See — Temperature of the Sun and Ages of Stars and Nebxdae. 39 



condensed that gravity is intense, the outer atmosphere ought 

 to be of hydrogen, such as we observe in the Sirian stars. 

 The heavier elements in the Sirian stars are pressed down by 

 gravity, and their spectral lines are either faint, or entirely 

 absent. Now if our Sun had already passed through the 

 Sirian stage, and the temperature was falling, the hydrogen 

 atmosphere which had been separated from the other ele- 

 ments by the effects of gravity ought still to surround its 

 globe. As all the elements in the Sun are fairly evenly 

 mixed, such heavy vapors as calcium and iron mixing freely 

 with those of light elements like hydrogen and helium, we 

 infer that our Sun has not yet passed through the Sirian 

 stage of development. The lower temperature of solar stars 

 thus indicates an earlier condition than that met with in the 

 Sirian stars. 



(d.) /Stars of the third class are at a still earlier stage 

 of development. This inference is based upon well 

 known spectral phenomena which connect classes I, II, 

 and III. If the first class stars are related to the second 

 class stars as stated above, the continuity of spectral 

 lines show that the third class stars are still younger, and 

 further from the maximum of their temperature curves. It 

 is a fact of great significance that the Milky Way, presumably 

 the oldest part of the visible creation, is composed almost 

 wholly of Sirian stars. On the other hand, the solar stars 

 and to a greater extent the orange stars, seem to cluster about 

 the poles of the galaxy. The orange stars are in fact rela- 

 tively thickest in those regions of the sky which are poor in 

 stars, like Hydra, Microscopium, etc. This depth of color of 

 the stars remote from the Milky Way frequently attracted the 

 attention of the writer while occupied with the survey of the 

 Southern hemisphere. If the reddish stars have a larger bulk 

 than the older more condensed stars, they would naturally 

 receive more accretions of dark matter from surrounding 

 space, the chance of collision at periastron being thereby in- 

 creased, and one might naturally explain in this way the 

 greater variability of the third class stars. 



(e.) Present and Past Temperatures of the Sun. If we 

 adopt the effective temperature of the Solar photosphere ex- 



