308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
These are: spectral analysis of the sun and stars, chemical analysis of 
meteorites, and the isotopic composition of the elements. 
THE COMPOSITION OF THE SUN AND STARS 
Spectral analysis of the light of stars and distant nebulae is un- 
doubtedly the most direct way to determine the relative abundances 
of the elements in the universe asa whole. The intensity of the absorp- 
tion lines, the so-called Frauenhofer lines, in the spectrum depends on 
the concentration of the atoms causing the absorption. In general it 
is not difficult to identify the element that causes an observed absorp- 
tion line. In order to calculate the correlation of line intensity with 
atomic concentration, a number of physical properties of the absorbing 
atoms and the thermodynamic state of the absorbing stellar matter 
have to be known in detail. 
Unfortunately, the experimental determinations of these properties 
are not yet complete and one has to rely in many cases on theoretical 
calculations which often give only crude approximations. Further- 
more, various other quantities such as optical depth of the layer in 
which the absorption occurs, thermal velocity of the absorbing atoms, 
their macroscopic turbulent motion, and other characteristics have to be 
known before the exact functional dependence of line intensity and 
atomic concentration can be calculated. 
The first abundance data based on special analyses were obtained 
by Miss Payn in 1925 and by Russel in 1929. Since then the work 
of these authors has been improved and extended by many in- 
vestigators, and it was found that the chemical composition of the 
universe is indeed remarkably uniform, although the most recent 
investigations indicate definite systematic variations in the composi- 
tion of stars, depending on their age and their position in the galaxies. 
The most important result of the astronomical investigations is 
the discovery that hydrogen is by far the most abundant element in 
the universe. Next to hydrogen in abundance is helium. In the sun 
helium constitutes about one-fourth of the atoms. All the other ele- 
ments make up only about 1 percent of the sun’s total mass. The 
most prominent of these other elements are carbon, nitrogen, oxygen, 
and neon. Among the metals sodium, magnesium, aluminum, cal- 
cium, and iron are the most abundant ones. 
COMPOSITION OF METEORITES 
Another set of empirical abundance data can be derived from chemi- 
cal analysis of meteorites. It is generally assumed that meteoritic 
matter has undergone less chemical fractionation than any terrestrial 
material found on the surface of the earth since the time it was 
formed from solar material. The main type of fractionation recog- 
nizable in meteorites is that of a separation of the elements into three 
chemical phases: a metal phase, a sulfide phase, and a silicate phase. 
