nature. We have had occasion to see that this sun of ours is a
magnificent globe immensely larger than the greatest of his planets,
while the greatest of these planets is immensely larger than this
earth; but now we are to learn that our sun is, indeed, only a star
not nearly so bright as many of those which shine over our heads every
night. We are comparatively close to the sun, so that we are able to
enjoy his beautiful light and cheering heat. Each of those other
myriads of stars is a sun, and the splendor of those distant suns is
often far greater than that of our own. We are, however, so enormously
far from them that they appear dwindled down to insignificance. To
judge impartially between our sun or star and such a sun or star as
Sirius we should stand halfway between the two; it is impossible to
make a fair estimate when we find ourselves situated close to one star
and a million times as far from the other. After allowance is made for
the imperfections of our point of view, we are enabled to realize the
majestic truth that the sun is no more than a star, and that the other
stars are no less than suns. This gives us an imposing idea of the
extent and magnificence of the universe in which we are situated. Look
lip at the sky at night--you will see a host of stars; try to think
that every one of them is itself a sun. It may probably be that those
suns have planets circling round them, but it is hopeless for us to
expect to see such planets. Were you standing on one of those stars
and looking towards our system, you would not perceive the sun to be
the brilliant and gorgeous object that we know so well. If you could
see him at all, he would merely seem like a star, not nearly as bright
as many of those you can see at night. Even if you had the biggest of
telescopes to aid your vision, you could never discern from one of
these bodies the planets which surround the sun, no astronomer in the
stars could see Jupiter, even if his sight were a thousand times as
powerful as any sight or telescope that we know. So minute an object
as our earth would, of course, be still more hopelessly beyond the
possibility of vision.
The Number of the Stars.
To count the stars involves a task which lies beyond the power of man
to accomplish. Even without the aid of any telescope, we can see a
great multitude of stars from this part of the world. There are also
many constellations in the southern hemisphere which never appear
above our horizon. If, however, we were to go to the equator, then, by
waiting there for a twelve-month, all the stars in the heavens would
have been successively exposed to view. An astronomer, Houzeau, with
the patience to count them, enumerated about six thousand. This is the
naked-eye estimate of the star-population of the heavens; but if
instead of relying on unaided vision, you get the assistance of a
little telescope, you will be astounded at the enormous multitude of
stars which are disclosed.
[Illustration: FIG 1. THE GREAT BEAR AND THE POLE.]
An ordinary opera-glass or binocular is a very useful instrument for
looking at the stars in the heavens. If you employ an instrument of
this sort, you will be amazed to find that the heavens teem with
additional hosts of stars that your unaided vision would never have
given you knowledge of. Any part of the sky may be observed; but, just
to give an illustration, I shall take one special region, namely, that
of the Great Bear (Fig. 1). The seven well-known stars are here shown,
four of which form a sort of oblong, while the other three represent
the tail. I would like you to make this little experiment. On a fine
clear night, count how many stars there are within this oblong; they
are all very faint, but you will be able to see a few, and, with good
sight, and on a clear night, you may see perhaps ten. Next take your
opera-glass and sweep it over the same region; if you will carefully
count the stars it shows, you will find fully two hundred; so that
the opera-glass has, in this part of the sky, revealed nearly twenty
times as many stars as could be seen without its aid. As six thousand
stars can be seen by the eye all over the heavens, we may fairly
expect that twenty times that number--that is to say, one hundred and
twenty thousand stars--could be shown by the opera-glass over the
entire sky. Let us go a step further, and employ a telescope, the
object-glass of which is three inches across. This is a useful
telescope to have, and, if a good one, will show multitudes of
pleasing objects, though an astronomer would not consider it very
powerful. An instrument like this, small enough to be carried in the
hand, has been applied to the task of enumerating the stars in the
northern half of the sky, and three hundred and twenty thousand stars
were counted. Indeed, the actual number that might have been seen with
it is considerably greater, for when the astronomer Argelander made
this memorable investigation he was unable to reckon many of the stars
in localities where they lay very close together. This grand count
only extended to half the sky, and, assuming that the other half is as
richly inlaid with stars, we see that a little telescope like that we
have supposed will, when swept over the heavens, reveal a number of
stars which exceeds that of the population of any city in England
except London. It exhibits more than one hundred times as many stars
as our eyes could possibly reveal. Still, we are only at the beginning
of the count; the very great telescopes add largely to the number.
There are multitudes of stars which in small instruments we cannot
see, but which are distinctly visible from our great observatories.
That telescope would be still but a comparatively small one which
would show as many stars in the sky as there are people living in the
mighty city of London; and with the greatest instruments, the tale of
stars has risen to a number far greater than that of the entire
population of Great Britain.
In addition to those stars which the largest telescopes show us, there
are myriads which make their presence evident in a wholly different
way. It is only in quite recent times that an attempt has been made to
develop fully the powers of photography in representing the celestial
objects. On a photographic plate which has been exposed to the sky in
a great telescope the stars are recorded by thousands. Many of these
may, of course, be observed with a good telescope, but there are not a
few others which no one ever saw in a telescope, which apparently no
one ever could see, though the photograph is able to show them. We do
not, however, employ a camera like that which the photographer uses
who is going to take your portrait. The astronomer's plate is put into
his telescope, and then the telescope is turned towards the sky. On
that plate the stars produce their images, each by its own light. Some
of these images are excessively faint, but we give a very long
exposure of an hour or two hours; sometimes as much as four hours'
exposure is given to a plate so sensitive that a mere fraction of a
second would sufficiently expose it during the ordinary practice of
taking a photograph in daylight. We thus afford sufficient time to
enable the fainter objects to indicate their presence upon the
sensitive film. Even with an exposure of a single hour a picture
exhibiting sixteen thousand stars has been taken by Mr. Isaac Roberts,
of Liverpool. Yet the portion of the sky which it represents is only
one ten-thousandth part of the entire heavens. It should be added that
the region which Mr. Roberts has photographed is furnished with stars
in rather exceptional profusion.
Here, at last, we have obtained some conception of the sublime scale
on which the stellar universe is constructed. Yet even these plates
cannot represent all the stars that the heavens contain. We have every
reason for knowing that with larger telescopes, with more sensitive
plates, with more prolonged exposures, ever fresh myriads of stars
will be brought within our view.
You must remember that every one of these stars is truly a sun, a
lamp, as it were, which doubtless gives light to other objects in its
neighborhood as our sun sheds light upon this earth and the other
planets. In fact, to realize the glories of the heavens you should try
to think that the brilliant points you see are merely the luminous
points of the otherwise invisible universe.
Standing one fine night on the deck of a Cunarder we passed in open
ocean another great Atlantic steamer. The vessel was near enough for
us to see not only the light from the mast-head but also the little
beams from the several cabin ports; and we could see nothing of the
ship herself. Her very existence was only known to us by the twinkle
of these lights. Doubtless her passengers could see, and did see, the
similar lights from our own vessel, and they probably drew the correct
inference that these lights indicated a great ship.
Consider the multiplicity of beings and objects in a ship: the
captain and the crew, the passengers, the cabins, the engines, the
boats, the rigging, and the stores. Think of all the varied interests
there collected and then reflect that out on the ocean, at night, the
sole indication of the existence of this elaborate structure was given
by the few beams of light that happened to radiate from it. Now raise
your eyes to the stars; there are the twinkling lights. We cannot see
what those lights illuminate, we can only conjecture what untold
wealth of non-luminous bodies may also lie in their vicinity; we may,
however, feel certain that just as the few gleaming lights from a ship
are utterly inadequate to give a notion of the nature and the contents
of an Atlantic steamer, so are the twinkling stars utterly inadequate
to give even the faintest conception of the extent and the interest of
the universe. We merely see self-luminous bodies, but of the
multitudes of objects and the elaborate systems of which these bodies
are only the conspicuous points we see nothing and we know very
little. We are, however, entitled to infer from an examination of our
own star--the sun--and of the beautiful system by which it is
surrounded, that these other suns may be also splendidly attended.
This is quite as reasonable a supposition as that a set of lights seen
at night on the Atlantic Ocean indicates the existence of a fine ship.
The Clusters of Stars.
On a clear night you can often see, stretching across the sky, a track
of faint light, which is known to astronomers as the "Milky Way." It
extends below the horizon, and then round the earth to form a girdle
about the heavens. When we examine the Milky Way with a telescope we
find, to our amazement, that it consists of myriads of stars, so small
and so faint that we are not able to distinguish them individually; we
merely see the glow produced from their collective rays. Remembering
that our sun is a star, and that the Milky Way surrounds us, it would
almost seem as if our sun were but one of the host of stars which form
this cluster.
There are also other clusters of stars, some of which are exquisitely
beautiful telescopic spectacles. I may mention a celebrated pair of
these objects which lies in the constellation of Perseus. The sight of
them in a great telescope is so imposing that no one who is fit to
look through a telescope could resist a shout of wonder and admiration
when first they burst on his view. But there are other clusters. Here
is a picture of one which is known as the "Globular Cluster in the
Centaur" (Fig. 2). It consists of a ball of stars, so far off that,
however large these several suns may actually be, they have dwindled
down to extremely small points of light. A homely illustration may
serve to show the appearance which a globular cluster presents in a
good telescope. I take a pepper-caster, and on a sheet of white paper
I begin to shake out the pepper until there is a little heap at the
centre and other grains are scattered loosely about. Imagine that
every one of those grains of pepper was to be transformed into a tiny
electric light, and then you have some idea of what a cluster of stars
would look like when viewed through a telescope of sufficient power.
There are multitudes of such groups scattered through the depths of
space. They require our biggest telescopes to show them adequately. We
have seen that our sun is a star, being only one of a magnificent
cluster that forms the Milky Way. We have also seen that there are
other groups scattered through the length and depth of space. It is
thus we obtain a notion of the rank which our earth holds in the
scheme of things celestial.
[Illustration: FIG. 2. GLOBULAR CLUSTER IN THE CENTAUR.]
The Rank of the Earth as a Globe in Space.
Let me give an illustration with the view of explaining more fully the
nature of the relation which the earth bears to the other globes which
abound through space, and you must allow me to draw a little upon my
imagination. I shall suppose that the mails of our country extend not
only over this globe, but that they also communicate with other
worlds; that postal arrangements exist between Mars and the earth,
between the sun and Orion--in fact, everywhere throughout the whole
extent of the universe. We shall consider how our letters are to be
addressed. Let us take the case of Mr. John Smith, merchant, who lives
at 1001, Piccadilly; and let us suppose that Mr. John Smith's business
transactions are of such an extensive nature that they reach not only
all over this globe, but away throughout space. I shall suppose that
the firm has a correspondent residing--let us say in the constellation
of the Great Bear; and when this man of business wants to write to Mr.
Smith from these remote regions, what address must he put upon the
letter, so that the Postmaster-General of the universe shall make no
mistake about its delivery? He will write as follows:--
MR. JOHN SMITH,
1001 Piccadilly,
London,
England,
Europe,
Earth,
Near the Sun,
Milky Way,
The Universe.
Let us now see what the several lines of this address mean. Of course
we put down the name of Mr. John Smith in the first line, and then we
will add "1001 Piccadilly" for the second; but as the people in the
Great Bear are not likely to know where Piccadilly is, we shall add
"London" underneath. As even London itself cannot be well known
everywhere, it is better to write "England." This would surely find
Mr. John Smith from any post-office on this globe. From other globes,
however, the supreme importance of England may not be so immediately
recognized, and therefore it is as well to add another line, "Europe."
This ought to be sufficient, I think, for any post-office in the solar
system. Europe is big enough to be visible from Mars or Venus, and
should be known to the post-office people there, just as we know and
have names for the continents on Mars. But further away there might be
a little difficulty; from Uranus and Neptune the different regions on
our earth can never have been distinguished, and therefore we must add
another line to indicate the particular globe of the solar system
which contains Europe. Mark Twain tells us that there was always one
thing in astronomy which specially puzzled him, and that was to know
how we found out the names of the stars. We are, of course, in
hopeless ignorance of the name by which this earth is called among
other intelligent beings elsewhere who can see it. I can only adopt
the title of "Earth," and therefore I add this line. Now our address
is so complete that from anywhere in the solar system--from Mercury,
from Jupiter, or Neptune--there ought to be no mistake about the
letter finding its way to Mr. John Smith. But from his correspondent
in the Great Bear this address would be still incomplete; they cannot
see our earth from there, and even the sun himself only looks like a
small star--like one, in fact, of thousands of stars elsewhere.
However, each star can be distinguished, and our sun may, for
instance, be recognized from the Great Bear by some designation. We
shall add the line "Near the Sun," and then I think that from this
constellation, or from any of the other stars around us, the address
of Mr. John Smith may be regarded as complete. But Mr. Smith's
correspondence may be still wider. He may have an agent living in the
cluster of Perseus or on some other objects still fainter and more
distant; then "Near the Sun" is utterly inadequate as a concluding
line to the address, for the sun, if it can be seen at all from
thence, will be only of the significance of an excessively minute
star, no more to be designated by a special name than are each of the
several leaves on the trees of a forest. What this distant
correspondent will be acquainted with is not the earth or the sun but
only the cluster of stars among which the sun is but a unit. Again we
use our own name to denote the cluster, and we call it the "Milky
Way." When we add this line, we have made the address of Mr. John
Smith as complete as circumstances will permit. I think a letter
posted to him anywhere ought to reach its destination. To perfect it,
however, we will finish up with one line more--"The Universe."
The Distances of the Stars.
I must now tell you something about the distances of the stars. I
shall not make the attempt to explain fully how astronomers make such
measurements, but I will give you some notion of how it is done. You
may remember I showed you how we found the distance of a globe that
was hung from the ceiling. The principle of the method for finding the
distance of a star is somewhat similar, except that we make the two
observations not from the two ends of a table, not even from opposite
sides of the earth, but from two opposite points on the earth's orbit,
which are therefore at a distance of one hundred and eighty-six
million miles. Imagine that on Midsummer Day, when standing on the
earth here, I measure with a piece of card the angle between the star
and the sun. Six months later, on Midwinter Day, when the earth is at
the opposite point of its orbit, I again measure the angle between the
same star and the sun, and we can now determine the star's distance by
making a triangle. I draw a line a foot long, and we will take this
foot to represent one hundred and eighty-six million miles, the
distance between the two stations; then placing the cards at the
corners, I rule the two sides and complete the triangle, and the star
must be at the remaining corner; then I measure the sides of the
triangle, and how many feet they contain, and recollecting that each
foot corresponds to one hundred and eighty-six million miles, we
discover the distance of the star. If the stars were comparatively
near us, the process would be a very simple one; but, unfortunately,
the stars are so extremely far off that this triangle, even with a
base of only one foot, must have its sides many miles long. Indeed,
astronomers will tell you that there is no more delicate or
troublesome work in the whole of their science than that of
discovering the distance of a star.
In all such measurements we take the distance from the earth to the
sun as a conveniently long measuring-rod, whereby to express the
results. The nearest stars are still hundreds of thousands of times as
far off as the sun. Let us ponder for a little on the vastness of
these distances. We shall first express them in miles. Taking the
sun's distance to be ninety-three million miles, then the distance of
the nearest fixed star is about twenty millions of millions of
miles--that is to say, we express this by putting down a 2 first, and
then writing thirteen ciphers after it. It is, no doubt, easy to speak
of such figures, but it is a very different matter when we endeavor to
imagine the awful magnitude which such a number indicates. I must try
to give some illustrations which will enable you to form a notion of
it. At first I was going to ask you to try and count this number, but
when I found it would require at least three hundred thousand years,
counting day and night without stopping, before the task was over, it
became necessary to adopt some other method.
When on a visit in Lancashire I was once kindly permitted to visit a
cotton mill, and I learned that the cotton yarn there produced in a
single day would be long enough to wind round this earth twenty-seven
times at the equator. It appears that the total production of cotton
yarn each day in all the mills together would be on the average about
one hundred and fifty-five million miles. In fact, if they would only
spin about one-fifth more, we could assert that Great Britain produced
enough cotton yarn every day to stretch from the earth to the sun and
back again! It is not hard to find from these figures how long it
would take for all the mills in Lancashire to produce a piece of yarn
long enough to reach from our earth to the nearest of the stars. If
the spinners worked as hard as ever they could for a year, and if all
the pieces were then tied together, they would extend to only a small
fraction of the distance; nor if they worked for ten years, or for
twenty years, would the task be fully accomplished. Indeed, upwards of
four hundred years would be necessary before enough cotton could be
grown in America and spun in this country to stretch over a distance
so enormous. All the spinning that has ever yet been done in the world
has not formed a long enough thread!
There is another way in which we can form some notion of the immensity
of these sidereal distances. You will recollect that, when we were
speaking of Jupiter's moons, I told you of the beautiful discovery
which their eclipses enabled astronomers to make. It was thus found
that light travels at the enormous speed of about one hundred and
eighty-five thousand miles per second. It moves so quickly that within
a single second a ray would flash two hundred times from London to
Edinburgh and back again.
We said that a meteor travels one hundred times as swiftly as a
rifle-bullet; but even this great speed seems almost nothing when
compared with the speed of light, which is ten thousand times as
great. Suppose some brilliant outbreak of light were to take place in
a distant star--an outbreak which would be of such intensity that the
flash from it would extend far and wide throughout the universe. The
light would start forth on its voyage with terrific speed. Any
neighboring star which was at a distance of less than one hundred and
eighty-five thousand miles would, of course, see the flash within a
second after it had been produced. More distant bodies would receive
the intimation after intervals of time proportioned to their
distances. Thus, if a body were one million miles away, the light
would reach it in from five to six seconds, while over a distance as
great as that which separates the earth from the sun the news would be
carried in about eight minutes. We can calculate how long a time must
elapse ere the light shall travel over a distance so great as that
between the star and our earth. You will find that from the nearest of
the stars the time required for the journey will be over three years.
Ponder on all that this involves. That outbreak in the star might be
great enough to be visible here, but we could never become aware of it
till three years after it had happened. When we are looking at such a
star to-night we do not see it as it is at present, for the light that
is at this moment entering our eyes has travelled so far that it has
been three years on the way. Therefore, when we look at the star now
we see it as it was three years previously. In fact, if the star were
to go out altogether, we might still continue to see it twinkling for
a period of three years longer, because a certain amount of light was
on its way to us at the moment of extinction, and so long as that
light keeps arriving here, so long shall we see the star showing as
brightly as ever. When, therefore, you look at the thousands of stars
in the sky to-night, there is not one that you see as it is now, but
as it was years ago.
I have been speaking of the stars that are nearest to us, but there
are others much farther off. It is true we cannot find the distances
of these more remote objects with any degree of accuracy, but we can
convince ourselves how great that distance is by the following
reasoning. Look at one of the brightest stars. Try to conceive that
the object was carried away further into the depths of space, until it
was ten times as far from us as it is at present, it would still
remain bright enough to be recognized in quite a small telescope; even
if it were taken to one hundred times its original distance it would
not have withdrawn from the view of a good telescope; while if it
retreated one thousand times as far as it was at first it would still
be a recognizable point in our mightiest instruments. Among the stars
which we can see with our telescopes, we feel confident there must be
many from which the light has expended hundreds of years, or even
thousands of years, on the journey. When, therefore, we look at such
objects, we see them, not as they are now, but as they were ages ago;
in fact, a star might have ceased to exist for thousands of years, and
still be seen by us every night as a twinkling point in our great
telescopes.