CHAPTERS ON THE STARS. 137 



Companion of Sirius 1.2 



Companion of Procyon 0.2 



It will now be interesting to compare the brightness of these bodies 

 with that which the Sun would have if seen at their distance. In a 

 former chapter we showed how this could be done. The results are: 



At the distance of Procyon the apparent magnitude of the Sun 

 would be 2 m .8. At the distance of Sirius, it would be 2 m .3. Supposing 

 the Sun to be changed in size, its density remaining unchanged, until 

 it had the same mass as the respective companions of Procyon and 

 Sirius, its magnitudes would be: 



For companion of Procyon 3.9 



For companion of Sirius 2.9 



The actual magnitudes of these companions cannot be estimated with 

 great precision, owing to the effect of the brilliancy of the star. From 

 the estimate of the companion of Sirius, by Professor Pickering, its 

 magnitude was about the eighth. It is probable that the magnitude 

 of the companion of Procyon is not very different. It will be seen 

 that these magnitudes are very different from those which they would 

 have were the companions models of the Sun. What is very curious is 

 that they differ in the opposite direction from the stars in general, and 

 especially from their primaries. Either they have a far less surface bril- 

 liancy than the Sun or their density is much greater. There can be no 

 doubt that the former rather than the latter is the case. 



This great mass of the two companions as compared with their bril- 

 liancy suggests the question whether they may not shine, in part at 

 least, by the light of their primaries. A very little consideration will 

 show that this cannot be the case. A simple calculation will show that, 

 to shine as brightly as they do, the diameter of the companion of Sirius 

 would have to be enormous, at least 1-30 its distance from Sirius. 

 Moreover, its apparent brightness would vary so widely in different 

 parts of its orbit that we should see it almost as well when near Sirius 

 as when distant from it. The most likely cause of the small bright- 

 ness is the low temperature of the body. 



Gaseous Constitution of the Stars. 



The results of the last chapter point to the conclusion that the 

 stars, or at least the brighter among them, are masses of gas, more or 

 less compressed in their interior by the action of gravitation upon their 

 more superficial parts. We have now to show how this result was ar- 

 rived at, at least in the case of the San, from different considerations, 

 before the spectroscope had taught us anything of the constitution of 

 these bodies. 



We must accept, as one of the obvious conclusions of modern science, 



