458 ASTROPHYSICS 



the mass must be nearly a maximum, for the low density indicates 

 a constitution that is still gaseous. 



Passing time brings a lowering of average temperature. The color 

 passes from yellow to the red, in consequence of lower radiating tem- 

 peratures and increasing general absorption by the atmosphere. The 

 hydrogen lines become indistinct, metallic absorption remains pro- 

 minent, and broad absorption bands are introduced. In one type, of 

 which Alpha Herculis * is an example, these bands are of unknown 

 origin; in another, illustrated by 19 Piscium, 2 they have been de- 

 finitely identified as of carbon origin. The relation between the two 

 types is not clear. It has even been advocated that the evolutionary 

 process divides shortly after passing the solar stage : that the reddish 

 stars with absorption bands sharply terminated on the violet edges 

 are on one branch, and that the very red stars with absorption bands 

 sharply defined on the red edges are on the other branch. This plan 

 of overcoming a difficulty seems to me to introduce a greater difficulty ; 

 and I do not doubt that systematic investigation will supply the con- 

 nections now missing. That the denser edges of the bands in Type 

 iv 3 Secchi should occupy the same positions as the denser edges of 

 absorption bands in Type in 4 can hardly be without significance; 

 and Keeler's view that the carbon absorption bands in Type iv are 

 matched by carbon radiation, in some stars, at least, of Type in, 

 suggests a most promising line of investigation for powerful instru- 

 ments. 



There is scarcely room for doubt that these types of stars are ap- 

 proaching the last stages of stellar development. Surface tempera- 

 tures have lowered to the point of permitting more complex chemical 

 combinations than those in the sun. The development of "sun-spots " 

 on a large scale is quite probable, and the first struggles to form a 

 crust may be enacted. Type in includes the several hundred long- 

 period variable stars of the Omicron Ceti 5 class, whose spectra at 

 maximum brilliancy show several bright lines of hydrogen and other 

 elements. The hot gases and vapors seem to be alternately impris- 

 oned and released. It is significant that the dull red stars are all very 

 faint, there are none brighter than the 5$ magnitude. Their effect- 

 ive radiating power is undoubtedly very low. 



The period of development succeeding the red-star age of Type 

 iv has illustrations near at hand, in the planets Jupiter and the 

 earth ; invisible save by borrowed light. When the interior heat of a 

 body shall have become impotent, the future promises nothing save 

 the slow leveling influence of its own gravitation and meteorological 



1 Huggins, An Atlas of Representative Stellar Spectra, plate xii. 



2 Clerke's Problems in Astro-Physics, p. 218. 



3 Frost's Scheiner, p. 312. 



4 Ibid. p. 300. 



5 Clerke's Problems in Astro-Physics, p. 224. 



