18G8.] 



Constitution of the Sun and Stars. 



49 



the force of gravity upon the outer atmosphere the same as it is upon the 

 sun, the star will he equally white. The class of more brilliantly white 

 stars with an almost violet gleam, such as Sirius and a Lyree, are those with 

 masses too great in proportion to their temperatures for this adjustment. 

 And, on the other hand, those whose masses fall short of what the fore- 

 going condition assigns, or, on the other side, whose temperatures are in 

 excess, will, in proportion as they deviate from its fulfilment, have spectra 

 more and more closed in upon that part in which the spectra of two incan- 

 descent bodies differ least in brightness when the luminous bodies are at 

 nearly, but not quite, the same temperature — that is, upon the green, yel- 

 low, orange, and red rays, uniting into a tint which always inclines to either 

 yellow, orange, scarlet, or crimson. The minute crimson stars which are 

 met with here and there in the sky seem to be either very small stars, or 

 stars enormously distended by heat. It is very desirable that the proper 

 motion and parallax of these bodies should be inquired into when practi- 

 cable, on the chance that some of them may be found to owe their colour 

 to being very small, and therefore very close to us. 



84. I need not say with what fidelity these many consequences of a 

 change in the force of gravity in passing from star to star reproduce them- 

 selves in Mr. Huggins's observations. But before making the comparison, 

 it will be well to consider rapidly what interfering causes may have to be 

 taken into account. 



85. "VVe have hitherto spoken of the effects of the intensity of gravity in 

 a star upon substances giving the same lines as we see in the sun. Oar 

 results are therefore subject to modification wherever the system of lines is 

 itself changed. If, for example, the elements which give rise to the more 

 prominent lines of the sun's spectrum are wanting in the stars, other lines, 

 which perhaps are not, like those of the sun, pretty evenly spread over the 

 whole spectrum, may take their place. If this should happen, some co- 

 lours will be more absorbed by them than others ; and this will tend to give 

 to the star the complementary tint. In such cases the resultant effect will 

 be mixed; the effect of the cause just mentioned being blended with that 

 strengthening of the lines at the blue end of the spectrum which operates 

 most when gravity on a star is weak. We shall presently find that this 

 state of things, which would be improbable in solitary stars, may have been 

 brought about in the case of the companions of some double stars. Or, 

 again, elements which in the sun are free may in the stars be found only in 

 a state of combination, and be either absent from the star's atmosphere or 

 give rise in it to an entirely new set of lines. This* has perhaps been the 



* Or the whole of the free hydrogen may have been thrown off into rings. If the 

 star's rotation were such as to cast off the upper layer of hydrogen at his equator, and if 

 the hydrogen could remain uncombined under these circumstances, fresh hydrogen dif- 

 fusing upwards or flowing in from the poles would constantly fill the void ; and each sup- 

 ply, as it arrived, would be in turn flirted off, until in the end the whole of the free hydrogen 

 of the star would be in this way drained away. The explanation in the text is, however, 

 on many accounts the more probable one, 



VOL. XVII. E 



