Constitution and Evolution of the Stars 89
under its own gravitation. The weight of the overlying
layers produces a pressure increasing steadily from the
surface to the centre, which must at any point be balanced
by the expansive tendency of the gas, arising from its high
temperature. The temperature, too, is greatest at the
centre, and decreases towards the surface. Hence, heat
must flow continually through the star’s substance, down
the temperature gradient, till it escapes by radiation at the
surface. The supply of heat must be kept up in some
way; and one obvious process, as Helmholtz suggested
long ago, is the slow contraction of the star. The work
done by the gravitational forces in pulling the outer parts
of the star toward the centre, reappears as heat produced
by the compression, and maintains the star as a going con-
cern. As the star contracts, its density must increase; and
the pressure will increase too, for the various parts of the
mass are nearer one another, and attract one another
more strongly. When the star has shrunk to half
its original diameter, the mean density will be eight times
as great.
If the star, after contraction, continues to be “built on the
same model,” so to speak—that is, if the law according
to which the density increases proportionally toward the
centre remains the same, except for the altered scale of
miles provided by the shortened radius, the density at any
point, after contraction, will also be eight times the orig-
inal density at the corresponding point (distant from the
centre by the same fraction of the radius).
How will the pressures at the two points compare? The
portion of the star nearer the centre than the point under
consideration is compressed by the weight of the overlying
portions. After the contraction, every part of these is
twice as near the centre as before, and will, therefore, be