The Interpretation of Normal, Main-Sequence, Early Type Spectra

Abstract

The problem facing astrophysicists is to build up an exact physical picture of the conditions in a stellar atmosphere by analysis of the part of the radiation field from the star which is accessible to observation. In practice the most readily obtained suitable observational material has been high-dispersion spectrograms covering the spectral range 3000 A to 6800 A. In fact most studies have been done using a restricted spectral range from about 3500 A to 5000 A. The observations yield relative intensities within the spectrum over short wave-length regions, that is line profiles, and the relative strengths of the lines, that is equivalent widths. Each observational fact is expressed relative to the intensity of an adopted datum line which is called the continuum of the star. Little or no use is made in the analysis of the relative intensity of the continuum at different wave lengths because such information, on an absolute scale, cannot be easily obtained from normal stellar spectrograms. The profiles and equivalent widths of the spectral lines are obtained by the methods described in Chapter X. Most methods of analysis are concerned with equivalent widths only, since reliable line profiles can only be obtained for a few strong lines.