FORMATION OF STARS—SPITZER 159 
photons striking from the opposite sides push the grains toward each 
other. As a result, there is an effective force of attraction between 
grains which is several thousand times as great as the gravitational 
force between them. 
STAR FORMATION 
The farther we go away from observational data the more uncertain 
our theories become. The mechanism of star formation, which is the 
ultimate objective of much of the work described above, is still in a 
rather speculative state. However, putting all the above information 
together does provide a reasonable preliminary picture for the process 
by which stars can be formed from interstellar matter. 
The process may be assumed to start with an interstellar gas, 
formed at the same time as the rest of the universe. The first step 
in the process is then the slow condensation of interstellar particles 
from the gas. After these particles have reached a certain size, the 
radiative attraction between them forces them together and they drift 
toward each other, forming an obscuring cloud in a time of about 10 
million years. In a cloud, where the density of grains is high, the 
temperature tends to be low. In the surrounding region the high 
temperature produces high pressure, and the low-temperature, low- 
pressure cloud therefore becomes compressed. In this way the density 
of gas within a cloud will be increased, corresponding to the observed 
result that a cloud of grains is also a cloud of gas. 
Currents produced by differences of temperature and also by the 
general rotation of the galaxy will tend to tear some of these clouds to 
pieces. On the other hand, the forces of condensation will pull them 
together and some clouds may be expected to go on contracting. The 
radiative force becomes ineffective when the clouds become so opaque 
that light does not penetrate into them very far. At this point 
gravitation takes over and tends to produce a further contraction. 
In this stage a cloud has a diameter of a light-year or less. Small 
opaque clouds of this type, called globules, have been known for some 
time, and are shown in plate 1, figure 1. 
One of the chief problems concerns the angular momentum of this 
prestellar globule, or protostar. According to Newton’s laws of 
motion, the angular momentum, which is proportional to the product 
of the radius and the rotational velocity, remains constant; as the 
radius decreases the rotational speed increases. Since the radius of a 
typical cloud is some 10 million times the radius of a supergiant star, 
this increase in rotational speed can be quite impressive. Unless 
some way can be found to dispose of the angular momentum, a proto- 
star would hurl itself to pieces by centrifugal force. The possibility 
that turbulent motions in the gas may carry the angular momentum 
