PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
SOME SPECTRAL CHARACTERISTICS OF CEPHEID VARIABLES 
By W. S. Adams and A. H. Joy 
Mt. Wilson Solar Observatory, Carnegie Institution of Washington 
Communicated February 25, 1918 
In a recent investigation of the absolute magnitudes of 500 stars 1 we have 
shown that in a large number of cases the intensity of the hydrogen lines is 
abnormally great in relation to the spectral type as derived from the more 
general characteristics of the spectrum. The effect is most striking in the 
case of giant M-type stars such as a Orionis, but it is marked for many K and 
G-type spectra as well. The suggestion was made that abnormal intensity 
of the hydrogen lines is a general characteristic of the giant stars of at least 
some of the spectral types. A discussion of this question with reference to 
certain of the variable stars of the 5 Cephei type is the object of this com- 
munication. 
It has been shown by the investigations of Hertzsprung 2 and others that the 
Cepheid variables are stars of very high intrinsic luminosity, with an average 
absolute magnitude, as derived from parallactic motion, more than seven 
magnitudes brighter than the sun. Directly measured parallaxes of five 
stars yield a value of about five magnitudes. Whichever result is accepted 
it is evident that these stars are exceedingly luminous and form most interest- 
ing material for a study of the question of the intensity of the hydrogen lines. 
The spectrum of the Cepheid variables has been studied by many observers, 
and it is quite impossible to make adequate reference to their results in this 
place. In general, however, most of the work of Albrecht 3 , Duncan 4 , Shapley 5 , 
and many others has dealt with variations in the character of the spectrum 
at maximum and minimum of light. For the purpose we have in mind a 
direct comparison is instituted between the spectra of the several variables 
and that of the sun, which is selected as a typical star of type Go. Photographs 
of the spectra, all of which were made with a slit spectrograph, taken at 
maximum and minimum of light are also compared directly with one another. 
Furthermore typical stars having spectra of types F 0 , F 5 , and G5 have been 
Volume 4 
MAY 15, 1918 
Number 5 
