Basic Techniques of Making Measurements on the Continuum and Determination of the Relative Intensities of Stellar Spectral Lines

Abstract

The accurate measurement of intensities in stellar spectra, both relative and absolute, must be made in all observable spectral regions in order to obtain the basic data for the computation of model stellar atmospheres. The effects of interstellar and atmospheric extinction, absorption, and scattering in the instrument must be especially considered in the work of absolute spectrophotometry. Differential atmospheric extinction and instrumental absorption may be neglected if the various points on a line are measured relative to the continuum. The determination of the continuum, however, involves many problems. Sensitive photographic and photoelectric techniques record a stellar spectrum in order to allow the determination of the distribution of energy with wavelength. The low intensity level of most stellar sources has encouraged the development of photoelectric spectrophotometry, which in certain applications obtains better results than photographic methods. However, the present slow rate of scan, wavelength by wavelength, even with several photocells simultaneously in use, indicates that photographic procedures will undoubtedly be the most used for several years to come. The methods of calibration of the spectrograms, the use of the “densitometer” (generally called “microphotometer” by astronomers) and the correction of its reproduction of intensity data are the basic techniques applied to gain the fundamental data for different examples of stars and stellar systems. The determination of line profiles, both theoretically and by measurement, their importance to the theory of model atmospheres, the importance of high-dispersion studies of selected line profiles, and related procedures are further problems. The intensity distribution of the continuous spectrum for stars of different spectral types, the classification of stars by spectra, the theoretical interpretation of the measurements, the problems resulting from uncertainties in the determination of the continuum and stellar equivalent widths to approximately 10%—a permissible accuracy—completes the acquisition of data for the description of the atmosphere’s physical structure.