THE SUN
The sun is the chief member of our system. It controls the motions of
the planets by its immense gravitative power. Besides this it is the
most important body in the entire universe, so far as we are concerned;
for it pours out continually that flood of light and heat, without which
life, as we know it, would quickly become extinct upon our globe.
Light and heat, though not precisely the same thing, may be regarded,
however, as next-door neighbours. The light rays are those which
directly affect the eye and are comprised in the visible spectrum. We
_feel_ the heat rays, the chief of which are beyond the red portion of
the spectrum. They may be investigated with the _bolometer_, an
instrument invented by the late Professor Langley. Chemical rays--for
instance, those radiations which affect the photographic plate--are for
the most part also outside the visible spectrum. They are, however, at
the other end of it, namely, beyond the violet.
Such a scale of radiations may be compared to the keyboard of an
imaginary piano, the sound from only one of whose octaves is audible to
us.
The brightest light we know on the earth is dull compared with the light
of the sun. It would, indeed, look quite dark if held up against it.
It is extremely difficult to arrive at a precise notion of the
temperature of the body of the sun. However, it is far in excess of any
temperature which we can obtain here, even in the most powerful electric
furnace.
A rough idea of the solar heat may be gathered from the calculation that
if the sun's surface were coated all over with a layer of ice 4000 feet
thick, it would melt through this completely in one hour.
The sun cannot be a hot body merely cooling; for the rate at which it is
at present giving off heat could not in such circumstances be kept up,
according to Professor Moulton, for more than 3000 years. Further, it is
not a mere burning mass, like a coal fire, for instance; as in that case
about a thousand years would show a certain drop in temperature. No
perceptible diminution of solar heat having taken place within historic
experience, so far as can be ascertained, we are driven to seek some
more abstruse explanation.
The theory which seems to have received most acceptance is that put
forward by Helmholtz in 1854. His idea was that gravitation produces
continual contraction, or falling in of the outer parts of the sun; and
that this falling in, in its turn, generates enough heat to compensate
for what is being given off. The calculations of Helmholtz showed that a
contraction of about 100 feet a year from the surface towards the centre
would suffice for the purpose. In recent years, however, this estimate
has been extended to about 180 feet. Nevertheless, even with this
increased figure, the shrinkage required is so slight in comparison with
the immense girth of the sun, that it would take a continual
contraction at this rate for about 6000 years, to show even in our
finest telescopes that any change in the size of that body was taking
place at all. Upon this assumption of continuous contraction, a time
should, however, eventually be reached when the sun will have shrunk to
such a degree of solidity, that it will not be able to shrink any
further. Then, the loss of heat not being made up for any longer, the
body of the sun should begin to grow cold. But we need not be distressed
on this account; for it will take some 10,000,000 years, according to
the above theory, before the solar orb becomes too cold to support life
upon our earth.
Since the discovery of radium it has, on the other hand, been suggested,
and not unreasonably, that radio-active matter may possibly play an
important part in keeping up the heat of the sun. But the body of
scientific opinion appears to consider the theory of contraction as a
result of gravitation, which has been outlined above, to be of itself
quite a sound explanation. Indeed, the late Lord Kelvin is said to have
held to the last that it was amply sufficient to account for the
underground heat of the earth, the heat of the sun, and that of all the
stars in the universe.
One great difficulty in forming theories with regard to the sun, is the
fact that the temperature and gravitation there are enormously in excess
of anything we meet with upon our earth. The force of gravity at the
sun's surface is, indeed, about twenty-seven times that at the surface
of our globe.
The earth's atmosphere appears to absorb about one-half of the
radiations which come to us from the sun. This absorptive effect is very
noticeable when the solar orb is low down in our sky, for its light and
heat are then clearly much reduced. Of the light rays, the blue ones are
the most easily absorbed in this way; which explains why the sun looks
red when near the horizon. It has then, of course, to shine through a
much greater thickness of atmosphere than when high up in the heavens.
What astonishes one most about the solar radiation, is the immense
amount of it that is apparently wasted into space in comparison with
what falls directly upon the bodies of the solar system. Only about the
one-hundred-millionth is caught by all the planets together. What
becomes of the rest we cannot tell.
That brilliant white body of the sun, which we see, is enveloped by
several layers of gases and vaporous matter, in the same manner as our
globe is enveloped by its atmosphere (see Fig. 10, p. 131). These are
transparent, just as our atmosphere is transparent; and so we see the
white bright body of the sun right through them.
This white bright portion is called the _Photosphere_. From it comes
most of that light and heat which we see and feel. We do not know what
lies under the photosphere, but, no doubt, the more solid portions of
the sun are there situated. Just above the photosphere, and lying close
upon it, is a veil of smoke-like haze.
Next upon this is what is known as the _Reversing Layer_, which is
between 500 and 1000 miles in thickness. It is cooler than the
underlying photosphere, and is composed of glowing gases. Many of the
elements which go to make up our earth are present in the reversing
layer in the form of vapour.
The _Chromosphere_, of which especial mention has already been made in
dealing with eclipses of the sun, is another layer lying immediately
upon the last one. It is between 5000 and 10,000 miles in thickness.
Like the reversing layer, it is composed of glowing gases, chief among
which is the vapour of hydrogen. The colour of the chromosphere is, in
reality, a brilliant scarlet; but, as we have already said, the
intensely white light of the photosphere shines through it from behind,
and entirely overpowers its redness. The upper portion of the
chromosphere is in violent agitation, like the waves of a stormy sea,
and from it rise those red prominences which, it will be recollected,
are such a notable feature in total solar eclipses.
[Illustration: FIG. 10.--A section through the Sun, showing how the
prominences rise from the chromosphere.]
The _Corona_ lies next in order outside the chromosphere, and is, so
far as we know, the outermost of the accompaniments of the sun. This
halo of pearly-white light is irregular in outline, and fades away into
the surrounding sky. It extends outwards from the sun to several
millions of miles. As has been stated, we can never see the corona
unless, when during a total solar eclipse, the moon has, for the time
being, hidden the brilliant photosphere completely from our view.
The solar spectrum is really composed of three separate spectra
commingled, _i.e._ those of the photosphere, of the reversing layer, and
of the chromosphere respectively.
If, therefore, the photosphere could be entirely removed, or covered up,
we should see only the spectra of those layers which lie upon it. Such a
state of things actually occurs in a total eclipse of the sun. When the
moon's body has crept across the solar disc, and hidden the last piece
of photosphere, the solar spectrum suddenly becomes what is technically
called "reversed,"--the dark lines crossing it changing into bright
lines. This occurs because a strip of those layers which lie immediately
upon the photosphere remains still uncovered. The lower of these layers
has therefore been called the "reversing layer," for want of a better
name. After a second or two this reversed spectrum mostly vanishes, and
an altered spectrum is left to view. Taking into consideration the rate
at which the moon is moving across the face of the sun, and the very
short time during which the spectrum of the reversing layer lasts, the
thickness of that layer is estimated to be not more than a few hundred
miles. In the same way the last of the three spectra--namely, that of
the chromosphere--remains visible for such a time as allows us to
estimate its depth at about ten times that of the reversing layer, or
several thousand miles.
When the chromosphere, in its turn during a total eclipse, has been
covered by the moon, the corona alone is left. This has a distinct
spectrum of its own also; wherein is seen a strange line in the green
portion, which does not tally with that of any element we are acquainted
with upon the earth. This unknown element has received for the time
being the name of "Coronium."