3o8 POPULAR SCIENCE MONTHLY. 



origin; in another, illustrated by 19 Piscium, they have been definitely 

 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 over- 

 coming a difficulty seems to me to introduce a greater difficulty; and 

 I do not doubt that systematic investigation will supply the connec- 

 tions now missing. That the denser edges of the bands in Type IV 

 (Secchi) should occupy the same positions as the denser edges of ab- 

 sorption bands in Type III, 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 III suggests a most 

 promising line of investigation for powerful instruments. 



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 III includes the several hundred long- 

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

 maximum brilliancy show several bright lines of hydrogen and other 

 elements. The hot gases and vapors seem to be alternately imprisoned 

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

 faint; — there are none brighter than the 5y 2 magnitude. Their 

 effective 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 ele- 

 ments. It is true that a collision may occur to transform a dark body's 

 energy of motion into heat, sufficient to convert it into a glowing nebula, 

 and start it once more over the long path of evolution. This is a 

 beautiful theory, but the facts of observation do not give it satisfactory 

 support. There is little doubt that the principal novas of recent years 

 have been the results of collisions, either between two massive dark 

 bodies, or between a massive body and an invisible nebula. The sud- 

 denness with which intense brilliancy is generated would seem to call 

 for the former, but the latter is much more probable, in view of many 

 facts. The nebular spectra of the novas are generated in a few months ; 

 but in every case thus far observed the bright nebular bands grow 

 faint very rapidly, and in the course of a few years leave a continuous 

 spectrum, — apparently that of an ordinary star. Either the masses 



