new interest. There is a bright star, Vega, or Alpha Lyræ, a beautiful
gem, so far off that the light from it which now reaches our eyes
started before many of my audience were born. Suppose that there are
astronomers residing on worlds amid the stars, and that they have
sufficiently powerful telescopes to view this globe, what do you think
they would observe? They will not see our earth as it is at present;
they will see it as it was years (and sometimes many years) ago. There
are stars from which if England could now be seen, the whole of the
country would be observed at this present moment to be in a great
state of excitement at a very auspicious event. Distant astronomers
might notice a great procession in London, and they could watch the
coronation of a youthful queen amid the enthusiasm of a nation. There
are other stars still further, from which, if the inhabitants had good
enough telescopes, they would now see a mighty battle in progress not
far from Brussels. One splendid army could be beheld hurling itself
time after time against the immovable ranks of the other. They would
not, indeed, be able to hear the ever-memorable "Up, Guards, and at
them!" but there can be no doubt that there are stars so far away that
the rays of light which started from the earth on the day of the
battle of Waterloo are only just arriving there. Further off still,
there are stars from which a bird's-eye view could be taken at this
very moment of the signing of Magna Charta. There are even stars from
which England, if it could be seen at all, would now appear, not as
the great England we know, but as a country covered by dense forests,
and inhabited by painted savages, who waged incessant war with wild
beasts that roamed through the island. The geological problems that
now puzzle us would be quickly solved could we only go far enough into
space and had we only powerful enough telescopes. We should then be
able to view our earth through the successive epochs of past
geological time; we should be actually able to see those great animals
whose fossil remains are treasured in our museums tramping about over
the earth's surface, splashing across its swamps, or swimming with
broad flippers through its oceans. Indeed, if we could view our own
earth reflected from mirrors in the stars, we might still see Moses
crossing the Red Sea, or Adam and Eve being expelled from Eden.
So important is the subject of star distance that I am tempted to give
one more illustration in order to bring before you some conception of
how vast such distances are. I shall take, as before, the nearest of
the stars so far as known to us, and I hope to be forgiven for taking
an illustration of a practical and a commercial kind instead of one
more purely scientific. I shall suppose that a railway is about to be
made from London to Alpha Centauri. The length of that railway, of
course, we have already stated: it is twenty billions of miles. So I
am now going to ask your attention to the simple question as to the
fare which it would be reasonable to charge for the journey. We shall
choose a very cheap scale on which to compute the price of a ticket.
The parliamentary rate here is, I believe, a penny for every mile. We
will make our interstellar railway fares much less even than this; we
shall arrange to travel at the rate of one hundred miles for every
penny. That, surely, is moderate enough. If the charges were so low
that the journey from London to Edinburgh only cost fourpence, then
even the most unreasonable passenger would be surely contented. On
these terms how much do you think the fare from London to this star
ought to be? I know of one way in which to make our answer
intelligible. There is a National Debt with which your fathers are,
unhappily, only too well acquainted; you will know quite enough about
it yourselves in those days when you have to pay income tax. This debt
is so vast that the interest upon it is about sixty thousand pounds a
day, the whole amount of the National Debt being six hundred and
thirty-eight millions of pounds.
If you went to the booking-office with the whole of this mighty sum in
your pocket--but stop a moment; could you carry it in your pocket?
Certainly not, if it were in sovereigns. You would find that after you
had as many sovereigns as you could conveniently carry there would
still be some left--so many, indeed, that it would be necessary to get
a cart to help you on with the rest. When the cart had as great a load
of sovereigns as the horse could draw there would be still some more,
and you would have to get another cart; but ten carts, twenty carts,
fifty carts, would not be enough. You would want five thousand of
these before you would be able to move off towards the station with
your money. When you did get there and asked for a ticket at the rate
of one hundred miles for a penny, do you think you would get any
change? No doubt some little time would be required to count the
money, but when it was counted the clerk would tell you that there was
not enough--that he must have nearly two hundred millions of pounds
more.
That will give some notion of the distance of the nearest star, and we
may multiply it by ten, by one hundred, and even by one thousand, and
still not attain to the distance of some of the more remote stars that
the telescope shows us.
On account of the immense distances of the stars we can only perceive
them to be mere points of light. We can never see a star to be a globe
with marks on it like the moon, or like one of the planets--in fact,
the better the telescope the smaller does the star seem, though, of
course, its brightness is increased with every addition to the
light-grasping power of the instrument.
The Brightness and Color of Stars.
Another point to be noticed is the arrangement of stars in classes,
according to their lustre. The brightest stars, of which there are
about twenty, are said to be of the first magnitude. Those just
inferior to the first magnitude are ranked as the second; and those
just lower than the second are estimated as the third; and so on. The
smallest points that your unaided eyes will show you are of about the
sixth magnitude. Then the telescope will reveal stars still fainter
and fainter, down to what we term the seventeenth or eighteenth
magnitudes, or even lower still. The number of stars of each magnitude
increases very much in the classes of small ones.
Most of the stars are white, but many are of a somewhat ruddy hue.
There are a few telescopic points which are intensely red, some
exhibit beautiful golden tints, while others are blue or green.
There are some curious stars which regularly change their brilliancy.
Let me try to illustrate the nature of these variables. Suppose that
you were looking at a street gas-lamp from a very long distance, so
that it seemed a little twinkling light; and suppose that some one was
preparing to turn the gas-cock up and down. Or, better still, imagine
a little machine which would act regularly so as to keep the light
first of all at its full brightness for two days and a half, and then
gradually turn it down until in three or four hours it declines to a
feeble glimmer. In this low state the light remains for twenty
minutes; then during three or four hours the gas is to be slowly
turned on again until it is full. In this condition the light will
remain for two days and a half, and then the same series of changes is
to recommence. This would be a very odd form of gas-lamp. There would
be periods of two days and a half during which it would remain at its
full; these would be separated by intervals of about seven hours, when
the gradual turning down and turning up again would be in progress.
The imaginary gas-lamp is exactly paralleled by a star Algol, in the
constellation of Perseus (Fig. 3), which goes through the series of
changes I have indicated. Ordinarily speaking, it is a bright star of
the second magnitude, and, whatever be the cause, the star performs
its variations with marvellous uniformity. In fact, Algol has always
arrested the attention of those who observed the heavens, and in early
times was looked on as the eye of a demon. There are many other stars
which also change their brilliancy. Most of them require much longer
periods than Algol, and sometimes a new star which nobody has ever
seen before will suddenly kindle into brilliancy. It is now known that
the bright star Algol is attended by a dark companion. This dark star
sometimes comes between Algol and the observer and cuts off the light.
Thus it is that the diminution of brightness is produced.
[Illustration: FIG. 3. PERSEUS AND ITS NEIGHBORING STARS INCLUDING
ALGOL.]
Double Stars.
Whenever you have a chance of looking at the heavens through a
telescope, you should ask to be shown what is called _a double star_.
There are many stars in the heavens which present no remarkable
appearance to the unaided eye, but which a good telescope at once
shows to be of quite a complex nature. These are what we call double
stars, in which two quite distinct stars are placed so close together
that the unaided eye is unable to separate them. Under the magnifying
power of the telescope, however, they are seen to be distinct. In
order to give some notion of what these objects are like, I shall
briefly describe three of them. The first lies in that best known
constellation, the Great Bear. If you look at his tail, which consists
of three stars, you will see that near the middle one of the three a
small star is situated; we call this little star Alcor, but it is the
brighter one near Alcor to which I specially call your attention. The
sharpest eye would never suspect that it was composed of two stars
placed close together. Even a small telescope will, however, show this
to be the case, and this is the easiest and the first observation that
a young astronomer should make when beginning to turn a telescope to
the heavens. Of course you will not imagine that I mean Alcor to be
the second component of the double star; it is the bright star near
Alcor which is the double. Here are two marbles, and these marbles are
fastened an inch apart. You can see them, of course, to be separate;
but if the pair were moved further and further away, then you would
soon not be able to distinguish between them, though the actual
distance between the marbles had not altered. Look at these two wax
tapers which are now lighted; the little flames are an inch apart. You
would have to view them from a station a third of a mile away if the
distance between the two flames were to appear the same as that
between the two components of this double star. Your eye would never
be able to discriminate between two lights only an inch apart at so
great a distance; a telescope would, however, enable you to do so, and
this is the reason why we have to use telescopes to show us double
stars.
You might look at that double star year after year throughout the
course of a long life without finding any appreciable change in the
relative positions of its components. But we know that there is no
such thing as rest in the universe; even if you could balance a body
so as to leave it for a moment at rest, it would not stay there, for
the simple reason that all the bodies round it in every direction are
pulling at it, and it is certain that the pull in one direction will
preponderate, so that move it must. Especially is this true in the
case of two suns like those forming a double star. Placed
comparatively near each other they could not remain permanently in
that position; they must gradually draw together and come into
collision with an awful crash. There is only one way by which such a
disaster could be averted. That is by making one of these stars
revolve around the other just as the earth revolves around the sun, or
the moon revolves around the earth. Some motion must, therefore, be
going on in every genuine double star, whether we have been able to
see that motion or not.
Let us now look at another double star of a different kind. This time
it is in the constellation of Gemini. The heavenly twins are called
Castor and Pollux. Of these, Castor is a very beautiful double star,
consisting of two bright points, a great deal closer together than
were those in the Great Bear; consequently a better telescope is
required for the purpose of showing them separately. Castor has been
watched for many years, and it can be seen that one of these stars is
slowly revolving around the other; but it takes a very long time,
amounting to hundreds of years, for a complete circuit to be
accomplished. This seems very astonishing, but when you remember how
exceedingly far Castor is, you will perceive that that pair of stars
which appear so close together that it requires a telescope to show
them apart must indeed be separated by hundreds of millions of miles.
Let us try to conceive our own system transformed into a double star.
If we took our outermost planet--Neptune--and enlarged him a good
deal, and then heated him sufficiently to make him glow like a sun, he
would still continue to revolve round our sun at the same distance,
and thus a double star would be produced. An inhabitant of Castor who
turned his telescope towards us would be able to see the sun as a
star. He would not, of course, be able to see the earth, but he might
see Neptune like another small star close to the sun. If generations
of astronomers in Castor continued their observations of our system,
they would find a binary star, of which one component took a century
and a half to go round the other. Need we then be surprised that when
we look at Castor we observe movements that seem very slow?
There is often so much diffused light about the bright stars seen in a
telescope, and so much twinkling in some states of the atmosphere,
that stars appear to dance about in rather a puzzling fashion,
especially to one who is not accustomed to astronomical observations.
I remember hearing how a gentleman once came to visit an observatory.
The astronomer showed him Castor through a powerful telescope as a
fine specimen of a double star, and then, by way of improving his
little lesson, the astronomer mentioned that one of these stars was
revolving around the other. "Oh, yes," said the visitor, "I saw them
going round and round in the telescope." He would, however, have had
to wait for a few centuries with his eye to the instrument before he
would have been entitled to make this assertion.
Double stars also frequently delight us by giving beautifully
contrasted colors. I dare say you have often noticed the red and the
green lights that are used on railways in the signal lamps. Imagine
one of those red and one of those green lights away far up in the sky
and placed close together, then you would have some idea of the
appearance that a colored double star presents, though, perhaps, I
should add that the hues in the heavenly bodies are not so vividly
different as are those which our railway people find necessary. There
is a particularly beautiful double star of this kind in the
constellation of the Swan. You could make an imitation of it by boring
two holes, with a red-hot needle, in a piece of card, and then
covering one of these holes with a small bit of the topaz-colored
gelatine with which Christmas crackers are made. The other star is to
be similarly colored with blue gelatine. A slide made on this
principle placed in the lantern gives a very good representation of
these two stars on the screen. There are many other colored doubles
besides this one; and, indeed, it is noteworthy that we hardly ever
find a blue or a green star by itself in the sky; it is always as a
member of one of these pairs.
How We Find What the Stars are Made of.
Here is a piece of stone. If I wanted to know what it was composed of,
I should ask a chemist to tell me. He would take it into his
laboratory, and first crush it into powder, and then, with his test
tubes, and with the liquids which his bottles contain, and his
weighing scales, and other apparatus, he would tell all about it;
there is so much of this, and so much of that, and plenty of this, and
none at all of that. But now, suppose you ask this chemist to tell you
what the sun is made of, or one of the stars. Of course, you have not
a sample of it to give him; how, then, can he possibly find out
anything about it? Well, he can tell you something, and this is the
wonderful discovery that I want to explain to you. We now put down the
gas, and I kindle a brilliant red light. Perhaps some of those whom I
see before me have occasionally ventured on the somewhat dangerous
practice of making fire-works. If there is any boy here who has ever
constructed sky-rockets, and put the little balls into the top which
are to burn with such vivid colors when the explosion takes place, he
will know that the substance which tinged that fire red must have been
strontium. He will recognize it by the color; because strontium gives
a red light which nothing else will give. Here are some of these
lightning papers, as they are called; they are very pretty and very
harmless; and these, too, give brilliant red flashes as I throw them.
The red tint has, no doubt, been produced by strontium also. You see
we recognized the substance simply by the color of the light it
produced when burning.
Perhaps some of you have tried to make a ghost at Christmas by
dressing up in a sheet, and bearing in your hand a ladle blazing with
a mixture of common salt and spirits of wine, the effect produced
being a most ghastly one. Some mammas will hardly thank me for this
suggestion, unless I add that the ghost must walk about cautiously,
for otherwise the blazing spirit would be very apt to produce
conflagrations of a kind more extensive than those intended. However,
by the kindness of Professor Dewar, I am enabled to show the
phenomenon on a splendid scale, and also free from all danger. I
kindle a vivid flame of an intensely yellow color, which I think the
ladies will unanimously agree is not at all becoming to their
complexions, while the pretty dresses have lost their variety of
colors. Here is a nice bouquet, and yet you can hardly distinguish the
green of the leaves from the brilliant colors of the flowers, except
by trifling differences of shade. Expose to this light a number of
pieces of variously colored ribbon, pink and red and green and blue,
and their beauty is gone; and yet we are told that this yellow is a
perfectly pure color; in fact, the purest color that can be produced.
I think we have to be thankful that the light which our good sun sends
us does not possess purity of that description. There is one substance
which will produce that yellow light; it is a curious metal called
sodium--a metal so soft that you can cut it with a knife, and so light
that it will float on water; while, still more strange, it actually
takes fire the moment it is dropped on the water. It is only in a
chemical laboratory that you will be likely to meet with the actual
metallic sodium, yet in other forms the substance is one of the most
abundant in nature. Indeed, common salt is nothing but sodium closely
united with a most poisonous gas, a few respirations of which would
kill you. But this strange metal and this noxious gas, when united,
become simply the salt for our eggs at breakfast. This pure yellow
light, wherever it is seen, either in the flame of spirits of wine
mixed with salt or in that great blaze at which we have been looking,
is characteristic of sodium. Wherever you see that particular kind of
light, you know that sodium must have been present in the body from
which it came.
We have accordingly learned to recognize two substances, namely,
strontium and sodium, by the different lights which they give out when
burning. To these two metals we may add a third. Here is a strip of
white metallic ribbon. It is called magnesium. It seems like a bit of
tin at the first glance, but indeed it is a very different substance
from tin; for, look, when I hold it in the spirit-lamp, the strip of
metal immediately takes fire, and burns with a white light so dazzling
that it pales the gas-flames to insignificance. There is no other
substance which will, when kindled, give that particular kind of light
which we see from magnesium. I can recommend this little experiment as
quite suitable for trying at home; you can buy a bit of magnesium
ribbon for a trifle at the opticians; it cannot explode or do any
harm, nor will you get into any trouble with the authorities provided
you hold it when burning over a tray or a newspaper, so as to prevent
the white ashes from falling on the carpet.
There are, in nature, a number of simple bodies called elements.
Every one of these, when ignited under suitable conditions, emits a
light which belongs to it alone, and by which it can be distinguished
from every other substance. I do not say that we can try the
experiments in the simple way I have here indicated. Many of the
materials will yield light which will require to be studied by much
more elaborate artifices than those which have sufficed for us. But
you will see that the method affords a means of finding out the actual
substances present in the sun or in the stars. There is a practical
difficulty in the fact that each of the heavenly bodies contains a
number of different elements; so that in the light it sends us the
hues arising from distinct substances are blended into one beam. The
first thing to be done is to get some way of splitting up a beam of
light, so as to discover the components of which it is made. You might
have a skein of silks of different hues tangled together, and this
would be like the sunbeam as we receive it in its unsorted condition.
How shall we untangle the light from the sun or a star? I will show
you by a simple experiment. Here is a beam from the electric light;
beautifully white and bright, is it not? It looks so pure and simple,
but yet that beam is composed of all sorts of colors mingled together,
in such proportions as to form white light. I take a wedge-shaped
piece of glass called a prism, and when I introduce it into the course
of the beam, you see the transformation that has taken place (Fig. 4).
Instead of the white light you have now all the colors of the
rainbow--red, orange, yellow, green, blue, indigo, violet, marked by
their initial letters in the figure. These colors are very beautiful,
but they are transient, for the moment we take away the prism they
all unite again to form white light. You see what the prism has done;
it has bent all the light in passing through it; but it is more
effective in bending the blue than the red, and consequently the blue
is carried away much further than the red. Such is the way in which we
study the composition of a heavenly body. We take a beam of its light,
we pass it through a prism, and immediately it is separated into its
components; then we compare what we find with the lights given by the
different elements, and thus we are enabled to discover the substances
which exist in the distant object whose light we have examined. I do
not mean to say that the method is a simple one; all I am endeavoring
to show is a general outline of the way in which we have discovered
the materials present in the stars. The instrument that is employed
for this purpose is called the spectroscope. And perhaps you may
remember that name by these lines, which I have heard from an
astronomical friend:--
"Twinkle, twinkle, little star,
Now we find out what you are,
When unto the midnight sky,
We the spectroscope apply."
[Illustration: FIG. 4. HOW A RAY OF LIGHT IS SPLIT UP.]
I am sure it will interest everybody to know that the elements which
the stars contain are not altogether different from those of which the
earth is made. It is true there may be substances in the stars of
which we know nothing here; but it is certain that many of the most
common elements on the earth are present in the most distant bodies. I
shall only mention one, the metal iron. That useful substance has been
found in some of the stars which lie at almost incalculable distances
from the earth.
The Nebulæ.