Pope. — On the Southern Stars and other Celestial Objects. 171 



y Crucis. — It has been customary for astronomers to catalogue this 

 star — the " Head of the Cross " — as a double star ; but the proper motion 

 of the large orange-coloured star is rapidly carrying it away from its five- 

 and-a-half-magnitude blue companion. The spectrum of y Orucis is perhaps 

 the finest of all stellar spectra. The groups of lines are so numerous and 

 so well marked that this spectrum may be observed under almost any at- 

 mospheric conditions, if the star can be seen at all. y Orucis is a typical 

 star of Secchi's third class, which are all orange colour verging towards red. 

 In their spectra there are numerous, easily-seen, close groups of lines; but 

 the hydrogen lines are either very indistinct or altogether absent, a Ononis 

 and a Herculis are good specimens of the two principal subdivisions of this 

 class. In the spectrum of y Crucis there are at least eight broad groups of 

 lines, and some of these occupy the parts of the spectrum at which sodium, 

 iron, magnesium, and calcium lines are found in the solar spectrum. 

 But, because they are groups, it is much more difficult to say whether they 

 contain the lines belonging to those elements or not, than it is in the case 

 of a first or second-class spectrum. Still, I anticipate that careful measure- 

 ments wiU confirm my opinion that ii-on and magnesium lines, especially 

 the latter, are present in the spectrum of this star ; the sodium line is pro- 

 bably there too. There is, also, a fine line just at the part where the green 

 merges into the blue of the spectrum. This is possibly the F. hydrogen 

 line. 



There is one very significant feature in this spectrum, so at least it seems 

 to me. It is well known that when the Sun is near the horizon, especially 

 in damp weather, his spectrum contains certain groups of lines which are 

 due to the aqueous vapoiu' in our own atmosphere, and that, as he reaches 

 a greater altitude, these lines become faint or disappear. Now, two at 

 least of the groups in the spectrum of y Crucis appear to occupy the same 

 position as two of the principal groups of atmospheric lines. Now this 

 being verified, important conclusions might follow. Secchi, on grounds of 

 this sort, infers the existence of aqueous vapours in the neighbourhood of 

 sun-spots. The spectroscope knows nothing, so to speak, about distance, 

 except indeed where motion of approach or recession is concerned. If 

 these aqueous vapour-lines are produced in spots on the sun, may they not 

 be produced in much the same way in y Crucis, the principal difference bemg 

 that on the distant star the cause is more general and the effect greater than 

 it is on om'own Sun. If I am not mistaken, the existence of these spectrum 

 lines should enable us to read a certain portion of the "life history" of a 

 star. 



This history might be something like this : Let us suppose that, count- 

 less ages ago, y Crucis was a white star, like Sirius, It was then far more 



