Close Binary Systems. iii 



Although, however, all variable stars of the Algol type, that is, 

 close binar)- stars revolving in an orbit coincident, or nearly so, with 

 the plane of sight, will conform to a general type of variation, there 

 will be individual differences in the character and magnitude of the 

 light changes of each star, marking it out unmistakably from its 

 fellows. The two stars forming a system may be equal in bright- 

 ness, or thev may be \er\ unequal : they may be equal in size, or 

 the disparity may be very great: they may move round one another 

 in contact, or two or three diameters apart : they may circle exactly 

 in a plane coincident with the plane of sight or at an appreciable 

 inclination to it : their orbit may be circular or elliptical. Thus, 

 while the general type of variation is due to the common fact of 

 revolution, the individual distinctive features of each system are due 

 to the relative size, brightness and movements of the component 

 stars. 



Each Algol variable, therefore, will have a separate light curve of 

 its own. possessing certainly the general family features of all Algol 

 stars, but, also, at the same time possessing individual characteristics 

 which mark it out distinctly from all other variable stars of the same 

 class. 



Perhaps this common family likeness, and yet individual dis- 

 tinctive difference, will be best illustrated by exhibiting the light 

 curves of five tvpical southern Algol variable stars. These five 

 typical stars are : — 



Star, 



K.A. 1900. 



1. S Velorum 



2. K Arae 



3. CPD— 414511 



4. X Carinae 



5. KK Cctitauri .. 



H. M. S. 



9—29—27 



16—31—26 



10 — 17—48 

 S— 29— 7 



14- 9^55 



An exhaustive examination of even the minutest details of these 

 five typical light curves. Fig. r. would prove of no ordinary interest. 

 It is beyond the province, however, of this paper to enter upon 

 such an investigation. I would confine myself rather to a consider- 

 ation of the outstanding features of each type, and will endeavour 

 to exhibit the principal conditions of matter and motion which lie 

 behind these features. 



It will be observed that in the first two light curves that of the 

 binary systems .S Velonmi and R Arae. there is only one depression. 

 That is, in the case of S Velorum, R Arae, and all stars of this tvpe 

 of variation, there is only one eclipse every revolution. These 

 binary systems are composed of two stars, one bright, the other quite 

 dark. When the dark star gets in front of the bright one we have 

 the observed eclipse : when the bright star gets in front of the dark 

 one there is of course no change in the brightness of the svstem. 



