1888.] 



of the various Species of Heavenly Bodies. 



91 



components of a physical double are identical, both had their origin 

 in the same nebulosity. 



Tf the magnitudes are nearly equal, but the colours and spectra 

 different, it may be that the one with the most advanced spectrum has 

 the smaller mass, and if the advance is in duo proportion, we are 

 justified in regarding them as having had a common origin. 



If the magnitudes are very unequal, we may take the one with the 

 smaller magnitude as having the smaller mass, and if it is propor- 

 tionately in advance, as indicated by its spectrum or colour, we may 

 regard both components as having had a common origin. If the 

 smaller one be less advanced than the larger one, as most generally 

 happens, we have to regard it as a late addition to the system. 



If the two stars are of equal mass and revolve round their common 

 centre of gravity they have in all probability done so from the nebu- 

 lous stage, and therefore they will have arrived at the same stage 

 along the evolution road, and their colours and spectra will be 

 identical. 



If, however, the masses are very different, then the smaller mass 

 will run through its changes at a much greater rate than the larger 

 one. In this way it is possible that the stars seen so frequently 

 associated with globular nebulas may be explained ; while the nebula 

 with a larger mass remains still in the nebulous condition, the smaller 

 one may bs advanced to any point, and may indeed even be totally 

 invisible, while the parent nebula is still a nebula. This condition 

 may be stated most generally by pointing to those double stars in 

 which the companions are small and red, although we know nothing 

 for certain with regard to their masses. But if we pass to the other 

 category in which the companion is added afterwards, the most 

 extreme form would be a nebula revolving round a completely formed 

 star; a less extreme form would be a bright line star, or a star of the 

 second group, revolving round it. In this case the colour would be 

 blue or greenish-blue or gray ; now this is the greatly preponderating 

 condition, as I have gathered from a discussion of the colours of the 

 small companions given in Smyth's ' Celestial Cycle ' ; and accepting 

 these colours alone, we should be led to think that most of the small 

 companions of our present stars were not companions originally, but 

 represent later additions to the systems. 



It is obvious that there are very many other questions of great 

 interest lying round these considerations, but it is not necessary that 

 I should refer at greater length to them on this occasion, as my 

 present object is only to show that a consideration of the colours of 

 double stars really adds weight to the a cause of variability which I 

 have suggested. 



