THE STELLAR UNIVERSE. 



359 



distant regions of the universe where, as Sir John Herschel observes, but for 

 such evidences we might conclude all to be lifeless. Our own sun requires 

 nine times the period of this star to make a single revolution on its axis, and 

 an opaque body sufficiently large to produce a similar temporary obscuration 

 of it to a distant observer, would require to revolve round it in less than four- 

 teen hours. 



The star called x Cygni, situate in the neck of the Swan, nearly equidistant 

 from (3 and y Cyvni, affords another interesting example of this class. The 

 period of this star was discovered by Kirch in 1687. When it has its great- 

 est brightness, it appears to be of the sixth magnitude, and when least, it be- 

 comes a telescopic star of the eleventh magnitude. Its total period is 306 

 days and 21 hours. It retains its maximum magnitude for a fortnight. It 

 then decreases gradually for three months arid a half, and afterward increases 

 gradually during an equal time. It does not always attain the same maximum 

 brightness, the greatest magnitude varying between the fifth and the seventh. 



The following table of periodic stars, exhibiting specimens of every variety 

 of period, has been given by Sir John Herschel : 



What, then, it will be asked, are the probable or possible causes of these 

 singular phenomena 1 Several explanations, more or less plausible, have been 

 proposed. 



1 . The phenomena of the spots on the sun have afforded ground for the 

 supposition that the stars, being distant suns, may have patches more or less 

 opaque on their surfaces, which being successively presented toward the earth 

 by the rotation of the star upon an axis, produce the effect of periodical varia- 

 tion in brightness or apparent magnitude. " Such a motion of a star," says 

 Sir William Herschel, " may be as evidently proved, as the diurnal motion of 

 the earth. Dark spots, or large portions of the surface less luminous than 

 the rest, turned alternately in certain directions, either toward or from us, will 

 account for all the phenomena of periodical changes in the lustre of the stars 

 so satisfactorily, that we certainly need not look for any other cause." 



The analogy of the spots on the sun, however, is subject to an objection. 

 They certainly would not render the sun a periodic star to the observers of a 

 distant system ; for, to say nothing of their inconsiderable magnitude, com- 

 pared with the entire solar disk, their want of permanence and the irregu- 

 larity of their appearance and disappearance would entirely preclude such an 



* The.=e letters, B., FL, and M., refer to the catalogues of Bode, Flamsteed, and Mayer. 



