CHAPTERS ON THE STARS. 507 



confine ourselves to a few illustrations of spectra of the familiar types 

 described by Secchi and Vogel. 



There are many star spectra which cannot be included in any of the 

 classes we have described. Up to the present time these are generally 

 described as stars of peculiar spectra. 



As the present chapter is confined to the more general side of the 

 subject, we shall not attempt any description of special spectra. These, 

 especially the peculiar spectra of the nebulae, of new stars, of variable 

 stars, etc., will be referred to, so far as necessary, in the chapters relat- 

 ing to those objects. 



The most interesting conclusion drawn from observations with the 

 spectroscope is that the stars are composed, in the main, of elements 

 similar to those found in our sun. As the latter contains most of the 

 elements found on the earth and few or none not found there, we may 

 say that earth and stars seem to be all made out of like matter. It is, 

 however, not yet easy to say that no elements unknown on the earth 

 exist in the heavens. It would scarcely he safe to assume that, because 

 the line of some terrestrial substance is found in the spectrum of a 

 star, it is produced by that substance. It is quite possible that an un- 

 known substance might show a line in appreciably the same position as 

 that of some substance known to us. The evidence becomes conclu- 

 sive only in the case of those elements of which the spectral lines are so 

 numerous that when they all coincide with lines given by a star, there 

 can be no doubt of the identity. 



PROPER MOTIONS OF THE STARS. 



We may assume that the stars are all in motion. It is true that only a 

 comparatively small number of stars have been actually seen to be in 

 motion; but as some motion exists in nearly every case where observa- 

 tions would permit of its being determined, we may assume the rule to 

 be universal. Moreover, if a star were at rest at one time it would be 

 set in motion by the attraction of other stars. 



Statements of the motion from different points of view illustrate in 

 a striking way the vast distance of the stars and the power of modern 

 telescopic research. If Hipparchus or Ptolemy should rise from their 

 sleep of 2,000 years — nay, if the earliest priests of Babylon should come 

 to life again and view the heavens, they would not perceive any change 

 to have taken place in the relative positions of the stars. The general 

 configurations of the constellations would be exactly that to which they 

 were accustomed. Had they been very exact observers they might notice 

 a slight difference in the position of Arcturus; but as a general rule the 

 unchangeability would have been manifest. 



Tn dealing- with the subject, the astronomer commonly expresses the 



