1868,] 



Constitution of the Sun and Stars. 



55 



not even swell his bulk perceptibly *. But upon his satellite the conse- 

 quences will be very remarkable. Hydrogen was the first gas to go ; then, 

 in order, sodium, magnesium, calcium, chromium, manganese, iron, If 

 the process has gone far enough to distil away all of these gases in the 

 free state, the spectrum of the companion has been robbed of the principal 

 lines found in solitary stars, to be replaced by an entirely new system ema- 

 nating from substances of higher vapour-density, which, to judge from the 

 spectra of the few coloured double stars that Mr. Huggins has succeeded 

 in examining, are crowded abnormally over the scarlet, orange, yellow, and 

 part of the green, giving to the companions of double stars those blue, violet, 

 or greenish tints which are met with nowhere else. If the process be con- 

 tinued still further, more gases will be swept away, and the photosphere 

 laid nearly bare ; as a consequence, the smaller star will appear white and 

 nearly destitute of lines. This may have furnished that numerous class of 

 double stars of which the companions are small and white. 



94. No double star can come forth unless unequal pressure has acted so 

 effectually on the smaller constituent as to communicate to it a swift 

 motion of rotation. It is likely that cases may occur where the forces that 

 accomplish this act with such inordinate strength that the cohesion of the 

 smaller star is unable to withstand them, and there result two or more 

 fragments spinning violently, and destined thenceforward to traverse slightly 

 separate paths. This seems a not improbable account of such a multiple 

 system as y Andromedse. 



95. Upon the primary the consequences of the same violence would 

 probably be entirely different. They would compel him to rotate at a 

 great speed, perhaps so rapidly as to fling off his own equatorial parts f. 

 These would form rings about him of the elliptic section which was inves- 

 tigated by Laplace ; at least, they would assume this form if they consisted 

 only of gas, or of gas with cloud dispersed through it which is constantly 

 dissolving and reforming, so as to keep always in a state of minute division, 

 — so long, in fact, as the gaseous pressure caused by any accidental conden- 



* A moment's consideration will make this plain. In fact, if the quantities of all the 

 gases in the earth's atmosphere were doubled, it would add only 3^ miles, or, more ex- 

 actly, 5534 metres to its height. The result, after all disturbance had quieted down, 

 would be the same as if a denser stratum of air of this trifling thickness were slipped in 

 between the present atmosphere and the ground. To spectators from without, who 

 would judge of our atmosphere chiefly as one which reaches upwards to a distance of 

 about 200 kilometres (the height at which meteors begin to glow), the effect would be 

 wholly insensible. 



t This would be most likely to occur when the friction had acted chiefly on the super- 

 ficial parts of the larger star, since under these circumstances a star might be enormously 

 dilated without any considerable increase of its moment of gyration ; so that, cceteris 

 paribus, such a star would rotate swifter than one whose bulk was due to the equal expan- 

 sion of all parts. 



Sirius may have been an instance of such a star (see footnote, p. 53). We have 

 perhaps some reason, judging from the existing areolar momentum of the parts of the 

 solar system, to suspect that it was in a considerable degree the case of our sun also. 



