302 PHYSICAL SCIENCE 



of our own little, probably abnormal, system, let 

 us now look at the more usual life of a star. 



We have seen that a gaseous nebula will 

 spin faster as it contracts under its own gravita- 

 tion, but other changes will also occur. It 

 radiates heat, but, owing to the fall of its outer 

 layers towards the centre, more heat is developed 

 from this loss of mechanical energy, and the 

 nebula or star grows hotter. As the tempera- 

 ture rises, radiation pressure reinforces gaseous 

 pressure, and these two causes oppose gravitation. 

 All the time the star is growing denser, and the 

 possibility of further shrinkage, and therefore of 

 heat development, less. Hence a maximum 

 temperature must be reached, after which the 

 star, having passed middle age, gets slowly older 

 and colder. 



These mathematical predictions are well sup- 

 ported by astronomical evidence. The earliest 

 classification of stars was made on a scale of 

 apparent brightness. Hipparchus chose about 

 twenty of the brightest stars as of the first 

 magnitude, and classed the faintest stars he 

 could see as of the sixth magnitude. In the 

 modern form of this grouping, a star of one 

 magnitude gives 2.5 times as much light as one 

 of the next lower magnitude, and a difference 

 from the first magnitude to the sixth corresponds 

 to a ratio in brightness of a hundred to one. 



When photography was applied to this 

 problem, a new scale of brightness was obtained, 

 for the ordinary plate is more sensitive than the 

 eye to blue light and less sensitive to red. 

 Hence the number found by subtracting the 

 visual magnitude from the photographic is a 



