202 LECTURE ON THE EESULTS OF SPECTEUM ANALYSIS 



and violet parts of the spectrum, and tlie orange rays therefore, wliicli are com- 

 paratively free from lines, predominate. 



In the delicate blue companion, the strongest groups of lines ai'e found in the 

 yellow, orange, and in part of the red. In the arrangement of these groups of 

 lines we have a sufficient cause for the predominance of the other portions of 

 the spectrum which unite in the eye to give the blue purple color of the light 

 of this star. 



We have, therefore, shown that the colors of the stars are produced by the 

 vapors existing in their atmosphere. The chemical constitution of a star's 

 atmosphere will depend upon the elements existing in the star and upon its tem- 

 perature. 



VARIABLE STARS. 



The brightness of many of the stars is found to be variable. From night to 

 night, from month to month, or from season to season, their light may be ob- 

 served to be continually changing, at one time increasing, at another time dimin- 

 ishing. The careful study of these variable stars by numerous observers has 

 shown that their continual changes do not take place in an uncertain or irregular 

 manner. The greater part of these remarkable objects wax and wane in accord- 

 ance with a fixed law of periodic variations which is peculiar to each. 



We have been seebiug for some time to throw light upon this strange phe- 

 nomenon by means of observation of their spectra. If in any case the periodic 

 variation of brightness is associated with physical changes occurring in the star, 

 we might obtain some information by means of the prism. Again, if the diminu- 

 tion in brightness of a star should be caused, by the interposition of a dark body, 

 then, in that case, if the dark body be surrounded with an atmosphere, its presence 

 might possibly be revealed to us by the appearance of additional lines of ab- 

 sorption in the spectrum of the star when at its minimum. One such change in 

 the spectrum of a variable star we believe we have already observed. 



Betelgeux is a star of a moderate degree of variability. When this star was 

 at its maximum brilliancy in February last, we missed a group of lines, the ex- 

 act position of which we had determined with great accuracy by micrometric 

 measurement some two years before. 



We have observed the spectra of several variable stars at different phases of 

 their periodic variation, but our results are not yet complete.- 



It is worthy of notice that the variable stars which have a ruddy or an orange 

 tint possess spectra analogous to that of Betelgeux and /? Pegasi. 



As an example of this group of variable stars, Mr. Ladd will throw upon the 

 screen the spectrum of /x Oephei when at its maximum. 



TEMPORARY STARS. 



With the variable stars modern opinion would associate the remarkable phe- 

 nomena of the so-called new stars which occasionally, but at long intervals, have 

 suddenly appeared in the sky. But in no case has a permanently bright star 

 been added to the heavens. The splendor of all these objects was temporary 

 only, though whether they died out or still exist as extremely faint stars is 

 uncertain. In case of the two modern temporary stars, that seen by Mr. Hind 

 in 1845, and the bright star recently observed in Corona, though they have lost 

 their ephemeral glory, they still continue as stars of the tenth and eleventh 

 magnitudes. 



'ihe old theories respecting these strange objects must be rejected. We can- 

 not believe with Tycho Brahe that objects so ephemeral ai-e new creations, nor 

 with Riccioli that they are stars brilliant on one side only, which have been 

 suddenly turned round by the Deity. The theory that they have suddenly 

 darted towards us with a velocity greater than that of light, from a region of 

 remote invisibility, will not now find supporters. 



