164 COSMOS. 



often disfiguia the very best catalogues. The disappearance 

 of a heavenly "body from the pJace in which it had before 

 been distinctly seen, may be the result of its own mHion 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 sight. 

 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 : Stellas 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 tiling 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 undeterminable 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 tim.e 

 The latter class of phenomena (of which 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- 

 certaitied. 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 Lyi'a (Vega Lyrae). Ptolemy 

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

 isms of Eratosthenes (cap. 25), Vega is called levKov kqI Xafnrprw. 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 Lyre i^Fidicula of Pliny 

 xviii., 25) became a star of the first magnitude. 



