502 POPULAR SCIENCE MONTHLY. 



solid. It is therefore not strictly correct to say, as is sometimes done, 

 that an incandescent gas always gives a spectrum of bright lines. It will 

 give such a spectrum only when it is transparent through and through.* 

 A gaseous mass, so large as to be opaque, would, if it were of the 

 same temperature inside and out, give a continuous spectrum, without 

 any dark lines. But the laws of temperature in such a mass show that 

 it will be cooler at the surface than in the interior. This cooler en- 

 velope will absorb the rays emanating from the interior as in the case 

 when the latter is solid. We conclude, therefore, that the fact that the 

 great majority of stars show a spectrum like that of the sun, namely, a 

 continuous one crossed by dark lines, does not throw any light on the 

 question whether the matter composing the body of the star is in a solid, 

 liquid or gaseous state. The fact is that the most plausible theories of 

 the constitution of the sun lead to the conclusion that its interior mass 

 is really gaseous. Only the photosphere may be to a greater or less 

 extent solid or liquid. The dark lines that we see in the solar spectrum 

 are produced by the absorption of a comparatively thin and cool layer 

 of gas resting upon the photosphere. Analogy as well as the general 

 similarity of the spectra lead us to believe that the constitution of most 

 of the stars is similar to that of the sun. 



CLASSIFICATION OF STELLAB SPECTRA. 



When the spectra of thousands of stars were recorded for study, such 

 a variety was found that some system of classification was necessary. 

 The commencement of such a system was made by Secchi in 1863. It 

 was based on the observed relation between the color of a star and the 

 general character of its spectrum. 



Arranging the stars in a regular series, from blue in tint through 

 white to red, it was found that the number and character of the spectral 

 lines varied in a corresponding way. The blue stars, like Sirius, Vega 

 and a Aquilse, though they had the F lines strong, as well as the two 

 violet lines H, had otherwise only extremely fine lines. On the other 

 hand, the red stars, like oc Orionis and a Scorpii, show spectra with 

 several broad bands. Secchi was thus led to recognize three types of 

 spectra, as follows: 



The first type is that of the white or slightly blue stars, like Sirius, 

 Vega, Altair, Eigel, etc. The typical spectrum of these stars shows all 

 seven spectral colors, interrupted by four strong, dark lines, one in the 



* As this principle is not universally understood, it may be well to remark that it results 

 immediately from Kirchoff's law of the proportionality between the radiating and absorbing 

 powers of all bodies for light of each separate wave-length. When a body, even if gaseous in 

 form, is of such great size and density that light of no color can pass entirely through it, then 

 the consequent absorption by the body of light of all colors shows that throughout the region 

 where the absorption occurs there must be an emission of light of these same colors. Thus light 

 from all parts of the spectrum will be emitted by the entire mass. 



