158 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
FORCES IN INTERSTELLAR SPACE 
To discuss in detail how stuff in space can condense to form new 
stars we must determine the physical conditions of matter in space. 
In particular, we must combine the observational evidence described 
above with our knowledge of basic physical principles to investigate 
the different forces that are at work on the different particles. Only 
in this way can we predict how the interstellar medium will behave 
under various widely different conditions. 
In the immense vacuum between the stars, an interstellar particle 
spends most of its time moving in a straight line without interruption. 
Occasionally, one of two things may happen to it: an encounter with 
another interstellar particle, or an encounter with a light wave, or 
photon. The information which physicists have obtained on such 
processes is not so complete as astronomers would like, but is sufficient 
for an approximate evaluation of the effects which these various col- 
lisions will produce. 
The collisions of the interstellar atoms and grains with each other 
help determine the temperature of matter in space. In most cases, 
the collisions are elastic and the kinetic energy of the different par- 
ticles is exchanged back and forth; as a result, the distribution of 
velocities corresponds to that in thermal equilibrium at some particular 
temperature. Photoemission of energetic electrons from hydrogen 
atoms and grains, on absorption of photons, tends to keep the tem- 
perature high, but inelastic collisions between atoms and grains tend 
to give a low temperature. Near a very hot and very bright star the 
gas will be heated up to about 10,000° K., but in other regions a tem- 
perature of about 100° K. seems likely. This difference of tempera- 
ture between different regions is believed to produce cosmic currents, 
or winds, in the same way as the winds on earth are produced. 
In some cases the interstellar particles stick to each other on col- 
lision. ‘Thus atoms stick together to form molecules, molecules stick 
together to form larger molecules, and grains grow by slow accretion. 
This process was analyzed during the war by a number of Dutch as- 
tronomers, who were able to show that the interstellar grains have 
probably been formed by this evolutionary process within the last 
few billion years. More accurate physical information on collisions 
between particles at low energies is required to make this theory 
more quantitative. 
Collisions between grains and photons are important in star build- 
ing. It is well known that light exerts pressure. Since starlight ina 
galaxy comes from all directions in the galactic plane, a single grain 
will be knocked this way or that by photon collisions, without any net 
motion resulting. However, when several grains are present, the 
shadow of each ove on the other unbalances the radiative force, and 
