156 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
matter between the stars as there is inside the stars. This association 
between obscuring clouds and supergiant stars is strengthened by the 
fact that in the closest galaxy, the great nebula in Andromeda, super- 
giant stars are observed to occur in exactly those regions where the 
obscuring clouds are most prominent. Thus the observational evi- 
dence indicating a physical connection between clouds and super- 
giant stars is very strong. 
Before we can accept the hypothesis that supergiant stars have in 
fact formed from these clouds we must investigate whether or not 
there is some process which could cause interstellar matter to condense 
into stars. In this way we are led to consider the physical nature of 
the stuff between the stars, and the forces which operate onit. Thirty- 
five years ago the very existence of interstellar matter was not fully 
realized but recently extensive information on this topic has been 
obtained. 
ATOMS IN SPACE 
The dominant constituents of interstellar matter are believed to be 
individual atoms. These atoms absorb or emit light of particular 
wave lengths, which can be measured accurately by use of the spectro- 
scope. In some regions, where the gas is at a high temperature, 
bright emission lines of hydrogen, oxygen, and nitrogen are observed. 
Measurements of the intensities of these lines show that the density 
of the interstellar gas is about one hydrogen atom in each cubic 
centimeter, with other elements present as slight impurities. The 
interstellar medium is a much better vacuum than is ever obtained in 
a terrestrial laboratory. If a fly were to breathe a single breath 
into a vacuum chamber as big as the Empire State Building, the re- 
sulting density of the air would still be much greater than the density 
of the interstellar gas. 
In other regions of space the interstellar gas is cool, and no emission 
lines are produced. Instead, the atoms absorb the light from distant 
stars, producing absorption lines at particular wave lengths. The 
absorption lines of the abundant gases, hydrogen, helium, nitrogen, 
oxygen, etc., when these are cool, lie far out in the ultraviolet, where 
they cannot be detected. Interstellar absorption lines of sodium, 
calcium, titanium, and iron lie within the observable spectrum and 
have been observed in the spectra of bright stars a few thousand 
light-years away. These lines are very sharp, and can usually be 
distinguished from the lines produced by the atoms in a stellar at- 
mosphere, where the high temperature and pressure give wide lines. 
Recent work has been concerned with the detailed distribution of 
interstellar gas. Measurement of the strongest absorption lines, with 
the most powerful spectrographs available at the 100-inch telescope of 
