6o6 



NATURE 



\April 26, 1888 



SUGGESTIONS ON THE CLASSIFICA TION OF 

 THE VARIOUS SPECIES OF HEAVENLY 

 BODIES} 



II. 



II.— CLASSIFICATION. 



I. Former Classifications of Stars. 



T N the various classifications of the celestial bodies 

 ■*■ which have been attempted from time to time, nebulae 

 and comets have been regarded as things apart from the 

 stars ; but from what I have stated in the first part of 

 this paper, relating to the origin of the various groups of 

 heavenly bodies, it is clear that it is not only unnecessary 

 but unphilosophical to make such a separation ; and 

 indeed, if any such separation were needed, such a result 

 would seem to indicate that the line of evolution is by no 

 means so simple and clear as it really seems to be. But 

 although it is no longer necessary to draw this distinction, 

 it is important that I should state the various spectro- 

 scopic classifications which have been attempted in the 

 case of the stars. With this information before us, we 

 shall be better able to see the definite lines on which any 

 new classification must be based to include all celestial 

 forms. 



Fraunhofer, RMtherfurd, and Secchi. 



When we inquire into the various labours upon which 

 our present knowledge of the spectra of the various orders 

 of " stars " are based, the first we come across are those of 

 Fraunhofer, who may be said to have founded this branch 

 of scientific inquiry in the year 1814. 



Fraunhofer not only instituted the method of work 

 which now is found to be the most effective, but his ob- 

 servations at that time were so excellent that he had no 

 difficulty in finding coincidences between lines in the sun 

 and in Venus. 



Fraunhofer's reference to his observations runs as 

 follows : — 



" I have also made several observations on some of the 

 brightest fixed stars. As their light was much fainter 

 than that of Venus, the brightness of their spectrum was 

 consequently still less. I have nevertheless seen, without 

 any illusion, in the spectrum of the light of Sirius, three 

 large lines, which apparently have no resemblance with 

 those of the sun's light. One of them is in the green, 

 and two in the blue space. Lines are also seen in the 

 spectrum of other fixed stars of the first magnitude ; but 

 these stars appear to be different from one another in 

 relation to these lines. As the object-glass of the tele- 

 scope of the theodolite has only thirteen lines of aperture, 

 these experiments may be repeated, with greater precision, 

 by means of an object-glass of greater dimensions." "- 



He did not attempt to classify his observations on 

 stellar spectra, but, as pointed out by Prof. Dun^r (" Les 

 Etoiles k Spectres de la Troisieme Classe," p. 3), those 

 that he most particularly mentions are really remarkably 

 diverse in their characteristics. 



In these researches Fraunhofer was followed by Ruther- 

 furd, who, in the year 1863, was the first to indicate that 

 the various stellar spectra which he had then observed were 

 susceptible of being arranged into different groups. His 

 paper was published in Sillimafi's Journal (vol. xxxv. 

 p. 71), and, after giving an account of the observations 

 actually made, continues as follows : — 



"The star spectra present such varieties that it is 

 difficult to point out any mode of classification. For the 

 present, I divide them into three groups :— First, those 

 having many lines and bands, and mostly resembling the 

 sun, viz. Capella, ^ Geminorum, a Ononis, Aldebaran, 

 y Leonis, Arcturus, and ^3 Pegasi. These are all reddish 

 or golden stars. The second group, of which Sirius is 



The Bakerian Lecture, delivered at the Royal Society on April 12, by 

 J-^Norman Lockyer, F.R.S. Continued from p. 590. 



.'^ "On the Refractive and Dispersive Power of Different Species of Glass, 

 with an Account of the Lines Which cross the Spectrum," Fraunhofer, trans, 

 lated in Edin. PhilosoJ>hic Journal, vol. x., October to April, 1823-24, p. 39 



the type, presents spectra wholly unlike that of the sun, 

 and are white stars. The third group, comprising 

 a Virginis, Rigel, &c., are also white stars, but show no 

 lines ; perhaps they contain no mineral substance, or are 

 incandescent without flame." 



Soon afterwards Secchi carried on the inquiry, and 

 began in 1865 by dividing the objects he had then 

 observed into two types. These two types were sub- 

 sequently expanded in 1867 into three (" Cataloge delle 

 Stelle di cui si e determinate lo Spettro Luminoso," 

 Secchi, Parigi, 1867) : first, white stars, like a Lyrae ; 

 secondly, yellow stars, like Arcturus ; and, thirdly, deeply 

 coloured stars, like a Herculis and u Orionis. The order 

 of these types was not always as stated, but I have not 

 been able to find the exact ^date at which the order was 

 changed (Duner, " Sur les Etoiles," p. 128). Secchi sub- 

 sequently added a fourth type, in which the flutings 

 were less numerous. There is little doubt that Secchi 

 was led to these types not so much by any considerations 

 relating to the chemical constitution of the atmospheres 

 of these bodies as in relation to their colours. His first 

 classifications, in fact, simply separated the white stars 

 from the coloured ones (see on this point " Le Scopirte 

 Spettroscopiche," P. A. Secchi, Roma, 1865). 



The fourth type included, therefore, stars of a deeper red 

 colour than those of the third, and Secchi pointed out that 

 this was accompanied by a remarkable change in the 

 spectrum ; in fact, of Secchi's four types thus established, 

 the first and second had line spectra and the third and 

 fourth had fluted ones. At that time the important dis- 

 tinction to be drawn between line- and fluted-spectra was 

 not so well recognized as it is at present ; and further the 

 relation of spectra to temperature was not so fully con- 

 sidered. Secchi, as a result of laboratory work, however, 

 at once showed an undoubted connection between the ab- 

 sorption flutings in the stars of the fourth type and those 

 seen in the spectrum of carbon under certain conditions ; 

 and although this conclusion has been denied, it has since 

 been abundantly confirmed by Vogel and others (see 

 Vogel, Publicationen, Potsdam, No. 14, 1884, p, 31). 



Relation to Temperature. 



At the time that Secchi was thus classifying the stars, 

 the question was taken up also by Zollner, who in 1865 

 first threw out the suggestion that the spectra might pro- 

 bably enable us to determine somewhat as to the relative 

 ages of these bodies ; and he suggested that the yellow and 

 red light of certain stars were indications of a reduction 

 of temperature (Zollner, " Photometrische Untersuch- 

 ungen," p. 243). 



In 1868 this subject occupied the attention of Angstrom 

 with special reference to the contrasted spectra of lines 

 and flutings. On this he wrote as follows, showing that 

 temperature considerations might help us in the matter of 

 variable stars (" Recherches sur le Spectre solaire," 

 Upsala, 1868):— 



" D'apresles observations faites par MM. Secchi et Hug- 

 gins, les raies d'absorption dans les spectres stellaires sont 

 de deux especes : chez I'une, le spectre est raye de lignes 

 tres-fines, comme le spectre solaire ; chez I'autre, les 

 raies constituent des groupes entiers a espaces dgaux ou 

 des bandes nuancdes. Ces derniers groupes appartiennent 

 vraisemblablement aux corps composes, etje mentionnerai, 

 en particulier, que ceux trouves dans le spectre de a 

 Orionis ressemblent fort aux bandes lumineuses que 

 donne la spectre de I'oxyde de manganese. Suppose que 

 ma theorie soit juste, I'apparition de ces bandes doit done 

 indiquer que la temperature de I'etoile est devenue assez 

 basse pour que de telles combinaisons chimiques puissent 

 se former et se conserver. 



" Entre ces deux limites de temperature chez les etoiles, 

 limites que Ton peut caractdriser par la presence de I'une 

 ou de I'autre espece des raies d'absorption, on peut 

 s'imaginer aussi un dtat intermediaire, dans lequel les gaz 



