suddenly affected this star had its origin from without. At the same
time, I cannot agree with Meyer and Klein in considering that the cause
of the heat was either the downfall of a planetary mass on the star, or
the collision of the star with a star-cloudlet, or nebula, traversing
space in one direction while the star swept onwards in another. A planet
could not very well come into final conflict with its sun at one fell
swoop. It would gradually draw nearer and nearer, not by the narrowing
of its path, but by the change of the path's shape. The path would, in
fact, become more and more eccentric; until, at length, at its point of
nearest approach, the planet would graze its primary, exciting an
intense heat where it struck, but escaping actual destruction that time.
The planet would make another circuit, and again graze its sun, at or
near the same part of the planet's path. For several circuits this would
continue, the grazes not becoming more effective each time, but rather
less. The interval between them, however, would grow continually less
and less. At last the time would come when the planet's path would be
reduced to the circular form, its globe touching its sun's all the way
round, and then the planet would very quickly be reduced to vapour, and
partly burned up, its substance being absorbed by its sun. But all the
successive grazes would be indicated to us by accessions in the star's
lustre, the period between each seeming outburst being only a few months
at first, and becoming gradually less and less (during a long course of
years, perhaps even of centuries), until the planet was finally
destroyed. Nothing of this sort has happened in the case of any
so-called new star.
As for the rush of a star through a nebulous mass, that is a theory
which would scarcely be entertained by any one acquainted with the
enormous distances separating the gaseous star-clouds properly called
nebulae. There may be small clouds of the same sort scattered much more
densely through space; but we have not a particle of evidence that this
actually is the case. All we certainly _know_ about star-cloudlets
suggest that the distances separating them from each other are
comparable with those which separate star from star, in which case the
idea of a star coming into collision with a star-cloudlet, and still
more the idea of this occurring several times in a century, is wild in
the extreme.
On the whole, the theory seems more probable than any of these, that
enormous flights of large meteoric masses travel around those stars
which thus occasionally break forth in conflagration, such flights
travelling on exceedingly eccentric paths, and requiring enormously long
periods to complete each circuit of their vast orbits. In conceiving
this, we are not imagining anything new. Such a meteoric flight would
differ only in degree not kind from meteoric flights which are known to
circle around our own sun. I am not sure, indeed, that it can be
definitely asserted that our sun has no meteoric appendages of the same
nature as those which, if this theory be true, excite to intense
periodic activity the sun round which they circle. We know that comets
and meteors are closely connected, every comet being probably (many
certainly) attended by flights of meteoric masses. The meteors which
produce the celebrated November showers of falling stars follow in the
track of a comet invisible to the naked eye. May we not reasonably
suppose, then, that those glorious comets which have not only been
visible but conspicuous, shining even in the day-time, and brandishing
round tails which, like that of the 'wonder in heaven, the great
dragon,' seemed to 'draw the third part of the stars of heaven,' are
followed by much denser flights of much more massive meteors? Now some
among these giant comets have paths which carry them very close to our
sun. Newton's comet, with its tail a hundred millions of miles in
length, all but grazed the sun's globe. The comet of 1843, whose tail,
says Sir J. Herschel, 'stretched half-way across the sky,' must actually
have grazed the sun, though but lightly, for its nucleus was within
80,000 miles of his surface, and its head was more than 160,000 miles in
diameter. And these are only two among the few comets whose paths are
known. At any time we might be visited by a comet mightier than either,
travelling on an orbit intersecting the sun's surface, followed by
flights of meteoric masses enormous in size and many in number, which,
falling on the sun's globe with the enormous velocity corresponding to
their vast orbital range and their near approach to the sun--a velocity
of some 360 miles per second--would, beyond all doubt, excite his whole
frame, and especially his surface regions, to a degree of heat far
exceeding what he now emits.
We have had evidence of the tremendous heat to which the sun's surface
would be excited by the downfall of a shower of large meteoric masses.
Carrington and Hodgson, on September 1, 1859, observed (independently)
the passage of two intensely bright bodies across a small part of the
sun's surface--the bodies first increasing in brightness, then
diminishing, then fading away. It is generally believed that these were
meteoric masses raised to fierce heat by frictional resistance. Now so
much brighter did they appear, or rather did that part of the sun's
surface appear through which they had rushed, that Carrington supposed
the dark glass screen used to protect the eye had broken, and Hodgson
described the brightness of this part of the sun as such that the part
shone like a brilliant star on the background of the glowing solar
surface. Mark, also, the consequences of the downfall of those two
bodies only. A magnetic disturbance affected the whole frame of the
earth at the very time when the sun had been thus disturbed. Vivid
auroras were seen not only in both hemispheres, but in latitudes where
auroras are very seldom witnessed. 'By degrees,' says Sir J. Herschel,
'accounts began to pour in of great auroras seen not only in these
latitudes, but at Rome, in the West Indies, in the tropics within
eighteen degrees of the equator (where they hardly ever appear); nay,
what is still more striking, in South America and in Australia--where,
at Melbourne, on the night of September 2, the greatest aurora ever seen
there made its appearance. These auroras were accompanied with unusually
great electro-magnetic disturbances in every part of the world. In many
places the telegraph wires struck work. They had too many private
messages of their own to convey. At Washington and Philadelphia, in
America, the electric signal-men received severe electric shocks. At a
station in Norway the telegraphic apparatus was set fire to; and at
Boston, in North America, a flame of fire followed the pen of Bain's
electric telegraph, which writes down the message upon chemically
prepared paper.' Seeing that where the two meteors fell the sun's
surface glowed thus intensely, and that the effect of this accession of
energy upon our earth was thus well marked, can it be doubted that a
comet, bearing in its train a flight of many millions of meteoric
masses, and falling directly upon the sun, would produce an accession of
light and heat whose consequences would be disastrous? When the earth
has passed through the richer portions (not the actual nuclei, be it
remembered) of meteor systems, the meteors visible from even a single
station have been counted by tens of thousands, and it has been computed
that millions must have fallen upon the whole earth. These were meteors
following in the train of very small comets. If a very large comet
followed by no denser a flight of meteors, but each meteoric mass much
larger, fell directly upon the sun, it would not be the outskirts but
the nucleus of the meteoric train which would impinge upon him. They
would number thousands of millions. The velocity of downfall of each
mass would be more than 360 miles per second. And they would continue to
pour in upon him for several days in succession, millions falling every
hour. It seems not improbable that, under this tremendous and
long-continued meteoric hail, his whole surface would be caused to glow
as intensely as that small part whose brilliancy was so surprising in
the observation made by Carrington and Hodgson. In that case, our sun,
seen from some remote star whence ordinarily he is invisible, would
shine out as a new sun, for a few days, while all things living on our
earth, and whatever other members of the solar system are the abode of
life, would inevitably be destroyed.
The reader must not suppose that this idea has been suggested merely in
the attempt to explain outbursts of stars. The following passage from a
paper of considerable scientific interest by Professor Kirkwood, of
Bloomington, Indiana, a well-known American astronomer, shows that the
idea had occurred to him for a very different reason. He speaks here of
a probable connection between the comet of 1843 and the great sun-spot
which appeared in June 1843. I am not sure, however, but that we may
regard the very meteors which seem to have fallen on the sun on
September 1, 1859, as bodies travelling in the track of the comet of
1843--just as the November meteors seen in 1867-8, 9, etc., until 1872,
were bodies certainly following in the track of the telescopic comet of
1866. 'The opinion has been expressed by more than one astronomer,' he
says, speaking of Carrington's observation, 'that this phenomenon was
produced by the fall of meteoric matter upon the sun's surface. Now, the
fact may be worthy of note that the comet of 1843 actually grazed the
sun's atmosphere about three months before the appearance of the great
sun-spot of the same year. Had it approached but little nearer, the
resistance of the atmosphere would probably have brought its entire mass
to the solar surface. Even at its actual distance it must have produced
considerable atmospheric disturbance. But the recent discovery that a
number of comets are associated with meteoric matter, travelling in
nearly the same orbits, suggests the inquiry whether an enormous
meteorite following in the comet's train, and having a somewhat less
perihelion distance, may not have been precipitated upon the sun, thus
producing the great disturbance observed so shortly after the comet's
perihelion passage.'
There are those, myself among the number, who consider the periodicity
of the solar spots, that tide of spots which flows to its maximum and
then ebbs to its minimum in a little more than eleven years, as only
explicable on the theory that a small comet having this period, and
followed by a meteor train, has a path intersecting the sun's surface.
In an article entitled 'The Sun a Bubble,' which appeared in the
'Cornhill Magazine' for October 1874, I remarked that from the observed
phenomena of sun-spots we might be led to suspect the existence of some
as yet undetected comet with a train of exceptionally large meteoric
masses, travelling in a period of about eleven years round the sun, and
having its place of nearest approach to that orb so close to the solar
surface that, when the main flight is passing, the stragglers fall upon
the sun's surface. In this case, we could readily understand that, as
this small comet unquestionably causes our sun to be variable to some
slight degree in brilliancy, in a period of about eleven years, so some
much larger comet circling around Mira, in a period of about 331 days,
may occasion those alternations of brightness which have been described
above. It may be noticed in passing, that it is by no means certain that
the time when the sun is most spotted is the time when he gives out
least light. Though at such times his surface is dark where the spots
are, yet elsewhere it is probably brighter than usual; at any rate, all
the evidence we have tends to show that when the sun is most spotted,
his energies are most active. It is then that the coloured flames leap
to their greatest height and show their greatest brilliancy, then also
that they show the most rapid and remarkable changes of shape.
Supposing there really is, I will not say danger, but a possibility,
that our sun may one day, through the arrival of some very large comet
travelling directly towards him, share the fate of the suns whose
outbursts I have described above, we might be destroyed unawares, or we
might be aware for several weeks of the approach of the destroying
comet. Suppose, for example, the comet, which might arrive from any part
of the heavens, came from out that part of the star-depths which is
occupied by the constellation Taurus--then, if the arrival were so timed
that the comet, which might reach the sun at any time, fell upon him in
May or June, we should know nothing of that comet's approach: for it
would approach in that part of the heavens which was occupied by the
sun, and his splendour would hide as with a veil the destroying enemy.
On the other hand, if the comet, arriving from the same region of the
heavens, so approached as to fall upon the sun in November or December,
we should see it for several weeks. For it would then approach from the
part of the heavens high above the southern horizon at midnight.
Astronomers would be able in a few days after it was discovered to
determine its path and predict its downfall upon the sun, precisely as
Newton calculated the path of _his_ comet and predicted its near
approach to the sun. It would be known for weeks then that the event
which Newton contemplated as likely to cause a tremendous outburst of
solar heat, competent to destroy all life upon the surface of our earth,
was about to take place; and, doubtless, the minds of many students of
science would be exercised during that interval in determining whether
Newton was right or wrong. For my own part, I have very little doubt
that, though the change in the sun's condition in consequence of the
direct downfall upon his surface of a very large comet would be but
temporary, and in that sense slight--for what are a few weeks in the
history of an orb which has already existed during thousands of millions
of years?--yet the effect upon the inhabitants of the earth would be by
no means slight. I do not think, however, that any students of science
would remain, after the catastrophe, to estimate or to record its
effects.
Fortunately, all that we have learned hitherto from the stars favours
the belief that, while a catastrophe of this sort may be possible, it is
exceedingly unlikely. We may estimate the probabilities precisely in the
same way that an insurance company estimates the chance of a railway
accident. Such a company considers the number of accidents which occur
among a given number of railway journeys, and from the smallness of the
number of accidents compared with the largeness of the number of
journeys estimates the safety of railway travelling. Our sun is one
among many millions of suns, any one of which (though all but a few
thousands are actually invisible) would become visible to the naked eye,
if exposed to the same conditions as have affected the suns in flames
described in the preceding pages. Seeing, then, that during the last
two thousand years or thereabouts, only a few instances of the kind,
certainly not so many as twenty, have been recorded, while there is
reason to believe that some of these relate to the same star which has
blazed out more than once, we may fairly consider the chance exceedingly
small that during the next two thousand, or even the next twenty
thousand years, our sun will be exposed to a catastrophe of the kind.
We might arrive at this conclusion independently of any considerations
tending to show that our sun belongs to a safe class of system-rulers,
and that all, or nearly all, the great solar catastrophes have occurred
among suns of a particular class. There are, however, several
considerations of the kind which are worth noting.
In the first place, we may dismiss as altogether unlikely the visit of a
comet from the star-depths to our sun, on a course carrying the comet
directly upon the sun's surface. But if, among the comets travelling in
regular attendance upon the sun, there be one whose orbit intersects the
sun's globe, then that comet must several times ere this have struck the
sun, raising him temporarily to a destructive degree of heat. Now, such
a comet must have a period of enormous length, for the races of animals
now existing upon the earth must all have been formed since that comet's
last visit--on the assumption, be it remembered, that the fall of a
large comet upon the sun, or rather the direct passage of the sun
through the meteoric nucleus of a large comet, would excite the sun to
destructive heat. If all living creatures on the earth are to be
destroyed when some comet belonging to the solar system makes its next
return to the sun, that same comet at its last visit must have raised
the sun to an equal, or even greater intensity of heat, so that either
no such races as at present exist had then come into being, or, if any
such existed, they must at that time have been utterly destroyed. We
may fairly believe that all comets of the destructive sort have been
eliminated. Judging from the evidence we have on the subject, the
process of the formation of the solar system was one which involved the
utilisation of cometic and meteoric matter; and it fortunately so
chanced that the comets likely otherwise to have been most
mischievous--those, namely, which crossed the track of planets, and
still more those whose paths intersected the globe of the sun--were
precisely those which would be earliest and most thoroughly used up in
this way.
Secondly, it is noteworthy that all the stars which have blazed out
suddenly, except one, have appeared in a particular region of the
heavens--the zone of the Milky Way (all, too, on one half of that zone).
The single exception is the star in the Northern Crown, and that star
appeared in a region which I have found to be connected with the Milky
Way by a well-marked stream of stars, not a stream of a few stars
scattered here and there, but a stream where thousands of stars are
closely aggregated together, though not quite so closely as to form a
visible extension of the Milky Way. In my map of 324,000 stars this
stream can be quite clearly recognised; but, indeed, the brighter stars
scattered along it form a stream recognisable with the naked eye, and
have long since been regarded by astronomers as such, forming the stars
of the Serpent and the Crown, or a serpentine streak followed by a loop
of stars shaped like a coronet. Now the Milky Way, and the outlying
streams of stars connected with it, seem to form a region of the stellar
universe where fashioning processes are still at work. As Sir W.
Herschel long since pointed out, we can recognise in various parts of
the heavens various stages of development, and chief among the regions
where as yet Nature's work seems incomplete, is the Galactic
zone--especially that half of it where the Milky Way consists of
irregular streams and clouds of stellar light. As there is no reason for
believing that our sun belongs to this part of the galaxy, but on the
contrary good ground for considering that he belongs to the class of
insulated stars, few of which have shown signs of irregular variation,
while none have ever blazed suddenly out with many hundred times their
former lustre, we may fairly infer a very high degree of probability in
favour of the belief that, for many ages still to come, the sun will
continue steadily to discharge his duties as fire, light, and life of
the solar system.
VII.
_THE RINGS OF SATURN._
The rings of Saturn, always among the most interesting objects of
astronomical research, have recently been subjected to close scrutiny
under high telescopic powers by Mr. Trouvelot, of the Harvard
Observatory, Cambridge, U.S. The results which he has obtained afford
very significant evidence respecting these strange appendages, and even
throw some degree of light on the subject of cosmical evolution. The
present time, when Saturn is the ruling planet of the night, seems
favourable for giving a brief account of recent speculations respecting
the Saturnian ring-system, especially as the observations of Mr.
Trouvelot appear to remove all doubt as to the true nature of the rings,
if indeed any doubt could reasonably be entertained after the
investigations made by European and American astronomers when the dark
inner ring had but recently been recognised.
It may be well to give a brief account of the progress of observation
from the time when the rings were first discovered.
In passing, I may remark that the failure of Galileo to ascertain the
real shape of these appendages has always seemed to me to afford
striking evidence of the importance of careful reasoning upon all
observations whose actual significance is not at once apparent. If
Galileo had been thus careful to analyse his observations of Saturn, he
could not have failed to ascertain their real meaning. He had seen the
planet apparently attended by two large satellites, one on either side,
'as though supporting the aged Saturn upon his slow course around the
sun.' Night after night he had seen these attendants, always similarly
placed, one on either side of the planet, and at equal distances from
it. Then in 1612 he had again examined the planet, and lo, the
attendants had vanished, 'as though Saturn had been at his old tricks,
and had devoured his children.' But after a while the attendant orbs had
reappeared in their former positions, had seemed slowly to grow larger,
until at length they had presented the appearance of two pairs of mighty
arms encompassing the planet. If Galileo had reasoned upon these changes
of appearance, he could not have failed, as it seems to me, to interpret
their true meaning. The three forms under which the rings had been seen
by him sufficed to indicate the true shape of the appendage. Because
Saturn was seen with two attendants of apparently equal size and always
equi-distant from him, it was certain that there must be some appendage
surrounding him, and extending to that distance from his globe. Because
this appendage disappeared, it was certain that it must be thin and
flat. Because it appeared at another time with a dark space between the
arms and the planet, it was certain that the appendage is separated by a
wide gap from the body of the planet. So that Galileo might have
concluded--not doubtfully, but with assured confidence--that the
appendage is a thin flat ring nowhere attached to the planet, or, as
Huyghens said some forty years later, Saturn '_annulo cingitur tenui,
plano, nusquam cohaerente_.' Whether such reasoning would have been
accepted by the contemporaries of Galileo may be doubtful. The
generality of men are not content with reasoning which is logically
sound, but require evidence which they can easily understand. Very
likely Huyghens' proof from direct observation, though in reality not a
whit more complete and far rougher, would have been regarded as the
first true proof of the existence of Saturn's ring, just as Sir W.
Herschel's observation of one star actually moving round another was
regarded as the first true proof of the physical association of certain
stars, a fact which Michell had proved as completely and far more neatly
half a century earlier, by a method, however, which was 'caviare to the
general.'