416
Conclusion
[CH. XVII
distribution of matter in this plane would be unstable, gravitational
instability would again cause condensations to form and the shed matter
would ultimately break up into a series of separate detached bodies. Calcula
tion shews that these would each have a mass of the same order as the masses
of the stars. This makes it exceedingly likely that we have here found the
birthplaces of the stars. It is significant that stars are observed in abundance
in the outer regions of the spiral nebulae, but none in the inner lenticular
regions, or in those spherical or elliptical nebulae which, having stopped short
of the critical lenticular configurations, have shed no matter out into their
equatorial planes. Thus stars are found precisely in those regions in which
theory predicts that they should be formed by gravitational instability and
nowhere else.
Normally the reign of gravitational instability must end with the birth of
stars. The masses of the stars are too small for further shrinkage to carry
them again along sequence ( 6 ), so that if such shrinkage occurs, the stars
must follow sequence (a) and may finally break up into binary systems.
In a few exceptional cases, however, gravitational instability may come into
action again. On rare occasions it may happen that one star passes so near to
another that it draws out long arms of matter, these being in effect exaggerated
tides caused by the near proximity of the two stars. When this occurs, the
matter in these arms is a fit subject for the operation of gravitational instability,
and calculation shews that the matter would condense into detached bodies,
each of mass about equal to that of the planets. In this way we conjecture
that our earth and the other planets were born out of the sun. Such planets
as do not liquefy or solidify at once may in their turn be caused to eject long
arms of matter which the operation of gravitational instability will break up
into detached masses, the satellites of the planets. Our moon forms a rather
exceptional case, having a mass far more nearly equal to its primary than is
found anywhere else in the solar system. We have found that this indicates
that the earth must have been partially liquefied before the moon was born.
The action of gravitational instability must finally end with the birth of
satellites. To escape the fate of dissipating away into space these must con
dense into either liquid or solid form immediately after birth, and when they
have done this, gravitational instability can obtain no further hold over them.
We have found, however, that gravitational instability accounts for the birth
of four generations of astronomical bodies in succession, of nebulae out of
chaos, of stars out of nebulae, of planets out of stars and of satellites out of
planets. Our conclusion that these successive generations are born by gravi
tational instability demands no hypotheses beyond the presence of forces which
are already known to exist, namely gravitation and gas-pressure, and it survives
at every step the test of numerical computation.
387. Another section of cosmogony deals with the time required for these
processes to occur and, as a consequence, with the total age of the universe.