COMPOSITION OF OUR UNIVERSE — BROWN 201 



fixed by their relative cross sections for proton capture, which will 

 depend in turn upon the temperature condition within the stars. 



Thus one would expect to find major differences in the composi- 

 tion of stars with respect to all elements which can undergo thermo- 

 nuclear reactions at the temperatures which exist in stellar interiors. 

 But there are limitations to the temperatures which exist in stars, 

 and as a result one would not expect elements heavier than oxygen 

 to undergo thermonuclear reaction to any appreciable extent. Does 

 this mean that stars may also differ appreciably from one another 

 with respect to the abundances of their heavier nonreactive constitu- 

 ents? It is very diflBcult to compare the abundances of elements in 

 stars of widely differing spectral characteristics with any great pre- 

 cision. However, stars of similar spectral type can be compared. 

 Recently, Greenstein compared the abundances of several elements 

 in a number of F-type stars which possess widely different luminos- 

 ities. He found that for the ordinary stars of this type no well- 

 established abundance difference within a factor of two exists. In 

 other words, it appears that stars possess nearly identical compositions 

 with respect to elements heavier than oxygen. 



If this result is correct for our own galaxy, is it true of the billions 

 of galaxies which are visible to us ? Unfortunately the data are too 

 meager to permit us to draw such a sweeping conclusion. Neverthe- 

 less, the probability of such an assumption being correct appears to 

 be considerable. 



INTERSTELLAR MATTER 



It is well recognized that in certain regions of our own and other 

 galaxies as much as 50 percent of the mass exists in the form of finely 

 divided matter distributed throughout interstellar space. Although 

 this matter is extremely dilute, the tremendous distances between our 

 sun and other stars result in there being sufficient gas between some 

 stars and the earth to produce definite absorption lines, the intensities 

 of which can be measured. If one studies the spectrum of a distant 

 star which has a large motion either toward or away from the sun, the 

 absorption lines produced by the reversing layer will be shifted owing 

 to tlie Doppler effect. Superimposed upon the spectrum of the star 

 one will see undisplaced lines corresponding to the absorption lines 

 of various elements. The locations of these stationary lines are found 

 to be independent of the velocity of the star relative to the earth, and 

 can only be attributed to the existence of matter between the star 

 and the earth. 



The first estimates of the relative abundances of elements in inter- 

 stellar material were made by T. Dunham, Jr. ( 1939) and by O. Struve 

 (1941). Recently B. Stromgren has succeeded in establishing with 



