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We have seen photographs of atoms and free electrons. Now we want a photograph of X-rays to complete
the stellar population. We cannot quite manage that, but we can very nearly. Photographs by X-rays
are common enough; but a photograph of X-rays is a different matter. I have already said that electrons
can be broken away from atoms by X-rays colliding with them. When this happens the free electron is
usually shot off with high velocity so that it is one of the express electrons which can be photographed.
In Fig. 5 you see four electrons shot off in this way. You notice that they all start from points in the
same line, and it does not require much imagination to see in your mind a mysterious power travelling
along this line and creating the explosions. That power is the X-rays which were directed in a narrow
beam along the line (from right to left) when the photograph was taken. Although the X-rays are left to
your imagination, the photograph at any rate shows the process of ionization which is so important in
the stellar interior the freeing of electrons from the atoms by the incidence of X-rays. You notice that it
is just a chance whether the X-ray ionizes an atom when it meets it. There are trillions of atoms lying
about (of which the photograph takes no notice); but, nevertheless, the X-rays travel a long way before
meeting the atom which they choose to operate on.
- Fig 6. Ions produced by Collision of a Beta particle (C. T. R. Wilson) [Note:Primarily it is the electric
charge and not the high speed of particles which determines their appearance in these photographs.
But a highspeed particle leaves behind it a trail of electrically charged particles -- the victims of its furious
driving -- so that it is shown indirectly by its line of victims.]
Finally I can show you the other method of ionizing atoms by battering of a more mechanical kind -- in
this case by the collision of a fast electron. In Fig. 6 a fast electron was travelling nearly horizontally,
but the tiny water-drops that should mark its track are so spread out that you do not at first trace the
connexion. Notice that the drops occur in pairs. This is because the fast electron battered some of the
atoms along its track, wrenching away an electron from each. You see at intervals along the track a
broken atom and a free electron lying side by side, though you cannot tell which is which. Occasionally
the original fast electron was too vigorous and there is more of a mix up, but usually you can see clearly
the two fragments resulting from the smash.
A cynic might remark that the interior of a star is a very safe subject to talk about because no one can
go there and prove that you are wrong. I would plead in reply that at least I am not abusing the unlimited
opportunity for imagination; I am only asking you to allow in the interior of the star quite homely objects
and processes which can be photographed. Perhaps now you will turn round on me and say, 'What
right have you to suppose that Nature is as barren of imagination as you are? Perhaps she has hidden
in the star something novel which will upset all your ideas.' But I think that science would never have
achieved much progress if it had always imagined unknown obstacles hidden round every corner. At
least we may peer gingerly round the corner , and perhaps we shall find there is nothing very formidable
after all. Our object in diving into the interior is not merely to admire a fantastic world with conditions
transcending ordinary experience; it is to get at the inner mechanism which makes stars behave as they
do. If we are to under stand the surface manifestations, if we are to understand why 'one star differeth
from another star in glory', we must go below -- to the engine-room -- to trace the beginning of the stream
of heat and energy which pours out through the surface. Finally, then, our theory will take us back to
the surface and we shall be able to test by comparison with observation whether we have been badly
misled. Meanwhile, although we naturally cannot prove a general negative , there is no reason to anticipate
anything which our laboratory experience does not warn us of.
The X-rays in a star are the same as the X-rays experimented on in a laboratory, but they are enormously
more abundant in the star. We can produce X-rays like the stellar X-rays, but we cannot produce them
in anything like stellar abundance. The photograph (Fig. 5) showed a laboratory beam of X-rays which
had wrenched away four electrons from different atoms; these would be speedily recaptured. In the star
you must imagine the intensity multiplied many million-fold, so that electrons are being wrenched away
as fast as they settle and the atoms are kept stripped almost bare. The nearly complete mutilation of
the atoms is important in the study of the stars for two main reasons.
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By PanEris
using Melati.
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