THE SUN'S PLACE AMONG THE STARS ^ 



By Walter S. Adams 

 Mount Wilson Observatory of the Carnegie Institution of Washington 



[With 5 plates] 



In previous lectures in this series you have had described to you 

 the many skillful and important investigations that have given us 

 such extensive knowledge of the sun as a source of light and heat, 

 of its composition, physical nature, and characteristics, and of the 

 far-flung gravitational attraction by which the sun holds the planets 

 in their courses. I should like to consider with you the sun in its 

 relation to the stars, how it compares in size and brightness and mass 

 with the vast number of other suns by which it is surrounded, and 

 what from analogy and comparison with the stars we may reason- 

 ably expect its future life history to be. 



It will perhaps be of interest at the outset to interpret our subject 

 literally and to define according to the best of our knowledge the 

 geographical location of the sun among the stars. The stars in the 

 observable universe are grouped into systems scattered like islands 

 throughout space and separated by enormous distances, which light 

 requires millions of years to traverse. These systems of stars, the 

 extra-galactic nebulae of the astronomer (pi. 1, fig. 1), extend out 

 to the limits of the largest telescopes and, generally speaking, seem 

 to be distributed uniformly; that is, the same volume of space in 

 all directions and at all distances contains about the same number of 

 stellar systems or nebulae. Occasionally clusters of nebulae are 

 found in which hundreds of these objects appear upon a single 

 photograph, but from a statistical point of view these clusters do not 

 affect the uniformity of the distribution seriously. The space be- 

 tween the nebulae seems to be singularly free from matter ; probably 

 some traces of gas and of cosmic dust are present here and there, but 

 observations of the light of the most distant nebulae show that even 

 throughout these immense distances there can be but very little 

 obscuring material. 



* The fourth Arthur lecture, deUvered at the Smithsonian Institution on Dec. 18, 1934. 



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