deliberation. The radiation of the heat actually present at any
moment in a large helium star would probably not require many
tens of thousands of years, but this quantity of heat is
negligible in comparison with the quantity generated within the
star during and by the processes of condensation from the
helium age down to the Class M state. We know that the
compression of any body against resistance generates or
releases heat. Now a gaseous star at any instant is in a state
of equilibrium. Its internal heat and the centrifugal force due
to its rotation about an axis are trying to expand it. Its own
gravitational power is trying to draw all of its materials to
the center. Until there is a loss of heat no contraction can
occur; but just as soon as there is such a loss gravity
proceeds to diminish the stellar volume. Contraction will
proceed more slowly than we should at first thought expect,
because in the process of contraction additional heat is
generated and this becomes a factor in resisting further
compression. Contraction is resisted vastly more by the heat
generated in the process of contraction than it is by the store
of heat already in evidence. The quantity of heat in our Sun,
now existing as heat, would suffice to maintain its present
rate of outflow only a few thousands of years. The heat
generated in the process of the Sun's shrinkage under gravity,
however, is so extensive as to maintain the supply during
millions of years to come. Helmholtz has shown that the
reduction of the Sun's radius at the rate of 45 meters per year
would generate as much heat within the Sun as is now radiated.
This rate of shrinkage is so slow that our most refined
instruments could not detect a change in the solar diameter
until after the lapse of 4,000 or 5,000 years. Again, there are