WHAT LIES BETWEEN THE STARS — ADAMS 143 



Publications of the Society .^ Modern observations with blue and 

 red color filters show the remarkable degree to which the presence 

 of such obscuring clouds modifies the appearance of great areas of 

 the sky, especially in the region of the southern Milky Way, and 

 illustrate the use made by astronomers of the power of red light to 

 penetrate cosmic dust. 



We now know at least three forms of solid matter in interstellar 

 space. There are probably dark stars, that is, stars with temperatures 

 so low that they give out little or no visible light. If we knew 

 where to look we might be able to detect some of them with sensitive 

 heat-measuring devices — which will measure the heat given out by a 

 candle at a distance of many miles — but as it is we can only infer 

 their existence. We know that in the descending scale of stellar 

 temperature we find cooler and cooler stars until finally we observe 

 objects which give out only a faint red light. It seems reasonable 

 to assume that there may be many others with still lower temperatures 

 which have become invisible and are gradually approaching the con- 

 dition of cold bodies like our planets or asteroids. They are probably 

 small stars which have gone through the successive temperature stages 

 of stellar development at a rather rapid rate. 



In addition to these occasional dark stars there are in the spaces 

 between the visible stars great numbers of smaller masses of matter, 

 "chunks" as Dr. Hubble has called them, such as now and then fall 

 upon the earth in the form of meteorites. They are cold bodies with 

 the chill of the depths of space upon them, commonly ranging in 

 mass from a few pounds to a few tons. 



Finally and most important, we have the dust of space, often 

 gathered into huge cosmic clouds which weaken or even blot out the 

 stars behind them and give us much of the variegated pattern of the 

 Milky Way. 



There is, however, another form in which we find matter existing 

 in space, matter not in the solid state but in the form of gas, consist- 

 ing of molecules, atoms, and even portions of atoms, the tiny elec- 

 trons of the physicist. Much of our knowledge of this subject is 

 of very recent date, and because it is new and because it is certain 

 to afi^ect our views of the conditions in interstellar space I should 

 like to discuss it in a simple way this evening. 



This brings us at once to a consideration of a few elementary facts 

 about the spectrum, for it is from the spectrum that we gain essen- 

 tially all our knowledge of matter in the gaseous state. As you all 

 know, white light is a mixture of several primary colors and the 

 eye combines them into an impression which we call white. The 



» Pnbl. Astron. Soc. Pacific, vol. 52, p. 80, 1940. 



