THE COLOURS OF THE STARS
Has it ever occurred to you that the stars are not all of the same
colour? It is true that, just glancing at them casually, you might say
they are all white; but if you examine them more carefully you cannot
help seeing that some shine with a steely blue light, while others are
reddish or yellowish. These colours are not easy to distinguish with the
naked eye, and might not attract any attention at all unless they were
pointed out; yet when attention is drawn to the fact, it is impossible
to deny the redness of some, such as Aldebaran. But though we may admit
this, we might add that the colours are so very faint and inconspicuous,
that they might be, after all, only the result of imagination.
To prove that the colours are constant and real we must use a telescope,
and then we need have no further doubt of their reality, for instead of
disappearing, the colours of some stars stand out quite vividly beyond
the possibility of mistake. Red stars are a bright red, and they are the
most easily seen of all, though the other colours, blue and yellow and
green, are seen very decidedly by some people. The red stars have been
described by various observers as resembling 'a drop of blood on a black
field,' 'most magnificent copper-red,' 'most intense blood-red,' and
'glowing like a live coal out of the darkness of space.' Some people see
them as a shining red, like that of a glowing cloud at sunset. Therefore
there can be no doubt that the colours are genuine enough, and are
telling us some message. This message we are able to read, for we have
begun to understand the language the stars speak to us by their light
since the invention of the spectroscope. The spectroscope tells us that
these colours indicate different stages in the development of the stars,
or differences of constitution--that is to say, in the elements of which
they are made. Our own sun is a yellow star, and other yellow stars are
akin to him; while red and blue and green stars contain different
elements, or elements in different proportions.
Stars do not always remain the same colours for an indefinite time; one
star may change slowly from yellow to white, and another from red to
yellow; and there are instances of notable changes, such as that of the
brilliant white Sirius, who was stated in old times by many different
observers to be a red star. All this makes us think, and year by year
thought leads us on to knowledge, and knowledge about these distant suns
increases. But though we know a good deal now, there are still many
questions we should like to ask which we cannot expect to have answered
for a long time yet, if ever.
The star colours have some meanings which we cannot even guess; we can
only notice the facts regarding them. For instance, blue stars are never
known to be solitary--they always have a companion, but why this should
be so passes our comprehension. What is it in the constitution of a blue
star which holds or attracts another? Whatever it may be, it is
established by repeated instances that blue stars do not stand alone. In
the constellation of Cygnus there are two stars, a blue and a yellow
one, which are near enough to each other to be seen in the same
telescope at the same time, and yet in reality are separated by an
almost incredible number of billions of miles. But as we know that a
blue star is never seen alone, and that it has often as its companion a
yellowish or reddish star, it is probable that these two, situated at an
enormous distance from one another, are yet in some mysterious way
dependent on each other, and are not merely seen together because they
happen to fall in the same field of view.
Many double stars show most beautifully contrasted colours: among them
are pairs of yellow and rose-red, golden and azure, orange and purple,
orange and lilac, copper-colour and blue, apple-green and cherry-red,
and so on. In the Southern Hemisphere there is a cluster containing so
many stars of brilliant colours that Sir John Herschel named it 'the
Jewelled Cluster.'
I expect most of you have seen an advertisement of Pear's Soap, in which
you are asked to stare at some red letters, and then look away to some
white surface, such as a ceiling, when you will see the same letters in
green. This is because green is the complementary or contrasting colour
to red, and the same thing is the case with blue and yellow. When any
one colour of either of these pairs is seen, it tends to make the other
appear by reaction, and if the eye gazed hard at blue instead of red, it
would next see yellow, and not green. Now, many people to whom this
curious fact is known argue that perhaps the colours of the double
stars are not real, but the effect of contrast only; for instance, they
say a red star near a companion white one would tend to make the
companion appear green, and so, of course, it would. But this does not
account for the star colours, which are really inherent in the stars
themselves, as may be proved by cutting off the light of one star, and
looking only at the other, when its colour still appears unchanged.
Another argument equally strong against the contrast theory is that the
colours of stars in pairs are by no means always those which would
appear if the effect was only due to complementary colours. It is not
always blue and yellow or red and green pairs that we see, though these
are frequent, but many others of various kinds, such as copper and blue,
and ruddy and blue.
We have therefore come to the conclusion that there are in this
astonishing universe numbers of gloriously coloured suns, some of which
apparently lie close together. What follows? Why, we want to know, of
course, if these stars are really pairs connected with each other, or if
they only appear so by being in the same line of sight, though one is
infinitely more distant than the other. And that question also has been
answered. There are now known thousands of cases in which stars,
hitherto regarded as single, have been separated into two, or even more,
by the use of a telescope. Of these thousands, some hundreds have been
carefully investigated, and the result is that, though there are
undoubtedly some in which the connexion is merely accidental, yet in by
far the greater number of cases the two stars thus seen together have
really some connexion which binds them to one another; they are
dependent on one another. This has been made known to us by the working
of the wonderful law of gravitation, which is obeyed throughout the
whole universe. We know that by the operation of this law two mighty
suns will be drawn toward each other with a certain pull, just as surely
as we know that a stone let loose from the hand will fall upon the
earth; so by noting the effect of two mighty suns upon each other many
facts about them may be found out. By the most minute and careful
measurements, by the use of the spectroscope, and by every resource
known to science, astronomers have, indeed, actually found out with a
near approach to exactness how far some of these great suns lie from
each other, and how large they are in comparison with one another.
The very first double star ever discovered was one which you have
already seen, the middle one in the tail of the Great Bear. If you look
at it you will be delighted to find that you can see a wee star close to
it, and you will think you are looking at an example of a double star
with your very own eyes; but you will be wrong, for that wee star is
separated by untold distances from the large one to which it seems so
near. In fact, any stars which can be seen to be separate by the naked
eye must lie immeasurably far apart, however tiny seems the space
between them. Such stars may possibly have some connexion with each
other, but, at any rate in this case, such a connexion has not been
proved. No, the larger star itself is made up of two others, which can
only be seen apart in a telescope. Since this discovery double stars
have been plentifully found in every part of the sky. The average space
between such double stars as seen from our earth is--what do you think?
It is the width of a single hair held up thirty-six feet from our eyes!
This could not, of course, be seen without the use of a telescope or
opera-glasses. It serves to give some impression of star distances when
we think that the millions and millions of miles lying between those
stars have shrunk to that hair's-breadth seen from our point of view.
Twin stars circle together round a common centre of gravity, and are
bound by the laws of gravitation just as the planets are. Our sun is a
solitary star, with no companion, and therefore such a state of things
seems to us to be incredible. Fancy two gigantic suns, one topaz-yellow
and the other azure-blue, circling around in endless movement! Where in
such a system would there be room for the planets? How could planets
exist under the pull of two suns in opposite directions? Still more
wonders are unfolded as the inquiry proceeds. Certain irregularities in
the motions of some of these twin systems led astronomers to infer that
they were acted upon by another body, though this other body was not
discernible. In fact, though they could not see it, they knew it must be
there, just as Adams and Leverrier knew of the existence of Neptune,
before ever they had seen him, by the irregularities in the movements of
Uranus. As the results showed, it was there, and was comparable in size
to the twin suns it influenced, and yet they could not see it. So they
concluded this third body must be dark, not light-giving like its
companions. We are thus led to the strange conclusion that some of these
systems are very complicated, and are formed not only of shining suns,
but of huge dark bodies which cannot be called suns. What are they,
then? Can they be immense planets? Is it possible that life may there
exist? No fairy tale could stir the imagination so powerfully as the
thought of such systems including a planetary body as large or larger
than its sun or suns. If indeed life exists there, what a varied scene
must be presented day by day! At one time both suns mingling their
flashing rays may be together in the sky; at another time only one
appears, a yellow or blue sun, as the case may be. The surface of such
planets must undergo weird transformations, the foliage showing one day
green, the next yellow, and the next blue; shadows of azure and orange
will alternate! But fascinating as such thoughts are, we can get no
further along that path.
To turn from fancy to facts, we find that telescope and spectroscope
have supplied us with quite enough matter for wonder without calling
upon imagination. We have discovered that many of the stars which seem
to shine with a pure single light are double, and many more consist not
only of two stars, but of several, some of which may be dark bodies. The
Pole Star was long known to be double, and is now discovered to have a
third member in its system. These multiple systems vary from one
another in almost every case. Some are made up of a mighty star and a
comparatively small one; others are composed of stars equal in
light-giving power--twin suns. Some progress swiftly round their orbits,
some go slowly; indeed, so slowly that during the century they have been
under observation only the very faintest sign of movement has been
detected; and in other systems, which we are bound to suppose double,
the stars are so slow in their movements that no progress seems to have
been made at all.
The star we know as the nearest to us in the heavens, Alpha Centauri, is
composed of two very bright partners, which take about eighty-seven
years to traverse their orbit. They sometimes come as near to each other
as Saturn is to the sun. In the case of Sirius astronomers found out
that he had a companion by reason of his irregularities of movement
before they discovered that companion, which is apparently a very small
star, only to be discerned with good telescopes. But here, again, it
would be unwise to judge only by what we see. Though the star appears
small, we know by the influence it exercises on Sirius that it is very
nearly the same size as he is. Thus we judge that it is poor in
light-giving property; in fact, its shining power is much less than that
of its companion, though its size is so nearly equal. This is not
wonderful, for Sirius's marvellous light-giving power is one of the
wonders of the universe; he shines as brilliantly as twenty-nine or
thirty of our suns!
In some cases the dark body which we cannot see may even be larger than
the shining one, through which alone we can know anything of it. Here we
have a new idea, a hint that in some of these systems there may be a
mighty earth with a smaller sun going round it, as men imagined our sun
went around the earth before the real truth was found out.
So we see that, when we speak of the stars as suns comparable with our
sun, we cannot think of them all as being exactly on the same model.
There are endless varieties in the systems; there are solitary suns like
ours which may have a number of small planets going round them, as in
the solar system; but there are also double suns going round each other,
suns with mighty dark bodies revolving round them which may be planets,
and huge dark bodies with small suns too. Every increase of knowledge
opens up new wonders, and the world in which we live is but one kind of
world amid an infinite number.
In this chapter we have learnt an altogether new fact--the fact that the
hosts of heaven comprise not only those shining stars we are accustomed
to see, but also dark bodies equally massive, and probably equally
numerous, which we cannot see. In fact, the regions of space may be
strewn with such dark bodies, and we could have no possible means of
discovering them unless they were near enough to some shining body to
exert an influence upon it. It is not with his eyes alone, or with his
senses, man knows of the existence of these great worlds, but often
solely by the use of the powers of his mind.