i8o 



POPULAR SCIENCE MONTHLY 



Recent advances in methods of research have also made possible 

 the study of various other phenomena, in addition to the variability in 

 brightness. All available information will be needed to assist us in 

 finding the true explanation of the changes. Especially must we study 

 the spectra of these stars, and the changes in the spectra at different 

 phases of the light-curve, as well as the motions of the stars in the line 

 of sight. For a long time it has been known that the radial motion of 

 any bright body may be studied from the shifting positions of the 

 spectral lines. This principle is proving of great importance in dif- 

 ferent branches of astronomy. Only recently, however, and in few 

 cases has this crucial method been applied to the problem of variable 

 stars ; yet it appears that the true solution of the difficulties must await, 

 in many cases, the application of this method of research. 



The determination of these different phenomena — the light-curve, 

 the velocity-curve and the spectrum — is often carried on without 

 special reference to the physical causes which produce them. But it 





LlGHT-CDRVES OF VARIABLE STARS. 



will be convenient in what follows to refer to the phenomena and the 

 probable causes together. No final classification of variable stars is 

 possible at the present time, since such a classification would doubtless 

 be based on the physical causes which underly the phenomena, and 

 these are known in comparatively few cases. The division proposed 

 by Professor Pickering, in 1881, is as convenient as any for our pur- 

 poses. He placed them all in the following classes : 



I. New stars. 



II. Long-period variables, undergoing great variations in light. 



III. Stars undergoing slight changes, according to laws as yet 

 little understood. 



IV. Short-period variables of the (3 Lyra? type. 



V. Algol stars. 



