VARIABLE STARS. 167 



That the periods of the variable stars are very irregular 

 has been long known ; but that this variability, with all its 

 apparent irregularity, is subject to certain definite laws, was 

 first established by Argelander. This he hopes to be able 

 to demonstrate in a longer and independent treatiee of liis 

 own. In the case of ;:^ Cygni, he considers that two perturb- 

 ations in the period — the one of 100, the other of 8^ — are 

 more probable than a single period of 108. "Whether such 

 disturbances arise from changes in the process of light wliich 

 is going on in the atmosphere of the star itself, or from the 

 periodic times of some planet which revolves round the fixed 

 star or sun x Cygni, and by attraction influences the form of 

 its photosphere, is still a doubtful question. The greatest 

 irregularity in change of intensity has unquestionably been 

 exhibited by the variabilis Scuti (Sobieski's shield) ; for this 

 star diminishes from the 5 •4th down to the nmth magnitude ; 

 and, moreover, according to Pigott, it once totally disappeared 

 at the end of the last century. At other times the fluctua- 

 tions in its brightness have been only from the 6'5tli to the 

 sixth magnitude. The maximum of the variations of ;:^ Cygni 

 have been between the 6* 7th and fourth magnitude ; of Mira, 

 from the fourth to the 2* 1st magnitude. On the other hand, 

 in the duration of its periods 6 Cephei shows an extraordi- 

 7iary, and, indeed, of all variable stars, the greatest regularity, 

 as is proved by the 87 minima observed between the 10th 

 of October, 1840, and 8th of Januar}^, 1848, and even later. 

 In the case of e Aurigce, the variation of its brilliancy, dis- 

 covered by that indefatigable observer, Heis, of Aix-la-Cha 

 pelle,^ extends only from the 3*4th to the 4*oth magnitude. 



A great difference in the maximum of brightness is exhib- 

 ited by Mira Ceti. In the year 1779, for instance (on the 

 6th of November), Mira was only a little dimmer than Alde- 

 baran, and, indeed, not unfrequently brighter than stars ot" 

 the second magnitude ; w^hereas at other times this variable 

 Btar scarcely attained to the intensity of the light of 6 Ceti, 

 which is of the fourth magnitude. Its mean la'ightness is 

 equal to that of y Ceti (third magnitude). If we designate 

 by the brightness of the faintest star visible to the naked 

 eye, and that of Aldebaran by 50, then Mira has varied in 

 its maximum from 20 to 47. Its probable brightness ma}'^ be 

 expressed by 30 : it is oftener below than above this limit 

 The measure of its excess, hovv'^ever, when it does occur, ia 



* Aro;elander iu Sclmmaclier's Astron. Nachr.,hc , xxvi. (IRJS), No 

 «2 J, s. 3G9. 



