268 ANNUAL BEPORT SMITHSONIAN INSTITUTION, 19 31 



prism of our spectrograph, ends up in a different place on the photo- 

 graph. 



Now to go back to the star. The hot, deep parts of the star are send- 

 ing out light darts of every conceivable wave length. Except for a 

 few which are stopped in the atmosphere of the star these light darts 

 leave the star, and after traveling for years through empty space a 

 small part of them strike the earth, and a still smaller part fall upon 

 the lens or mirror of one of our telescopes, and are collected together 

 and sent through a spectrograph (fig. 3). 



The spectrograph will sort these darts out according to their wave 

 length, sending the short, blue ones to one end of the photographic 

 plate, the longer, green ones to the middle, and the still longer, red 



A -\/W» -WV* 



C ^W\^ ^ = (J) 



:♦ = ©■ 



ft -VNA/* -v-W* 



Figure 3. — The formation of a continuous spectrum with dark absorption lines 

 Light darts of every conceivable wave length and fi'equency of vibration are emitted 

 by the hot interiors of stars. If these could pass to the earth unobstructed, our 

 spectrograph would sort them into a continuous band on the photographic plate, the 

 light darts of longer wave length (red light) going to one end of the plate and 

 those of shorter wave length (violet light) going to the other end. The two atoms 

 shown in the diagram are supposed to be in the atmospliere of a star and to be 

 capable of becoming excited through the absorption of quantities of energy exactly 

 corresponding to the quantities carried by the light darts C and O. As a result, 

 these particular light darts are absorbed in the stellar atmosphere and are missing 

 from the otherwise continuous band of color which falls upon the plate. The 

 spectrum shows a bright background crossed by two absorption lines. 



<ines to the other end of the plate. Since the deep layers of the star 

 are giving out light darts of every possible wave length, after pass- 

 ing through the prism, there will be a solid band of color. We caU 

 this a continuous spectrum. 



But the interesting thing is that some of these light darts do not 

 escape from the star. Some of them of particular wave lengths are 

 sidetracked by atoms which absorb them in the outer layers of the 

 star, and never get to the earth at all. Where they would have fallen 

 on the plate there are dark empty spaces. This is how we first dis- 

 covered that there are atoms in the stars. These dark spaces on the 

 plate we know as spectral lines and they make up our code message 

 from the stars. 



