August 1, 189fl ] 



KNOWLEDGE. 



147 



actually take placo. These extraordinary objects display a 

 continuous, in iiddition to a bright-line spectrum. Their 

 constitution would thus seem to bo almost identical with 

 that of " white prominences." They nnist be enormously 

 hot — hotter, certainly, than the photosphere, since certain 

 ingredients of their light stand out vividly against the 

 background furnished by it. Prof. Hale has, in fact, 

 succeeded in photographing them, through their H and K 

 emanations, on all parts of the snn's disc. Their unexpected 

 importance has thus been directly brought into view. Far 

 from bemg confined to the vicinity of spots, facuhe are now 

 perceived to cover the spot-zones with an almost unbroken 

 network, and to occur sporadically even to the very poles. 

 They show no marks of absorption by the cooler chromo- 

 spheric gases, and may hence be inferred either to attain 

 to a higher level, or to be in some other unknown way 

 protected against their inroads. For it must be admitted 

 that Kirehhoft's law, although true so far as it goes, does 

 not at all completely express the relations of emission to 

 absorption in celestial bodies. 



The effectiveness of facular light in modifying the 

 spectrum of the sun, taken as a whole, obviously depends 

 upon the proportionate amount of it present. Increase it 

 sufficiently, and its bright-line ingredient wdl eventually 

 become first distinctly perceptible, and at last conspicuous. 

 Under these circumstances, then, of augmented facular 

 development, the sun observed, for example, from Sii'ius 

 would appear as a bright-line star. And it might very 

 well be that his appearances under that aspect would be 

 periodical ; for the faculte emblazoning his surface become 

 much more numerous and intense with the growth of sun- 

 spots, and attain, concurrently with them, a maximum once 

 in about eleven years. Hence the periodicity of the sun 

 might, with a slight intensification of existing conditions, 

 be ascertained from distances at which he would be reduced 

 to the status of an insignificant pomt of light. 



This is no mere theoretical inference. On occasions 

 when facuhe were unusually numerous and lustrous, M. 

 Deslandres has actually succeeded, by treating the sun as a 

 star — that is, by admitting into his apparatus the whole of 

 its rays simultaneously — in photographing vivid reversals of 

 the calcium-lines. The sun wa? then, pro fantn, a bright- 

 line star. The aggregate of his light, in other words, con- 

 tained evidence of gaseous emissions. But other stars of 

 the solar type are presumably subject to similar vicissitudes. 

 I'apella, Arcturus, Aldebaran, undergo, we are entitled to 

 suppose, cyclical agitations marked by the fiuctuating 

 encroachments upon their photospheres of spotted and 

 faculous areas. These ought, at least in some cases, to 

 betray themselves spectrographically by the creeping-in of 

 fine lines of light into the dense l)ands of calcium-absorption 

 characterizing all solar stars. But the anticipated appear- 

 ance might have to be long waited for. Numberless 

 negative results would probably put a severe strain upon 

 the patience of the investigator. A single positive one 

 would, however, be no trivial success. It would imply the 

 opening of a hitherto closed door. It would give the 

 means of determining the flow of change in suns incon- 

 ceivably distant and immeasurably vast. Through the 

 prosperous issue of such a research the spot-maxima, say, 

 of Arcturus might come to be known more accurately than 

 those of our own "bright particular star." The study of 

 solar periodicity, moreover, could hardly fail to reap 

 advantage from comparisons of corresponding phenomena 

 in sun and stars ; and the problem of stellar variability 

 might even be brought a step nearer to a solution. 



In Mira Ceti, and probably in other periodical stars of 

 the same class, H and K are no less dark and broad than 

 they are in the sun ; but they appear to originate under 



very different conditions. The evidence regarding these 

 depends upon the circumstance that li is double. Both 

 hydrogen and calcium have commonly a share in its pro- 

 duction. A member of the hydrogen series, in short, falls 

 so near the calcium ray that when either is much widened 

 it conceals the other. Thus the diffuse H in the spectrum 

 of white stars like Vega appertains to hydrogen. The 

 presence and state of the nearly (coincident calcium-line 

 can only be inferred from the rule of its approximate 

 correspondence with its twin K, which, in Vega, has 

 shrunk into insignificance. Conversely, the hydrogen H, 

 if present at all in the sun's absorption-spectrum, is 

 plunged so deep in the shadow of the calcium-band as to 

 be undistinguishable. In order, then, to avoid these 

 ambiguities, K is taken as the representative of the calcium 

 pair ; and its value as an index to stellar constitution, 

 pointed out thirteen years ago by Dr. Huggins, has been 

 in no small degree heightened as the result of recent 

 discoveries. 



Now the Harvard College " spectrograms" of Wira show 

 six brilliant hydrogen-rays, namely, CI, /i, with four of the 

 ultra-violet sequence. But the intermediate H is invisible. 

 Few, however, will venture to assert that the continuity 

 of the harmonical priigression is really broken. The 

 gaseous molecules cannot conceivably be agitated in such 

 a manner as to cause them to drop out this particular 

 quality of vibration. There is, moreover, an obvious 

 reason for its apparent absence.* It is obliterated by the 

 strong calcium-band falling near its place in the spectrum. 

 This implie.s that the absorbing calcium-vapour is situated 

 between our eyes and the glowing hydrogen, the radiations 

 of which, so far as they come within its grasp, are cut off by it. 

 In other words, the region of this particular kind of absorp- 

 tion, which, in the sun, is sunk below the region of bright- 

 line emission, overlies it in Mira. There is, apparently, 

 no escape from this conclusion. Wo are then forced 

 to admit that what might have been taken for the natural 

 arrangement in order of vapour-density does not invariably 

 prevail in stellar atmosphere.s. What the counteracting 

 influences may be, we know not ; we can only note the 

 effects of their exertion. These certainly deserve very 

 close attention ; for there are few facts in astro-physics 

 more curious than the seemingly inverted positions of 

 the absorbing and emitting strata in the sun and Mira. 

 Yet the subject has attracted up to the present singularly 

 little attention. Data for its profitable discussion are 

 much needed, but are not forthcoming. They could, 

 nevertheless, be easily procured with the aid of very 

 moderate spectrographic appliances. A few photographs 

 of the spectra of long-period variables showing bright 

 Hues, such as y^ Cygni, R Andromeda;, R Leonis, and 

 many more, would at least tell whether the eclipse of the 

 hydrogen H by the calcium H is a peculiarity common to 

 the class. A few additional photographs would answer 

 the further question as to whether the eclipse is a per- 

 manent condition, or merely a transient phase in each 

 individual star. 



Nova Aurigre was unique among sidereal objects in its 

 display of bright calcium-lines. Nor did they persist 

 even here. They became extinct during the temporary 

 invisibility of the star during the summer of 189'2. Yet, 

 considering the predominance of H and K in the spectra 

 of solar appendages, and their easy reversibility, their 

 inclusion as vivid rays might have been anticipated in a 

 great number of emission-spectra. They remain, never- 

 theless, hitherto unrecorded in nebulie, bright-line stars, 



* See an article b\ the present writer in The Obnervalory, 

 Vol. xi., 1). 84. 



