18.1: Variations in Density
Though very tenuous everywhere, the cloud cannot have a uniform density
throughout. Let us consider in broad outline how the density must vary
from the centre outwards.
As in the earth's atmosphere, any gas in the cloud that bears the
weight of higher layers must be compressed. So the greatest density must
be at the centre of the cloud.
A space traveller who moved outwards from this centre would be
rising against the cloud's gravitational field and would be entering an
increasingly rarified atmosphere as he did so. After a while he would find
himself climbing up the inner slope of the ridge that formed the lip of the
crater around the astronomical summit. This ridge or reversal zone, it
will be remembered, is moving outwards as the cloud grows in mass. A
little earlier the place where the space traveller is at the moment was on an
outward slope. Gas was falling away from it into further space and is now
falling in the reverse direction towards the core. So it is safe to assume that
none of the gas that was there originally can have remained. The only
gas that the space traveller encounters must be new particles that have
originated but recently. On the inner slope of the crater the gas density
must be as near to zero as it can be anywhere in space.
On crossing the ridge the space traveller reaches a very gentle gradient
that slopes downwards away from the centre of the cloud. During the first
stage of growth gas is originating in this region faster than it falls down the
slope and the traveller enters once again into a region that is less depleted.
The density is here much as it was on the summit at the still earlier stage,
when the cloud first began to form as a small sphere.
As the ridge moves steadily outwards, the picture changes. What has
just been on the top of the ridge finds itself on the inner slope and falls
backwards on to the core in the hollow of the crater. The change goes on
until the ridge has reached the fringe of the cloud and the first stage of
growth has been completed.
Thus there must be a period during the first stage of growth when the
cloud has a central core, which may already be fairly dense at its centre and
is very tenuous in its outer layers. Around this there must be a region where
there would be no gas whatever if it were not for new origins within the
region. It is easy to show from the inverse square law that the potential
gradient must be relatively steep in this region and so the new origins fall
quickly out of it.
Beyond the region of the near-perfect vacuum there must be more
cloud. It must extend a long way out into space and be very tenuous; for
its density must be below the critical value for cloud formation. The
potential gradient becomes steeper as one proceeds from the crater outwards and so the gas density must also diminish with distance from the
It will be found later that the large vacuous region around the core
plays an important part in the evolution of galaxies and so it needs to have
a name. I propose to call it the 'depleted region'.
18.2: Rotation of the Core
Stars and galaxies are known to rotate, but it is far from easy to say
why. I doubt whether it is generally appreciated how difficult it is to find an
explanation. Any explanation that is found must conform to established
principles of mechanics and it is desirable that it should not require the
invention of ad hoc hypotheses. I doubt whether a satisfying explanation
can be based at all on accepted notions about gravitation. The difficulty,
together with a tentative solution of the problem, will be discussed in
Chapter 28. It cannot be usefully discussed until after a discussion of the
nature of gravitation has been undertaken. In the meantime it must
suffice to note the bare fact that galaxies are observed to rotate.
18.3. The Physics of the Core
It would be a mistake to seek much resemblance between the core of
the extragalactic cloud and a star. Apart from the facts that both spin and
consist mostly of hydrogen, they can have little in common.
The core is very new at the time we are considering, i.e. before the reversal zone reaches the rim of the cloud. The time that it takes for the
zone to traverse to this has been found to be probably no more than some
hundreds of thousands of years. The time taken to travel far enough to
leave a core behind must then be reckoned, perhaps, in tens of thousands.
The extent of the core is enormous; it could contain many millions of
stars. So the journey travelled by those molecules that contribute to the
shrinking is a long one. After a hundred thousand years or so the core
must still be large enough to be very tenuous. It can have no property
that one could think of as rigidity. Collisions between molecules must be
rare by terrestrial standards. The quantity of heat generated per unit
volume by collisions must be small.
In such a core one should not expect the conditions to exist that cause
the synthesis of hydrogen into helium; this can only happen with pressures
and temperatures such as occur inside the sun. The heat generated inside
the core does not seem to have any other source at this time than the
gravitational potential energy of the molecules of hydrogen, which is very
small compared with the energy released by the synthesis of helium. The
temperature in the core can therefore only be the result of the small amount
of heat generated by collisions. The core must be a very cold body.
This must have a significant effect on the rate of shrinking. In a star
the helium synthesis produces intense internal radiation. Its pressure is
so great that it balances the star's own gravitational field. In other words,
pressure of radiation supports the weight of the outer shell and prevents
it from falling towards the centre. When this pressure ceases, as sometimes
happens, the star collapses rapidly and with a great increase in the rate of
As it does not seem possible to postulate the restraint of internal
pressure of radiation for the core during its early life, one must assume that
the penetration of molecules into the core is restricted only by collisions;
and as, in view of the core's tenuousness, these are rare, penetration must
be deep. The rate of shrinking must therefore be great.
18.4: Retardation of Spin
From this one might at first be inclined to infer that the angular
velocity of the core would rise to the point where centrifugal force balanced
gravitation. If this were to happen, there would be no shrinking. But there
is also a retarding influence that we have to consider.
While the cloud is shrinking and becoming more dense by reduction
in its volume, it is also becoming more dense by the acquisition of new
matter, and this from two distinct sources.
The first of these is new origins within the region occupied by the
cloud. The new particles would appear to an observer to be at rest relative
to a given frame of reference. The existing particles must collide with the
new ones and entrain them. Hence the new particles come to participate
in the general rotatory motion. The angular momentum of the core
remains constant, but the angular velocity becomes less as the mass
The other source of new matter is the gas that finds itself within the
reversal zone as this moves outwards. This gas, which has hitherto been
falling away from the centre of the cloud, now falls towards it across the
depleted region. It has to fall a long way across this steep part of the inner
slope where, as has been mentioned above, there is a high vacuum. During
this falling it acquires a substantial momentum and must penetrate deep
into the core. Like the originating matter, it is entrained and causes
It would be interesting to know more about the physics of the core.
How does its density vary from the centre, where the only force is the
weight of the gas around it, to the periphery, where centrifugal force
opposes the weight? How does the angular velocity vary with time? Does
it increase at first up to a maximum and then decrease as the retarding
effect of additional mass becomes more pronounced? To what extent is
the core flattened by rotation? Is it disc-shaped at first, and does it become
spherical later when pressure of radiation from helium synthesis blows
out, as it were? When, if at all, does helium synthesis begin? But such
questions must be put to various specialists in astrophysics. They are
beyond the scope of this elementary study.
18.5: The Spokes
It has been pointed out in Chapter 10, entitled 'The Astronomical
Landscape', that the potential gradient around an astronomical summit
cannot be at all uniform. There must be shoulders and valleys and these
must stretch out into space in several directions in all three dimensions
The shoulders descend gently for a long while and eventually reach those
places that have been called astronomical passes. The valleys descend more
steeply and end in the deep wells that represent the neighbouring galaxies
The number of pronounced shoulders, it will be remembered, cannot
be great. A typical number is likely to be six; and these will spread out
from the summit like rods pushed through the faces of a cube. If all
surrounding galaxies were arranged in a regular cubic pattern, the six
astronomical shoulders would be at right-angles to each other.
The cloud can only form where the potential gradient is below this
critical value. This must occur along a gentle shoulder at a time when the
valley next to it is so steep that no gas can accumulate there. The cloud
cannot therefore have the form of a simple sphere. Its shape must be very
different. Hydrogen must be accumulating on a shoulder while it is rapidly
pouring down the side into the deeper valley below. It must also be pouring
down this towards the neighbouring galaxy.
This shows that the inner cloud must, during its early stages, be
surrounded by a collection of spokes that extend away from the summit
in all directions and far out into space. The spokes must be roughly
cylindrical and must become more tenuous towards their tips, where the
potential gradient is greatest.
As space expands, the landscape is flattening in all directions and so
the spokes must grow in diameter as well as in length. They must also
acquire a gravitational field of their own. In other words, they must
carve out shallow troughs for themselves along the shoulders. They may
be thought of as lying in these and attracting hydrogen towards their axes,
Hydrogen in the outer regions of a spoke must fall inwards towards its
axis and thereby collide with hydrogen that is already there. It will give
rise to some turbulence. But I should not like to claim that the turbulence
would be enough to have any significant effect. To all intents and purposes
one may perhaps regard the spokes like inert slugs lying motionless along
their supporting shoulders.
Though I have called them spokes, they are not spatially arranged like
the spokes of a wheel. They are not all in one plane but point in each
direction of three dimensional space; and they are not connected to a
central hub. The wide depleted region separates them from the inner core
of the cloud.
The picture that we have been led to infer differs substantially, as I have
said already, from that presented by the spiral nebulae.
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