402 A Short History of Astronomy [Cn. xin. 



differences inferences as to the relative " ages," or at any 

 rate the stages of developn. t, of different stars. 



Many of the dark lines in the spectra of stars have been 

 identified, first by Sir William Huggins in 1864, with the 

 lines of known terrestrial elements, such as hydrogen, iron, 

 sodium, calcium ; so that a certain identity between the 

 materials of which our own earth is made and that of 

 bodies so remote as the fixed stars is thus established. 



In addition to the classes of stars already mentioned, the 

 spectroscope has shewn the existence of an extremely in- 

 teresting if rather perplexing class of stars, falling into 

 several subdivisions, which seem to form a connecting 

 link between ordinary stars and nebulae, for, though in- 

 distinguishable telescopically from ordinary stars, their 

 spectra shew bright lines either periodically or regularly. 

 A good many stars of this class are variable, and several 

 " new " stars which have appeared and faded away of late 

 years have shewn similar characteristics. 



313. The first application to the fixed stars of the spectro- 

 scopic method ( 302) of determining motion towards or away 

 from the observer was made by Sir William Huggins in 1868. 

 A minute displacement from its usual position of a dark 

 hydrogen line (F) in the spectrum of Sirius was detected, 

 and interpreted as shewing that the star was receding from 

 the solar system at a considerable speed. A number of 

 other stars were similarly observed in the following year, 

 and the work has been taken up since by a number of 

 other observers, notably at Potsdam under the direction 

 of Professor H. C. Vogel, and at Greenwich. 



314. A very remarkable application of this method to 

 binary stars has recently been made. If two stars are 

 revolving round one another, their motions towards and 

 away from the earth are changing regularly and are differ- 

 ent ; hence, if the light from both stars is received in the 

 spectroscope, two spectra are formed one for each star 

 the lines of which shift regularly relatively to one another. 

 If a particular line, say the F line, common to the spectra 

 of both stars, is observed when both stars are moving 

 towards (or away from) the earth at the same rate which 

 happens twice in each revolution only one line is seen ; 

 but when they are moving differently, if the spectroscope 



