415 
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