THE EXPANDING UNIVERSE — HUBBLE 121 



gion is homogeneous, very much the same everywhere and in all 

 directions. 



We may now present a rough sketch of our sample of the universe. 

 The faintest nebulae that can be detected with the largest telescope in 

 oi^eration (the 100-inch reflector on Mount Wilson) are about 2 mil- 

 lion times fainter than the faintest star that can be seen with the naked 

 eye. Since we know the average candlepower of these nebulae, we 

 can estimate their average distance — 500 million light-years. A 

 sphere with this radius defines the observable region of space. 

 Throughout the sphere are scattered about 100 million nebulae, at vari- 

 ous stages of their evolutional development. These nebulae average 

 about 100 million times brighter than the sun and several thousand 

 million times more massive. Our own stellar system is a giant nebula, 

 and is presumably a well-developed, open spiral. The nebulae are 

 found, as has been said, singly, in groups and in clusters but, on the 

 grand scale, these local irregularities average out and the observable 

 region as a whole is approximately homogeneous. The average in- 

 terval between neighboring nebulae is about 2 million light-years, and 

 the internebular space is sensibly transparent. 



THE LAW OF RED t-IIIFTS 



Another general characteristic of the observable region has been 

 found in the law of red shifts, sometimes called the velocity-distance 

 relation. Tliis feature introduces the subject of spectrum analysis. 

 It is well known that, in general, light from any source is a composite 

 of man}^ individual colors or wave lengths. When the composite beam 

 passes through a glass prism or other suitable device, the individual 

 colors are separated out in an ordered rainbow sequence, known as a 

 spectrum. The prism bends the light waves according to the wave 

 length. The deflections are least for the long waves of the red and 

 are greatest for the short waves of the violet. Hence position in the 

 spectrum indicates the wave length of the light falling at any par- 

 ticular place in the sequence. 



Incandescent solids, and certain other sources, radiate light of all 

 possible wave lengths, and their spectra are continuous. Incandes- 

 cent gases, however, radiate only certain particular wave lengths, and 

 their spectra, called emission spectra, consist of various isolated colors 

 separated by blank spaces. The patterns are well known, hence gases 

 in a distant light source can be identified by their spectra. 



The sun presents a third kind of spectrum, known as an absorption 

 spectrum. The main body of the sun furnishes a continuous spec- 

 trum. The heavy atmosphere surrounding the main body is gaseous 

 and would normally exhibit an emission spectrum. Actually, the at- 

 mosphere, because it is cooler than the main body, absorbs from the 



