THE STARS
In the foregoing chapters we have dealt at length with those celestial
bodies whose nearness to us brings them into our especial notice. The
entire room, however, taken up by these bodies, is as a mere point in
the immensities of star-filled space. The sun, too, is but an ordinary
star; perhaps quite an insignificant one[28] in comparison with the
majority of those which stud that background of sky against which the
planets are seen to perform their wandering courses.
Dropping our earth and the solar system behind, let us go afield and
explore the depths of space.
We have seen how, in very early times, men portioned out the great mass
of the so-called "fixed stars" into divisions known as constellations.
The various arrangements, into which the brilliant points of light fell
as a result of perspective, were noticed and roughly compared with such
forms as were familiar to men upon the earth. Imagination quickly saw in
them the semblances of heroes and of mighty fabled beasts; and, around
these monstrous shapes, legends were woven, which told how the great
deeds done in the misty dawn of historical time had been enshrined by
the gods in the sky as an example and a memorial for men. Though the
centuries have long outlived such fantasies, yet the constellation
figures and their ancient names have been retained to this day, pretty
well unaltered for want of any better arrangement. The Great and Little
Bears, Cassiopeia, Perseus, and Andromeda, Orion and the rest, glitter
in our night skies just as they did centuries and centuries ago.
Many persons seem to despair of gaining any real knowledge of astronomy,
merely because they are not versed in recognising the constellations.
For instance, they will say:--"What is the use of my reading anything
about the subject? Why, I believe I couldn't even point out the Great
Bear, were I asked to do so!" But if such persons will only consider for
a moment that what we call the Great Bear has no existence in fact, they
need not be at all disheartened. Could we but view this familiar
constellation from a different position in space, we should perhaps be
quite unable to recognise it. Mountain masses, for instance, when seen
from new directions, are often unrecognisable.
It took, as we have seen, a very long time for men to acknowledge the
immense distances of the stars from our earth. Their seeming
unchangeableness of position was, as we have seen, largely responsible
for the idea that the earth was immovable in space. It is a wonder that
the Copernican system ever gained the day in the face of this apparent
fixity of the stars. As time went on, it became indeed necessary to
accord to these objects an almost inconceivable distance, in order to
account for the fact that they remained apparently quite undisplaced,
notwithstanding the journey of millions of miles which the earth was now
acknowledged to make each year around the sun. In the face of the
gradual and immense improvement in telescopes, this apparent immobility
of the stars was, however, not destined to last. The first ascertained
displacement of a star, namely that of 61 Cygni, noted by Bessel in the
year 1838, definitely proved to men the truth of the Copernican system.
Since then some forty more stars have been found to show similar tiny
displacements. We are, therefore, in possession of the fact, that the
actual distances of a few out of the great host can be calculated.
To mention some of these. The nearest star to the earth, so far as we
yet know, is Alpha Centauri, which is distant from us about 25 billions
of miles. The light from this star, travelling at the stupendous rate of
about 186,000 miles per second, takes about 4-1/4 years to reach our
earth, or, to speak astronomically, Alpha Centauri is about 4-1/4 "light
years" distant from us. Sirius--the brightest star in the whole sky--is
at twice this distance, _i.e._ about 8-1/2 light years. Vega is about 30
light years distant from us, Capella about 32, and Arcturus about 100.
The displacements, consequent on the earth's movement, have, however,
plainly nothing to say to any real movements on the part of the stars
themselves. The old idea was that the stars were absolutely fixed; hence
arose the term "fixed stars"--a term which, though inaccurate, has not
yet been entirely banished from the astronomical vocabulary. But careful
observations extending over a number of years have shown slight changes
of position among these bodies; and such alterations cannot be ascribed
to the revolution of the earth in its orbit, for they appear to take
place in every direction. These evidences of movement are known as
"proper motions," that is to say, actual motions in space proper to the
stars themselves. Stars which are comparatively near to us show, as a
rule, greater proper motions than those which are farther off. It must
not, however, be concluded that these proper motions are of any very
noticeable amounts. They are, as a matter of fact, merely upon the same
apparently minute scale as other changes in the heavens; and would
largely remain unnoticed were it not for the great precision of modern
astronomical instruments.
One of the swiftest moving of the stars is a star of the sixth magnitude
in the constellation of the Great Bear; which is known as "1830
Groombridge," because this was the number assigned to it in a catalogue
of stars made by an astronomer of that name. It is popularly known as
the "Runaway Star," a name given to it by Professor Newcomb. Its speed
is estimated to be at least 138 miles per second. It may be actually
moving at a much greater rate, for it is possible that we see its path
somewhat foreshortened.
A still greater proper motion--the greatest, in fact, known--is that of
an eighth magnitude star in the southern hemisphere, in the
constellation of Pictor. Nothing, indeed, better shows the enormous
distance of the stars from us, and the consequent inability of even such
rapid movements to alter the appearance of the sky during the course of
ages, than the fact that it would take more than two centuries for the
star in question to change its position in the sky by a space equal to
the apparent diameter of the moon; a statement which is equivalent to
saying that, were it possible to see this star with the naked eye, which
it is not, at least twenty-five years would have to elapse before one
would notice that it had changed its place at all!
Both the stars just mentioned are very faint. That in Pictor is, as has
been said, not visible to the naked eye. It appears besides to be a very
small body, for Sir David Gill finds a parallax which makes it only as
far off from us as Sirius. The Groombridge star, too, is just about the
limit of ordinary visibility. It is, indeed, a curious fact that the
fainter stars seem, on the average, to be moving more rapidly than the
brighter.
Investigations into proper motions lead us to think that every one of
the stars must be moving in space in some particular direction. To take
a few of the best known. Sirius and Vega are both approaching our system
at a rate of about 10 miles per second, Arcturus at about 5 miles per
second, while Capella is receding from us at about 15 miles per second.
Of the twin brethren, Castor and Pollux, Castor is moving away from us
at about 4-1/2 miles per second, while Pollux is coming towards us at
about 33 miles per second.
Much of our knowledge of proper motions has been obtained indirectly by
means of the spectroscope, on the Doppler principle already treated of,
by which we are enabled to ascertain whether a source from which light
is coming is approaching or receding.
The sun being, after all, a mere star, it will appear only natural for
it also to have a proper motion of its own. This is indeed the case; and
it is rushing along in space at a rate of between ten and twelve miles
per second, carrying with it its whole family of planets and satellites,
of comets and meteors. The direction in which it is advancing is towards
a point in the constellation of Lyra, not far from its chief star Vega.
This is shown by the fact that the stars about the region in question
appear to be opening out slightly, while those in the contrary portion
of the sky appear similarly to be closing together.
Sir William Herschel was the first to discover this motion of the sun
through space; though in the idea that such a movement might take place
he seems to have been anticipated by Mayer in 1760, by Michell in 1767,
and by Lalande in 1776.
A suggestion has been made that our solar system, in its motion through
the celestial spaces, may occasionally pass through regions where
abnormal magnetic conditions prevail, in consequence of which
disturbances may manifest themselves throughout the system at the same
instant. Thus the sun may be getting the credit of _producing_ what it
merely reacts to in common with the rest of its family. But this
suggestion, plausible though it may seem, will not explain why the
magnetic disturbances experienced upon our earth show a certain
dependence upon such purely local facts, as the period of the sun's
rotation, for instance.
One would very much like to know whether the movement of the sun is
along a straight line, or in an enormous orbit around some centre. The
idea has been put forward that it may be moving around the centre of
gravity of the whole visible stellar universe. Mädler, indeed,
propounded the notion that Alcyone--the chief star in the group known as
the Pleiades--occupied this centre, and that everything revolved around
it. He went even further to proclaim that here was the Place of the
Almighty, the Mansion of the Eternal! But Mädler's ideas upon this point
have long been shelved.
To return to the general question of the proper motion of stars.
In several instances these motions appear to take place in groups, as if
certain stars were in some way associated together. For example, a large
number of the stars composing the Pleiades appear to be moving through
space in the same direction. Also, of the seven stars composing the
Plough, all but two--the star at the end of its "handle," and that one
of the "pointers," as they are called, which is the nearer to the pole
star--have a common proper motion, _i.e._ are moving in the same
direction and nearly at the same rate.
Further still, the well-known Dutch astronomer, Professor Kapteyn, of
Groningen, has lately reached the astonishing conclusion that a great
part of the visible universe is occupied by two vast streams of stars
travelling in opposite directions. In both these great streams, the
individual bodies are found, besides, to be alike in design, alike in
chemical constitution, and alike in the stage of their development.
A fable related by the Persian astronomer, Al Sufi (tenth century, A.D.)
shows well the changes in the face of the sky which proper motions are
bound to produce after great lapses of time. According to this fable the
stars Sirius and Procyon were the sisters of the star Canopus. Canopus
married Rigel (another star,) but, having murdered her, he fled towards
the South Pole, fearing the anger of his sisters. The fable goes on to
relate, among other things, that Sirius followed him across the Milky
Way. Mr. J. E. Gore, in commenting on the story, thinks that it may be
based upon a tradition of Sirius having been seen by the men of the
Stone Age on the opposite side of the Milky Way to that on which it now
is.
Sirius is in that portion of the heavens _from_ which the sun is
advancing. Its proper motion is such that it is gaining upon the earth
at the rate of about ten miles per second, and so it must overtake the
sun after the lapse of great ages. Vega, on the other hand, is coming
towards us from that part of the sky _towards_ which the sun is
travelling. It should be about half a million years before the sun and
Vega pass by one another. Those who have specially investigated this
question say that, as regards the probability of a near approach, it is
much more likely that Vega will be then so far to one side of the sun,
that her brightness will not be much greater than it is at this moment.
Considerations like these call up the chances of stellar collisions.
Such possibilities need not, however, give rise to alarm; for the stars,
as a rule, are at such great distances from each other, that the
probability of relatively near approaches is slight.
We thus see that the constellations do not in effect exist, and that
there is in truth no real background to the sky. We find further that
the stars are strewn through space at immense distances from each other,
and are moving in various directions hither and thither. The sun, which
is merely one of them, is moving also in a certain direction, carrying
the solar system along with it. It seems, therefore, but natural to
suppose that many a star may be surrounded by some planetary system in a
way similar to ours, which accompanies it through space in the course of
its celestial journeyings.
[28] Vega, for instance, shines one hundred times more brightly than the
sun would do, were it to be removed to the distance at which that star
is from us.