THE SPECTRA OF STARS AND NEBULAS* 



I'd 



lighl that it emits, so that if such a vapour lies in limit <»i* ;i 

 source at higher temperature giving a continuous spectrum, the 

 result is a continuous spectrum crossed by dark lines. This is 

 called an absorption spectrum, and Kirchhoff's observation is 

 of fundamental importance in astronomy, because the spectrum 

 of the sun and the spectra of nearly all the stars show dark 

 lines on a bright, continuous background. The experiment 

 shows that we can identify the substances which produce such 

 dark lines, just as surely as if they were bright, by the process 

 of matching them by emission spectra artificially produced. 



Such, then, are the main principles of spectrum analysis. We 

 may next consider their application to celestial bodies, begin- 

 ning with the sun, which may properly be regarded as the 

 nearest star. 



The dark lines which are characteristic of the spectrum of 

 sunlight were first accurately mapped by the German physicist 

 Fraunhofer in 1814, and have since been known as the Fraun- 

 hofer lines. In more recent times the magnificent photographs 

 obtained by Eowland exhibit not less than 20,000 of these 

 lines, which have been carefully entered in a great catalogue, 

 showing their relative intensities and their positions on the 

 scale of wave-lengths of the vibrations which produce them. 

 The. chemical significance of a great number of these lines has 

 been determined by the application of Kirchhoff's principle of 

 the reversal of lines, by Kirchhoff himself, and subsequently by 

 Lockyer, Eowland, and others. The great majority of the more 

 prominent lines have, in fact, already been matched by spectra 

 produced in the laboratory, largely from common substances 

 such as hydrogen, sodium, magnesium, iron, and calcium. 



In accordance with Kirchhoff 's experiment, we interpret the 

 dark lines of the solar spectrum as indicating that the bright 

 central ball of the sun — which of itself would give a continuous 

 spectrum — is surrounded by luminous gases and vapours which 

 produce the dark lines by their absorption. At ordinary times 

 this atmosphere is not visible, because it is not so bright as the 

 diffused light of the sky, but its existence is fully confirmed by 

 observations during total eclipses of the sun, when the glare of 

 the surrounding sky is shut off by the moon's shadow. Under 

 these conditions the direct emission spectrum of the sun's 

 atmosphere may be observed or photographed. In place of the 

 usual dark Fraunhofer lines the expected multitude of bright 

 lines is then observed in the spectrum at the sun's edge during 

 the few seconds that this comparatively shallow "reversing 

 layer " or " flash stratum " remains uncovered by the moon. It 



