A deeper understanding of early stellar evolution must begin by examining the rarefied medium out of which stars form. In this chapter, accordingly, we describe the overall physical properties of interstellar matter. We include both the gaseous constituent and the solid grains. For the former, we emphasize the physical state of the hydrogen and defer discussion of interstellar molecules until Chapter 5. Our treatment of the dust naturally brings in elements of radiation transport theory. The concepts introduced here will be extensively used throughout the remainder of this book.
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