August 25, 1 881] 



NATURE 



391 



SOLAR PHYSICS— THE CHEMISTRY OF THE 



SUN"- 

 Tests afforded by the Stars 

 "Y^E will now see how the views which have been put forward are 

 ' borne out by the facts which are presented to us by the stars. 

 There is no need to occupy much time, in fact reference need 

 only be made to Dr. Huggins' paper which was communicated 

 to the Royal Society in the course of last year, and with that 

 paper we may compare some earlier writings. It was as early 

 as 1864 that Dr. Huggins, who was then associated with the 

 late Dr. Miller, called attention to the intensely strong lines of 

 hydrogen visible in the hottest stars.- In this paper they pointed 

 out at the same time that other metallic lines associated with 

 those lines of hydrogen were thin and faint. It has been 

 already mentioned that, as we have independent evidence that 

 these stars are hotter than our sun, we had strong grounds for 

 believing that here we were in presence of a result brought 

 about by a higher temperature, associated as it was with a simpler 

 spectrum, and, therefore, presumably with simpler constituents. 

 We need not stop now to discuss the objection which has been 

 put forward by an ingenious person ignorant of tlie facts, that 

 the broadening of these lines may not be due to an increase of 

 temperature at all, but really to a very rapid equatorial rotation 

 of the star. This is a fah sample of one of the classes of 

 objections one has to meet. Of course it is at once put out of 

 court by the fact, also stated by Dr. Huggins, that, associated 

 with the thick line.=, are excessively thin lines. Any enormous 

 equatorial velocity of the star should have made all the lines 



thick, but this is not the fact. Now we have only two lines ia 

 the solar spectrum at all comparable in thickness with these 

 hydrogen lines in the hottest stars, taking Sirius and a Lyrse 

 as types. 



In a paper communicated to the Koyal Society in 1876' it was 

 remarked that laboratory work indicated the possibility that line- 

 spectra might, after all, really not result from the vibration of 

 f similar molecules ; and at that time the evidence seemed to be so 

 clear in the case of calcium that it was pointed out that the time 

 had arrived when evidence touching calcium itself ought, if pos- 

 sible, to be obtained from the stars by means, of course, of pho- 

 tography, because the part of the spectrum in question — the 

 region of H and K — is exceedingly faint in the case of the stars. 



Why, it may be asked, was it important to get this evi- 

 dence from the stars ? I will read an extract from a book,' 

 published some years ago, which puts this view forth: — "It 

 is abundantly clear that if the so-called elements, or, more 

 properly speaking, their finest atoms, those that give us line- 

 spectra, are really compounds, the compounds must have been 

 formed at a very high temperature. It is easy to imagine that 

 there may be no superior limit to temperature, and, therefore, 

 no superior limit beyond which such combinations are possible, 

 because the atoms which have the power of combining together 

 at these transcendental stages of heat do not exist as such, or 

 rather they exist combined with other similar atoms at all lower 

 temperatures. Hence the association will be a combination of 

 more complex molecules as temperature is reduced, and of 

 dissociation, therefore, with increased temperature there may 

 be no end." 



That was one point. 



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Here is the next point which made an appeal to the stars so 

 necessary. " We are justified in supposing that our terrestrial cal- 

 cium once formed is a distinct entity, whether it be an element or 

 not, and, therefore, by working at terrestrial calcium alone we 

 shall never know, even if its dissociation be granted, v. hether the 

 temperature produces a simpler form, a more atomic condition 

 of the s.ime thing, or whether w e are unable to break it up into 

 .V + y, because in our terrestrial calcium, assuming all calcium 

 to be alike, neither .v nor y will ever vary ; but if calcium be 

 the product of the condition of relatively low temperature, then 

 in stars hot enough to enable its constituents to exist uncom- 

 pounded we may expect these constituents to vary in quantity ; 

 there may be more -r in one star, and more y in another. If 

 this be so, then the H and K lines would vary in thickness, and 

 the extremest limit of variation will be that we shall have only H 

 represented, or x in one star, or only K represented or y in 

 another, and intermediately between those extreme conditions 

 we have cases in which, though both H and K are visible, H is 

 thicker in some and K is thicker in others. " 



What, then, are the results of this appeal to the stars which 

 Dr. Huggins has made with such splendid success ? We have 

 in the hottest stars a spectrum so regular, so rhythmic, that it 

 seems imj: ossible not to consider it as produced either by the same 



' Lectures in the Cours. 

 Revised from shorthand 1 



= " On the Spectra of ; 

 p. 242). 



1 Solar Physics at South Kensington (see p. 



s. Continued from p. 370. 



e of the Fixed Stars " (Pnv. Roy. Soc, : 



ISO). 



spectra (Huggtns\ 



substance or by substances closely allied. Is itby^mere accident 

 ; that some of the least refrangible lines coincide w ith those of 

 ' hydrogen ? is it by mere accident that the most refrai gible lines 

 I have never been seen except in these stars ? One of them coin- 

 cides with H, one of the lines which still remain thick in the sun, 

 and with nhich we find a fine line of hydrogen to be coincident. 

 Fig. 43 is a copy of Dr. Huggins' diagram, to which reference 

 has been made. At the top is a portion of the solar spectrum in 

 the violet and ultra-violet, and next is the spectrum of the hottest 

 star, a Lyrje. This spectrum, it will be seen,' is simpler even than 

 the spectrum of the solar prominences, and not only is there 

 this wonderful simplicity , but note the exquisite rhythm by 

 which the distance between the lines gradually increases as we 

 go from one end of the spectrum to the other. Note also that 

 the least refrangible line shown on the di,agram is coincident 

 with /; in the violet part of the solar spectnim, and that the next 

 line is coincident with the line H, to which reference has been 

 made in the notes I have read. Note also the relative intensi- 

 ties of the lines H and K in the sun, in which their intensities 

 are about equal, and in i; UrsK Majoris, in w hich K is alto- 

 gether absent. These are the first points in this diagram to 

 which attention must be drawn. There will be other points as 

 we proceed further. 



But in descending from the general to the particular _Dr. 



^ " Preliminary Note on the Compound Nature of the Line Spectra'of 

 Elementarj- Bodies" (froc. Roy. Soc, No. 168, 1876). 

 2 " Studies in Spectrum Analysis," p. 196. 



