164 COSMOS. 



often disfigure the very best catalogues. The disappearance 

 of a heavenly body from the place in which it had before 

 been distinctly seen, may be the result of its own m jtion as 

 much as of any such diminution of its photometric process 

 (whether on its surface or in its photosphere), as would ren- 

 der the waves of light too weak to excite our organs of sfght. 

 What we no longer see is not necessarily annihilated. The 

 idea of destruction or combustion, as applied to disappearing 

 stars, belongs to the age of Tycho Brahe. Even Pliny, in 

 the fine passage where he is speaking of Hipparchus, makes 

 i a question : Stellse an obirent nascerenturve ? The ap- 

 parent eternal cosmical alternation of existence and destruc- 

 tion is not annihilation ; it is merely the transition of matter 

 into new forms, into combinations which are subject to new 

 processes. Dark cosmical bodies may by a renewed process 

 of light again become luminous. 



Periodically variable Stars. — Since all is in motion in 

 the vault of heaven, and every thing is variable both in space 

 and time, we are led by analogy to infer that as the fixed 

 stars universally have not merely an apparent, but also a 

 proper motion of their own, so their surfaces or luminous at- 

 mospheres are generally subject to those changes which re- 

 cur, in the great majority, in extremely long, and, therefore, 

 unmeasured and probably undetennhiahle periods, or which, 

 in a few, occur without being periodical, as it were, by a 

 sudden revolution, either for a shorter or for a longer time. 

 The latter class of phenomena (of Avhich a remarkable in- 

 stance is furnished in our own days by a large star in Argo) 

 will not be here discussed, as our proper subject is those fixed 

 stars whose 'periods have already been investigated and as- 

 certaiiied. It is of importance here to make a distinction 

 between three great sidereal phenomena, whose connection 

 has not as yet been demonstrated ; namely, variable stars of 

 known periodicity ; the instantaneous lighting up in the heav- 

 ens of so-called neio stars ; and sudden changes in the lu- 

 minosity of long-known fixed stars, which previously shone 



(Deneb) is little inferior in brilliancy to Lyra (Vega Lj'ra?). Ptolemy 

 classes Vega among stars of the first magnitude, and in the Cataster 

 isms of Eratosthenes (cap. 25), Vega is called Aew/coy kol "kau-np^v. Con 

 sidering the many inaccuracies of" a poet, who never himself observed 

 the stars, one is not much disposed to give credit to the assertion that it 

 was only between the years 272 and 127 B.C., i. e., between the times 

 of Aratus and Hipparchus, that the star Vega Lyrae (^Fidicnla of Pliny, 

 xviii., 2.")) became a star of the first magnitude. 



