WHAT LIES BETWEEN THE STARS 



By Walter. S. Adams 

 Carnegie Institution of Washington, Mount Wilson Ohservatory, Pasadena, Calif. 



[With 4 plates] 



We are accustomed to think of the material upon which the astron- 

 omer works as consisting mainly of the sun and its planetary system, 

 occasional comets, and the vast array of stars and nebulae which dot 

 our skies at night. In other words the astronomer is largely con- 

 cerned with matter in a sufficiently condensed form either to radiate 

 light like the hot sun and stars or to reflect light like the cool planets 

 and satellites. In recent years, however, new information has obliged 

 us to consider more seriously what lies between the stars, and it is 

 this subject which I should like to discuss briefly with you this 

 evening. 



In the first place it is interesting to realize how much space there 

 really is in our stellar universe and how little of it is actually occu- 

 pied by the stars. If this room represented an average portion of 

 space and we let a floating speck of dust represent a star, we could 

 not allow another speck within the room to represent another star 

 because no matter where we put it the two would be too near each 

 other. The star nearest to the sun is about 25 million million miles 

 away. Another way of realizing how much of space is compar- 

 atively empty is through its average density. If we put together 

 everything we can observe directly, such as the stars and nebulae, 

 in the general neighborhood of our sun, and divide the total by the 

 volume of the space in which it lies, we find for each cubic inch 1 

 grain of matter divided by 1 followed by 22 ciphers. At the center 

 of our galaxy the density is probably 10 times greater. These values 

 may perhaps be in error by a factor of 10 but we need not feel the 

 deep concern of the individual who thought the lecturer gave 1 

 billion instead of 10 billion years for the possible life of our sun 

 and was enormously relieved when he discovered his error. 



1 Alexander F. Morrison Lecture. Reprinted by permission from Publications of the 

 Astronomical Society of the Pacific, vol. 53, No. 312, April 1941. 



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