THE UNITY AND EVOLUTION OF THE STAR SYSTEM
The very condensed sketch now given of such of the discoveries of recent
Astronomy as relate to the subject we are discussing will, it is hoped,
give some idea both of the work already done and of the number of
interesting problems yet remaining to be solved. The most eminent
astronomers in every part of the world look forward to the solution of
these problems not, perhaps, as of any great value in themselves, but as
steps towards a more complete knowledge of our universe as a whole. Their
aim is to do for the star-system what Darwin did for the organic world, to
discover the processes of change that are at work in the heavens, and to
learn how the mysterious nebulæ, the various types of stars, and the
clusters and systems of stars are related to each other. As Darwin solved
the problem of the origin of organic species from other species, and thus
enabled us to understand how the whole of the existing forms of life have
been developed out of pre-existing forms, so astronomers hope to be able to
solve the problem of the evolution of suns from some earlier stellar types,
so as to be able, ultimately, to form some intelligible conception of how
the whole stellar universe has come to be what it is. Volumes have already
been written on this subject, and many ingenious suggestions and hypotheses
have been advanced. But the difficulties are very great; the facts to be
co-ordinated are excessively numerous, and they are necessarily only a
fragment of an unknown whole. Yet certain definite conclusions have been
reached; and the agreement of many independent observers and thinkers on
the fundamental principles of stellar evolution seems to assure us that we
are progressing, if slowly yet with some established basis of truth,
towards the solution of this, the most stupendous scientific problem with
which the human intellect has ever attempted to grapple.
THE UNITY OF THE STELLAR UNIVERSE
During the latter half of the nineteenth century the opinion of astronomers
has been tending more and more to the conception that the whole of the
visible universe of stars and nebulæ constitutes one complete and
closely-related system; and during the last thirty years especially the
vast body of facts accumulated by stellar research has so firmly
established this view that it is now hardly questioned by any competent
authority.
The idea that the nebulæ were far more remote from us than the stars long
held sway, even after it had been given up by its chief supporter. When Sir
William Herschel, by means of his then unapproached telescopic power,
resolved the Milky Way more or less completely into stars, and showed that
numerous objects which had been classed as nebulæ were really clusters of
stars, it was natural to suppose that those which still retained their
cloudy appearance under the highest telescopic powers were also clusters or
systems of stars, which only needed still higher powers to show their true
nature. This idea was supported by the fact that several nebulæ were found
to be more or less ring-shaped, thus corresponding on a smaller scale to
the form of the Milky Way; so that when Herschel discovered thousands of
telescopic nebulæ, he was accustomed to speak of them as so many distinct
universes scattered through the immeasurable depths of space.
Now, although any real conception of the immensity of the one stellar
universe, of which the Milky Way with its associated stars is the
fundamental feature, is, as I have shown, almost unattainable, the idea of
an unlimited number of other universes, almost infinitely remote from our
own and yet distinctly visible in the heavens, so seized upon the
imagination that it became almost a commonplace of popular astronomy and
was not easily given up even by astronomers themselves. And this was in a
large part due to the fact that Sir William Herschel's voluminous writings,
being almost all in the Philosophical Transactions of the Royal Society,
were very little read, and that he only indicated his change of view by a
few brief sentences which might easily be overlooked. The late Mr. Proctor
appears to have been the first astronomer to make a thorough study of the
whole of Herschel's papers, and he tells us that he read them all over five
times before he was able thoroughly to grasp the writer's views at
different periods.
But the first person to point out the real teaching of the facts as to the
distribution of the nebulæ was not an astronomer, but our greatest
philosophical student of science in general, Herbert Spencer. In a
remarkable essay on 'The Nebular Hypothesis' in the _Westminster Review_ of
July, 1858, he maintained that the nebulæ really formed a part of our own
Galaxy and of our own stellar universe. A single passage from his paper
will indicate his line of argument, which, it may be added, had already
been partially set forth by Sir John Herschel in his _Outlines of
Astronomy_.
'If there were but one nebula, it would be a curious coincidence were this
one nebula so placed in the distant regions of space as to agree in
direction with a starless spot in our own sidereal system. If there were
but two nebulæ, and both were so placed, the coincidence would be
excessively strange. What, then, shall we say on finding that there are
thousands of nebulæ so placed? Shall we believe that in thousands of cases
these far-removed galaxies happen to agree in their visible positions with
the thin places in our own galaxy? Such a belief is impossible.'
He then applies the same argument to the distribution of the nebulæ as a
whole:--'In that zone of celestial space where stars are excessively
abundant, nebulæ are rare, while in the two opposite celestial spaces that
are farthest removed from this zone, nebulæ are abundant. Scarcely any
nebulæ lie near the galactic circle (or plane of the Milky Way); and the
great mass of them lie round the galactic poles. Can this also be mere
coincidence?' And he concludes, from the whole mass of the evidence, that
'the proofs of a physical connection become overwhelming.'
Nothing could be more clear or more forcible; but Spencer not being an
astronomer, and writing in a comparatively little read periodical, the
astronomical world hardly noticed him; and it was from ten to fifteen years
later, when Mr. R.A. Proctor, by his laborious charts and his various
papers read before the Royal and Royal Astronomical Societies from 1869 to
1875, compelled the attention of the scientific world, and thus did more
perhaps than any other man to establish firmly the grand and far-reaching
principle of the essential unity of the stellar universe, which is now
accepted by almost every astronomical writer of eminence in the civilised
world.
THE EVOLUTION OF THE STELLAR UNIVERSE
Amid the enormous mass of observations and of suggestive speculation upon
this great and most interesting problem, it is difficult to select what is
most important and most trustworthy. But the attempt must be made, because,
unless my readers have some knowledge of the most important facts bearing
upon it (besides those already set forth), and also learn something of the
difficulties that meet the inquirer into causes at every step of his way,
and of the various ideas and suggestions which have been put forth to
account for the facts and to overcome the difficulties, they will not be in
a position to estimate, however imperfectly, the grandeur, the marvel, and
the mystery of the vast and highly complex universe in which we live and of
which we are an important, perhaps the most important, if not the only
permanent outcome.
THE SUN A TYPICAL STAR
It being now a recognised fact that the stars are suns, some knowledge of
our own sun is an essential preliminary to an inquiry into their nature,
and into the probable changes they have undergone.
The fact that the sun's density is only one-fourth that of the earth, or
less than one and a half times that of water, demonstrates that it cannot
be solid, since the force of gravity at its surface being twenty-six and a
half times that at the earth's surface, the materials of a solid globe
would be so compressed that the resulting density would be at least twenty
times greater instead of four times less than that of the earth. All the
evidence goes to show that the body of the sun is really gaseous, but so
compressed by its gravitative force as to behave more like a liquid. A few
figures as to the vast dimensions of the sun and the amount of light and
heat emitted by it will enable us better to understand the phenomena it
presents, and the interpretation of those phenomena.
Proctor estimated that each square inch of the sun's surface emitted as
much light as twenty-five electric arcs; and Professor Langley has shown by
experiment that the sun is 5300 times brighter, and eighty-seven times
hotter than the white-hot metal in a Bessemer converter. The actual amount
of solar heat received by the earth is sufficient, if wholly utilised, to
keep a three-horse-power engine continually at work on every square yard of
the surface of our globe. The size of the sun is such, that if the earth
were at its centre, not only would there be ample space for the moon's
orbit, but sufficient for another satellite 190,000 miles beyond the moon,
all revolving inside the sun. The mass of matter in the sun is 745 times
greater than that of all the planets combined; hence the powerful
gravitative force by which they are retained in their distant orbits.
What we see as the sun's surface is the photosphere or outer layer of
gaseous or partially liquid matter kept at a definite level by the power of
gravitation. The photosphere has a granular texture implying some diversity
of surface or of luminosity; although the even contour of the sun's margin
shows that these irregularities are not on a very large scale. This surface
is apparently rent asunder by what are termed sun-spots, which were long
supposed to be cavities, showing a dark interior; but are now thought to be
due to downpours of cooled materials driven out from the sun, and forming
the prominences seen during solar eclipses. They appear to be black, but
around their margin is a shaded border or penumbra formed of elongated
shining patches crossing and over-lapping, something like heaps of straw.
Sometimes brilliant portions overhang the dark spots, and often completely
bridge them over; and similar patches, called faculæ, accompany spots, and
in some cases almost surround them.
Sun-spots are sometimes numerous on the sun's disc, sometimes very few,
and they are of such enormous size that when present they can easily be
seen with the naked eye, protected by a piece of smoked glass; or, better
still, with an ordinary opera-glass similarly protected. They are found to
increase in number for several years, and then to decrease; the maxima
recurring after an average period of eleven years, but with no exactness,
since the interval between two maxima or minima is sometimes only nine and
sometimes as much as thirteen years; while the minima do not occur midway
between two maxima, but much nearer to the succeeding than to the preceding
one. What is more interesting is, that variations in terrestrial magnetism
follow them with great accuracy; while violent commotions in the sun,
indicated by the sudden appearance of faculæ, sun-spots, or prominences on
the sun's limb, are always accompanied by magnetic disturbances on the
earth.
WHAT SURROUNDS THE SUN
It has been well said that what we commonly term the sun is really the
bright spherical nucleus of a nebulous body. This nucleus consists of
matter in the gaseous state, but so compressed as to resemble a liquid or
even a viscous fluid. About forty of the elements have been detected in the
sun by means of the dark lines in its spectrum, but it is almost certain
that all the elements, in some form or other, exist there. This semi-liquid
glowing surface is termed the photosphere, since from it are given out the
light and heat which reach our earth.
Immediately above this luminous surface is what is termed the 'reversing
layer' or absorbing layer, consisting of dense metallic vapours only a few
hundred miles thick, and, though glowing, somewhat cooler than the surface
of the photosphere. Its spectrum, taken, at the moment when the sun is
totally darkened, through a slit which is directed tangentially to the
sun's limb, shows a mass of bright lines corresponding in a large degree to
the dark lines in the ordinary solar spectrum. It is thus shown to be a
vaporous stratum which absorbs the special rays emitted by each element and
forming its characteristic coloured lines, changing them into black lines.
But as coloured lines are not found in this layer corresponding to all the
black lines in the solar spectrum, it is now held that special absorption
must also occur in the chromosphere and perhaps in the corona itself. Sir
Norman Lockyer, in his volume on _Inorganic Evolution_, even goes so far as
to say, that the true 'reversing layer' of the sun--that which by its
absorption produced the dark lines in the solar spectrum--is now shown to
be _not_ the chromosphere itself but a layer above it, of lower
temperature.
Above the reversing layer comes the chromosphere, a vast mass of rosy or
scarlet emanations surrounding the sun to a depth of about 4000 miles. When
seen during eclipses it shows a serrated waving outline, but subject to
great changes of form, producing the prominences already mentioned. These
are of two kinds: the 'quiescent,' which are something like clouds of
enormous extent, and which keep their forms for a considerable time; and
the 'eruptive,' which shoot out in towering tree-like flames or
geyser-like eruptions, and while doing so have been proved to reach
velocities of over 300 miles a second, and subside again with almost equal
rapidity. The chromosphere and its quiescent prominences appear to be truly
gaseous, consisting of hydrogen, helium, and coronium, while the eruptive
prominences always show the presence of metallic vapours, especially of
calcium. Prominences increase in size and number in close accordance with
the increase of sun-spots. Beyond the red chromosphere and prominences is
the marvellous white glory of the corona, which extends to an enormous
distance round the sun. Like the prominences of the chromosphere, it is
subject to periodical changes in form and size, corresponding to the
sun-spot period, but in inverse order, a minimum of sun-spots going with a
maximum extension of the corona. At the total eclipse of July 1878, when
the sun's surface was almost wholly clear, a pair of enormous equatorial
streamers stretched east and west of the sun to a distance of ten millions
of miles, and less extensions of the corona occurred at the poles. At the
eclipses of 1882 and 1883, on the other hand, when sun-spots were at a
maximum, the corona was regularly stellate with no great extensions, but of
high brilliancy. This correspondence has been noted at every eclipse, and
there is therefore an undoubted connection between the two phenomena.
The light of the corona is believed to be derived from three sources--from
incandescent solid or liquid particles thrown out from the sun, from
sunlight reflected from these particles, and from gaseous emissions. Its
spectrum possesses a green ray, which is peculiar to it, and is supposed to
indicate a gas named 'coronium'; in other respects the spectrum is more
like that of reflected sunlight. The enormous extensions of the corona into
great angular streamers seem to indicate electrical repulsive forces
analogous to those which produce the tails of comets.
Connected with the sun's corona is that strange phenomenon, the zodiacal
light. This is a delicate nebulosity, which is often seen after sunset in
spring and before sunrise in autumn, tapering upwards from the sun's
direction along the plane of the ecliptic. Under very favourable conditions
it has been traced in the eastern sky in spring to 180° from the sun's
position, indicating that it extends beyond the earth's orbit.
Long-continued observations from the summit of the Pic du Midi show that
this is really the case, and that it lies almost exactly in the plane of
the sun's equator. It is therefore held to be produced by the minute
particles thrown off the sun, through those coronal wings and streamers
which are visible only during solar eclipses.
The careful study of the solar phenomena has very clearly established the
fact that none of the sun's envelopes, from the reversing layer to the
corona itself, is in any sense an atmosphere. The combination of enormous
gravitative force with an amount of heat which turns all the elements into
the liquid or gaseous state, leads to consequences which it is difficult
for us to follow or comprehend. There is evidently constant internal
movement or circulation in the interior of the sun, resulting in the
faculæ, the sun-spots, the intensely luminous photosphere, and the
chromosphere with its vast flaming coruscations and eruptive protuberances.
But it seems impossible that this incessant and violent movement can be
kept up without some great and periodical or continuous inrush of fresh
materials to renew the heat, keep up the internal circulation, and supply
the waste. Perhaps the movement of the sun through space may bring him into
contact with sufficiently large masses of matter to continually excite that
internal movement without which the exterior surface would rapidly become
cool and all planetary life cease. The various solar envelopes are the
result of this internal agitation, uprushes, and explosions, while the vast
white corona is probably of little more density than comets' tails,
probably even of less density, since comets not unfrequently rush through
its midst without suffering any loss of velocity. The fact that none of the
solar envelopes are visible to us until the light of the photosphere is
completely shut off, and that they all vanish the very instant the first
gleam of direct sunlight reaches us, is another proof of their extreme
tenuity, as is also the sharply defined edge of the sun's disc. The
envelopes therefore consist partly of liquid or vaporous matter, in a very
finely divided state, driven off by explosions or by electrical forces, and
this matter, rapidly cooling, becomes solidified into minutest particles,
or even physical molecules. Much of this matter continually falls back on
the sun's surface, but a certain quantity of the very finest dust is
continually driven away by electrical repulsion, so as to form the corona
and the zodiacal light. The vast coronal streamers and the still more
extensive ring of the zodiacal light are therefore in all probability due
to the same causes, and have a similar physical constitution with the tails
of comets.
As the whole of our sunlight must pass through both the reversing layer and
the red chromosphere, its colour must be somewhat modified by them. Hence
it is believed that, if they were absent, not only would the light and heat
of the sun be considerably greater, but its colour would be a purer white,
tending towards bluish rather than towards the yellowish tinge it actually
possesses.
THE NEBULAR AND METEORITIC HYPOTHESES
As the constitution of the sun, and its agency in producing magnetism and
electricity in the matter and orbs around it, afford us our best guide to
the constitution of the stars and nebulæ, and to their possible action on
each other, and even upon our earth, so the mode of evolution of the sun
and solar system, from some pre-existing condition, is likely to help us
towards gaining some knowledge of the constitution of the stellar universe
and the processes of change going on there.
At the very commencement of the nineteenth century the great mathematician
Laplace published his Nebular Theory of the Origin of the Solar System; and
although he put it forth merely as a suggestion, and did not support it
with any numerical or physical data, or by any mathematical processes, his
great reputation, and its apparent probability and simplicity, caused it to
be almost universally accepted, and to be extended so as to apply to the
evolution of the stellar universe. This theory, very briefly stated, is,
that the whole of the matter of the solar system once formed a globular or
spheroidal mass of intensely heated gases, extending beyond the orbit of
the outermost planet, and having a slow motion of revolution about an axis.
As it cooled and contracted, its rate of revolution increased, and this
became so great that at successive epochs it threw off rings, which, owing
to slight irregularities, broke up, and, gravitating together, formed the
planets. The contraction continuing, the sun, as we now see it, was the
result.
For about half a century this nebular hypothesis was generally accepted,
but during the last thirty years so many objections and difficulties have
been suggested, that it has been felt impossible to retain it even as a
working hypothesis. At the same time another hypothesis has been put forth
which seems more in accordance with the facts of nature as we find them in
our own solar system, and which is not open to any of the objections
against the nebular theory, even if it introduces a few new ones.
A fundamental objection to Laplace's theory is, that in a gas of such
extreme tenuity as the solar nebula must have been, even when it extended
only to Saturn or Uranus, it could not possibly have had any cohesion, and
therefore could not have given off whole rings at distant intervals, but
only small fragments continuously as condensation went on, and these,
rapidly cooling, would form solid particles, a kind of meteoric dust, which
might aggregate into numerous small planets, or might persist for
indefinite periods, like the rings of Saturn or the great ring of the
Asteroids.
Another equally vital objection is, that, as the nebula when extending
beyond the orbit of Neptune could have had a mean density of only about the
two-hundred millionth of our air at sea level, it must have been many
hundred times less dense than this at and near its outer surface, and would
there be exposed to the cold of stellar space--a cold that would solidify
hydrogen. It is thus evident that the gases of all the metallic and other
solid elements could not possibly exist as such, but would rapidly, perhaps
almost instantaneously, become first liquid and then solid, forming
meteoric dust even before contraction had gone far enough to produce such
increased rotation as would throw off any portion of the gaseous matter.
Here we have the foundations of the meteoritic hypothesis which is now
steadily making its way. It is supported by the fact that we everywhere
find proofs of such solid matter in the planetary spaces around us. It
falls continually upon the earth. It can be collected on the Arctic and
Alpine snows. It occurs everywhere in the deepest abysses of the ocean
where there are not sufficient organic deposits to mask it. It constitutes,
as has now been demonstrated, the rings of Saturn. Thousands of vast rings
of solid particles circulate around the sun, and when our earth crosses any
of these rings, and their particles enter our atmosphere with planetary
velocity, the friction ignites them and we see falling stars. Comets'
tails, the sun's corona, and the zodiacal light are three strange
phenomena, which, though wholly insoluble on any theory of gaseous
formation, receive their intelligible explanation by means of excessively
minute solid particles--microscopic cosmic dust--driven outward by the
tremendous electrical repulsions that emanate from the sun.
Having these and other proofs that solid matter, ranging in size, perhaps,
from the majestic orbs of Jupiter and Saturn down to the inconceivably
minute particles driven millions of miles into space to form a comet's
tail, does actually exist everywhere around us, and by collisions between
the particles or with planetary atmospheres can produce heat and light and
gaseous emanations, we find a basis of fact and observation for the
meteoritic hypothesis which Laplace's nebular, and essentially gaseous,
theory does not possess.
During the latter half of the nineteenth century several writers suggested
this idea of the possible formation of the Solar System, but so far as I am
aware, the late R.A. Proctor was the first to discuss it in any detail, and
to show that it explained many of the peculiarities in the size and
arrangement of the planets and their satellites which the nebular
hypothesis did not explain. This he does at some length in the chapter on
meteors and comets in his _Other Worlds than Ours_, published in 1870. He
assumed, instead of the fire-mist of Laplace, that the space now occupied
by the solar system, and for an unknown distance around it, was occupied by
vast quantities of solid particles of all the kinds of matter which we now
find in the earth, sun, and stars. This matter was dispersed somewhat
irregularly, as we see that all the matter of the universe is now
distributed; and he further assumed that it was all in motion, as we now
know that all the stars and other cosmical masses are, and must be, in
motion towards or around some centre.
Under these conditions, wherever the matter was most aggregated, there
would be a centre of attraction through gravitation, which would
necessarily lead to further aggregation, and the continual impacts of such
aggregating matter would produce heat. In course of time, if the supply of
cosmic matter was ample (as the result shows that it must have been,
whatever theory we adopt), our sun, thus formed, would approximate to its
present mass and acquire sufficient heat by collision and gravitation to
convert its whole body into the liquid or gaseous condition. While this was
going on, subordinate centres of aggregation might form, which would
capture a certain proportion of the matter flowing in under the attraction
of the central mass, while, owing to the nearly uniform direction and
velocity with which the whole system was revolving, each subordinate centre
would revolve around the central mass, in somewhat different planes, but
all in the same direction.
Mr. Proctor shows the probability that the largest outside aggregation
would be at a great distance from the central mass, and this having once
been formed, any centres farther away from the sun would be both smaller
and very remote, while those inside the first would, as a rule, become
smaller as they were nearer the centre. The heated condition of the earth's
interior would thus be due, not to the primitive heat of matter in a
gaseous state out of which it was formed--a condition physically
impossible--but would be acquired in the process of aggregation by the
collisions of meteoric masses falling on it, and by its own gravitative
force producing continuous condensation and heat.
On this view Jupiter would probably be formed first, and after him at very
great distances, Saturn, Uranus, and Neptune; while the inner aggregations
would be smaller, as the much greater attractive power of the sun would
give them comparatively little opportunity of capturing the meteoric matter
that was continuously flowing towards him.
THE METEORITIC NATURE OF THE NEBULÆ
Having thus reached the conclusion that wherever apparently nebulous matter
exists within the limits of the solar system it is not gaseous but consists
of solid particles, or, if heated gases are associated with the solid
matter they can be accounted for by the heat due to collisions either with
other solid particles or with accumulations of gases at a low temperature,
as when meteorites enter our atmosphere, it was an easy step to consider
whether the cosmic nebulæ and stars may not have had a similar origin.
From this point of view the nebulæ are supposed to be vast aggregations of
meteorites or cosmic dust, or of the more persistent gases, revolving with
circular or spiral motions, or in irregular streams, and so sparsely
scattered that the separate particles of dust may be miles--perhaps
hundreds of miles--apart; yet even those nebulæ, only visible by the
telescope, may contain as much matter as the whole solar system. From this
simple origin, by steps which can be observed in the skies, almost all the
forms of suns and systems can be traced by means of the known laws of
motion, of heat-production, and of chemical action. The chief English
advocate of this view at the present time is Sir Norman Lockyer, who, in
numerous papers, and in his works on _The Meteoritic Hypothesis_ and
_Inorganic Evolution_, has developed it in detail, as the result of many
years' continuous research, aided by the contributory work of continental
and American astronomers. These views are gradually spreading among
astronomers and mathematicians, as will be seen by the very brief outline
which will now be given of the explanations they afford of the main groups
of phenomena presented by the stellar universe.
DR. ROBERTS ON SPIRAL NEBULÆ
Dr. Isaac Roberts, who possesses one of the finest telescopes constructed
for photographing stars and nebulæ, has given his views on stellar
evolution, in _Knowledge_ of February 1897, illustrated by four beautiful
photographs of spiral nebulæ. These curious forms were at first thought to
be rare, but are now found to be really very numerous when details are
brought out by the camera. Many of the very large and apparently quite
irregular nebulæ, like the Magellanic Clouds, are found to have faint
indications of spiral structure. As more than ten thousand nebulæ are now
known, and new ones are continually being discovered, it will be a long
time before these can all be carefully studied and photographed, but
present indications seem to show that a considerable proportion of them
will exhibit spiral forms.
Dr. Roberts tells us that all the spiral nebulæ he has photographed are
characterised by having a nucleus surrounded by dense nebulosity, most of
them being also studded with stars. These stars are always arranged more or
less symmetrically, following the curves of the spiral, while outside the
visible nebula are other stars arranged in curves strongly suggesting a
former greater extension of the nebulous matter. This is so marked a
feature that it at once leads to a possible explanation of the numerous
slightly curved lines of stars found in every part of the heavens, as being
the result of their origin from spiral nebulæ whose material substance has
been absorbed by them.
Dr. Roberts proposes several problems in relation to these bodies: Of what
materials are spiral nebulæ composed? Whence comes the vortical motion
which has produced their forms? The material he finds in those faint clouds
of nebulous matter, often of vast extent, that exist in many parts of the
sky, and these are so numerous that Sir William Herschel alone recorded the
positions of fifty-two such regions, many of which have been confirmed by
recent photographs. Dr. Roberts considers these to be either gaseous or
with discrete solid particles intermixed. He also enumerates smaller
nebulous masses undergoing condensation and segregation into more regular
forms; spiral nebulæ in various stages of condensation and of aggregation;
elliptic nebulæ; and globular nebulæ. In the last three classes there is
clear evidence, on every photograph that has been taken, that condensation
into stars or star like forms is now going on.
He adopts Sir Norman Lockyer's view that collisions of meteorites within
each swarm or cloud would produce luminous nebulosity; so also would
collisions between separate swarms of meteorites produce the conditions
required to account for the vortical motions and the peculiar distribution
of the nebulosity in the spiral nebulæ. Almost any collision between
unequal masses of diffused matter would, in the absence of any massive
central body round which they would be forced to revolve, lead to spiral
motions. It is to be noted that, although the stars formed in the spiral
convolutions of the nebulæ follow those curves, and retain them after the
nebulous matter has been all absorbed by them, yet, whenever such a nebula
is seen by us edgewise, the convolutions with their enclosed stars will
appear as straight lines; and thus not only numbers of star groups arranged
in curves, but also those which form almost perfect straight lines, may
possibly be traced back to an origin from spiral nebulæ.
Motion being a necessary result of gravitation, we know that every star,
planet, comet, or nebula must be in motion through space, and these
motions--except in systems physically connected or which have had a common
origin--are, apparently, in all directions. How these motions originated
and are now regulated we do not know; but there they are, and they furnish
the motive power of the collisions, which, when affecting large bodies or
masses of diffused matter, lead to the formation of the various kinds of
permanent stars; while when smaller masses of matter are concerned those
temporary stars are formed which have interested astronomers in all ages.
It must be noted that although the motions of the single stars appear to be
in straight lines, yet the spaces through which they have been observed to
move are so small that they may really be moving in curved orbits around
some central body, or the centre of gravity of some aggregation of stars
bright and dark, which may itself be comparatively at rest. There may be
thousands of such centres around us, and this may sufficiently explain the
apparent motions of stars in all directions.
A SUGGESTION AS TO THE FORMATION OF SPIRAL NEBULÆ
In a remarkable paper in the Astrophysical Journal (July 1901), Mr. T.C.
Chamberlin suggests an origin for the spiral nebulæ, as well as of swarms
of meteorites and comets, which seems likely to be a true, although perhaps
not the only one.
There is a well-known principle which shows that when two bodies in space,
of stellar size, pass within a certain distance of each other, the smaller
one will be liable to be torn into fragments by the differential attraction
of the larger and denser body. This was originally proved in the case of
gaseous and liquid bodies, and the distance within which the smaller one
will be disrupted (termed the Roche limit) is calculated on the supposition
that the disrupted body is a liquid mass. Mr. Chamberlin shows, however,
that a solid body will also be disrupted at a lesser distance dependent on
its size and cohesive strength; but, as the size of the two bodies
increases, the distance at which disruption will occur increases also, till
with very large bodies, such as suns, it becomes almost as large as in the
case of liquids or gases.
The disruption occurs from the well-known law of differential gravitation
on the two sides of a body leading to tidal deformation in a liquid, and to
unequal strain in a solid. When the changes of gravitative force take place
slowly, and are also small in amount, the tides in liquids or strains in
solids are very small, as in the case of our earth when acted on by the sun
and moon, the result is a small tide in the ocean and atmosphere, and no
doubt also in the molten interior, to which the comparatively thin crust
may partially adjust itself. But if we suppose two dark or luminous suns
whose proper motions are in such a direction as to bring them near each
other, then, as they approach, each will be deflected towards the other,
and will pass round their common centre of gravity with immense velocity,
perhaps hundreds of miles in a second. At a considerable distance they will
begin to produce tidal elongation towards and away from each other, but
when the disruptive limit is nearly reached, the gravitative forces will be
increasing so rapidly that even a liquid mass could not adjust its shape
with sufficient quickness and the tremendous internal strains would produce
the effects of an explosion, tearing the whole mass (of the smaller of the
two) into fragments and dust.
But it is also shown that, during the entire process, the two elongated
portions of the originally spherical mass would be so acted upon by
gravity as to produce increasing rotation, which as the crisis approached
would extend the elongation, and aid in the explosive result. This rapid
rotation of the elongated mass would, when the disruption occurred,
necessarily give to the fragments a whirling or spiral motion, and thus
initiate a spiral nebula of a size and character dependent on the size and
constitution of the two masses, and on the amount of the explosive forces
set up by their approach.
There is one very suggestive phenomenon which seems to prove that this _is_
one of the modes of formation of spiral nebulæ. When the explosive
disruption occurs the two protuberances or elongations of the body will fly
apart, and having also a rapid rotatory movement, the resulting spiral will
necessarily be a double one. Now, it is the fact that almost all the
well-developed spiral nebulæ have two such arms opposite to each other, as
beautifully shown in M. 100 Comæ, M. 51 Canum, and others photographed by
Dr. I. Roberts. It does not seem likely that any other origin of these
nebulæ should give rise to a double rather than to a single spiral.
THE EVOLUTION OF DOUBLE STARS
The advance in knowledge of double and multiple stars has been wonderfully
rapid, numerous observers having devoted themselves to this special branch.
Many thousands were discovered during the first half of the nineteenth
century, and as telescopic power increased new ones continued to flow in by
hundreds and thousands, and there has been recently published by the
Yerkes Observatory a catalogue of 1290 such stars, discovered between 1871
and 1899 by one observer, Mr. S.W. Burnham. All these have been found by
the use of the telescope, but during the last quarter of a century the
spectroscope has opened up a new world of double stars of enormous extent
and the highest interest.
The telescopic binaries which have been observed for a sufficient time to
determine their orbits, range from periods of about eleven years as a
minimum up to hundreds and even more than a thousand years. But the
spectroscope reveals the fact that the many thousands of telescopic
binaries form only a very small part of the binary systems in existence.
The overwhelming importance of this discovery is, that it carries the times
of revolution from the minimum of the telescopic doubles downward in
unbroken series through periods of a few years, to those reckoned by
months, by days, and even by hours. And with this reduction of period there
necessarily follows a corresponding reduction of distance, so that
sometimes the two stars must be in contact, and thus the actual birth or
origin of a double star has been observed to occur, even though not
actually seen. This mode of origin was indeed anticipated by Dr. Lee of
Chicago in 1892, and it has been confirmed by observation in the short
space of ten years.
In a remarkable communication to _Nature_ (September 12th, 1901) Mr.
Alexander W. Roberts of Lovedale, South Africa, gives some of the main
results of this branch of inquiry. Of course all the variable stars are to
be found among the spectroscopic binaries. They consist of that portion of
the class in which the plane of the orbit is directed towards us, so that
during their revolution one of the pair either wholly or partially eclipses
the other. In some of these cases there are irregularities, such as double
maxima and minima of unequal lengths, which may be due to triple systems or
to other causes not yet explained, but as they all have short periods and
always appear as one star in the most powerful telescopes, they form a
special division of the spectroscopic binary systems.
There are known at present twenty-two variables of the Algol type, that is,
stars having each a dark companion very close to it which obscures it
either wholly or partially during every revolution. In these cases the
density of the systems can be approximately determined, and they are found
to be, on the average, only one-fifth that of water, or one-eighth that of
our sun. But as many of them are as large as our sun, or even considerably
larger, it is evident that they must be wholly gaseous, and, even if very
hot, of a less complex constitution than our luminary. Mr. A.W. Roberts
tells us that five out of these twenty-two variables revolve _in absolute
contact_ forming systems of the shape of a dumb-bell. The periods vary from
twelve days to less than nine hours; and, starting from these, we now have
a continuous series of lengthening periods up to the twin stars of Castor
which require more than a thousand years to complete their revolution.
During his observations of the above five stars, Mr. Roberts states that
one, X Carinæ, was found to have parted company, so that instead of being
actually united to its companion the two are now at a distance apart equal
to one-tenth of their diameters, and he may thus be said to have been
almost a witness of the birth of a stellar system.
A year later we find the record (in _Knowledge_, October 1902) of Professor
Campbell's researches at the Lick Observatory. He states that, out of 350
stars observed spectroscopically, one in eight is a spectroscopic binary;
and so impressed is he with their abundance that, as accuracy of
measurement increases, he believes that _the star that is not a
spectroscopic binary will prove to be the rare exception_! Professor G.
Darwin had already shown that the 'dumb-bell' was a figure of equilibrium
in a rotating mass of fluid; and we now find proofs that such figures
exist, and that they form the starting-point for the enormous and
ever-increasing quantities of spectroscopic binary star-systems that are
now known. The origin of these binary stars is also of especial interest as
giving support to Professor Darwin's well-known explanation of the origin
of the moon by disruption from the earth, owing to the very rapid rotation
of the parent planet. It now appears that suns often subdivide in the same
manner, but, owing perhaps to their intensely heated gaseous state they
seem usually to form nearly equal globes. The evolution of this special
form of star-system is therefore now an observed fact; though it by no
means follows that all double stars have had the same mode of origin.
CLUSTERS OF STARS AND VARIABLES
The clusters of stars, which are tolerably abundant in the heavens and
offer so many strange and beautiful forms to the telescopist, are yet
among the most puzzling phenomena the philosophic astronomer has to deal
with.
Many of these clusters which are not very crowded and of irregular forms,
strongly suggest an origin from the equally irregular and fantastic forms
of nebulæ by a process of aggregation like that which Dr. Roberts describes
as developing within the spiral nebulæ. But the dense globular clusters
which form such beautiful telescopic objects, and in some of which more
than six thousand stars have been counted besides considerable numbers so
crowded in the centre as to be uncountable, are more difficult to explain.
One of the problems suggested by these clusters is as to their stability.
Professor Simon Newcomb remarks on this point as follows: 'Where thousands
of stars are condensed into a space so small, what prevents them from all
falling together into one confused mass? Are they really doing so, and will
they ultimately form a single body? These are questions which can be
satisfactorily answered only by centuries of observation; they must
therefore be left to the astronomers of the future.'
There are, however, some remarkable features in these clusters which afford
possible indications of their origin and essential constitution. When
closely examined most of them are seen to be less regular than they at
first appear. Vacant spaces can be noted in them; even rifts of definite
forms. In some there is a radiated structure; in others there are curved
appendages; while some have fainter centres. These features are so exactly
like what are found, in a more pronounced form, in the larger nebulæ, that
we can hardly help thinking that in these clusters we have the result of
the condensation of very large nebulæ, which have first aggregated towards
numerous centres, while these agglomerations have been slowly drawn towards
the common centre of gravity of the whole mass. It is suggestive of this
origin that while the smaller telescopic nebulæ are far removed from the
Milky Way, the larger ones are most abundant near its borders; while the
star-clusters are excessively abundant on and near the Milky Way, but very
scarce elsewhere, except in or near vast nebulæ like the Magellanic Clouds.
We thus see that the two phenomena may be complementary to each other, the
condensation of nebulæ having gone on most rapidly where material was most
abundant, resulting in numerous star-clusters where there are now few
nebulæ.
There is one striking feature of the globular clusters which calls for
notice; the presence in some of them of enormous quantities of variable
stars, while in others few or none can be found. The Harvard Observatory
has for several years devoted much time to this class of observations, and
the results are given in Professor Newcomb's recent volume on 'The Stars.'
It appears that twenty-three clusters have been observed spectroscopically,
the number of stars examined in each cluster varying from 145 up to 3000,
the total number of stars thus minutely tested being 19,050. Out of this
total number 509 were found to be variable; but the curious fact is, the
extreme divergence in the proportion of variables to the whole number
examined in the several clusters. In two clusters, though 1279 stars were
examined, not a single variable was found. In three others the proportion
was from one in 1050 to one in 500. Five more ranged up to one in 100, and
the remainder showed from that proportion up to one in seven, 900 stars
being examined in the last mentioned cluster of which 132 were variable!
When we consider that variable stars form only a portion, and necessarily a
very small proportion, of binary systems of stars, it follows that in all
the clusters which show a large proportion of variables, a very much larger
proportion--in some cases perhaps all, must be double or multiple stars
revolving round each other. With this remarkable evidence, in addition to
that adduced for the prevalence of double stars and variables among the
stars in general, we can understand Professor Newcomb adding his testimony
to that of Professor Campbell already quoted, that 'it is probable that
among the stars in general, single stars are the exception rather than the
rule. If such be the case, the rule should hold yet more strongly among the
stars of a condensed cluster.'
THE EVOLUTION OF THE STARS
So long as astronomers were limited to the use of the telescope only, or
even the still greater powers of the photographic plate, nothing could be
learnt of the actual constitution of the stars or of the process of their
evolution. Their apparent magnitudes, their movements, and even the
distances of a few could be determined; while the diversity of their
colours offered the only clue (a very imperfect one) even to their
temperature. But the discovery of spectrum analysis has furnished the means
of obtaining some definite knowledge of the physics and chemistry of the
stars, and has thus established a new branch ofscience--Astrophysics--which
has already attained large proportions, and which furnishes the materials
for a periodical and some important volumes. This branch of the subject
is very complex, and as it is not directly connected with our present
inquiry, it is only referred to again in order to introduce such of its
results as bear upon the question of the classification and evolution
of the stars.
By a long series of laboratory experiments it has been shown that numerous
changes occur in the spectra of the elements when subjected to different
temperatures, ranging upwards to the highest attainable by means of a
battery producing an electric spark several feet long. These changes are
not in the relative position of the bands or dark lines, but in their
number, breadth, and intensity. Other changes are due to the density of the
medium in which the elements are heated, and to their chemical condition as
to purity; and from these various modifications and their comparison with
the solar spectrum and those of its appendages, it has become possible to
determine, from the spectrum of a star, not only its temperature as
compared with that of the electric spark and of the sun, but also its place
in a developmental series.
The first general result obtained by this research is, that the bluish
white or pure white stars, having a spectrum extending far towards the
violet end, and which exhibits the coloured bands of gases only, usually
hydrogen and helium, are the hottest. Next come those with a shorter
spectrum not extending so far towards the violet end, and whose light is
therefore more yellow in tint. To this group our sun belongs; and they are
all characterised like it by dark lines due to absorption, and by the
presence of metals, especially iron, in a gaseous state. The third group
have the shortest spectra and are of a red colour, while their spectra
contain lines denoting the presence of carbon. These three groups are often
spoken of as 'gaseous stars,' 'metallic stars,' and 'carbon stars.' Other
astronomers call the first group 'Sirian stars,' because Sirius, though not
the hottest, is a characteristic type; the second being termed 'solar
stars'; others again speak of them as stars of Class I., Class II., etc.,
according to the system of classification they have adopted. It was soon
perceived, however, that neither the colour nor the temperature of stars
gave much information as to their nature and state of development, because,
unless we supposed the stars to begin their lives already intensely hot
(and all the evidence is against this), there must be a period during which
heat increases, then one of maximum heat, followed by one of cooling and
final loss of light altogether. The meteoritic theory of the origin of all
luminous bodies in the heavens, now very widely adopted, has been used, as
we have seen, to explain the development of stars from nebulæ, and its
chief exponent in this country, Sir Norman Lockyer, has propounded a
complete scheme of stellar evolution and decay which may be here briefly
outlined:
Beginning with nebulæ, we pass on to stars having banded or fluted
spectra, indicating comparatively low temperatures and showing bands or
lines of iron, manganese, calcium, and other metals. They are more or less
red in colour, Antares in the Scorpion being one of the most brilliant red
stars known. These stars are supposed to be in the process of aggregation,
to be continually increasing in size and heat, and thus to be subject to
great disturbances. Alpha Cygni has a similar spectrum but with more
hydrogen, and is much hotter. The increase of heat goes on through Rigel
and Beta Crucis, in which we find mainly hydrogen, helium, oxygen,
nitrogen, and also carbon, but only faint traces of metals. Reaching the
hottest of all--Epsilon Orionis and two stars in Argo--hydrogen is
predominant, with traces of a few metals and carbon. The cooling series is
indicated by thicker lines of hydrogen and thinner lines of the metallic
elements, through Sirius, to Arcturus and our sun, thence to 19 Piscium,
which shows chiefly flutings of carbon, with a few faint metallic lines.
The process of further cooling brings us to the dark stars.
We have here a complete scheme of evolution, carrying us from those
ill-defined but enormously diffused masses of gas and cosmic dust we know
as nebulæ, through planetary nebulæ, nebulous stars, variable and
double-stars, to red and white stars and on to those exhibiting the most
intense blue-white lustre. We must remember, however, that the most
brilliant of these stars, showing a gaseous spectrum and forming the
culminating point of the ascending series, are not necessarily hotter than,
or even so hot as, some of those far down on the descending scale; since
it is one of the apparent paradoxes of physics that a body may become
hotter during the very process of contraction through loss of heat. The
reason is that by cooling it contracts and thus becomes denser, that a
portion of its mass falls towards its centre, and in doing so produces an
amount of heat which, though absolutely less than the heat lost in cooling,
will under certain conditions cause the reduced surface to become hotter.
The essential point is, that the body in question must be wholly gaseous,
allowing of free circulation from surface to centre. The law, as given by
Professor S. Newcomb, is as follows:--
'_When a spherical mass of incandescent gas contracts through the loss of
its heat by radiation into space, its temperature continually becomes
higher as long as the gaseous condition is retained._'
To put it in another way: if the compression was caused by external force
and no heat was lost, the globe would get hotter by a calculable amount for
each unit of contraction. But the heat lost in causing a similar amount of
contraction is so little more than the increase of heat produced by
contraction, that the slightly diminished total heat in a smaller bulk
causes the temperature of the mass to increase.
But if, as there is reason to believe, the various types of stars differ
also in chemical constitution, some consisting mainly of the more permanent
gases, while in others the various metallic and non-metallic elements are
present in very different proportions, there should really be a
classification by constitution as well as by temperature, and the course of
evolution of the differently constituted groups may be to some extent
dissimilar.
With this limitation the process of evolution and decay of sun through a
cycle of increasing and decreasing temperature, as suggested by Sir Norman
Lockyer, is clear and suggestive. During the ascending series the star is
growing both in mass and heat, by the continual accretion of meteoritic
matter either drawn to it by gravitation or falling towards it through the
proper motions of independent masses. This goes on till all the matter for
some distance around the star has been utilised, and a maximum of size,
heat, and brilliancy attained. Then the loss of heat by radiation is no
longer compensated by the influx of fresh matter, and a slow contraction
occurs accompanied by a slightly increased temperature. But owing to the
more stable conditions continuous envelopes of metals in the gaseous state
are formed, which check the loss of heat and reduce the brilliancy of
colour; whence it follows that bodies like our sun may be really hotter
than the most brilliant white stars, though not giving out quite so much
heat. The loss of heat is therefore reduced; and this may serve to account
for the undoubted fact that during the enormous epochs of geological time
there has been very little diminution in the amount of heat we have
received from the sun.
On the general question of the meteoritic hypothesis one of our first
mathematicians, Professor George Darwin, has thus expressed his views: 'The
conception of the growth of the planetary bodies by the aggregation of
meteorites is a good one, and perhaps seems more probable than the
hypothesis that the whole solar system was gaseous.' I may add, that one of
the chief objections made to it, that meteorites are too complex to be
supposed to be the primitive matter out of which suns and worlds have been
made, does not seem to me valid. The primitive matter, whatever it was, may
have been used up again and again, and if collisions of large solid globes
ever occur--and it is assumed by most astronomers that they must sometimes
occur--then meteoric particles of all sizes would be produced which might
exhibit any complexity of mineral constitution. The material universe has
probably been in existence long enough for all the primitive elements to
have been again and again combined into the minerals found upon the earth
and many others. It cannot be too often repeated that no explanation--no
theory--can ever take us to the beginning of things, but only one or two
steps at a time into the dim past, which may enable us to comprehend,
however imperfectly, the processes by which the world, or the universe, as
it is, has been developed out of some earlier and simpler condition.