228 



KNOWLEDGE. 



[Oc 



1903. 



stars have reached in an evolutionary process, and there is 

 abundant evidence to show that the nearest relations of 

 the Orion stars are stars of the first type. Including such 

 stars as y Argus, which show bright as well as dark lines 

 in their spectra, Lockyer classifies the Orion stars in eight 

 groups, and suggests their arrangeiueut in the following 

 evolutionary order : — 



Argonian (y Argus). 

 Alnitamiau (e Orion is). 

 Crucian (a Crucis). I Achernian (a Eridani). 



Taurian ( ? Tauri). Algolian (/i Persei). 



Eigelian (j3 Orionis). | Markabian (a Pegasi). 



Oil tlie left are the stars believed to be of increasing 

 temperature, following a. Cygni in the evolutionary scheme, 

 and on the left those of decreasing temperature, which are 

 immediately succeeded by stars of the Sirian group. The 

 maximum of stellar temperature is considered to lie 

 represented by the two groups which head the list. 



It is not necessary here to describe in detail the differ- 

 ences between the several groups. Suffice it to say that in 

 passing from the Eigelian to the Alnitamian group there is 

 a general dimiuHtion in the strength of the enhanced 

 metallic lines, accompanied by an increase in the intensities 

 of the lines of helium, while in passing from the Alnitamian 

 to the Markabian group there is a similar change in the 

 inverse order. Stars on the down grade are distinguished 

 from those of similar temperature which are becoming hotter 

 bv the greater intensity of the lines of hydrogen, and the 

 lower intensities of the additional lines as a whole. 



The Alnitamian group is especially distinguished by the 

 presence of a series of lines of hydrogen first discovered in 

 X Puppis by Prof. Pickering, the wave-lengths of which 

 bear a simj^le numerical relationship to the series of lines 

 of that gas with which we are familiar in the Sirian stars 

 and in the spectra obtained from Geissler tubes. This new 

 series has not yet been experimentally reproduced, but the 

 similarity with other spectra which form "series" justifies 

 the belief that it is produced by hydrogen in a particular 

 but, at present, uukuown state; Miss Gierke distinguishes 

 this form of the gas from the ordinary form by the appro- 

 priate title "cosmic hydrogen," while Sir Nonnau Lockyer 

 calls it " proto-hydrogen." Another important line at 

 A. 4(JS7, as Eydberg has shown, is in all probability a line 

 of the " principal " series of hydrogen (the others forming 

 the two " subordinate " series), and the only one which 

 comes within our present range of stellar observation. As 

 in the case of the Pickering series, all efforts to reproduce 

 this line in the laboratory have so far been fruitless, owing 

 probably to lack of means to raise the gas to a sufficiently 

 high temperature. 



The Crucian variety of the Orion stars is by far the 

 most numerous, at least among the brighter stars which 

 have been investigated in sufficient detail for their proper 

 classification. Among the l)est examples in the northern 

 hemisphere are y Orionis and i) Ursa; Majoris, and in the 

 southern heavens a and /3 Crucis. The .spectrum of the 



(The longitudinal streaks are due to irregularities in the 

 driving clock, the photograph having been taken with a 

 prismatic camera.) 



The wave-lengths of the lines marked in the photograph 

 are as follow : — 



From the detailed tables of stellar lines which have 

 been published by Sir Norman Lockyer, we gather that 

 the chemical elements hitherto identified in four typical 

 stars of the Orion class are as follow : — 



The paucity of substances actually represented by 

 spectrum lines in these stars is not necessarily at variance 

 with the view that all stars are fundamentally of the same 

 composition. We know from laboratory experience that 

 the elements contained in a mixture are not always equally 

 effective in making their presence known in the spectrum. 

 Again, it is usually only the reversing layer that gives any 

 intelligible information as to the composition of a star, 

 and it may be that many substances fail to show themselves 

 in the absorption spectrum just as the Dj line of helium 

 writes no record in the dark line spectrum of the sun. 

 Or, as Sir Norman Lockyer supposes from the stellar 

 evidence, metallic substances such as iron, submitted to 



H. M5 



Fig. 19. — The Spectrum of B Cracis, showing the origins of some of the principal lines. 



latter, in which the lines of oxygen are developed to an 

 exceptional degree, is reproduced in Fig. 10 from a photo- 

 graph which we owe to the kindness of Dr. McClean. 



the transcendental temperatures of the hottest stars, may 

 be dissociated into hydrogen and helium. In one or all of 

 these ways the absence of legible indications of so many 



