THE CONTENTS OF INTERSTELLAR SPACE ' 



By C. G. Abbot 



Secretary, Smithsonian Institution 



[With 3 plates] 



Many years ago the late Professor Barnard, whose devotion to 

 celestial photography was extraordinary, found many regions among 

 the multitudes of stars in the Milky Way where stars seem to be 

 absent. Plates 1 and 2 show some outstanding instances of this. To 

 explain these vacancies we must suppose either that there are no 

 stars in these directions, so that we see clear through our system to 

 the void beyond, or else that dark clouds of matter intercept the light 

 of the stars which lie beyond them. Such clouds, as they are supposed 

 to be, are not connected in any way to our earth, as rain clouds are, 

 for however many times one photographs these parts of the heavens 

 the dark starless regions remain unchanged. The results of recent 

 studies, described in what follows, go to show that the dark regions 

 are in fact caused by condensations of a rare medium which fills all 

 space within our galaxy. 



Although in the opinion of astronomers the dark-cloud hypothesis 

 was more probable than the star-vacancy hypothesis, it was some 

 years before the proof came. In order to make it clear we must 

 recall that when the light of the sun or of a star is made to pass 

 through a spectroscope, not only does the rainbow band of colors 

 appear, but this band is crossed by numerous dark (or sometimes 

 bright) lines. These lines are the identifying marks of the chemical 

 elements. (See pi. 3.) Hydrogen, for instance, has a line in the 

 red, one in the blue, one in the indigo, and one in the violet, all of 

 which are very conspicuous. Calcium (the metallic part of the 

 familiar compound, lime) has 1 line in the green, 2 in the indigo, 

 2 very notable ones (often called " H " and " K ") in the violet, and 



^ This article is basod iip.oii two tocliiiiciil papers, one by Otto Struve (Popular Astron- 

 omy, vol. 41, no. 8, p. 423, 19.3.';). the other by J. S. Plaskett and .T. A. Pearce (Publica- 

 tions of tlie Dominion Astrophysical Observatory, vol. 5, no. 3, p. 1G7, 1931!). Although 

 parts of the article are larj^ely in the words of these authors, It was necessary In adapting 

 the material for use in the Smithsonian Report to malio sli;;lit verbal chau;;is throu,t;hout, 

 and for that reason no quotation marks are used. Pages 212 to 215 are based on 

 Plaskett and Pearce, pages 216 to 218 on Struve. 



72774—35 15 211 



