Traditionally the absorption line spectrum of a star has been explained as originating in a (cooler) layer, which surrounds the (hotter) stellar photosphere. The observation of many (unexpected) emission lines from various kinds of objects contradicts such a simple picture. Through the improved observational facilities that have become available in recent decades we now have a much clearer picture of the outer layers of the stars. Since these layers were first observed in the sun, the best introduction is to start with a description of what happens in the sun. This book is certainly not the place for a detailed discussion of the chromosphere and the corona. We shall provide only some information concerning matters that are of interest for the study of stellar spectra. The reader can find more detailed treatment of the solar chromosphere (and also the corona) in many books, like for instance Astrophysics of the Sun by Zirin (1988) and Spectroscopy of Astrophysical Plasmas edited by Dalgarno and Layzer (1987).

The solar chromosphere

The solar chromosphere is the layer enclosing the sun's photosphere. Temperatures diminish in the outer layers of the sun and reach a minimum at what is conventionally called the photosphere. Beyond this layer, temperatures rise again with distance above the sun – first slowly and later very rapidly, until a standstill is reached at about 300000 kelvin. The first part of the atmosphere, where the temperature rise is slow, is called the ‘chromosphere’. The part where temperature rises rapidly is called the ‘transition region’. Grosso modo the chromosphere extends up to 10000 kelvin and the transition region up to 300000 kelvin.