506 POPULAR SCIENCE MONTHLY. 



shall discuss hereafter, is that they are nearly all situated very near the 

 central line of the Milky Way. 



Vogel proposed a modification of Secchi's classification, by subdivid- 

 ing each of his three types into two or three others, and including the 

 Wolf-Rayet stars under the second type. His definitions are as follows: 



Type I is distinguished by the intensity of the light in the more 

 refrangible end of the spectrum, the blue and violet. The type may be 

 divided into three subdivisions, designated a, b and c: 



In la the metallic lines are very faint, while the hydrogen lines are 

 distinguished by their breadth and strength. 



In lb the hydrogen lines are wanting. 



In Ic the lines of hydrogen and helium both show as bright lines. 

 Stars showing this spectrum are now known as helium stars. 



According to Vogel, the spectra of type II are distinguished by 

 having the metallic lines well-marked and the more refrangible end of 

 the spectrum much fainter than in the case of type I. He recognizes 

 two subdivisions: 



In Ha the metallic lines are very numerous, especially in the yellow 

 and green. The hydrogen lines are -strong, but not so striking as in la. 



In II& are found dark lines, bright lines and faint bands. In this 

 subdivision he includes the Wolf-Eayet stars, more generally classified 

 as of the fifth type. 



The distinguishing mark of the third type is that, besides dark lines, 

 there are numerous dark bands in all parts of the spectrum, and the 

 more refrangible end of the latter is almost wanting. There are two 

 subdivisions of this type: 



In Ilia the broad bands nearest the violet end are sharp, dark and 

 well-defined, while those near the red end are ill-defined and faint. In 

 III& the bands near the red end are sharp and well-defined; those 

 toward the violet faint and ill-defined. The character of the bands is 

 therefore the reverse of that in subdivision a. 



This classification of Vogel is still generally followed in Germany 

 and elsewhere. It is found, however, that there are star spectra of types 

 intermediate to all these defined. Moreover, in each type the individual 

 differences are so considerable that there is no well-defined limit to the 

 number of classes that may be recognized. At the Harvard Observatory 

 a classification quite different from that of Vogel has been used, but it is 

 too detailed for presentation here. The stars of type II are frequently 

 termed Capellan stars, or Solar stars. Certain stars of type I are termed 

 Orion stars, owing to the number of stars of the type found in that con- 

 stellation. The stars which show the lines of helium are known as 

 helium stars. We mention these designations because they frequently 

 occur in literature. It would, however, be outside the object of the 

 present work to describe all these classifications in detail. We therefore 



