HTELLAK EVOLUTION. 157 



If the lilies are dark he. knows that the lioht of the star after orioinat- 

 \n<x in an interior incaudest'ent bod}' lias passed throiig-h a mass of 

 cooler vapors, and that during- its transmission some of the light has 

 suffered al)sorption. If, on the other hand, the lines are })right, he 

 knows that the region wliere they are produced is hotter than that lying 

 below. Thus a single glance at the spectrum of a star is suffici(Mit to 

 give important information regarding the physical condition of its 

 atmosphere. 



But the spectral lines are a])le to tell a far more complete story of 

 stellar conditions. If their exact position in the spectrum can be 

 measured it becomes possible to detei mine tlie chemical composition 

 of the star's atmosphere. And here the spectroscopist may ])e said to 

 have the advantage of the archeologist, in that the key to stidlar 

 hieroglyphs is a master key. capal)le of interpreting not nierel}^ the 

 language of a single people or a single age, but of laying l)are the 

 secrets of the most distant portions of the universe and applying with 

 e(iual force to the primitive and to the most highly developed forms of 

 celestial phenomena. If we ttike a piece of iron wire and turn it into 

 vapor in the intense heat of an electric arc lamp we tind that the light 

 which the glowing iron vapor emits, when spread out into a spectrum 

 b}'^ a prism, consists of a series of lines characteristically spaced and 

 always occupying the same relative positions. In the same way every 

 other element when transformed into vapor l)y a sufhciently intense 

 heat emits characteristic radiations, consisting of groups of lines 

 occupying definite positions in the spectrum. It is thus easy to see how 

 the presence of iron vapor can be detected in the atmosphere of Sirius 

 or in that of the sun. In the spectrum of each of these stars we find 

 a group of lines occupying the same relative positions as the lines fur- 

 nished by the iron vapor in an electric arc. Hydrogen gives an even 

 more characteristic group of lines, which grow closer and closer together 

 as we pass from the red end of the spectrum toward the violet. This 

 group occurs in the spectra of thousands of stars and serves as an 

 important guide in determining a star's place in a general scheme of 

 stellar evolution. 



The practical means of carrying out this method of research may 

 be illustrated by a reference to the stellar spectroscope employed with 

 the 4()-inch Yerkes telescope. The spectroscope is rigidly attached to 

 the lower end of the telescope tube. The image of a star formed by 

 the 4U-inch lens passes into the spectroscope through a slit aliout one 

 one-thousandth of an inch wide. After analysis by a train of three 

 prisms an image of the resulting spectrum is formed liy a suitable lens 

 upon a photographic plate. In making the photograph it is only 

 necessar}^ to keep the image of a star exactly on the slit throughout 

 the exposure, which may occupy from one minute to several hours, the 

 duration dep3nding upon the brightness of the star. 



