Status and Future Prospects
• SOME OUTSTANDING ISSUES
• FUTURE PROSPECTS
• REFERENCES