FORMATION OF STARS—SPITZER 157 
the Mount Wilson Observatory, shows that a single line is frequently 
made up of several components. Each separate component is pro- 
duced by absorption in a single cloud of gas, the different components 
being separated in wave length by the difference in Doppler effect 
produced by the different cloud velocities. These clouds, each one 
about 20 light-years across, are moving through space at speeds of 
some 10 or 20 miles a second. A more detailed understanding of the 
nature of these clouds is desirable before one can discuss in detail how 
interstellar matter can form new stars. Further work along these 
lines is now in progress. 
SOLID PARTICLES 
In addition to the separate atoms drifting about in space, small 
solid particles, or grains, are also present. Each grain is about one 
hundred-thousandth of an inch in diameter; 10,000 placed end to 
end would make a line about as long as a period on this page. Since 
the size of these grains is just about equal to the wave length of visible 
light, these particles are of the size which is most effective in absorbing 
and scattering light waves. These particles are responsible for the 
general obscuration produced by the clouds shown in plate 2. Par- 
ticles of smaller size are presumably also present, but these do not 
produce such a noticeable effect, and can therefore not be detected. 
The properties of these particles have been determined from accu- 
rate measurements of the obscuration which they produce in light of 
different wave lengths. This obscuration is greater for blue light 
than for red light, which proves that the particles cannot be much 
larger in size than the wave length of light. On the other hand, the 
obscuration varies inversely only as the first power of the wave length, 
instead of as the fourth power which is observed for scattering by the 
molecules of the atmosphere. From this one can conclude that the 
grains are not very much smaller in size than the wave length of light. 
In this way a particle size of about the wave length of light has been 
determined. From the fact that the grains seem to scatter more 
than they absorb it seems likely that they are dielectric rather than 
metallic in composition. If, as seems likely, these grains were pro- 
duced by the sticking together of individual atoms, the enormous 
abundance of hydrogen relative to other elements would be expected 
to produce solid hydrogen compounds, in particular, ordinary ice. 
However, impurities of all other elements would also be present. 
Studies of the distribution of these grains have indicated that the 
clouds in which these grains are concentrated are apparently identical 
with the gaseous clouds already described. Thus whatever pushes 
atoms into clouds also pushes the grains together. 
