THE GLITTERING HEAVENS
On a clear moonless night the stars appear uncountable. You see them
twinkling through the leafless trees, and covering all the sky from the
zenith, the highest point above your head, down to the horizon. It seems
as if someone had taken a gigantic pepper-pot and scattered them far and
wide so that some had fallen in all directions. If you were asked to
make a guess as to how many you can see at one time, no doubt you would
answer 'Millions!' But you would be quite wrong, for the number of stars
that can be seen at once without a telescope does not exceed two
thousand, and this, after the large figures we have been dealing with,
appears a mere trifle. With a telescope, even of small power, many more
are revealed, and every increase in the size of the telescope shows more
still; so that it might be supposed the universe is indeed illimitable,
and that we are only prevented from seeing beyond a certain point by our
limited resources. But in reality we know that this cannot be so. If
the whole sky were one mass of stars, as it must be if the number of
them were infinite, then, even though we could not distinguish the
separate items, we should see it bright with a pervading and diffused
light. As this is not so, we judge that the universe is not unending,
though, with all our inventions, we may never be able to probe to the
end of it. We need not, indeed, cry for infinity, for the distances of
the fixed stars from us are so immeasurable that to atoms like ourselves
they may well seem unlimited. Our solar system is set by itself, like a
little island in space, and far, far away on all sides are other great
light-giving suns resembling our own more or less, but dwindled to the
size of tiny stars, by reason of the great void of space lying between
us and them. Our sun is, indeed, just a star, and by no means large
compared with the average of the stars either. But, then, he is our own;
he is comparatively near to us, and so to us he appears magnificent and
unique. Judging from the solar system, we might expect to find that
these other great suns which we call stars have also planets circling
round them, looking to them for light and heat as we do to our sun.
There is no reason to doubt that in some instances the conjecture is
right, and that there may be other suns with attendant planets. It is
however a great mistake to suppose that because our particular family in
the solar system is built on certain lines, all the other families must
be made on the same pattern. Why, even in our own system we can see how
very much the planets differ from each other: there are no two the same
size; some have moons and some have not; Saturn's rings are quite
peculiar to himself, and Uranus and Neptune indulge in strange vagaries.
So why should we expect other systems to be less varied?
As science has advanced, the idea that these faraway suns must have
planetary attendants as our sun has been discarded. The more we know the
more is disclosed to us the infinite variety of the universe. For
instance, so much accustomed are we to a yellow sun that we never think
of the possibility of there being one of another colour. What would you
say then to a ruby sun, or a blue one; or to two suns of different
colours, perhaps red and green, circling round each other; or to two
such suns each going round a dark companion? For there are dark bodies
as well as shining bodies in the sky. These are some of the marvels of
the starry sky, marvels quite as absorbing as anything we have found in
the solar system.
It requires great care and patience and infinite labour before the very
delicate observations which alone can reveal to us anything of the
nature of the fixed stars can be accomplished. It is only since the
improvement in large telescopes that this kind of work has become
possible, and so it is but recently men have begun to study the stars
intimately, and even now they are baffled by indescribable difficulties.
One of these is our inability to tell the distance of a thing by merely
looking at it unless we also know its size. On earth we are used to
seeing things appear smaller the further they are from us, and by long
habit can generally tell the real size; but when we turn to the stars,
which appear so much alike, how are we to judge how far off they are?
Two stars apparently the same size and close together in the sky may
really be as far one from another as the earth is from the nearest; for
if the further one were very much larger than the nearer, they would
then appear the same size.
At first it was natural enough to suppose that the big bright stars of
what we call the first magnitude were the nearest to us, and the less
bright the next nearest, and so on down to the tiny ones, only revealed
by the telescope, which would be the furthest away of all; but research
has shown that this is not correct. Some of the brightest stars may be
comparatively near, and some of the smallest may be near also. The size
is no test of distance. So far as we have been able to discover, the
star which seems nearest _is_ a first magnitude one, but some of the
others which outshine it must be among the infinitely distant ones. Thus
we lie in the centre of a jewelled universe, and cannot tell even the
size of the jewels which cover its radiant robe.
I say 'lie,' but that is really not the correct word. So far as we have
been able to find out, there is no such thing as absolute rest in the
universe--in fact, it is impossible; for even supposing any body could
be motionless at first, it would be drawn by the attraction of its
nearest neighbours in space, and gradually gain a greater and greater
velocity as it fell toward them. Even the stars we call 'fixed' are all
hurrying along at a great pace, and though their distance prevents us
from seeing any change in their positions, it can be measured by
suitable instruments. Our sun is no exception to this universal rule.
Like all his compeers, he is hurrying busily along somewhere in
obedience to some impulse of which we do not know the nature; and as he
goes he carries with him his whole cortege of planets and their
satellites, and even the comets. Yes, we are racing through space with
another motion, too, besides those of rotation and revolution, for our
earth keeps up with its master attractor, the sun. It is difficult, no
doubt, to follow this, but if you think for a moment you will remember
that when you are in a railway-carriage everything in that carriage is
really travelling along with it, though it does not appear to move. And
the whole solar system may be looked at as if it were one block in
movement. As in a carriage, the different bodies in it continue their
own movements all the time, while sharing in the common movement. You
can get up and change your seat in the train, and when you sit down
again you have not only moved that little way of which you are
conscious, but a great way of which you are not conscious unless you
look out of the window. Now in the case of the earth's own motion we
found it necessary to look for something which does not share in that
motion for purposes of comparison, and we found that something in the
sun, who shows us very clearly we are turning on our axis. But in the
case of the motion of the solar system the sun is moving himself, so we
have to look beyond him again and turn to the stars for confirmation.
Then we find that the stars have motions of their own, so that it is
very difficult to judge by them at all. It is as if you were bicycling
swiftly towards a number of people all walking about in different
directions on a wide lawn. They have their movements, but they all also
have an apparent movement, really caused by you as you advance toward
them; and what astronomers had to do was to separate the true movements
of the stars from the false apparent movement made by the advance of the
sun. This great problem was attacked and overcome, and it is now known
with tolerable certainty that the sun is sweeping onward at a pace of
about twelve miles a second toward a fixed point. It really matters very
little to us where he is going, for the distances are so vast that
hundreds of years must elapse before his movement makes the slightest
difference in regard to the stars. But there is one thing which we can
judge, and that is that though his course appears to be in a straight
line, it is most probably only a part of a great curve so huge that the
little bit we know seems straight.
When we speak of the stars, we ought to keep quite clearly in our minds
the fact that they lie at such an incredible distance from us that it is
probable we shall never learn a great deal about them. Why, men have not
even yet been able to communicate with the planet Mars, at its nearest
only some thirty-five million miles from us, and this is a mere nothing
in measuring the space between us and the stars. To express the
distances of the stars in figures is really a waste of time, so
astronomers have invented another way. You know that light can go round
the world eight times in a second; that is a speed quite beyond our
comprehension, but we just accept it. Then think what a distance it
could travel in an hour, in a day; and what about a year? The distance
that light can travel in a year is taken as a convenient measure by
astronomers for sounding the depths of space. Measured in this way light
takes four years and four months to reach us from the nearest star we
know of, and there are others so much more distant that hundreds--nay,
thousands--of years would have to be used to convey it. Light which has
been travelling along with a velocity quite beyond thought, silently,
unresting, from the time when the Britons lived and ran half naked on
this island of ours, has only reached us now, and there is no limit to
the time we may go back in our imaginings. We see the stars, not as they
are, but as they were. If some gigantic conflagration had happened a
hundred years ago in one of them situated a hundred light-years away
from us, only now would that messenger, swifter than any messenger we
know, have brought the news of it to us. To put the matter in figures,
we are sure that no star can lie nearer to us than twenty-five billions
of miles. A billion is a million millions, and is represented by a
figure with twelve noughts behind it, so--1,000,000,000,000; and
twenty-five such billions is the least distance within which any star
can lie. How much farther away stars may be we know not, but it is
something to have found out even that. On the same scale as that we took
in our first example, we might express it thus: If the earth were a
greengage plum at a distance of about three hundred of your steps from
the sun, and Neptune were, on the same scale, about three miles away,
the nearest fixed star could not be nearer than the distance measured
round the whole earth at the Equator!
All this must provoke the question, How can anyone find out these
things? Well, for a long time the problem of the distances of the stars
was thought to be too difficult for anyone to attempt to solve it, but
at last an ingenious method was devised, a method which shows once more
the triumph of man's mind over difficulties. In practice this method is
extremely difficult to carry out, for it is complicated by so many other
things which must be made allowance for; but in theory, roughly
explained, it is not too hard for anyone to grasp. The way of it is
this: If you hold up your finger so as to cover exactly some object a
few feet distant from you, and shut first one eye and then the other,
you will find that the finger has apparently shifted very considerably
against the background. The finger has not really moved, but as seen
from one eye or the other, it is thrown on a different part of the
background, and so appears to jump; then if you draw two imaginary
lines, one from each eye to the finger, and another between the two
eyes, you will have made a triangle. Now, all of you who have done a
little Euclid know that if you can ascertain the length of one side of a
triangle, and the angles at each end of it, you can form the rest of the
triangle; that is to say, you can tell the length of the other two
sides. In this instance the base line, as it is called--that is to say
the line lying between the two eyes--can easily be measured, and the
angles at each end can be found by an instrument called a sextant, so
that by simple calculation anyone could find out what distance the
finger was from the eye. Now, some ingenious man decided to apply this
method to the stars. He knew that it is only objects quite near to us
that will appear to shift with so small a base line as that between the
eyes, and that the further away anything is the longer must the base
line be before it makes any difference. But this clever man thought that
if he could only get a base line long enough he could easily compute the
distance of the stars from the amount that they appeared to shift
against their background. He knew that the longest base line he could
get on earth would be about eight thousand miles, as that is the
diameter of the earth from one side to the other; so he carefully
observed a star from one end of this immense base line and then from the
other, quite confident that this plan would answer. But what happened?
After careful observations he discovered that no star moved at all with
this base line, and that it must be ever so much longer in order to make
any impression. Then indeed the case seemed hopeless, for here we are
tied to the earth and we cannot get away into space. But the astronomer
was nothing daunted. He knew that in its journey round the sun the earth
travels in an orbit which measures about one hundred and eighty-five
millions of miles across, so he resolved to take observations of the
stars when the earth was at one side of this great circle, and again,
six months later, when she had travelled to the other side. Then indeed
he would have a magnificent base line, one of one hundred and
eighty-five millions of miles in length. What was the result? Even with
this mighty line the stars are found to be so distant that many do not
move at all, not even when measured with the finest instruments, and
others move, it may be, the breadth of a hair at a distance of several
feet! But even this delicate measure, a hair's-breadth, tells its own
tale; it lays down a limit of twenty-five billion miles within which no
star can lie!
This system which I have explained to you is called finding the star's
parallax, and perhaps it is easier to understand when we put it the
other way round and say that the hair's-breadth is what the whole orbit
of the earth would appear to have shrunk to if it were seen from the
distance of these stars!
Many, many stars have now been examined, and of them all our nearest
neighbour seems to be a bright star seen in the Southern Hemisphere. It
is in the constellation or star group called Centaurus, and is the
brightest star in it. In order to designate the stars when it is
necessary to refer to them, astronomers have invented a system. To only
the very brightest are proper names attached; others are noted according
to the degree of their brightness, and called after the letters of the
Greek alphabet: alpha, beta, gamma, delta, etc. Our own word 'alphabet'
comes, you know, from the first two letters of this Greek series. As
this particular star is the brightest in the constellation Centaurus, it
is called Alpha Centauri; and if ever you travel into the Southern
Hemisphere and see it, you may greet it as our nearest neighbour in the
starry universe, so far as we know at present.