1889.] of the various Species of Heavenly Bodies. 253, 



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

 smaller one be less advanced than the larger one, 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 nebulae may be explained ; while the nebula with a larger 

 mass remains still in the nebulous condition, the smaller one may be 

 advanced to any point, 'and may indeed even be totally invisible 

 (Group VII), 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 it may be suggested that the companion is added 

 afterwards, the most extreme form would be a nebula revolving 

 round a completely formed star, like an enormous comet round the 

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

 second group, revolving round one of a higher group. In this case ; 

 the colour would be blue or greenish -blue or grey. 



, ,,-, .., ,..'....- -.,,. " " ..." ',." ' ' ;r 



III. Light Curves. 



I find that the best way of dealing with^this question is to represent 

 the life of each component by a curve, in which the ordinates repre- 

 sent time and the " magnitude " of the star. Then, if the colours and 

 magnitudes r are consistent with the curves beginning at the same 

 point, we are justified in regarding both as having condensed from, 

 the same nebulosity. If not, in all probability the companion would 

 be a later addition. 



The form of the light curve, which represents the effect of increase 

 and decrease of temperature, will probably be something like fig. 21. 

 We should expect the curve to be somewhat similar to the light curves 

 of the regular variables of Group II, where the increase in luminosity 

 is due to the collision of two meteor-swarms. Here there is a rapid 

 rise to maximum, and a steadier fall to minimum. This is confirmed 

 by the fact that there is apparently a greater number of stars of 

 Group V than of Group III, though on this point I cannot yet speal? 

 with any certainty. If this should turn out to be so, the fact would 

 appear to indicate that the time of existence of a body as a star of 

 Group V is probably longer than the time during which it exists as a 

 condensed meteor-swarm under the conditions of Group III. During 



VOL. XLV. S 



