Inferences, as I have pointed out already, may generate false conclusions
either because they are based on false premises or because they have proceeded by a faulty process of reasoning. Hence one seeks, wherever possible, to test inference by observation. Does any observation provide
evidence for the story that I have just unfolded about the early life history
of the nebulae?
Insofar as what is provides evidence for what has been, there is plenty
of it. It has been shown in the last few chapters how those details that we
observe in the nebulae are just what one should expect to observe if my
theory is true. But the theory that I have presented may not be the only
one that could serve to explain what is observed. It frequently happens
that rival explanations account for the same facts. There were in the past,
for instance, rival explanations for the observation that days and nights
succeed each other. One was that the earth rotates about its axis, the other
that the sun and stars rotate about the earth.
On such occasions one has to compare the two explanations and test
each for its logical consistency and its compatibility with established
knowledge. So it may be here. Perhaps it will be possible to find a different
evolutionary history for the nebulae, one that will equally well explain
their structure and rotation. True, this has not yet happened; my theory
cannot yet be rejected in favour of an alternative; it can only be rejected
in favour of nothing. It is, however, to be hoped that the search for
alternative explanations will be undertaken in due course, and with appropriate persistence. Meanwhile one would like, if possible, to be able to
observe a spiral nebula in the making.
Is there any hope of this? Let us consider where one should look for
such an event.
In my model new nebulae begin to form on all sides of an existing
nebula as soon as this is about 366 x 109 years old. There must always be
some nebulae that have recently reached this age and so there must be
some others, close to them, that are just now going through the early stage
of their development. It is in such places, if anywhere, that nascent
galaxies could be observed.
We cannot, however, expect such very young nebulae to be luminous.
We know from our own galaxy that luminosity comes from the stars in it
and not from the interstellar gas. We also know that its cause is the helium
synthesis that occurs within them. But this does not begin until the
temperature and density in the stars are very high, and it must take a long
time for these necessary conditions to be reached. When they are reached,
moreover, the rate of helium synthesis is very slow at first. It only gets
properly under way when a substantial amount of nuclear energy has
already been converted into heat. So the helium synthesis has to proceed
for some time before the surface brightness becomes conspicuous. Hence it
does not seem to be probable that the amount of energy converted into
radiation per unit volume can be enough to make a star become luminous
until the nebula has existed for a substantial portion of the 366 x 109 years
that represent the time from one generation to the next. But the happenings
that determine the shape and structure of the nebula occupy, according to
my account, only a minute fraction of this period.
If one cannot detect young nebulae by their luminosity, can one hope
to detect them by their opaqueness? I do not think it likely. Much of the
far denser gas in our own galaxy is fairly transparent. Only certain patches
are opaque. So the amount of light that a very young nebula intercepts is
probably quite small. The nebula might reveal its presence as a darkish
speck in the sky, but this is doubtful.
There remains, however, one further possibility. This is the radio-telescope. Something very violent, not to say cataclysmic, is happening at
about the time when the first stage of growth is being completed: vast
quantities of hydrogen are pouring out of the spokes into the core.
We have seen that in this process the gas falls from a great height. In
falling a large amount of potential energy is being converted into kinetic
energy. The gas in the place where the core thickens must be in a state of
turmoil. Collisions between molecules must be vigorous and must emit
some radiation. Its wavelength can hardly be in the optical part of the
spectrum, but may well be in the part to which the radio-telescope responds.
Although the rate of release of energy per unit volume must be small, the
volume from which it is released is very great, larger than that of a typical
nebula. There thus seems to be quite a good prospect that evidence of the
process by which the spiral arms are formed would be picked up in a
A few things can be predicted about this particular radiation and should
help to identify it.
Firstly, the radiation must come from places that are about midway
between existing nebulae, i.e. where optical telescopes show nothing.
Secondly, this radiation is only emitted during the comparatively short
time while the spokes are discharging their hydrogen into the core and the
substance of the core is piling up behind the resultant thickening. This is
only a very small fraction of the time that elapses between one generation
of nebulae and the next. Perhaps the fraction is 10-5, perhaps less. The
number of radio messages of this particular type must therefore be very
small in comparison with the number of observed nebulae.
Thirdly, it may be possible to calculate the wave-length of the radiation
that would result from violent collisions between molecules of hydrogen
in a very tenuous gas. The radiation to look for is of this wave-length.
Fourthly, a new generation of nebulae must begin on every side of an
existing one when this is about 366 x 109 years old. This shows that the
particular kind of radio message that is to be looked for will hardly occur
here and there in isolated places, but tend to occur in clusters surrounding
an existing nebula.
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