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[Vol. XVIII. No. 455 



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CELESTIAL PHYSICS/ 



(Continued from p. 215.) 



The spectra of the stars are almost iafinitely diversified, 

 yet they can be arranged, with some exceptions, in a series 

 in which the adjacent spectra, especially in the photographic 

 region, are scarcely distinguishable, passing from the bluish- 

 White stars like Sirius, through stars more or less solar in 

 character, to stars with banded spectra, which divide them- 

 selves into two apparently independent groups, according as 

 the stronger edge of the bands is towards the red or the blue. 

 In such an arrangement the sun's place is towards the mid- 

 dle of the series. 



At present a difference of opinion exists as to the direc- 

 tion in the series in which evolution is proceeding, whether 

 by further condensation white stars pass into the orange and 

 red stages, or whether these more colored stars are younger 

 and will become white by increasing age. The latter view 

 was suggested by Johnstone Stoney in 1867. 



About ten years ago Eitter, in a series of papers, discussed 

 the behavior of gaseous masses during condensation, and the 

 probable resulting constitution of the heavenly bodies. Ac- 

 cording to him, a star passes through the orange and red 

 stages twice; first during a comparatively short period of 

 increasing temperature, which culminates in the white stage, 

 and a second time during a more prolonged stage of gradual 

 cooling. He suggested that the two groups of banded stars 

 may correspond to these different periods, the young stars 

 being those in which the stronger edge of the dark band 

 is towards the blue, the other banded stars, which are rela- 

 tively less luminous and few in number, being those which 

 are approaching extinction through age. 



Recently a similar evolutional order has been suggested, 

 which IS based upon the hypothesis that the nebulse and stars 

 consist of colliding meteoric stones in different stages of 

 condensation. 



^ Inaugural address at the meeting of the British Association for the Ad- 

 -vancement of Science, at Cardiff, August, 1891, by William Huggins, president 

 of the association (^Nature, Aug. 20). 



More recently the view has been put forward that the di- 

 versified spectra of tlie stars do not represent the stages of 

 an evolutional progress, but are due for the most part to 

 differences of original constitution. 



The few minutes which can be given to this part of the 

 address are insufficient for a discussion of these different 

 views. I purpose, therefore, to state briefly, and with re- 

 serve, as the subject is obscure, some of the considerations 

 from the characters of their spectra which appeared to me to 

 he in favor of the evolutional order in which I arranged the 

 stars from their photographic spectra in 1879. This order is 

 essentially the same as Vogel had previously proposed in his 

 classification of the stars in 1874, in which the white stars, 

 which are most numerous, represent the early adult and 

 most persistent stage of stellar life, the solar condition that 

 of full maturity and of commencing age, while in the orange 

 and red stars with banded spectra we see the setting in and 

 advance of old age. But this statement must be taken 

 broailly, and not as asserting that all stars, however differ- 

 ent in mass and possibly to some small extent in original 

 constitution, exhibit one invariable succession of spectra. 



In the spectra of the white stars the dark metallic lines 

 are relatively inconspicuous, and occasionally absent, at the 

 .same time that the dark lines of hydrogen are usually strong, 

 and more or less broad, upon a continuous spectrum, which 

 is remarkable for its brilliancy at the blue end. In some of 

 these stars the hydrogen and some other lines are bright, and 

 sometimes variable. 



As the greater or less prominence of the hydrogen lines, 

 dark or bright, is characteristic of the white stars as a class, 

 and diminishes gradually with the incoming and increase in 

 strength of the other lines, we are probably justified in re- 

 garding it as due to some conditions which occur naturally 

 during the progress of stellar life, and not to a peculiarity 

 of original constitution. 



To produce a strong absorption-spectrum a substance must 

 be at the particular temperature at which it is no'ably ab- 

 sorptive; and. further, this temperature must be sufficiently 

 below that of the region behind from which the light comes 

 for the gas to appear, so far as its special rays are concerned, 

 as darkness upon it. Considering the high temperature to 

 which hydrogen must be raised before it can show its char- 

 acteristic emission and absorption, we shall probably be 

 right in attributing the relative feebleness or absence of the 

 other lines, not to the paucity of the metallic vapors, but 

 rather to their being so hot relatively to the substances be- 

 hind them as to show feebly, if at all, by reversion. Such 

 a state of things would more probably be found, it seems to 

 me, in conditions anterior to the solar stage. A considera- 

 ble cooling of the sun would probably give rise to banded 

 spectra due to compounds, or to more complex molecules, 

 which might form near the condensing points of the vapors. 



The sun and stars are generally regarded as consisting of 

 glowing vapors surrounded by a photosphere where conden-- 

 sation is taking place, the temperature of the photospheric 

 layer from which the greater part of the radiation comes be- 

 ing constantly renewed from the hotter matter within. 



At the surface the convection currents would be strong, 

 producing a considerable commotion, by which the different 

 gases would he mixed and not allowed to retain the inequal- 

 ity of proportions at different levels due to their vapor den- 

 sities. 



Now the conditions of the radiating photosphere and those 

 of the gases above it, on which the character of the spectrum 

 of a star depends, will be determined, not alone by tempera- 



