are two which especially baffle our powers of comprehension. They
lie at the opposite extremes of nature. One relates to objects
which are infinitely small, the other relates to objects which are
almost infinitely great. The microscope teaches us that there are
animals so minute that if a thousand of them were ranged abreast
they would easily swim without being thrown out of line through the
eye of the finest cambric needle. Each of those minute creatures is
a highly organized number of particles, capable of moving about,
of finding and devouring its food, and of behaving in all other
respects as becomes an animal as distinguished from an unorganized
piece of matter. The mind is capable of realizing the structure of
these little creatures, and of fully appreciating their marvelous
adaptation to the life they are destined to lead. If these animals
excite our astonishment by reason of their extreme minuteness,
there is an appeal made to conceptions of an entirely different
character when we learn the lessons which the telescope teaches. As
the microscope reveals the excessively minute, so does the telescope
disclose the sublimely great. In each case myriads of objects are
submitted to our astonished view, but while the microscope brings
before us creatures of which countless millions could swim about
freely in a thimbleful of water, the telescope conducts our vision
to uncounted legions of stars, many of them millions of times larger
than the earth.
The grandest truth in the whole of nature is conveyed in that first
lesson in astronomy which answers the question: What are the stars?
This is a question that a child will ask, and I have heard of a
child’s pretty idea that the stars were little holes in the sky to
let the glory of heaven shine through. The philosopher will replace
this explanation by another hardly less poetical, which will enable
us to form some more adequate notion of the real magnificence of the
universe. Each star that we see is, it is true, only a glittering
little point of light, but that is merely because we are a long way
from it. An electric light which will dazzle your eye when quite
close will be reduced to an agreeable illumination if it is at a
little distance, will become a faint light a mile away, and at no
great distance will become altogether invisible. We must remember
that out in space there is plenty of room—there are no bounds; and
therefore when we see light glistening in the far distant depths we
can not at once conclude that the light is a faint one because it
appears to us to be faint. It may be that the light is only faint
because it comes from such a tremendous distance. In fact, the
brightest light conceivable could be reduced to the insignificance of
a small star if only it were removed sufficiently far.
The most intense light we know of comes, of course, from the
light which rules by day, from our sun himself. The sun pours his
unrivaled beams around us in all directions with prodigal abundance,
notwithstanding his enormous distance of ninety-three millions of
miles. Let me describe an experiment with respect to our sun, an
experiment, it is needless to say, which could never be performed,
but the results to which it leads us are none the less certain.
Astronomers have demonstrated them in many other ways.
Suppose that the sun were gradually to be moved away further and
further into space; suppose that by this time to-morrow the great
luminary should be twice as far as it is now, and the next day should
be three times as far, and the day after that four times, and so on
until in a year’s time we should find that the sun was 365 times
the distance from us that it is at present. Let us now trace the
changes which we should see in the brilliancy of our orb of day.
When he had reached double his distance from us, we should find that
the light had decreased to a quarter of its present amount, and the
heat which we derived from his beams would have decreased in the same
proportion. In ten days we should find that the light had become so
feeble as to be only one-hundredth part of that which we enjoy now.
The apparent size of the sun would also be steadily decreasing, for
as the distance of a body increases its apparent dimensions diminish.
Sometimes the diminution of apparent size with distance is well
illustrated on a clock tower. You would hardly believe that the hands
and face of a clock like that at Westminster were so large until you
happen to see a man cleaning or repairing it, when he appears a mere
pigmy in comparison with the mighty dial which points out the hours.
In a similar way with every increase of distance, the apparent size
of the sun would decline, and in the lapse of a year the sunlight
would be reduced to a feeble twilight. The sun itself would remain
visible for many years, even if it were steadily moving away, though
its lustre would continually decline, and its size would continually
diminish, until at last it would have shrunk to the insignificance of
a small point of light, still visible as a glittering object, but too
minute to enable any definite form to be perceived.
Further still, the sun might recede until it passed beyond the reach
of vision of the unaided eye; the telescope would, however, be able
to pursue the retreating luminary until at last it sank into the
depths of space beyond the reach of any instrument whatever.
This little argument will prepare us for an explanation of the stars.
They merely appear to us to be points of light of varying degrees
of brightness, but we have seen that our own sun might be reduced
in lustre to that of the very dimmest of the stars if only it were
removed sufficiently far. If, therefore, the stars are at a great
enough distance from our system, it may indeed be that they also are
suns, possibly equaling, or possibly even surpassing, our own sun in
magnificence.
Here is indeed an imposing suggestion. Can it be that the host of
stars which adorn our midnight sky are actually suns themselves of an
importance comparable with that of our own? This is a great thought,
and we desire to test it by every means in our power. You will see
from the reasoning I have given that the whole question turns simply
on one point, and that is: How far off are the stars?
The tiniest point of light that is just seen as a glimmer in the
mightiest of telescopes may be indeed a sun as great, or indeed
a million times greater, than our sun, if only that star be
sufficiently far off. To find the distance of a star is a problem
which taxes the utmost powers of the painstaking astronomer; every
refinement of skill in making his measurements and of care in the
calculation of his observations have to be lavished on the operation.
Alas! it but too often happens that the astronomer’s labors prove
to be futile. The surveying navigator often has to mark on his
chart that no bottom could be found in the depths of the sea. His
appliances would not work, or work reliably, in those ocean abysses;
so, too, the astronomer, when he tries to sound the depths of space
to the distances of the stars, has also to mark, generally speaking,
“No bottom here,” as the result of most of his investigations. When
this is the case we know for certain that the star on which his
calculations have been made must be a gorgeous sun, because we are
assured of the greatness of its distance, even though we have not
been able to find out what that distance was. There are, however,
some few places through the sky where the astronomer’s sounding line
can, so to speak, touch bottom; there are a few stars of which we do
know the distance, and the result is not a little significant. Were
our sun to be withdrawn from us to a distance so great as that of
the very nearest of the stars, our magnificent ruler and benefactor
would certainly have lost all his splendor; he would, in fact, have
shrunk to the similitude of a little star not nearly so bright as
many of those which we see over our heads every night. Imagine the
sun’s light subdivided into two hundred thousand parts, each of which
would give us only a feeble illumination, and then imagine that each
of these parts was again divided into two hundred thousand parts
more, and it is one of these last fragments that would represent the
miserable lustre which the sun would then display.
From these considerations we can enunciate the magnificent truth
which astronomy discloses to us. I do not think that in the whole
range of nature there is any thought so magnificent or so imposing as
that which teaches us to regard every star of every constellation
as a sun. We can not indeed assert that they are all so great as
our sun, but we can affirm with certainty that many of them are far
greater and far more splendid. Considering that our sun presides
over a system of worlds of which the earth is one, that it gives
light and heat to those worlds, and guides them in their movements,
it would greatly enlarge our conceptions of the universe if we were
assured that there was even one more sun as large and as splendidly
attended as is our own. But now we find that not only is there one
additional sun, but that they teem in uncounted thousands through
space. Look, for example, on the next fine night at the Great Bear,
the best known of all our northern constellations, and there you see
seven stars forming the well-known feature. Figure in your mind’s eye
each one of those stars in the likeness of a majestic sun, as big,
warm, and bright as our sun, and look at other parts of the sky and
repeat the process with the other constellations, and your conception
of the magnificence of the starry system will begin to assume proper
proportions. But this is only the first step, you must next look at
the smaller stars, and reflect that they, too, are also suns, only
much further off as a general rule than the brighter stars, though
this is by no means invariably the case. Thus your estimate of the
number of suns in the universe will rise to thousands, but you will
not stop there, you will get a telescope to help you, and, to your
extreme delight and wonder, you will find that there are hosts of
stars—too faint to be visible to the eye, but which the telescope
will immediately disclose. You will get a more powerful instrument,
and then you will perceive that the stars are to be numbered by tens
of thousands, and even by millions, and with every fresh accession
of power in your telescope fresh troops and myriads of suns are
revealed. Suns in clusters, suns strewn thickly here and sparsely
there, so as to give us the notion that the only limit to the number
we can see is the power of the telescopes we are using. Attempts at
actual numeration are futile, for who can tell the number of the
stars?
We can, however, form an estimate, and by taking samples, so to
speak, of the sky here and other samples there, we have been enabled
to learn the overwhelming fact that our universe does contain at the
very least one hundred millions of suns.
In discussing the extent of the visible universe, it must always
be borne in mind that the further a source of light is from us the
fainter is the illumination which we receive from it. Suppose that
a star which just lies on the limits of naked-eye visibility were
somehow to be transported to a distance which is twice as great,
then the lustre of that star would be diminished to one-fourth of
its original amount. It would, therefore, be of course invisible to
the unaided eye, but could still be easily perceived by a telescope.
Indeed, the very word _telescope_ means an instrument for looking at
objects a long way off, and the effect of the telescope is to reduce
the apparent distance of the object.
The bulk of a grain of sand as compared with the bulk of a football
may illustrate the space accessible to our eyes when compared with
the space accessible to one of the great telescopes. The larger
of these spaces has a thousand times the diameter of the others;
therefore, the relative quantities of these spaces are to be
obtained by multiplying 1,000 by 1,000 and by 1,000 again. Thus we
finally learn that the amplitude of our vision is augmented to one
thousand million times its original extent by the use of our greatest
telescopes. It need, therefore, be no matter for surprise that the
number of stars visible through our great telescopes or recorded on
the sensitive films of photographic plates should number scores of
millions. In fact, it would sometimes seem surprising that the number
of telescopic stars is not even greater than it actually appears to
be. If we are able to explore one thousand million times as much
space, we might expect that the number of objects disclosed would be
also increased about a thousand million-fold, but this is certainly
not the case. The truth seems to be that our sun is but one star of
a mighty cluster of stars; we happen to lie near the middle of the
cluster, and the rest of the stars belonging to it form what we know
as the Milky Way. There are, of course, other clusters scattered
through the heavens, some of them, perhaps, as great as that body
of stars which forms the Milky Way. Owing to our residence in this
cluster we see the neighboring suns in multitudes, and thus we
receive the impression that the solar system lies in an exceptionally
rich part of the universe in as far as the distribution of stars is
concerned.
On the outskirts of the universe lie those faintest and dimmest of
objects which we can just perceive through our greatest telescopes.
We know that many of the stars around us would still remain visible
in great instruments, even though they were removed a thousand times
as far off. Among the myriads of faint stars which we see from
our observatories, there may be many, indeed there must be many,
which are fully a thousand times as distant as the bright stars
which twinkle in our comparative neighborhood. We thus obtain some
conception of the stupendous distance at which the outskirts of the
universe are situated.
There are different ways of illustrating this point, but I think the
simplest, as well as the most striking, is that which is founded on
the velocity of light. It is a remarkable fact that the beautiful
star known as Vega[3] has a distance from us so tremendous that its
light must have taken somewhere about eighteen years to travel hither
from thence. Notwithstanding that the light dashes along with such
inconceivable speed that it will cover 185,000 miles in every second,
notwithstanding that a journey at this pace will complete the entire
circuit of this globe seven or eight times between two successive
ticks of the clock, the light will, nevertheless, take eighteen years
to reach our eye from the time it leaves Vega. We do not, therefore,
see the star as it is at present; we see it as it was eighteen years
ago. For the light which this evening enters our eyes has been all
that time on its journey. Indeed, if Vega were actually to be
blotted out from existence it would still continue to shine out as
vividly as ever for eighteen years before all the light on its way
had reached us.
We have been led to the belief that among the more distant stars in
the universe there must be many which are fully a thousand times as
far from us as is Vega, hence we arrive at the startling conception
that the light they emit has been on its journey for 18,000 years
before it reached us. When we look at those lights to-night we are
actually viewing them as they were 18,000 years ago. In fact, those
stars might have totally vanished 17,000 years ago, though we and our
descendants may still see them glittering for yet another thousand
years.
We shall realize a little more fully what this reasoning involves if
we suppose that astronomers dwelt on such a star, and that they had
eyes and telescopes sufficiently keen not only to discern our little
earth, but even to scrutinize its surface with attention. Let us
suppose that the stellar astronomers looked at England: do you think
they would see a network of railways joining mighty and populous
cities, furnished with immense manufactories and with countless
institutions? Such would be the England of to-day. But from the
distance at which these astronomers are situated light takes 18,000
years for its journey, and, therefore, what they would see would be
England as it was 18,000 years ago. To them England would even now
appear as a country mainly covered with forests inhabited by bears
and wolves, and totally void of any trace of civilization. This
illustration will, at all events, serve to convey some conception
of the distance at which the outskirts of our visible universe are
plunged in the depths of space.
FOOTNOTES:
[3] Vega is the brightest star in the Lyre and is nearly always at
night directly overhead in our latitude.—E. S.
THE STARS.—AMÉDÉE GUILLEMIN
No sight is at once so awe-inspiring and so grand as that of the
heavens on a beautiful night. If care be taken to choose as a
standpoint for observation an open place, such as a plain or the
summit of a hill on land, or, again, the open sea, and if the
atmosphere, somewhat charged with dew, possesses all its transparency
and purity, we shall see thousands of luminous points twinkling
in all directions, accomplishing slowly and together their silent
march. The contrast of the obscurity which reigns on the surface of
the earth with the brightness of that resplendent vault gives an
indefinite depth to the celestial ocean that deepens over our heads.
But let us here leave the magnificence of the spectacle to study it
in its most minute details.
Let us commence with the appearances. A characteristic common to
all the stars is an incessant and very rapid change of brightness,
which has received the name of _scintillation_. This is accompanied
by variations of color equally rapid, due to the same cause as the
successive disappearances and reappearances. All stars scintillate,
whatever may be their brilliancy, at least in our temperate regions.
But the intensity of this luminous movement is not the same in all,
and it varies, moreover, both with the degree of purity of the sky,
the elevation of the stars above the horizon, and the temperature of
the night.
According to Arago, scintillation is due to the difference of
velocity of the various colored rays traversing the unequally warm,
unequally dense, unequally humid atmospheric strata. Thus, in
tropical regions, where the atmospheric strata are more homogeneous,
scintillation is rarely observed in stars the elevation of which
above the horizon is more than 15°, or the sixth of the distance of
the horizon from the zenith. “This circumstance,” says Humboldt,
“gives to the celestial vault of these countries a particularly calm
and soft character.”
Another specific character of the stars is that their diameters are
without appreciable dimensions. To the naked eye, this distinction
would be insufficient, since, the moon and the sun excepted, the
most considerable planets have not sensible diameters. But, while
the magnifying power of optical instruments shows us the principal
planets under the form of clearly defined disks, the most powerful
glasses only show a star as a luminous point. The distance which
separates us from these bodies is so great that there is nothing to
astonish us in such a result.
Wollaston affirms that the apparent diameter of the most brilliant
star in the heavens, Sirius, is not more than the fiftieth part of
a second of an arc. But let us hasten to say that this result still
leaves a good margin as to the real dimensions of the star, since, at
the distance of Sirius, an apparent diameter would represent a real
diameter of 11,000,000 miles; that is, twelve times the diameter of
our sun.
Let us add, lastly, that the absence of appreciable dimensions does
not suffice to distinguish absolutely the stars from the planets,
since a certain number of the latter, as we have before seen, appear
in telescopes only as simple luminous points. Let us come, then, to a
permanent specific characteristic, the knowledge of which will always
prevent us from confounding a star with one of the known or unknown
bodies which form part of our solar group. This characteristic is as
follows:
The stars, properly so called, preserve among themselves—nearly
enough for our present purpose—the same relative distances. They
form, then, on the celestial vault apparent groups, the configuration
of which is nearly invariable. Centuries must elapse to show a change
of form, unless we employ extremely delicate measures. A planet, on
the contrary, moves rapidly across these groups, to such a degree
that, in the interval of a night, or at most of a few nights, this
displacement is very perceptible; hence the old denomination of
_fixed stars_, in opposition to the _wandering_ ones, or planets.
We must be careful, however, to guard against assigning to this word
a rigidity which it does not possess, for the stars really move with
a velocity not inferior to that which animates the members of our
system. Their immense distance is the only cause of their apparent
immobility, which vanishes when precise observations, embracing a
sufficient interval of time—some years, for example—are made.
A fact which strikes every one is the great diversity of brightness
in the stars which people the heavens. All degrees of intensity
are remarked, from the resplendent light of Sirius to the scarcely
perceptible glimmer of those hardly visible to the naked eye.
Whence arises this difference of brightness? This question we can not
answer for any star in particular, but it is easy to imagine that it
may result from various circumstances, such as their less or greater
distance, the real and various dimensions of the bodies, and, lastly,
the intrinsic brightness of the light peculiar to each. However this
may be, astronomers without regard to the unknown causes which may
influence the intensity of the stellar light, have divided stars
into _classes_ or _magnitudes_; and when we speak of a star of the
first, second, or fifth magnitude, it is understood that this way of
speaking refers only to the apparent brightness, and that nothing
is affirmed either as to the real dimensions or distance, or even
intrinsic brightness.
Besides, as the stars, arranged in the order of their brightness,
would form a progression decreasing by imperceptible degrees, the
classes adopted are themselves conventional and arbitrary. The first
six magnitudes comprise all stars visible to the naked eye. But the
use of the most powerful telescopes brings to view stars of feebler
light, descending to the sixteenth and seventeenth magnitudes. In
truth, the progression has no inferior limit: it extends more and
more in proportion as the progress of the optician’s art increases
the penetrating power of our instruments.
To gain an idea of the respective intensities of the light emitted by
the stars of the first six magnitudes, following the scale adopted
by astronomers, the accompanying illustration (Fig. 1), should be
inspected; in it the stars are figured by disks, the surfaces of
which are in proportion to their brilliancy.
But, we repeat, it must not be thought that the stars ranked in the
same class are, on that account, of the same brightness. Thus the
light of Sirius is estimated at four times the star Alpha Centauri;
but both, nevertheless, are included by astronomers in the number of
the stars of the first magnitude.
[Illustration: Fig. 1.—Relative Brilliancy of Stars of the first Six
Magnitudes]
We here give the names of the twenty most brilliant stars of the two
hemispheres which it is usual to consider as forming the first class.
They are here arranged in the order of their brightness:
1. Sirius
2. Eta Argus
3. Canopus
4. Alpha Centauri
5. Arcturus
6. Rigel
7. Capella
8. Vega
9. Procyon
10. Betelgeuse
11. Achernar
12. Aldebaran
13. Beta Centauri
14. Alpha Crucis
15. Antares
16. Altair
17. Spica
18. Fomalhaut
19. Beta Crucis
20. Pollux
Lastly, Regulus, a bright star in the constellation of the Lion, is
also ranked by some astronomers in the first magnitude, while others
only admit in this class the first seventeen stars in the above list.
These divergences are of no importance.
In proportion as the scale of brilliancy or magnitude is descended,
the number of the stars contained in each class rapidly increases.
The number of second magnitude stars in the heavens is about 65;
of the third, about 200; of the fifth, 1,100; and of the sixth
magnitude, 3,200. Adding these numbers together, we obtain a few over
5,000 stars of the first six magnitudes, and these comprise very
nearly all those that can be seen with the naked eye.
The smallness of this number nearly always astonishes those who have
not tried to form an exact estimate of the number of stars which
shine in the celestial vault on the most favorable nights.