ON THE ORIGIN OF THE PLANETARY SYSTEM. 189 



they must be of colossal size and distance. The former 

 does not appear very probable, because small masses 

 very soon give out their heat, and hence we are left 

 to the second alternative, that they are of huge di- 

 mensions and distances. The same conclusion had 

 been originally drawn by Sir W. Herschel, on the 

 assumption that the nebulas were heaps of stars. 



With those nebulae which, besides the lines of gases, 

 also show the continuous spectrum of ignited denser 

 bodies, are connected spots which are partly irresolv- 

 able and partly resolvable into heaps of stars, which 

 only show the light of the latter kind. 



The countless luminous stars of the heavenly firma- 

 ment, whose number increases with each newer and 

 more perfect telescope, associate themselves with this 

 primitive condition of the worlds as they are formed. 

 They are like our sun in magnitude, in luminosity, and 

 on the whole also in the chemical condition of their 

 surface, although there may be differences in the quan- 

 tity of individual elements. 



But we find also in space a third stadium, that of 

 extinct suns ; and for this also there are actual evi- 

 dences. In the first place, there are, in the course of 

 history, pretty frequent examples of the appearance of 

 new stars. In 1572 Tycho Brahe observed such a one, 

 which, though gradually burning paler, was visible 

 for two years, stood still like a fixed star, and finally 



