3IO 



POPULAR SCIENCE MONTHLY. 



the sun's disk. When photographed with an instrument which excludes 

 from the sensitive plate all light except that which is characteristic of 

 the vapor of calcium, its surface is found to be dotted over with ex- 

 tensive luminous regions. Associated with these are the sun-spots, 

 the minute study of which has revealed some strikingly beautiful 

 phenomena, which have been most successfully drawn by Langley. 

 The surface of the sun in the regions devoid of spots is shown by the 

 photographs of Janssen to consist of brilliant granules separated by 

 darker spaces. Much might be said of the peculiar law of rotation 

 of the sun, which causes a point near the equator to complete an axial 

 rotation in much less time than a point nearer the poles. Much 

 might also be said of the periodicity of sun-spots, which at times are 



a Fig. 11. 6 



Eruptive prominence photographed in full sunlight at the Kenwood Observ- 

 atory, Chicago, March 25, 1895. a, at IOh. 40m. (height, 162.000 miles), b, at IOh. 58m 

 height, 281,000 miles). (Figs. 10 and 11 are reproduced on the same scale.) 



very numerous and again, as at present, are absent from the sun's disk 

 for weeks together. But enough has already been told to indicate some 

 of the chief characteristics of this central star of the solar system, 

 which has thousands of counterparts among other stars of the same 

 spectral class. 



We are now approaching the last chapters in the life history of a 

 star. After the solar stage has passed the color changes from yellow to 

 orange, and subsequently to red, as the temperature falls. The spectral 

 lines of hydrogen become fainter and fainter and finally disappear com- 

 pletely. The lines of the metallic elements, on the contrary, become more 

 and more complex and the changes in their relative intensities are those 

 which are characteristic of lower temperatures. But curiously enough. 



