WHAT LIES BETWEEN THE STARS ADAMS 147 



exposure times of about 4 hours. The results were conslusive. The 

 predicted lines of hydrocarbon gas CH all appeared in their correct 

 positions with their calculated intensities. In the case of cyanogen 

 gas CN, the evidence is based upon fewer lines but is equally strong. 

 Hence the existence of the gases CH and CN in interstellar space 

 may be regarded as practically certain. 



After this brief description of how these discoveries were made, a 

 few comments upon the meaning of the results may be of interest. 

 In the first place, we have learned that several of the common ele- 

 ments exist in space in the form of atoms; sodium, calcium, potas- 

 sium, and titanium have been identified, and it is very probable that 

 many if not all of the others could be recognized if only conditions 

 were favorable for the appearance of their spectra. Then we have 

 very recently found that two common gases, or at least two slightly 

 modified common gases, are present, cyanogen and hydrocarbon gas. 

 This is the first discovery of molecules in interstellar space. That 

 hydrogen, the most abundant element of all in the universe, has not 

 been discovered directly is due to the fact that the lines which it 

 could show under the conditions present in space are in an inaccessi- 

 ble part of the spectrum, and, like the lines of many other important 

 elements, are cut off by the ozone in the earth's atmosphere, which in 

 the words of Russell "lies like a black pall upon the dreams of the 

 astrophysicist." However, we do find hydrogen combined with 

 carbon in hydrocarbon gas and thus have ample evidence for its 

 presence. 



Although the lines of hydrocarbon gas are well marked and at 

 least one of them is fairly conspicuous, it is the enormous length of 

 the path of light from the stars rather than the density of the gas 

 which provides enough absorbing molecules. The actual density is 

 extraordinarily low. In a cubic mile of space there are probably 

 only a very few thousand molecules ; and when we remember that the 

 diameter of a molecule is less than one ten-millionth of an inch it is 

 easy to see that very little of the space is occupied. But if the path 

 is long enough, a good many molecules will be encountered by the 

 light from a distant star, and observable absorption lines will result. 

 The same reasoning holds true for lines originating from atoms such 

 as sodium and calcium. Dunham estimates that there is one sodium 

 atom in about 25 cubic yards of space, and yet in the spectra of very 

 distant stars the interstellar sodium lines are conspicuous. 



A calculation by Russell of the average density of interstellar gas 

 in general gives a value of about 2 preceded by 24 ciphers of the 

 density of water. So great are the distances, however, that in the 

 volume of space whose radius is equal to that of the nearest fixed 

 star, the mass of the interstellar gas amounts to about one-fourth 



430577 — J2 11 



