a sign has recently appeared in the heavens which has been interpreted in a way suggesting that many worlds like our own have undergone a terrible catastrophe, every living creature upon them being consumed as by fire. i propose briefly to consider some of the thoughts suggested by this strange event.

it is difficult when we look at the star-lit heavens, suggestive as they are of solemn peace, to conceive the stupendous energy, the fierce uproar and tumult, of which even the faintest visible star in reality tells us. pythagoras spoke of the harmony of the celestial spheres, which we are only prevented from hearing by its continuity. "there's not the smallest orb which thou beholdest," said the science of the middle ages,

"but in his motion like an angel sings,

still quiring to the young-eyed cherubim."

the science of our own time tells us a still stranger story. there's not the smallest orb which thou beholdest, she says, but in his motion throbs like a mighty heart, still pulsating life to the worlds which circle round it. but while our powers of vision are limited to the narrow range of our present telescopes, we cannot watch the action of these great centres of energy, nor can we hope that the uproar of those remote fires will ever reach mortal ears, though to the mind's ear clear and distinct. it is no longer a mere fancy that each star is a sun. science has made this an assured fact, which no astronomer thinks of doubting. we know that in certain general respects each star resembles our sun. each is glowing like our sun with an intense heat. around each, as around our sun, are the vapours of many elements. in each the fires are maintained, as they are maintained in our sun, in some way which may be partly mechanical, partly chemical, but which certainly does not in the least resemble combustion. we know that in each star processes resembling in violence those taking place in our own sun must be continually in progress, and that such processes must be accompanied by a noise and tumult compared with which all the forms of uproar known upon our earth are as absolute silence. the crash of the thunderbolt, the bellowing of the volcano, the awful groaning of the earthquake, the roar of the hurricane, the reverberating peals of loudest thunder, any of these, or all combined, are as nothing compared with the tumult raging over every square mile, every square yard, of the surface of each one among the stars.

if we remember this when we hear of stars varying in brightness, we shall perceive that the least change which could be recognised from our remote stand-point must represent an accession or falling off of energy corresponding to far more than all the energies existing on our earth, or indeed on all the members of the solar system taken together. astronomers recognise our sun as in one sense a variable star; for we can hardly suppose that he shines with the same degree of brilliancy when many spots mark his surface as when he is quite free from spots; and astronomers know that these changes in the sun's condition correspond to wonderful changes in his activity. when spots are most numerous, the coloured flames rage with fierce energy over his whole globe, metallic vapours are shot forth from below his visible surface with velocities of many miles per second. whereas, when he has no spots, the coloured flames sink down from their former height of tens of thousands of miles, till they are but a few thousand miles in height; while metallic vapours are seldom emitted, and never to the same height, or with the same velocity, as when the spots are most numerous. but though the sun thus varies in condition, and probably in his total brightness, we cannot suppose that such variations could be recognised from the distance of even the nearest among the fixed stars. what, then, must be the nature of changes taking place in a star, that we, at our enormous distance, should be able to recognise them! we may well believe that the entire aspect of such a star must be changed to the inhabitants, if such there are, of worlds circling around them.

if, however, the changes taking place in stars, whose variations of brightness can just be recognised, must be amazing, how stupendous must be the changes affecting a star which alternates from brightness to invisibility, like mira, the star wonderful, in the constellation of the whale! how destructive those affecting a star like eta, of the ship argo, which has varied from the fourth magnitude to a lustre nearly equalling that of sirius, and thence to the lowest limit of visibility, in the course of the last hundred years!

even these changes, however, though justly regarded as among the chief wonders and mysteries of the star-depths, seem in turn to sink into nothingness by comparison with the sudden appearance of a new star, as interpreted by modern scientific observations. of old, when a new star appeared, it was thought for awhile to be a fresh creation; a new sun set in the centre of a new system of worlds,—a thought which was not then so startling as in our own times it would be reckoned. when the new star was seen slowly to die out until at last it became invisible, men were content to regard it as a sign set in the heavens for a special purpose. nor did they find much difficulty in associating such a phenomenon with some event of importance occurring during its continuance, or soon after the new star had died out. such were the explanations offered respecting the exceedingly bright star which made its appearance in the constellation cassiopeia in the year 1572. the place in which it appeared is shown in fig. 7. it must have sprung into its full glory in a very short time, for tycho brahé, the celebrated astronomer, tells us that, returning on november 1, 1572, from his laboratory to his dwelling-house, he saw the new star, which he was certain had not been visible an hour before, shining more brightly than any before seen. it surpassed all the stars in the heavens in brilliancy, and even jupiter when that planet is at its brightest. only venus at her brightest was superior to the new star. for three weeks it shone with full lustre, after which it began slowly to decline. being situated in a part of the heavens always above the horizon (for european observatories), the star's entire history could be followed. it remained for sixteen months steadfast in its position like the other stars. as it decreased in size it varied in colour. "at first," says an old writer; "its light was white and extremely bright; it then became yellowish; afterwards of a ruddy colour like mars; and finished with a pale, livid white, resembling the colour of saturn."

fig. 7.—cassiopeia; showing where a new star appeared in the year 1572.

in passing it may be remarked that there are reasons for expecting the return of tycho brahé's star in the course of a few years. for other new stars have been recorded as seen in the same part of the heavens in the years 945 and 1264, and though the interval from 945 to 1264 (or 319 years) exceeds by 11 years the interval from 1264 to 1572 (or 308 years), yet the difference is but small by comparison with either entire interval; and we may not unreasonably believe that the three new stars seen in cassiopeia have been only three apparitions of one and the same star, which shines out, with superior lustre, for a few months, once in a period averaging about 313 years. it seems to me not at all unlikely that, some time during the next twenty years, astronomers will have an opportunity of examining, with the telescope and spectroscope, a star which last appeared before either instrument had been invented.

already facts are known respecting the so-called new stars which will not permit us to accept the explanations of old so readily offered and admitted, simply because so little was certainly known.

in the year 1866 a star appeared suddenly in the constellation of the northern crown, where no star had before been visible to the naked eye. it was a little below the arc of stars forming the celestial coronet.[7]

it shone as a second magnitude star when first seen, but very rapidly diminished in lustre. it increased our knowledge in two important respects.

first, on examining argelander's charts of the northern heavens, the new star was found to have been observed and charted as a tenth magnitude star, that is, four magnitudes below the lowest limit of naked eye vision. it was not, then, a new sun, though it might still truly be called a new star, in this sense, that it was a new member of the set of stars which adorn our skies as seen by ordinary vision.

in the second place, the star was subject to the searching scrutiny of spectroscopic analysis, with results of a most interesting character.

the reader is no doubt aware that when the light of a star is analysed into its component colours by the instrument called the spectroscope, it is found that all the colours of the rainbow are present, as in the case of solar light, but (also in the sun's case) not all the tints of these colours. certain dark lines athwart the rainbow-tinted streak, called the spectrum of the star, indicate the presence of absorbing vapours in the star's atmosphere. this general statement is true of every fixed star, though the dark lines of some stars differ in number and position from the dark lines of others, showing that other absorbing vapours are present. in the case of the new star in the crown, the usual stellar spectrum was shewn,—a rainbow-tinted streak crossed by a number of dark lines. but besides these, there were seen four very bright lines,—lines so bright that the rainbow-tinted streak appeared as a dark background. the meaning of this is well understood by spectroscopists. it signifies that besides the vapours which, being cooler than the star, absorbed a portion of its light, and produced the dark lines, some vapours were present in the star's atmosphere which were a great deal hotter than the star, and so produced bright lines. now two of the lines corresponded in position with two of the well known lines of the gas hydrogen, showing that this was one of the gases which had been raised to an unusual degree of heat.

it was inferred that there had been some tremendous disturbance in that remote star, by which the hydrogen and some other vapours present in its atmosphere had been intensely heated. but astronomers were unable to decide whether the disturbance was of the nature of a conflagration, the hydrogen actually burning, or whether the heat was occasioned in some other way, as by the downfall of some immense mass upon that remote sun. for burning hydrogen and glowing hydrogen, though either could give the observed bright lines, are very different things. in the former case a chemical change is taking place, as in the case of burning wood or coal; the latter case resembles that of redhot iron, which is not burning itself (not changing into a different form as everything does which burns), though it will burn other things,—in the ordinary, and incorrect, use of the expression.

the general belief was that there had been a downfall of matter on the star in the crown, by which the whole globe of that sun had been excited to an intense degree of heat, especially at the surface, near which lies the hydrogen atmosphere of the star.

i must leave, however, to the next part, the further consideration of the strange thoughts suggested by the outburst of this star. i wish to use the small space remaining at present to indicate the place where another new star burst forth last november, so that any readers of these pages who have telescopes may know where to look for a sun which is now dying out, but was shining a few weeks ago as a third magnitude star. fig. 8 presents a portion of the well-known constellation cygnus or the swan. any star atlas will indicate the place of the lettered stars shown in the figure. the constellation itself does not show at all well at this season of the year.[8] the part shown in the figure is close to the horizon, and directly under the pole-star, at about half-past ten in the middle of february; but a little higher up, between north and north-east, at midnight. professor schmidt, of the athens observatory, noticed a new star, in the place shown, on november 24th last. it must have shone out suddenly, for schmidt had been observing in that region on the night of november 22nd (the last preceding clear night). it has since gradually faded, until now a small telescope is required to show it, shining as a seventh magnitude star, with a well-marked orange tint.

fig. 8.—part of cygnus, showing the place of new star (november 24, 1876).

we have now to consider the history of this star, and discuss the general questions suggested by the sudden blazing out of suns which had for many years, and probably for many centuries, shone continuously with a far feebler lustre. it is clear that we have good reason to be interested in these questions, seeing that, for aught we know, our sun may be one of those exposed to sudden great increase of lustre.

it seems certain, in the first place, that this star leapt very suddenly to its full splendour. schmidt had been observing the same regions of the heavens only two evenings before, and is sure the star was not then shining visibly to the naked eye. again, astronomy is now studied by so many persons, and so many more who are not students of astronomy are now well acquainted with the constellations, that it is very difficult for a new star to shine many hours without being detected. for example, the new star in the crown, which appeared in may, 1866, though not so well placed for observation, was detected by many observers at widely distant stations within a few hours of each other. it is probable that the star acquired its full lustre in a few hours at the utmost, and quite possible that, had any one been watching the place where the star appeared, he would have been able to see the star grow into full brightness by visible change of lustre, just as the lustre of a revolving light in a distant lighthouse visibly waxes and wanes. it may be, of course, that the increase of the star from its ordinary lustre, up to the stage when first it was visible to the naked eye, occupied many days, or even many months or years; but it seems more likely that as the later stages of increase were rapid, so also was the entire development of the new lustre. in that case, if there were inhabited worlds circling around that remote sun, they had but brief warning of the fate in store for them, as presently to be described.

like the star in the northern crown, the new star in cygnus was subjected to the searching scrutiny of the spectroscope. the results, though similar in general respects, were even more interesting than in the case of the brighter new star. in the interval between 1866 and 1876 spectroscopic analysis has developed largely. it has thus become possible to analyse more completely the light even of faint stars than the light of bright stars could be analysed a decade of years since.

the spectrum of the new star as examined by m. cornu, of the paris observatory, showed the bright lines of hydrogen, indicating the presence of enormous quantities of glowing hydrogen, in a state of intense heat. but beside these bright lines, others also could be seen. one of these was an orange-yellow line. it will be understood that the faint spectrum of a star cannot be so readily lengthened by increasing the dispersion as a bright spectrum; for with too great dispersion the light fades out altogether. and though this is not strictly the case with the bright lines, which are merely thrown farther apart by dispersion, yet still it remains true that one cannot deal with a star spectrum even of bright lines as one can with the solar spectrum. so that m. cornu was not able to determine whether the orange-yellow line belonged to sodium, or to that other substance, whatever it may be, which produces the orange-yellow line seen in the spectrum of a solar prominence.[9] another bright line, green in colour, agreed in position with a triple line belonging to the metal magnesium. lastly, a bright yellowish-green line was seen, which is known to be present in the spectrum of the sun's corona and of the low-lying ruddy matter round the sun, called the sierra by some, and by others (apparently unfamiliar with the greek language) the chromosphere.

now all this agrees very well with what had been noticed in the case of the star in the northern crown. for, unquestionably, if a sun increases so much in heat and lustre that the hydrogen outside it glows more brightly than the body of the star, then other matter outside that sun might also be expected to share the great increase of heat. we see that, outside our own sun, hydrogen, a certain unknown vapour of an orange yellow colour, magnesium, and another unknown vapour of greenish-yellow colour are present in enormous quantities; and it seems, therefore, reasonable to believe that other suns have these gases extending far outside the rest of their substance. it is certain that, if our sun were caused to glow with far more than its present degree of heat, the gases whose increase of brightness would be most discernible from a distant station (as a world circling around some remote star) would be just those gases which were glowing so resplendency around the star in cygnus last november—or rather at the time when that light which reached us last november set out from the remote star in the swan.

when we view the outburst of that remote sun in this way the thoughts suggested are not altogether satisfactory. that sun shows far too much resemblance to our own, and behaved, so far as can be judged, far too much as our own sun would behave if roused to many times its present degree of heat and splendour. when we hear of a railway accident it is a matter of special interest to us (if we travel much) to learn whether the conditions under which the accident took place resembled those under which the trains proceed by which we chiefly travel. when an express train suffers in such a way as to show some special danger arising from great velocity, we find ourselves to some degree concerned personally in the investigation which follows, if we travel generally by quick trains. if a bridge breaks down, and we have often to traverse bridges in railway journeying, we are similarly concerned, especially if any of the bridges we have to cross resemble in structure the one which has given way. so also of many other special forms of danger in railway travelling. now, on the same principle, we cannot but regard with considerable interest the circumstance that, apparently, a catastrophe has taken place in the star in cygnus, which has not only affected a sun resembling our own very closely in constitution, but has produced effects very closely corresponding to those which would affect our own sun if, through any cause, he were excited to many times his present degree of heat.

let us pause a little to reflect upon the effects which would follow a great increase of the sun's lustre. a change in our own sun, such as affected the star in cygnus, or that other star in the northern crown, would unquestionably destroy every living creature on the face of this earth; nor could any even escape which may exist on the other planets of the solar system. the star in the northern crown shone out with more than 800 times its former lustre: the star in cygnus with from 500 to many thousand times its former lustre, according as we take the highest possible estimate of its brightness before the catastrophe, or consider that it may have been very much fainter. now, if our sun were to increase tenfold in brightness, all the higher forms of animal life and nearly all vegetable life would inevitably be destroyed on this earth. a few stubborn animalcules might survive, and, possibly, a few of the lowest forms of vegetation, but naught else. if the sun increased a hundredfold in lustre his heat would doubtless sterilise the whole earth. the same would happen in other planets. the heat falling on the remotest members of the solar system would not, indeed, be excessive according to our conceptions. but if we regard neptune, uranus, saturn, and jupiter as the abode of life (which, for my own part, i consider altogether improbable), we cannot but suppose the orders of living creatures in each of these planets to be well fitted to exist under the conditions subsisting around them. if this is so—as who can for a moment doubt?—a sudden enormous increase in the sun's heat, though not making the supply received by those planets much greater than, or even equal to, the supply which we receive from the sun, would prove as fatal to living creatures there as to living creatures on our earth.

if, then, the sun increased in splendour as the stars have increased which the astronomers call new stars or temporary stars, there would be an end of life upon this earth; and nothing short of either the spontaneous development of life, or of the creation of various forms of life, could people our earth afresh. science knows nothing of spontaneous generation, and believers in revelation reject the doctrine. science knows nothing of the creation of living forms, but believers in revelation accept the doctrine. certain it is that if our sun ever undergoes the baptism of fire which has affected some few among his brother suns, one or other of these processes (if creation can be called a process) must come into operation, or else our earth and her companion worlds would for ever after remain absolutely devoid of life.

but if our sun, without suffering so great a change, underwent a change of less degree, it might well happen that though there would be enormous destruction of life upon the earth and other planets, some life (presumably the strongest and best) would survive. in that case, after a long period of time, the earth would again be well peopled, and it might even be that the various races of terrestrial creatures would be improved, by the desolation which the great solar conflagration had wrought.

it is somewhat curious, considering how little there is in the ordinary progress of events to suggest the idea, that most of the ancient systems of cosmogony recognised the periodical destruction of living creatures on the earth by fire as well as by water. each form of destruction was supposed to be brought about by planetary influences. the ecpyrosis, or destruction by fire, was effected when all the planets were in conjunction with cancer; the cataclysm, or destruction by flood, when all the planets were in conjunction with capricorn. each form of destruction was supposed also to purify the human race. "towards the termination of each era," writes lyell, speaking of these old ideas, "the gods could no longer bear with the wickedness of men, and a shock of the elements or a deluge overwhelmed them; after which calamity astrea again descended on the earth, to renew the golden age." the greeks undoubtedly borrowed all such doctrines from the egyptians, who "believed the world to be subject to occasional conflagrations and deluges, whereby the gods arrested the career of human wickedness, and purified the earth from guilt. after each regeneration mankind was in a state of virtue and happiness, from which they gradually degenerated again into vice and immorality."

considering that we have every reason to believe the records of great floods to relate to events which actually occurred, however imperfectly remembered, it seems not unreasonable to believe that the tradition of great heats had its origin in observed phenomena. as neither ordinary conflagrations nor volcanic outbursts would have suggested traditions of the kind, it would seem not impossible that at certain times our sun may have acquired for a time unusual lustre and heat, causing great and widely spread destruction among all forms of animal and vegetable life.

this idea may possibly seem to many, especially at a first view, too wild to be entertained for a moment. our sun shines, so far as appears to ordinary observation, with steadfast lustre from year to year, and also from age to age. if an occasional hot season suggests for a while to some that the sun has grown hotter, or a cool season that he has grown cooler, the restoration of cool or warmer weather, as the case may be, causes the thought to be quickly cast on one side that a change of either kind has taken place. again, if we examine the historical records of past ages, we find little to suggest the idea, or even the possibility, that the sun in former times shone with greater splendour or with less than at present. the men of those days were formed like the men of our own day, and could not have supported any much greater degree of heat or of cold than men can support at present. any sudden accession (or diminution) of solar light and heat, such as we are considering, would certainly have attracted marked attention, and have been recorded for the benefit of future ages. the geologic record, again, does, indeed, suggest variations in the sun's emission of heat as constituting one among the few available explanations of the existence of tropical forms of life in certain strata and of arctic forms in other strata. but even if this explanation be the true one, which is by no means established, such variations must of necessity have been slow, the condition of increased heat continuing for many ages in succession, and the like with the condition of diminished heat. we have no evidence, historical or geological, of the occurrence of any sudden accession of solar heat, followed by a quick return to the normal temperature, unless we find such evidence in the tradition prevalent among egyptian, indian, and chinese cosmogonists, that at certain recurring epochs in the past our earth has undergone destruction and renovation by fire.

yet, as i shall now show, it appears that the one only natural interpretation which can be given of the outburst of a new or temporary sun indicates an event which might happen to our own sun, and an event which if it happened at all would happen periodically. moreover, while it will appear that there is no reason for fearing the possible occurrence (which would, in such case, be really the recurrence) of such a catastrophe in the case of our own sun as has affected the stars in the crown and in cygnus, there is no reason for rejecting as incredible the idea that catastrophes very serious in their character may have affected our sun; and there is abundant reason for believing that small alterations in the sun's total emission of light and heat take place very often, in some cases periodically; in others—so far as we can yet judge—periodically.

lastly, it will be seen that there is always a possibility that our own or any other sun may undergo precisely such a change as the stars in cygnus and the northern crown. some indeed, even among men of science (as the abbé moigno, for example) believe that it was an event of this sort which st. peter predicted when he wrote, that as the old world, being overflowed with water, perished, so "the heavens and the earth which are now, by the same word are kept in store, reserved unto fire." according to that view, the day of destruction will come "as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also, and the works that are therein shall be burned up."

let us consider how the sudden brightness of a new star may be explained.

i must confess that for my own part i do not attach much weight to the suggestion once made by mr. huggins, that an actual conflagration had taken place in the case of the new star in the northern crown. it does not seem to me that any process of mere burning could account for the enormous accession of light and heat which that sun underwent.

consider the case of our own sun. his heat is very far beyond that which would be given out by any matter known to us undergoing any known process of true combustion. that is to say, if a mass as large as the sun of any known substance were caused to burn, under any conditions we can imagine, the momentary emission of heat by that mass would be very much less than the momentary emission of heat by the sun.

now it is quite conceivable that by some great accession of combustible matter, some supply of fuel exceeding many times his entire mass, the sun's entire emission of heat might be very largely increased. but though such an idea is conceivable, it seems altogether far-fetched. the conception is, in fact, inadmissible as an explanation of the increase of heat of a temporary star, not because of the improbability of the sudden accession of so enormous a quantity of matter (though that improbability is very great), but because if so enormous a quantity of matter fell upon the sun, many times as much heat would be generated by the mechanical effect of the impact as by the combustion of the freshly received matter. so that even with the daring assumption here made, combustion would account for only a small portion of the increase of light and heat.

huggins' idea was indeed somewhat different. he supposed that in consequence of some great internal convulsion of the sun in the northern crown a large volume of hydrogen and other gases was evolved from the interior, the hydrogen then by burning giving out the light corresponding to the bright lines. at the same time, the mass of the sun would be intensely heated by the surrounding mass of glowing hydrogen. when the liberation of gas from the interior ceased the flame would die out, and the sun's surface would gradually cool. but if we judge by the case of our own sun, the heat of the burning hydrogen would be nothing near so great as the heat of the glowing hydrogen already outside and within the visible globe of a sun.

on the whole it seems altogether more probable that the accession of splendour observed in the case of temporary stars is due to the downfall of enormous masses of matter upon the surface of these suns. it is, no doubt, well known to most of my readers that the downfall of meteoric matter upon the surface of our own sun has been considered a sufficient explanation of the sun's entire emission of light and heat. the theory that the sun's heat and light are thus excited has long since been abandoned; but not because the cause would be insufficient. it has been abundantly proved that a downfall of meteors, not sufficient in quantity to add appreciably to the sun's size in many thousands of years, would generate more heat and light than he emits in that time. the meteoric theory has been abandoned simply because it has been shown that no such downfall is taking place.

the reason why meteoric impact would suffice to warm the sun to his present temperature if the meteoric showers were heavy, and to warm him far beyond his present temperature if for a few days very heavy meteoric showers fell upon him, is simply that his attraction upon matter approaching him from without is capable of generating a tremendous velocity. we know that when a cannon-ball strikes a metal target, with a velocity perhaps of some 400 yards per second, great heat is excited, and there is a momentary flash of light. if the velocity were doubled, the quantity of heat would be doubled also. conceive, then, the tremendous heat which would be excited if a cannon-ball could be caused to strike a target with a velocity exceeding that just named some 1500 times! the ball and target would both be vaporised by the shock, if—which, however, could never happen—the target resisted the blow and brought the ball to rest. now matter which reaches the sun from without, under the influence of his tremendous attraction, strikes his globe with a velocity 1500 times greater than that of a cannon ball striking a target at a distance of two or three hundred yards. the heat excited is, therefore, very intense; and if meteors were showering at all times and in dense flights upon the sun's surface, we should require no other explanation of the sun's heat.

but it appears that meteoric systems are neither so numerous nor so rich as to account for the sun's uniform emission of heat, though occasional meteoric showers upon the sun may be heavy enough to increase appreciably the amount of heat he emits. it would seem, from experiments which have been made by professor piazzi smyth, of the edinburgh observatory, and later by the astronomer royal at greenwich, that from time to time the sun's emission of heat really is greater than usual. it seems not at all improbable that the increase is due to the occasional fall of large masses of meteors in great numbers upon the sun.

again, it seems that such falls occur periodically, or rather that at regular intervals great meteoric streams pour upon the sun's surface. for instance, the periodic increase and decrease in the number of sun-spots is accompanied (so far as we can judge by the observations made at edinburgh and greenwich) by an accession and diminution of the solar heat; and if the change is attributed to the passage of a meteoric stream athwart the sun, we should have to assign to such a stream a period of rather more than eleven years. this, from what we know about the association between meteors and comets, would correspond simply to the existence of a comet whose path intersects the sun's globe, and which is followed by a train of millions of large meteoric masses, many of which are consumed at each passage of the rich portion of the train athwart the globe of the sun. this comet must of necessity be inconspicuous, since it has hitherto escaped detection. in fact, its head and nucleus must long since have been entirely destroyed. only the meteoric train, far more widely scattered, remains, simply because at each passage past the sun, though many are captured, far greater numbers get safely past.

i am careful to remind the reader that though i have, for convenience, used the indicative mood in describing these matters, i am in reality presenting merely a theory. it may be that the solar spots and the accessions of heat are produced in some other way. but i must admit i find strong reasons for regarding as probable the general theory, that the alternations of solar activity (not the solar activity itself be it noted) are excited from without. and since we know, as a matter of fact, that meteors exist in enormous numbers within the solar system, and that they aggregate with rapidly increasing density in the sun's neighbourhood, we must believe that they fall upon the sun in enormous numbers. we also perceive that the supply cannot be uniform, but must vary greatly from time to time; while what we know about the periodicity of meteoric showers on our own earth suggests the belief, we may almost say the certainty, that there must be periodic downfalls of very heavy meteoric showers upon the sun's surface. we have, then, strong probability in favour of the belief that events may occur which, if they occurred, might be expected, with a high degree of probability, to produce effects resembling those actually observed,—viz., the production of a heat more intense than usual, accompanied by signs of great disturbance like the sun-spots. it does, therefore, seem at least not improbable that these accessions of heat and these signs of great disturbance really are brought about in the way supposed.

a further argument in favour of the meteoric origin of solar alternations of heat is to be found in the fact that, on one occasion at least, a solar phenomenon, corresponding precisely to what we should expect to see, if great meteoric masses fell upon the sun, has been followed by precisely the same signs of terrestrial disturbance which accompany and follow the formation of great solar spots. i refer to the remarkable occurrence witnessed by carrington and hodgson (at different observatories) in september, 1859, when two intensely bright points of light were seen travelling beside each other at the rate of about 120 miles per second along a short arc of the sun's surface,—an arc only equal in length to some four-and-a-half times the diameter of our earth.

on that occasion the emission of solar heat may or may not have been increased in an appreciable degree for several minutes. my own belief is that it must have been; but we certainly have no means of proving that it was. what we do know certainly is, that on that day all the phenomena which usually accompany the existence of many and large sun-spots showed themselves with exaggerated intensity. the magnetic needle was greatly disturbed, auroras displayed their coloured streamers in both hemispheres, telegraphic communication was interrupted, and everything tended to show that a disturbance of the same general character as that which produces sun-spots, but much more active while it lasted, had affected the sun. it seems, then, altogether reasonable to infer that sun-spots are due to the same cause as the disturbance which then occurred. so that if we conclude, with most astronomers competent to form an opinion, that the disturbance witnessed by carrington and hodgson was due to the downfall of two very large meteoric masses upon the sun, it would follow that sun-spots are due to more wide-spread meteoric showers, not consisting of masses so large.

the reader will long since have guessed, no doubt, to what all this tends. if the periodical variations of the sun's surface are due to meteoric and cometic systems whose orbits intersect the sun's globe, their periods being short (that is, lasting but a few years), it may well be that more important meteoric and cometic systems intersecting the sun's globe exist, which have much longer periods. when next one of these makes its passage athwart the sun, far more important solar disturbances may take place than those which occur when the regularly recurring systems salute the sun. two or three times in the history of science comets have approached very close to the surface of the sun, as in 1680, and again in 1843, but without actually impinging upon it. very slight changes in the motions of those comets, owing to the disturbing influences of the planets, would cause their very nuclei to strike the sun, and their meteoric trains to pour afterwards in a full stream upon him for many days, or even for many months and years in succession.

now i do not think our sun would necessarily suffer very much from any of these known comets. they may long since have parted with the greater quantity of their substance. but it is quite possible that even one of those well-known comets of the solar system might cause very serious outbursts of solar heat and light; and it is certainly not only possible but extremely probable that other comets, such as have visited the solar system on paths fortunately not bringing them near to the sun, would have worked much mischief had their paths been differently situated.

we know that newton held this opinion. he considered the real danger from comets to reside, not in the possibility that one might strike our earth, but in the possibility that one, falling upon the sun, might excite that orb to a degree of heat so intense that all life on this earth would be destroyed. it is true that, in newton's time, physical laws were not so well understood as at present, and a considerable portion of newton's reasoning was consequently inexact. but nothing which is now known opposes itself to the belief which newton adopted on this subject. on the contrary, whereas newton only recognised the danger arising from the consumption of a comet as fuel for the sun, we now recognise a far more serious danger, from the force of meteoric impact, and the heat excited as the thermal equivalent of the destroyed velocities. of this part of the danger newton had no clear conception, the relations between mechanical energy and heat not having been established until quite recent times.

it appears to me, however, that the danger in the case of our own sun—or may we not say our danger?—arises only from the possibility that some one of the comets which visit us from the star-depths may make straight for the sun; and this danger is exceedingly small. almost certainly a comet which, leaving the domain of another sun, falls under the attractive influence of our own, would approach him on a path passing many millions of miles from his surface. the chances against a more direct approach are so great that they may be regarded as, to all intents and purposes, overwhelming. a comet might visit us from the star-depth on a destructive course, just as a single black ball might be drawn at the first trial from a bag containing a million white balls and only that single black one. but the danger is exceedingly small.

we see, indeed, that other suns have suffered in this way, assuming cometic downfall to be the true cause of stellar outbursts. there are so many millions of suns, however, in the region of space to which telescopic survey extends that the occurrence of ten or twelve such outbursts in the course of four or five centuries need not be regarded as implying any serious danger. moreover, all the suns which have thus suffered lie within a particular region of the heavens,—viz., in the milky way, and in that half of the milky way which is most irregular, one may almost say ragged, in structure. (with one exception—the star in the northern crown, which, nevertheless, lies on a faint outlying streamer of the milky way not discernible to ordinary vision.) if then our sun belongs to this region of space, the danger for him and for us is somewhat greater than my previous argument would indicate. for, in that case, we must compare the number of outbursts, not with the total number of stars within telescopic range, but with the number of those stars which lie within this particular region of space. on the other hand, if our sun does not lie within that region of space, the danger for him and for us is very much less; for instead of a certain small number of accidents among his fellow suns, there have been no such accidents, only accidents affecting other suns which must be differently classed.

the case may be compared to the estimation of the dangers, let us say, of travelling by ocean steamships on a particular route. if we take the total number of accidents, for instance, to steamships travelling between england and the united states, we should estimate the risk of the journey as very small, the number of passengers who have lost their lives being very small compared with the number who have made the journey. but even this small risk is diminished if we estimate the danger for a passenger by cunard steamships, simply because no passenger has yet lost his life through accident to one of these cunard vessels.

so in the case of our sun, the danger of an outburst such as has affected the stars in the northern crown and cygnus is small enough when we estimate it by comparing the number of such accidents with the total number of stars, but vanishes almost into nothingness when we note that no insulated star like our sun seems hitherto to have undergone one of these tremendous catastrophes.

but as regards the fate of worlds circling round suns which have suffered in this way, we can form but one opinion. beyond all doubt, if such worlds existed and were inhabited when their central orb blazed forth with many hundred times its former lustre, all life must have perished from their surface. we may believe, as many do, that no conditions are too unlike those we are familiar with on earth to render life impossible; that the creatures subsisting in a world exposed to the most fiery heat or to the most intense cold are adapted as perfectly to the conditions under which they subsist as we are to the circumstances of terrestrial life. but even adopting this view, though it seems to accord ill with what we know of our own earth,—where life ceases towards the polar and over large tracts of the equatorial regions,—we could not believe that creatures thus adapted to the conditions prevailing around them could endure an entire change of those conditions. with the accessions of heat in the stars in cygnus and the crown, such change must inevitably have taken place. therefore, as i think, we must regard the catastrophes affecting those remote suns as assuredly involving "the end of many worlds."

note.—what is stated in the latter portion of this chapter applies now only to the star in the northern crown; for the star in cygnus has not faded into a small star, but into a small nebula! for the further history of this star, the reader is referred to my forthcoming treatise entitled, "pleasant ways in science."