AN EVENING AMONG THE STARS
[Illustration]
"Do you love the stars?" asked the magician of his lads, as they crowded
round him on the college green, one evening in March, to look through
his portable telescope.
"Have you ever sat at the window on a clear frosty night, or in the
garden in summer, and looked up at those wondrous lights in the sky,
pondering what they are, and what purpose they serve?"
I will confess to you that when I lived in London I did not think much
about the stars, for in the streets very few can be seen at a time even
on a clear night; and during the long evenings in summer, when town
people visit the country, you must stay up late to see a brilliant
display of starlight. It is when driving or walking across country on a
winter's evening week after week, and looking all round the sky, that
the glorious suns of heaven force you to take notice of them; and Orion
becomes a companion with his seven brilliant stars and his magnificent
nebula, which appears as a small pale blue patch, to eyes accustomed to
look for it, when the night is very bright and clear. It is then that
Charles's Wain becomes quite a study in all its different positions, its
horses now careering upwards, now plunging downwards, while the waggon,
whether upwards or downwards, points ever true, by the two stars of its
tail-board, to the steadfast pole-star.
It is on such nights as these that, looking southward from Orion, we
recognise the dog-star Sirius, bright long before other stars have
conquered the twilight, and feast our eye upon his glorious white beams;
and then, turning northwards, are startled by the soft lustrous sheen of
Vega just appearing above the horizon.
But stop, I must remember that I have not yet introduced you to these
groups of stars; and moreover that, though we shall find them now in the
positions I mention, yet if you look for them a few hours later
to-night, or at the same hour later in the year, you will not find them
in the same places in the sky. For as our earth turns daily on its axis,
the stars _appear_ to alter their position hour by hour, and in the same
way as we travel yearly on our journey round the sun, they _appear_ to
move in the sky month by month. Yet with a little practice it is easy to
recognise the principal stars, for, as it is our movement and not theirs
which makes us see them in different parts of the sky, they always
remain in the same position with regard to each other. In a very short
time, with the help of such a book as Proctor's _Star Atlas_; you could
pick out all the chief constellations and most conspicuous stars for
yourselves.
One of the best ways is to take note of the stars each night as they
creep out one by one after sunset. If you take your place at the window
to-morrow night as the twilight fades away, you will see them gradually
appear, now in one part, now in another of the sky, as
"One by one each little star
Sits on its golden throne."
The first to appear will be Sirius or the dog-star (see Fig. 54), that
pure white star which you can observe now rather low down to the south,
and which belongs to the constellation _Canis Major_. As Sirius is one
of the most brilliant stars in the sky, he can be seen very soon after
the sun is gone at this time of year. If, however, you had any doubt as
to what star he was, you would not doubt long, for in a little while two
beautiful stars start into view above him more to the west, and between
them three smaller ones in a close row, forming the cross in the
constellation of Orion, which is always very easy to recognise. Now the
three stars of Orion's belt which make the short piece of the cross
always point to Sirius, while Betelgeux in his right shoulder, and Rigel
in his left foot (see Figs. 54 and 55), complete the long piece, and
these all show very early in the twilight. You would have to wait longer
for the other two leading stars, Bellatrix in the right shoulder and
[Greek: k] Orionis in the right leg, for these stars are feebler and
only seen when the light has faded quite away.
[Illustration: Fig. 54.
Some of the constellations seen when looking south in March from six to
nine o'clock.]
By that time you would see that there are an immense number of stars in
Orion visible even to the naked eye, besides the veil of misty, tiny
stars called the "Milky Way" which passes over his arm and club. Yet the
figure of the huntsman is very difficult to trace, and the seven bright
stars, the five of the cross and those in the left arm and knee, are all
you need remember.
No! not altogether all, for on a bright clear night like this you can
detect a faint greenish blue patch (N, Fig. 54) just below the belt, and
having a bright star in the centre. This is called the "Great Nebula" or
mist of Orion (see Frontispiece). With your telescopes it looks very
small indeed, for only the central and brightest part is seen. Really,
however, it is so widespread that our whole solar system is as nothing
compared to it. But even your telescopes will show, somewhere near the
centre, what appears to be a bright and very beautiful star (see Fig.
55) surrounded by a darker space than the rest of the nebula, while in
my telescope you will see many stars scattered over the mist.
[Illustration: Fig. 55.
Chief stars of Orion, with Aldebaran. (After Proctor.)]
Now first let me tell you that these last stars do not, so far as we
know, lie _in_ the nebula, but are scattered about in the heavens
between us and it, perhaps millions of miles nearer our earth. But with
the bright star in the centre it is different, for the spectroscope
tells us that the mist passes _over_ it, so that it is either behind or
in the nebula. Moreover, this star is very interesting, for it is not
really one star, but six arranged in a group (see Fig. 56). You can see
four distinctly through my telescope, forming a trapezium or four-sided
figure, and more powerful instruments show two smaller ones. So [Greek:
th] Orionis, or the Trapezium of Orion, is a multiple star, probably
lying in the midst of the nebula.
[Illustration: Fig. 56.
The trapezium, [Greek: th] Orionis, in the nebula of Orion. (Herschel.)]
The next question is, What is the mist itself composed of? For a long
time telescopes could give us no answer. At last one night Lord Rosse,
looking through his giant telescope at the densest part of the nebula,
saw myriads of minute stars which had never been seen before. "Then,"
you will say, "it is after all only a cluster of stars too small for our
telescopes to distinguish." Wait a bit; it is always dangerous to draw
hasty conclusions from single observations. What Lord Rosse said was
true as to that particular part of the nebula, but not the whole truth
even there, and not at all true of other parts, as the spectroscope
tells us.
For though the light of nebulae, or luminous mists, is so faint that a
spectrum can only be got by most delicate operations, yet Dr. Huggins
has succeeded in examining several. Among these is the nebula of Orion,
and we now know that when the light of the mist is spread out it gives,
not a continuous band of colour such as would be given by stars, but
_faint coloured lines_ on a dark ground (see Fig. 57). Such lines as
these we have already learnt are always given by gases, and the
particular bright lines thrown by Orion's nebula answer to those given
by nitrogen and hydrogen, and some other unknown gases. So we learn at
last that the true mist of the nebula is formed of glowing gas, while
parts have probably a great number of minute stars in them.
[Illustration: Fig. 57.
Nebula-spectrum.
Sun-spectrum.
Spectrum of Orion's Nebula, showing bright lines, with sun-spectrum
below for comparison.]
Till within a very short time ago only those people who had access to
very powerful telescopes could see the real appearance of Orion, for
drawings made of it were necessarily very imperfect; but now that
telescopes have been made expressly for carrying photographic
appliances, even these faint mists print their own image for us. In 1880
Professor Draper of America photographed the nebula of Orion, in March
1881 Mr. Common got a still better effect, and last year Mr. Isaac
Roberts succeeded in taking the most perfect and beautiful photograph[1]
yet obtained, in which the true beauty of this wonderful mist stands out
clearly. I have marked on the edge of our copy two points [Greek: th]
and [Greek: th]', and if you follow out straight lines from these points
till they meet, you will arrive at the spot where the multiple star
lies. It cannot, however, be seen here, because the plate was exposed
for three hours and a half, and after a time the mist prints itself so
densely as to smother the light of the stars. Look well at this
photograph when you go indoors and fix it on your memory, and then on
clear nights accustom your eye to find the nebula below the three stars
of the belt, for it tells a wonderful story.
[1] Reproduced in the Frontispiece with Mr. Roberts's kind permission.
The star-halo at the top of the plate is caused by diffraction of
light in the telescope, and comes only from an ordinary star.
More than a hundred years ago the great German philosopher Kant
suggested that our sun, our earth, and all the heavenly bodies might
have begun as gases, and the astronomer Laplace taught this as the most
likely history of their formation. After a few years, however, when
powerful telescopes showed that many of the nebulae were only clusters of
very minute stars, astronomers thought that Laplace's teaching had been
wrong. But now the spectroscope has revealed to us glowing gas actually
filling large spaces in the sky, and every year accurate observations
and experiments tell us more and more about these marvellous distant
mists. Some day, though perhaps not while you or I are here to know it,
Orion's nebula, with its glowing gas and minute star-dust, may give some
clue to the early history of the heavenly bodies; and for this reason I
wish you to recognise and ponder over it, as I have often done, when it
shines down on the rugged moor in the stillness of a clear frosty
winter's night.
But we must pass on for, while I have been talking, the whole sky has
become bespangled with hundreds of stars. That glorious one to the west,
which you can find by following (Fig. 54) a curved line upwards from
Betelgeux, is the beautiful red star Aldebaran or the hindmost; so
called by the Arabs, because he drives before him that well-known
cluster, the Pleiades, which we reach by continuing the curve westwards
and upwards. Stop to look at this cluster through your telescopes, for
it will delight you; even with the naked eye you can count from six to
ten stars in it, and an opera-glass will show about thirty, though they
are so scattered you will have to move the glass about to find them. Yet
though my telescope shows a great many more, you cannot even count all
the chief ones through it, for in powerful telescopes more than 600
stars have been seen in the single cluster! while a photograph taken by
Mr. Roberts shows also four lovely patches of nebula.
And now from the Pleiades let us pass on directly overhead to the
beautiful star Capella, which once was red but now is blue, and drop
down gently to the south-east, where Castor and Pollux, the two most
prominent stars in the constellation "Gemini" or the twins, show
brilliantly against the black sky. Pause here a moment, for I want to
tell you something about Castor, the one nearest to Capella. If you look
at Castor through your telescopes, some of you may possibly guess that
it is really two stars, but you will have to look through mine to see it
clearly. These two stars have been watched carefully for many years, and
there is now no doubt that one of them is moving slowly round the other.
Such stars as these are called "binary," to distinguish them from stars
that merely _appear_ double because they stand nearly in a line one
behind the other in the heavens, although they may be millions of miles
apart. But "binary" stars are actually moving in one system, and revolve
round each other as our earth moves round the sun.
I wonder if it strikes you what a grand discovery this is? You will
remember that it is gravitation which keeps the moon held to the earth
so that it moves round in a circle, and which keeps the earth and other
planets moving round the sun. But till these binary stars were
discovered we had no means of guessing that this law had any force
beyond our own solar system. Now, however, we learn that the same law
and order which reigns in our small group of planets is in action
billions of miles away among distant suns, so that they are held
together and move round each other as our earth moves round our sun. I
will repeat to you what Sir R. Ball, the Astronomer-Royal of Ireland,
says about this, for his words have remained in my mind ever since I
read them, and I should like them to linger in yours till you are old
enough to feel their force and grandeur. "This discovery," he writes,
"gave us knowledge we could have gained from no other source. From the
binary stars came a whisper across the vast abyss of space. That whisper
told us that the law of gravitation is not peculiar to the solar system.
It gives us grounds for believing that it is obeyed throughout the
length, the breadth, the depth, and the height of the entire
universe."[1]
[1] _The Story of the Heavens._
And now, leaving Castor and going round to the east, we pass through the
constellation Leo or the Lion, and I want you particularly to notice six
stars in the shape of a sickle, which form the front part of the lion,
the brightest, called Regulus, being the end of the handle.[1] This
sickle is very interesting, because it marks the part of the heavens
from which the brilliant shower of November meteors radiates once in
thirty-three years. This is, however, too long a story to be told
to-night, so we will pass through Leo, and turning northwards, look high
up in the north-east (Fig. 58), where "Charles's Wain" stretches far
across the sky. I need not point this out to you, for every country lad
knows and delights in it. You could not have seen it in the twilight
when Sirius first shone out, for these stars are not so powerful as he
is. But they come out very soon after him, and when once fairly bright,
the four stars which form the waggon, wider at the top than at the
bottom, can never be mistaken, and the three stars in front, the last
bending below the others, are just in the right position for the horses.
For this reason I prefer the country people's name of Charles's Wain or
Waggon to that of the "Plough," which astronomers generally give to
these seven stars. They really form part of an enormous constellation
called the "Great Bear" (Fig. 59), but, as in the case of Orion, it is
very difficult to make out the whole of Bruin in the sky.
[1] In Fig. 54 the sickle alone comes within the picture.
[Illustration: Fig. 58.
Some of the constellations seen when looking north in March from six to
nine o'clock.]
[Illustration: Fig. 59.
The Great Bear, showing the position of Charles's Wain, and also the
small binary star [Greek: x] in the hind foot, whose period has been
determined.]
Now, although most people know Charles's Wain when they see it, we may
still learn a good deal about it. Look carefully at the second star from
the waggon and you will see another star close to it, called by country
people "Jack by the second horse," and by astronomers "Alcor." Even in
your small telescopes you can see that Jack or Alcor is not so close as
he appears to the naked eye, but a long way off from the horse, while in
my telescope you will find this second horse (called Mizar) split up
into two stars, one a brilliant white and the other a pale emerald
green. We do not know whether these two form a binary, for they have not
yet been observed to move round each other.
Take care in looking that you do not confuse the stars one with another,
for you must remember that your telescope makes objects appear upside
down, and Alcor will therefore appear in it _below_ the two stars
forming the horse.
But though we do not know whether Mizar is binary, there is a little
star a long way below the waggon, in the left hind paw of the Great Bear
([Greek: x], Figs. 58 and 59), which has taught us a great deal, for it
is composed of two stars, one white and the other grey, which move right
round each other once in sixty years, so that astronomers have observed
more than one revolution since powerful telescopes were invented. You
will have to look in my telescope to see the two stars divided, but you
can make an interesting observation for yourselves by comparing the
light of this binary star with the light of Castor, for Castor is such
an immense distance from us that his light takes more than a hundred
years to reach us, while the light of this smaller star comes in
sixty-one years, yet see how incomparably brighter Castor is of the two.
This proves that brilliant stars are not always the nearest, but that a
near star may be small and faint and a far-off one large and bright.
[Illustration: Fig. 60.
The seven stars of Charles's Wain, showing the directions in which they
are travelling. (After Proctor.)]
There is another very interesting fact known to us about Charles's Wain
which I should like you to remember when you look at it. This is that
the seven stars are travelling onwards in the sky, and not all in the
same direction. It was already suspected centuries ago that, besides the
_apparent_ motion of all the stars in the heavens caused by our own
movements, they have each also a _real_ motion and are travelling in
space, though they are so inconceivably far off that we do not notice
it. It has now been proved, by very accurate observations with powerful
instruments, that three of the stars forming the waggon and the two
horses nearest to it, together with Jack, are drifting forwards (see
Fig. 60), while the top star of the tailboard of the waggon and the
leader of the horses are drifting the other way. Thus, thousands of
years hence Charles's Wain will most likely have quite altered its
shape, though so very slowly that each generation will think it is
unchanged.
One more experiment with Charles's Wain, before we leave it, will help
you to imagine the endless millions of stars which fill the universe.
Look up at the waggon and try to count how many stars you can see inside
it with the naked eye. You may, if your eye is keen, be able to count
twelve. Now take an opera-glass and the twelve become two hundred. With
your telescopes they will increase again in number. In my telescope
upstairs the two hundred become hundreds, while in one of the giant
telescopes, such as Lord Rosse's in Ireland, or the great telescope at
Washington in the United States, thousands of stars are brought into
view within that four-sided space!
Now this part of the sky is not fuller of stars than many others; yet at
first, looking up as any one might on a clear evening, we thought only
twelve were there. Cast your eyes all round the heavens. On a clear
night like this you may perhaps, with the naked eye, have in view about
3000 stars; then consider that a powerful telescope can multiply these
by thousands upon thousands, so that we can reckon about 20,000,000
where you see only 3000. If you add to these the stars that rise later
at night, and those of the southern hemisphere which never rise in our
latitude, you would have in all about 50,000,000 stars, which we are
able to see from our tiny world through our most powerful telescopes.
But we can go farther yet. When our telescopes fail, we turn to our
other magic seer, the photographic camera, and trapping rays of light
from stars invisible in the most powerful telescope, make them print
their image on the photographic plate, and at once our numbers are so
enormously increased that if we could photograph the whole of the
heavens as visible from our earth, we should have impressions of at
least 170,000,000 stars!
These numbers are so difficult to grasp that we had better pass on to
something easier, and our next step brings us to the one star in the
heavens which never appears to move, as our world turns. To find it we
have only to draw a line upwards through the two stars in the tailboard
of the waggon and on into space. Indeed these two stars are called "the
Pointers," because a line prolonged onwards from them will, with a very
slight curve, bring us to the "Pole-star" (see Fig. 58). This star,
though not one of the largest, is important, because it is very near
that spot in the sky towards which the North Pole of our earth points.
The consequence is, that though all the other stars appear to move in a
circle round the heavens, and to be in different places at different
seasons, this star remains always in the same place, only appearing to
describe a very tiny circle in the sky round the exact spot to which our
North Pole points.
Month after month and year after year it shines exactly over that
thatched cottage yonder, which you see now immediately below it; and
wherever you are in the northern hemisphere, if you once note a certain
tree, or chimney, or steeple which points upwards to the Pole-star, it
will guide you to it at any hour on any night of the year, though the
other constellations will be now on one side, now on the other side of
it.
The Pole-star is really the front horse of a small imitation of
Charles's Wain, which, however, has never been called by any special
name, but only part of the "Little Bear." Those two hind stars of the
tiny waggon, which are so much the brightest, are called the "Guards,"
because they appear to move in a circle round the Pole-star night after
night and year after year like sentries.
[Illustration: Fig. 61.
The constellation of Cassiopeia, and the heavenly bodies which can be
found by means of it.[1]]
[1] For Almach see Fig. 58, it has been accidentally omitted from this
figure.
Opposite to them, on the farther side of the Pole-star, is a well-marked
constellation, a widespread W written in the sky by five large stars;
the second V of the W has rather a longer point than the first, and as
we see it now the letter is almost upside down (see Fig. 58).
These are the five brightest stars in the constellation Cassiopeia, with
a sixth not quite so bright in the third stroke of the W. You can never
miss them when you have once seen them, even though they lie in the
midst of a dense layer of the stars of the Milky Way, and if you have
any difficulty at first, you have only to look as far on the one side of
the Pole-star as the top hind star of Charles's Wain is on the other,
and you must find them. I want to use them to-night chiefly as guides to
find two remarkable objects which I hope you will look at again and
again. The first is a small round misty patch not easy to see, but which
you will find by following out the _second_ stroke of the first V of the
W. Beginning at the top, and following the line to the point of the V,
continue on across the sky, and then search with your telescope till you
catch a glimpse of this faint mist (_c_, Fig. 58; star-cluster, Fig.
61). You will see at once that it is sparkling all over with stars, for
in fact you have actually before you in that tiny cluster more stars
than you can see with the naked eye all over the heavens! Think for a
moment what this means. One faint misty spot in the constellation
Perseus, which we should have passed over unheeded without a telescope,
proves to be a group of more than 3000 suns!
The second object you will find more easily, for it is larger and
brighter, and appears as a faint dull spot to the naked eye. Going back
to Cassiopeia, follow out the _second_ V in the W from the top to the
point of the V and onwards till your eye rests upon this misty cloud,
which is called the Great Nebula of Andromeda, and has sometimes been
mistaken for a comet (Figs. 58 and 61). You will, however, be
disappointed when you look through the telescope, for it will still only
appear a mist, and you will be able to make nothing of it, except that
instead of being of an irregular shape like Orion, it is elliptical; and
in a powerful telescope two dark rifts can be seen separating the
streams of nebulous matter. These rifts are now shown in a photograph
taken by Mr. Roberts, 1st October 1888, to be two vast dusky rings lying
between the spiral stream of light, which winds in an ellipse till it
ends in a small nucleus at the centre.
Ah! you will say, this must be a cloud of gas like Orion's nebula, only
winding round and round. No! the spectroscope steps in here and tells us
that the light shows something very much like a continuous spectrum, but
not as long as it ought to be at the red end. Now, since gases give only
bright lines, this nebula cannot be entirely gaseous. Then it must be
made of stars too far off to see? If so, it is very strange that though
it is so dense and bright in some parts, and so spread out and clear in
others, the most powerful telescopes cannot break it up into stars. In
fact, the composition of the great nebula of Andromeda is still a
mystery, and remains for one of you boys to study when he has become a
great astronomer.
Still one more strange star we will notice before we leave this part of
the heavens. You will find it, or at least go very near it, by
continuing northwards the line you drew from Cassiopeia to the Star
Cluster (_c_, Fig. 58), and as it is a bright star, you will not miss
it. That is to say, it is bright to-night and will remain so till
to-morrow night, but if you come to me about nine o'clock to-morrow
evening I will show you that it is growing dim, and if we had patience
to watch through the night we should find, three or four hours later
still, that it looks like one of the smaller stars. Then it will begin
to brighten again, and in four hours more will be as bright as at first.
It will remain so for nearly three days, or, to speak accurately, 2
days, 20 hours, 48 minutes, and 55 seconds, and then will begin to grow
dull again. This star is called Algol the Variable. There are several
such stars in the heavens, and we do not know why they vary, unless
perhaps some dark globe passes round them, cutting off part of their
light for a time.
And now, if your eyes are not weary, let us go back to the Pole-star and
draw a line from it straight down the horizon due north. Shortly before
we arrive there you will see a very brilliant bluish-white star a little
to the east of this line. This is Vega, one of the brightest stars in
the heavens except Sirius. It had not risen in the earlier part of the
evening, but now it is well up and will appear to go on, steadily
mounting as it circles round the Pole-star, till at four o'clock
to-morrow morning it will be right overhead towards the south.
But beautiful as Vega is, a still more interesting star lies close to it
(see Fig. 58). This small star, called [Greek: e] Lyrae by astronomers,
looks a little longer in one direction than in the other, and even with
the naked eye some people can see a division in the middle dividing it
into two stars. Your telescopes will show them easily, and a powerful
telescope tells a wonderful story, for it reveals that each of these two
stars is again composed of two stars, so that [Greek: e] Lyrae (Fig. 62)
is really a double-double star. There is no doubt that each pair is a
binary star, that is, the two stars move round each other very slowly,
and possibly both pairs may also revolve round a common centre. There
are at least 10,000 double stars in the heavens; though, as we have
seen, they are not all binary. The list of binary stars, however,
increases every year as they are carefully examined, and probably about
one star in three over the whole sky is made up of more than one sun.
[Illustration: Fig. 62.
[Greek: e] Lyrae. A double-binary star. Each couple revolves, and the
couples probably also revolve round each other. (After Chambers.)]
Let us turn the telescope for a short time upon a few of the double
stars and we shall have a great treat, for one of the most interesting
facts about them is that both stars are rarely of the same colour. It
seems strange at first to speak of stars as coloured, but they do not by
any means all give out the same kind of light. Our sun is yellow, and so
are the Pole-star and Pollux; but Sirius, Vega, and Regulus are
dazzling white or bluish-white, Arcturus is a yellowish-white,
Aldebaran is a bright yellow-red, Betelgeux a deep orange-red, as you
may see now in the telescope, for he is full in view; while Antares, a
star in the constellation of the Scorpion, which at this time of year
cannot be seen till four in the morning, is an intense ruby red.
[Illustration: _Plate II._]
COLOURED DOUBLE STARS.
[Illustration: [Greek: g] _Andromedae_.]
[Illustration: [Greek: e] _Booetis_.]
[Illustration: [Greek: d] _Geminorum_.]
[Illustration: [Greek: a] _Herculis_.]
[Illustration: [Greek: b] _Cygni_.]
[Illustration: [Greek: e] _Cassiopeiae_.]
It appears to be almost a rule with double stars to be of two colours.
Look up at Almach ([Greek: g] Andromedae), a bright star standing next to
Algol the Variable in the sweep of four bright stars behind Cassiopeia
(see Fig. 58). Even to the naked eye he appears to flash in a strange
way, and in the telescope he appears as two lovely stars, one a deep
orange and the other a pale green, while in powerful telescopes the
green one splits again into two (Plate II.) Then again, [Greek: e]
Cassiopeae, the sixth star lying between the two large ones in the second
V of Cassiopeia, divides into a yellow star and a small rich purple one,
and [Greek: d] Geminorum, a bright star not far from Pollux in the
constellation Gemini, is composed of a large green star and a small
purple one. Another very famous double star ([Greek: b] Cygni), which
rises only a little later in the evening, lies below Vega a little to
the left. It is composed of two lovely stars; one an orange yellow and
the other blue; while [Greek: e] Booetis, just visible above the horizon,
is composed of a large yellow star and a very small green one.[1]
[1] The plate of coloured stars has been most kindly drawn to scale
and coloured for me by Mr. Arthur Cottam, F.R.A.S.
There are many other stars of two colours even among the few
constellations we have picked out to-night, as, for example, the star
at the top of the tailboard of Charles's Waggon and the second horse
Mizar. Rigel in Orion, and the two outer stars of the belt, [Greek: a]
Herculis, which will rise later in the evening, and the beautiful triple
star ([Greek: z] Cancer) near the Beehive (see Fig. 54), are all
composed of two or more stars of different colours.
Why do these suns give out such beautiful coloured light? The telescope
cannot tell us, but the spectroscope again reveals the secrets so long
hidden from us. By a series of very delicate experiments, Dr. Huggins
has shown that the light of all stars is sifted before it comes to us,
just as the light of our sun is; and those rays which are least cut off
play most strongly on our eyes, and give the colour to the star. The
question is a difficult one but I will try to give you some idea of it,
that you may form some picture in your mind of what happens.
We learnt in our last lecture (p. 131) that the light from our sun
passes through the great atmosphere of vapours surrounding him before it
goes out into space, and that many rays are in this way cut off; so that
when we spread out his light in a long spectrum there are dark lines or
spaces where no light falls.[1] Now in sunlight these dark lines are
scattered pretty evenly over the spectrum, so that about as much light
is cut off in one part as in another, and no one colour is stronger than
the rest.
[1] See No. 1 in Table of Spectra, Plate I.
Dr. Huggins found, however, that in coloured stars the dark spaces are
often crowded into particular parts of the long band of colour forming
the spectrum; showing that many of those light-rays have been cut off
in the atmosphere round the star, and thus their particular colours are
dimmed, leaving the other colour or colours more vivid. In red stars,
for example, the yellow, blue, and green parts of the spectrum are much
lined while the red end is strong and clear. With blue stars it is just
the opposite, and the violet end is most free from dark lines. So there
are really brilliantly coloured suns shining in the heavens, and in many
cases two or more of these revolve round each other.
And now I have kept your attention and strained your eyes long enough,
and you have objects to study for many a long evening before you will
learn to see them plainly. You must not expect to find them every night,
for the lightest cloud or the faintest moonlight will hide many of them
from view; and, moreover, though you may learn to use the telescope
fairly, you will often not know how to get a clear view with it. Still,
you may learn a great deal, and before we go in I want to put a thought
into your minds which will make astronomy still more interesting. We
have seen that the stronger our telescopes the more stars,
star-clusters, and nebulae we see, and we cannot doubt that there are
still countless heavenly bodies quite unknown to us. Some years ago
Bessel the astronomer found that Sirius, in its real motion through the
heavens, moves irregularly, travelling sometimes a little more slowly
than at other times, and he suggested that some unseen companion must be
pulling at him.
Twenty-eight years later, in 1862, two celebrated opticians, father and
son, both named Alvan Clark, were trying a new telescope at Chicago
University, when suddenly the son, who was looking at Sirius, exclaimed,
"Why, father, the star has a companion!" And so it was. The powerful
telescope showed what Bessel had foretold, and proved Sirius to be a
"binary" star--that is, as we have seen, a star which has another moving
round it.
It has since been proved that this companion is twenty-eight times
farther from Sirius than we are from our sun, and moves round him in
about forty-nine years. It is seven times as heavy as our sun, and yet
gives out so little light that only the keenest telescopes can bring it
into view.
Now if such a large body as this can give so very faint a light that we
can scarcely see it, though Sirius, which is close to it, shines
brightest of any star in the heavens, how many more bodies must there be
which we shall never see, even among those which give out light, while
how many there are dark like our earth, who can tell?
Now that we know each of the stars to be a brilliant sun, many of them
far, far brighter than ours, yet so like in their nature and laws, we
can scarcely help speculating whether round these glorious suns, worlds
of some kind may not be moving. If so, and there are people in them,
what a strange effect those double coloured suns must produce with red
daylight one day and blue daylight another!
Surely, as we look up at the myriads of stars bespangling the sky, and
remember that our star-sun has seven planets moving round it of which
one at least--our own earth--is full of living beings, we must picture
these glorious suns as the centres of unseen systems, so that those
twinkling specks become as suggestive as the faint lights of a great
fleet far out at sea, which tell us of mighty ships, together with
frigates and gunboats, full of living beings, though we cannot see them,
nor even guess what they may be like. How insignificant we feel when we
look upon that starlit sky and remember that the whole of our solar
system would be but a tiny speck of light if seen as far off as we see
the stars! If our little earth and our short life upon it were all we
could boast of we should be mites indeed.
But our very study to-night lifts us above these and reminds us that
there is a spirit within us which even now can travel beyond the narrow
bounds of our globe, measure the vast distances between us and the
stars, gauge their brightness, estimate their weight, and discern their
movements. As we gaze into the depths of the starlit sky, and travel
onwards and onwards in imagination to those distant stars which
photography alone reveals to us, do not our hearts leap at the thought
of a day which must surely come when, fettered and bound no longer to
earth, this spirit shall wander forth and penetrate some of the mystery
of those mighty suns at which we now gaze in silent awe.