110 VARIABLE STARS. 



briglit and dark lines, due to hydrogen; and tliese shift back and forth 

 with reference to each other in such a way as apparently to indicate 

 the presence of two or more bodies whirling- around each other, one of 

 them giving the ordinary dark-line si)ectrum and the other the bright- 

 line spectrum of the gas. 



The stars of the first and much the most numerous sub-class are 

 nearly all distinctly red and show in their spectra the bright lines of 

 hydrogen, but do not as yet furnish any certain evidence that their 

 periodicity is of a mechanical origin due; that is, to a revolution of 

 some sort. I'ossibly the explanation proposed by Lockyer may ulti- 

 mately be verified — that the sudden evolution of light is due to the 

 collision between two swarms of meteors which revolve around their 

 common center of gravity, and brush against each other as they pass 

 their perihelion point. But the periodicity of this class of variables is 

 hardly regular enough to suit this exphmation, and the sudden out- 

 burst of the bright lines in the si)ectrum of a star suggests rather some 

 such action, though on a vastly grander scale, as that which causes the 

 sun spots and the solar prominences, — causes operating within the star 

 itself. 



As to the " temi)(>rary stars" opinion is much divided. On the one 

 hand Lockyer and his numerous followers attribute tlie phenomena to 

 the collision of swarms of meteors, while on the other Huggins and 

 Vogel are disposed to assimilate them to the solar eruptions. The 

 latest instance of the kind, the " Nova Aurig* " already alluded to, 

 which appeared in February, 1892, has been most exhaustively studied 

 by a great number of observers. It is agreed on all hands that at 

 first its spectrum was full of bright lines, and dark lines also; and that 

 among these lines those of hydrogen, both bright and dark, were spec- 

 ially conspicuous, just as in the spectrum of Beta Lynv; and more- 

 over, that the bright lines were so shifted towards the red end of the 

 spectrum with respect to the dark lines as to indicate the tremendous 

 relative velocity of more than 500 miles a second in the two masses to 

 which the lines were respectively due. As to the other lines there was 

 dispute. Lockyer maintained that they were the same that are found 

 in the nebula'; other observers, no dou])t correctly, identified them 

 with certain lines which are conspicuous in the spectrum of the solar 

 chromosphere in every important eruption. The star faded to apparent 

 extinction in April, 1892, but most unexpectedly brightened up again 

 the following August, though not so as to be seen with the naked eye; 

 with the telescope it is still visible. Its spectrum, however, has under- 

 gone a remarkable transformation, many of the lines which were 

 formerly most coiisi)icuous having disappeared, while others have taken 

 their place, so that at present it is unquestionably a nebular si)ectrum. 

 As Mr. Campbell, of the Lick Observatory, who has perhaps the best 

 right to speak authoritatively, puts it, "The lines of the present 

 spectrum do not occuj^y the positions of the lines in the February 



