WHAT THE STARS ARE MADE OF
How can we possibly tell what the stars are made of? If we think of the
vast oceans of space lying between them and us, and realize that we can
never cross those oceans, for in them there is no air, it would seem to
be a hopeless task to find out anything about the stars at all. But even
though we cannot traverse space ourselves, there is a messenger that
can, a messenger that needs no air to sustain him, that moves more
swiftly than our feeble minds can comprehend, and this messenger brings
us tidings of the stars--his name is Light. Light tells us many
marvellous things, and not the least marvellous is the news he gives us
of the workings of another force, the force of gravitation. In some ways
gravitation is perhaps more wonderful than light, for though light
speeds across airless space, it is stopped at once by any opaque
substance--that is to say, any substance not transparent, as you know
very well by your own shadows, which are caused by your bodies stopping
the light of the sun. Light striking on one side of the earth does not
penetrate through to the other, whereas gravitation does. You remember,
of course, what the force of gravitation is, for we read about that very
early in this book. It is a mysterious attraction existing between all
matter. Every atom pulls every other atom towards itself, more or less
strongly according to distance. Now, solid matter itself makes no
difference to the force of gravitation, which acts through it as though
it were not there. The sun is pulling the earth toward itself, and it
pulls the atoms on the far side of the earth just as strongly as it
would if there were nothing lying between it and them. Therefore, unlike
light, gravitation takes no heed of obstacles in the way, but acts in
spite of them. The gravitation of the earth holds you down just the
same, though you are on the upper floor of a house, with many layers of
wood and plaster between you and it. It cannot pull you down, for the
floor holds you up, but it is gravitation that keeps your feet on the
ground all the same. A clever man made up a story about some one who
invented a kind of stuff which stopped the force of gravitation going
through it, just as a solid body stops light; when this stuff was made,
of course, it went right away off into space, carrying with it anyone
who stood on it, as there was nothing to hold it to the earth! That was
only a story, and it is not likely anyone could invent such stuff, but
it serves to make clear the working of gravitation. These two tireless
forces, light and gravitation, run throughout the whole universe, and
carry messages of tremendous importance for those who have minds to
grasp them. Without light we could know nothing of these distant worlds,
and without understanding the laws of gravity we should not be able to
interpret much that light tells us.
To begin with light, what can we learn from it? We turn at once to our
own great light-giver, the sun, to whom we owe not only all life, but
also all the colour and beauty on earth. It is well known to men of
science that colour lies in the light itself, and not in any particular
object. That brilliant blue cloak of yours is not blue of itself, but
because of the light that falls on it. If you cannot believe this, go
into a room lighted only by gas, and hey, presto! the colour is changed
as if it were a conjuring trick. You cannot tell now by looking at the
cloak whether it is blue or green! Therefore you must admit that as the
colour changes with the change of light it must be due to light, and not
to any quality belonging to the material of the cloak. But, you may
protest, if the colour is solely due to light, and light falls on
everything alike, why are there so many colours? That is a very fair
question. If the light that comes from the sun were of only one
colour--say blue or red--then everything would be blue or red all the
world over. Some doors in houses are made with a strip of red or blue
glass running down the sides. If you have one in your house like that,
go and look through it, and you will see an astonishing world made up of
different tones of the same colour. Everything is red or blue, according
to the colour of the glass, and the only difference in the appearance of
objects lies in the different shades, whether things are light or dark.
This is a world as it might appear if the sun's rays were only blue or
only red. But the sun's light is not of one colour only, fortunately for
us; it is of all the colours mixed together, which, seen in a mass, make
the effect of white light. Now, objects on earth are only either seen by
the reflected light of the sun or by some artificial light. They have no
light of their own. Put them in the dark and they do not shine at all;
you cannot see them. It is the sun's light striking on them that makes
them visible. But all objects do not reflect the light equally, and this
is because they have the power of absorbing some of the rays that strike
on them and not giving them back at all, and only those rays that are
given back show to the eye. A white thing gives back all the rays, and
so looks white, for we have the whole of the sun's light returned to us
again. But how about a blue thing? It absorbs all the rays except the
blue, so that the blue rays are the only ones that come back or rebound
from it again to meet our eyes, and this makes us see the object blue;
and this is the case with all the other colours. A red object retains
all rays except the red, which it sends back to us; a yellow object
gives back only the yellow rays, and so on. What an extraordinary and
mysterious fact! Imagine a brilliant flower-garden in autumn. Here we
have tall yellow sunflowers with velvety brown centres, clustering pink
and crimson hollyhocks, deep red and bright yellow peonies, slender
fairy-like Japanese anemones, great bunches of mauve Michaelmas daisies,
and countless others, and mingled with all these are many shades of
green. Yet it is the light of the sun alone that falling on all these
varied objects, makes that glorious blaze of colour; it seems
incredible. It may be difficult to believe, but it is true beyond all
doubt. Each delicate velvety petal has some quality in it which causes
it to absorb certain of the sun's rays and send back the others, and its
colour is determined by those it sends back.
Well then how infinitely varied must be the colours hidden in the sun's
light, colours which, mixed all together, make white light! Yes, this is
so, for all colours that we know are to be found there. In fact, the
colours that make up sunlight are the colours to be seen in the rainbow,
and they run in the same order. Have you ever looked carefully at a
rainbow? If not, do so at the next chance. You will see it begins by
being dark blue at one end, and passes through all colours until it gets
to red at the other.
We cannot see a rainbow every day just when we want to, but we can see
miniature rainbows which contain just the same colours as the real ones
in a number of things any time the sun shines. For instance, in the
cut-glass edge of an inkstand or a decanter, or in one of those
old-fashioned hanging pieces of cut-glass that dangle from the
chandelier or candle-brackets. Of course you have often seen these
colours reflected on the wall, and tried to get them to shine upon your
face. Or you have caught sight of a brilliant patch of colour on the
wall and looked around to see what caused it, finally tracing it to some
thick edge of shining glass standing in the sunlight. Now, the cut-glass
edge shows these colours to you because it breaks up the light that
falls upon it into the colours it is made of, and lets each one come out
separately, so that they form a band of bright colours instead of just
one ray of white light.
This is perhaps a little difficult to understand, but I will try to
explain. When a ray of white light falls on such a piece of glass, which
is known as a prism, it goes in as white light at one side, but the
three-cornered shape of the glass breaks it up into the colours it is
made of, and each colour comes out separately at the other side--namely,
from blue to red--like a little rainbow, and instead of one ray of white
light, we have a broad band of all the colours that light is made of.
Who would ever have thought a pretty plaything like this could have told
us what we so much wanted to know--namely, what the sun and the stars
are made of? It seems too marvellous to be true, yet true it is that for
ages and ages light has been carrying its silent messages to our eyes,
and only recently men have learnt to interpret them. It is as if some
telegraph operator had been going steadily on, click, click, click, for
years and years, and no one had noticed him until someone learnt the
code of dot and dash in which he worked, and then all at once what he
was saying became clear. The chief instrument in translating the message
that the light brings is simply a prism, a three-cornered wedge of
glass, just the same as those hanging lustres belonging to the
chandeliers. When a piece of glass like this is fixed in a telescope in
such a way that the sun's rays fall on it, then there is thrown on to a
piece of paper or any other suitable background a broad coloured band of
lovely light like a little rainbow, and this is called the sun's
spectrum, and the instrument by which it is seen is called a
spectroscope. But this in itself could tell us little; the message it
brings lies in the fact that when it has passed through the telescope,
so that it is magnified, it is crossed by hundreds of minute black
lines, not placed evenly at all, but scattered up and down. There may be
two so close together that they look like one, and then three far apart,
and then some more at different distances. When this remarkable
appearance was examined carefully it was found that in sunlight the
lines that appeared were always exactly the same, in the same places,
and this seemed so curious that men began to seek for an explanation.
Someone thought of an experiment which might teach us something about
the matter, and instead of letting sunlight fall on the prism, he made
an artificial light by burning some stuff called sodium, and then
allowed the band of coloured light to pass through the telescope; when
he examined the spectrum that resulted, he found that, though numbers of
lines to be found in the sun's spectrum were missing, there were a few
lines here exactly matching a few of the lines in the sun's spectrum;
and this could not be the result of chance only, for the lines are so
mathematically exact, and are in themselves so peculiarly distributed,
that it could only mean that they were due to the same cause. What could
this signify, then, but that away up there in the sun, among other
things, stuff called sodium, very well known to chemists on earth, is
burning? After this many other substances were heated white-hot so as to
give out light, in order to discover if the lines to be seen in their
spectra were also to be found in the sun's spectrum. One of these was
iron, and, astonishing to say, all the many little thread-like lines
that appeared in its spectrum were reproduced to a hair's-breadth, among
others, in the sun's spectrum. So we have found out beyond all
possibility of doubt some of the materials of which the sun is made. We
know that iron, sodium, hydrogen, and numerous other substances and
elements, are all burning away there in a terrific furnace, to which any
furnace we have on earth is but as the flicker of a match.
It was not, of course, much use applying this method to the planets, for
we know that the light which comes from them to us is only reflected
sunlight, and this, indeed, was proved by means of the spectroscope. But
the stars shine by their own light, and this opened up a wide field for
inquiry. The difficulty was, of course, to get the light of one star
separated from all the rest, because the light of one star is very faint
and feeble to cast a spectrum at all. Yet by infinite patience
difficulties were overcome. One star alone was allowed to throw its
light into the telescope; the light passed through a prism, and showed a
faint band of many colours, with the expected little black lines cutting
across it more or less thickly. Examinations have thus been made of
hundreds of stars. In the course of them some substances as yet unknown
to us on earth have been encountered, and in some stars one
element--hydrogen--is much stronger than in others; but, on the whole,
speaking broadly, it has been satisfactorily shown that the stars are
made on the same principles as our own sun, so that the reasoning of
astronomers which had argued them to be suns was proved.
[Illustration: THE SPECTRUM OF THE SUN AND SIRIUS.]
We have here in the picture the spectrum of the sun and the spectrum of
Arcturus. You can see that the lines which appear in the band of light
belonging to Sirius are also in the band of light belonging to the sun,
together with many others. This means that the substances flaming out
and sending us light from the far away star are also giving out light
from our own sun, and that the sun and Sirius both contain the same
elements in their compositions.
This, indeed, seems enough for the spectroscope to have accomplished; it
has interpreted for us the message light brings from the stars, so that
we know beyond all possibility of mistake that these glowing, twinkling
points of light are brilliant suns in a state of intense heat, and that
in them are burning elements with which we ourselves are quite familiar.
But when the spectroscope had done that, its work was not finished, for
it has not only told us what the stars are made of, but another thing
which we could never have known without it--namely, if they are moving
toward us or going away from us.