THE BIRTH OP THE SUN AND PLANETS 25 



But the evolutionary aspect of this wonderful universe 

 is, perhaps, more impressive still. We have seen that 

 the objects in it illustrate every phase of growth. Some 

 120,000 nebulae (calculated) show the earlier stages. 

 Dark stars, of which many are positively known, and the 

 total number is suspected to be very great, tell the end 

 of the story the fate of our sun. The nebulas range 

 from apparently motionless objectsthough nothing can 

 be absolutely motionless in the universe to great spiral 

 structures that express an infinite constructive energy. 

 In the case of the nebula in Orion a dark patch eats into 

 the heart of the filmy white cloud, and a system of six 

 stars is spread over the vacant space ; it seems clear that 

 the missing nebulous material has gone to the making of 

 these stars. Then there are nebulous stars, in which the 

 light of the growing centre struggles through as yet 

 uncondensed masses of gas. 



The spectroscope carries the story further. The 

 various colouring of the stars has always attracted 

 interest, but it remained for modern science to reveal its 

 meaning. Broadly speaking, the meaning is simple. A 

 bar of iron is hottest when it is white, cooler when it is 

 yellow, and still cooler when it is red. So your white or 

 bluish-white star is in the prime of life, your red star 

 sinking into old age, and your yellow star either before 

 or after its prime. But the astronomer does not rely on 

 this crude test of age. The spectroscopic analysis of 

 their light tells him not only how fast they are travelling 

 and of what material they are composed ; it also gives 

 most valuable indications of the condition of the elements 

 in the star, and therefore of its age. There are serious 

 difficulties with some classes of stars, into which we 

 cannot enter here ; but a sufficient number have been 

 classified to give us an idea of the life of a star. At first 

 its light shows a mass of metallic vapours at what we 



