WHAT LIES BETWEEN THE STARS — ADAMS 145 



a star like our sun contains not only sodium but a great variety of 

 other elements and so we get not only sodium lines but an immense 

 number of other lines as well — such as those of hydrogen, calcium, 

 iron, and some 60 other elements. 



It is hardly necessary to say that the spectra of the elements, these 

 characteristic patterns of bright lines which define them uniquely 

 and individually, have been studied with extraordinary care by physi- 

 cists and astronomers alike for many years. Maps have been made, 

 the intensities of the lines measured, and their positions determined 

 with an almost uncanny degree of precision. As a result astronomers 

 know almost every element which enters into the composition of the 

 sun and even the most distant stars, merely through comparison of 

 the positions and intensities of the dark lines produced in their at- 

 mospheres with the well-recognized bright lines of terrestrial 

 elements. 



One other point should be considered. When we observe a star, its 

 light comes to us through the earth's atmosphere which is itself com- 

 posed of various gases. These gases are cold and because they are 

 cold remain in the form of molecules. Intense heat will break up 

 molecules into atoms, and in the atmospheres of the hotter stars we 

 find only the lines due to atoms. Molecules, however, can also emit 

 and absorb light and give spectrum lines arranged in characteristic 

 patterns, the principal difference from those produced by atoms being 

 that molecules usually give an enormous number of closely packed 

 lines arranged in the form of bands. As a result when we observe the 

 spectrum of a star we find superposed upon it the bands of gases 

 such as oxygen, water vapor, and carbon dioxide in the atmosphere 

 of the earth. These bands lie mainly in the red and infared portion 

 of the spectrum. 



About 40 years ago two very narrow sharp lines were observed in 

 the violet part of the spectrum of a star in the constellation of Orion. 

 They were at once identified with well-known lines of calcium, but 

 their positions did not vary periodically as did those of the lines from 

 the star, and it was clear that they were not of stellar origin. They 

 were called provisionally "stationary" lines, and Sir Arthur Edding- 

 ton suggested the bold hypothesis that they originated in the absorp- 

 tion of the atoms of calcium gas in interstellar space. Some 20 years 

 later two more such lines were discovered at the Lick Observatory in 

 the yellow portion of the spectrum. These are due to sodium and 

 are the characteristic lines to which we have already referred. In 

 1936, observations with a spectroscope on the 100-inch telescope at 

 Mount Wilson led to the discovery of several additional lines, a few 

 of which were identified as due to titanium and potassium. By this 

 time the interstellar origin of all such lines had been fully established. 



