^ee — Temperature of the Sun and Ages of Stars and Nebulae. 37 



Thus the diffused nebulae are near the temperature of space, 

 or approximately — 273^ C. This may also be inferred from 

 other considerations. If such diffused masses were appre- 

 ciably heated, they would soon cool off; and, besides, mole- 

 cules on their outskirts having sensible molecular velocities, 

 would escape into interstellar space. How the light of such 

 masses is maintained is quite unknown, but it seems not 

 improbable that it is due to electric luminescence such as we 

 observe in the tails of comets, which also shine at tempera- 

 tures approaching the absolute zero. We may therefore sup- 

 pose the diffused and irregular nebulae, as well as the milky 

 nebulosity so abundantly scattered over the sky, to be 

 intensely cold. It is an impressive fact that hydrogen and 

 nebulium are the only elements recognized in the nebulae, and 

 all other elements presumably present are wholly non- 

 luminous. 



( b. ) Stars of the first class are at the maximum temperature 

 and already condensed to the smallest hulk consistent with the 

 laws of gaseous constitution. The high temperature of the 

 Sirian stars is inferred generally from the nature of the light 

 emitted by these bodies, and in the particular case of Sirius, 

 is proved by the enormous radiation of that star compared to 

 that of our Sun. Thus, while the mass of Sirius is only twice 

 that of our Sun, its radiation is shown to be forty or fifty 

 times the greater of the two bodies. It follows, therefore, 

 that the Sirian stars are intensely hot. By the above law of 

 temperature such heat can be developed and such radiation 

 maintained only when the radius of the condensing mass is 

 relatively small. The Sirian stars have therefore already 

 shrunk to small bulk, and the contention recently current 

 among astrophysicists, that the Sirian stars are of great bulk, 

 and resemble nebulae, can no longer be supported. 



Such tremendous radiation as we observe could not, it 

 appears, be maintained by the gravitational shrinkage of the 

 mass, except when the radius is small, arid the force of grav- 

 ity correspondingly enormous. As respects volume therefore 

 as well as temperature the Sirian stars are as far removed 

 from the nebular condition as possible; and any spectral 

 parallel between these two classes of objects should be ex- 



