252 Mr. J. N. Lockyer. On the Classification [Jan. 10, 



II. General Statement of Conditions. 



In discussing the question whether the components of a binary 

 star have condensed from the same nebulosity or not, a difficulty arises 

 on account of the fact that, according to iny theory of their constitu- 

 tion, there will be no constant relation between the mass of a swarm 

 and its brightness. When we see a " star " of a certain magnitude, 

 we cannot tell from its brightness alone whether it is a large faint 

 one or a small bright one ; for a large body at a low temperature may 

 be equalled, or even excelled in brightness, by a smaller body at a 

 higher temperature. But when we know the spectra of the bodies, 

 we also know their relative temperatures. In the absence of spectro- 

 scopic details, colour helps us to a certain extent, as I have shown. 



If a pair of stars of unequal masses have condensed from a double 

 nebula, the smaller one will be farther advanced along the tempera- 

 ture curve than the larger one ; the colours and spectra will be 

 different, but it is not imperative that the magnitudes shall be unequal. 

 The smaller swarm, because it must be in more rapid movement round 

 the common centre of gravity, will suffer more quasi-tidal action 

 and therefore collisions per unit volume ; it will therefore condense 

 more rapidly than the larger one ; it will soon become as luminous, 

 and afterwards will for a time be considerably hotter than the larger 

 one. 



If the masses be very unequal, the smaller one will have the smaller 

 magnitude for a longer time. "When there is a great difference in 

 magnitude, therefore, it 'is fair to assume that the one with the 

 smaller magnitude has also the smaller mass. 



Another difficulty in the discussion, in the absence of spectroscopic 

 details, is due to the similarity in colour of bodies at equal heights on 

 the opposite sides of the temperature curve. Thus, as already stated, 

 bodies in Group III have, as far as we at present know, exactly the 

 same colour, namely, yellow, as those in Group V. Again, many 

 of the members of Group II have the same colour as some in 

 Group VI. 



The general conditions with regard to this subject may be thus 

 briefly stated: If the magnitudes, colour, and spectra of the two 

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

 in the same nebulosity with two condensations, or in a double nebula. 



If 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 due 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 



