8 EVOLUTION OF THE EARTH 



star, a Centauri, would on this same scale be seen as two 

 spheres revolving about each other at a distance apart equal 

 to 2 miles, and each comparable in size to the sun. This 

 double star would be situated at a distance of about 25,000 

 miles from our planetary system with its sun, but the other 

 stars in this part of the stellar system would be separated 

 from each other on the average by more than twice this dis- 

 tance. The Galaxy, or Milky Way, is the cloud-like zone of 

 faint stars which extends as a belt around the sky. The stars 

 in it appear faint and close together because of their remote- 

 ness. They seem to constitute the outer zone of our stellar 

 system, and its dimensions are only vaguely known. On this 

 diminutive scale the Milky Way might be found to be encom- 

 passed by a circle of a hundred million miles diameter, or it 

 might be more or less. 



The nebula. All hypotheses of earth origin derive the 

 planets and the sun from an antecedent nebulous or meteoritic 

 state. The cloudy patches of light known as nebulas, which 

 are revealed especially by stellar photography, are, however, 

 of several very different natures and it is a vital question as 

 to which, if any, of these types, could have given birth to our 

 planetary system. 



First are the irregular nebulae, diffuse clouds of luminous 

 matter, pervading whole groups of stars as in Orion and the 

 Pleiades, shown in Plate I, A, denser about certain stars, but 

 nevertheless enormously attenuated. This kind of nebulosity 

 is associated with certain regions of the Milky Way. From 

 the characteristics of their spectra, the stars in such nebulae 

 are regarded as young stars and the nebulous matter may 

 represent the remains of an antecedent stage. 



The planetary nebulae are a distinct type, comparatively 

 few in number, and also found associated with the Milky Way. 

 They show in the telescope faint, greenish, circular discs from 

 which they derive their name rather than from any known 



