HOW THE HEAT IS KEPT UP. 37 



form of a star without being surrounded by a nebula to 

 any considerable extent. The aspect of the heavens will 

 also offer us many suggestions as to the future course of 

 the evolution through which the sun appears destined to 

 pass. There are many stars whose lustre seems declining 

 from what it must once have been. We are acquainted 

 with some stars which radiate quantities of light and 

 heat quite inconsiderable in proportion to their masses. 

 Such bodies as these have evidently advanced towards the 

 conditions of mere red-hot globes of solid matter, and 

 their further decline and ultimate extinction appear to be 

 impending. Unfortunately, however, our observations of 

 stars in stages still further advanced must necessarily be 

 of a very incomplete nature. Once a star has parted with 

 its lustre and become a mere mass of non-luminous 

 matter we are unable to see it. Of course I am speaking 

 here of stars properly so called and not of planets, which 

 are visible not from any light of their own, but in conse- 

 quence of the sun's light which falls upon them. The 

 stars are, however, usually speaking, millions of times as 

 far from us as are the planets. The beams of our sun are 

 utterly incapable of penetrating to the depths of space 

 with an intensity which would be sufficient to render dark 

 bodies visible. Our sun would, in fact, shed no more light 

 on a dark globe near the star Vega than Vega now sheds 

 on us. To Bee the dark stars is, therefore, impossible, but 

 we are not left entirely destitute of information in respect 

 to them. The dark stars, though invisible, are often very 

 massive ; they have the power of attracting objects in 

 their vicinity. It sometimes happens that the movements 

 of a bright star are influenced by the attraction of a dark 



