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385 , 386 ] The Action of Gravitational Instability
386 . The extra-galactic nebulae and star clouds are the most massive
astronomical formations known, their masses being of the order of a thousand
million suns. The masses of the rather enigmatical globular clusters are
probably distinctly smaller, but hardly of a different order of magnitude. After
these there is a great gap until we come to the stars with masses comparable
with the sun. In discussing still smaller masses we are perforce limited to the
solar system, since they could not be observed in more distant systems. Here
again a great gap appears; after the sun, the next most massive body is
Jupiter, whose mass is less than a thousandth part of that of the sun, and then
come the planets in general with masses of the order of a ten-thousandth part
of the sun’s mass. After these there is another great gap, and then a still
smaller system of bodies, the satellites, which have masses of the order of only
a ten-thousandth part of the masses of their primaries.
We have seen that an explanation of these discontinuities in the sequence
of masses is provided by the action of gravitational instability, which also
explains how one group of masses is formed out of another. This single concept
has proved capable of explaining the births of four successive generations of
astronomical bodies, each being born through the action of gravitational in
stability from the generation of more massive bodies immediately preceding it.
We have found that, as Newton first conjectured, a chaotic mass of gas of
approximately uniform density and of very great extent would be dynamically
unstable; nuclei would tend to form in it, around which the whole of the
matter would ultimately condense. We have obtained a formula which enables
us to calculate the average distance apart at which these nuclei would form
in a medium of given density, and this determines the average mass which
would ultimately condense round each.
If all the matter in those parts of the universe which are accessible to
our observation, a sphere of about 140 million light-years radius, were spread
out uniformly, it would form a gas of density 10 ~ 31 or thereabouts. We have
calculated that gravitational instability would cause such a medium to break
up into detached bodies whose distance apart would be of the same order as
the observed distance between the spiral nebulae; the mass of each such body
would accordingly be about equal to the mass of the average spiral nebula.
We may conjecture, although it is improbable that we shall ever be able to
prove, that the spiral nebulae were formed in this way. Any currents in the
primaeval chaotic medium would persist as rotations of the nebulae, and, as
these would be rotating with different speeds, they might be expected to shew
all the various types of configurations of our sequence ( b ), which is what is
actually observed.
Those nebulae whose rotational momentum was sufficient to carry them
past the critical lenticular shape in the course of their shrinkage would shed
a certain amount of matter in their equatorial plane in the manner indi
cated in the last two diagrams of sequence ( 6 ). Since a uniformly spread-out