# Popular Science Monthly/Volume 38/November 1890/The History of a Star

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* THE HISTORY OF A STAR.
By J. NORMAN LOCKYER.
IT is now exactly thirty years since the world rang with one of those discoveries which go down to the ages and at once insure the names of the makers of them being inscribed upon the muster-roll of the immortals. In the autumn of 1859, Kirchhoff and Bunsen announced that at last a way had been found of studying the chemical nature of bodies in space—nay, more, that they had already begun the work, and found that the sun, at all events, was built up of matter identical with that of which the earth is composed.
In physical science in most cases a new discovery means that by some new idea, new instrument, or some new and better use of an old one, Nature has been wooed in some new way. In this case it was a question of a new idea and an old instrument. The instrument was the spectroscope.
It forms no part of my present purpose to deal either with the principles involved in spectrum analysis or its history during the period which has elapsed since 1859. The task I have set myself in this article is a much more modest one.
First, I wish to point out that during the thirty years the method of work which Kirchhoff and Bunsen applied to the sun has been applied to the whole host of heaven. By this I do not mean that every star has been examined, but that many examples of each great class—nebula, comet, star, planet—have been studied. The same kind of information has been obtained with respect to these bodies as Kirchhoff and Bunsen gleaned with regard to the sun; and the great generalization to which I have referred has been found to hold good in the main for all. From nebulæ and stars existing in space in regions so remote that the observations have been of the utmost difficulty in consequence of the feebleness of their light; from comets careering through stretches of space almost at our doors, the same story has come of substances existing in them which are familiar to us here. In ascending thus from the particular to the general, from the sun to the most distant worlds, it is obvious that the field of observation has been enormously extended. Kirchhoff and Bunsen's view has been abundantly verified, as we have seen; but the question remains, Has this larger area of observation supplied us with facts which enable us to make a more general statement than theirs? It is possible that it has. Recent inquiry has suggested that if the study of meteorites be conjoined with that of the heavenly bodies, the story told by the spectroscope enables us to go a step further, ​and to say that not only have we the same matter everywhere, but all celestial bodies, including the earth, are due to an exquisitely simple evolution of matter in the form of meteoritic dust. We have no longer to rest content with the fact that all nature is one chemically: we have the cause.
Secondly, I propose to make as short and simple a statement as I can of the general idea of the new cosmogony suggested by the spectroscopic survey to which I have referred.
I must, in the first place, ask my readers to grant me the scientific use of their imagination; and in order that it may not be called upon to cope with questions as to whether space is infinite or not, or whether space and time ever had a beginning, we will not consider the possibility of the beginning of things or attempt to define the totality of space, but we will in imagination clear a certain part of space and then set certain possibilities at work.
How much space shall we clear? A very good idea of one of the units of space which is very convenient for me to employ here—mean the distance of the nearest star or one of the nearest stars—can be obtained by stating the time taken by light in performing the journey between the earth and the stars, knowing as we do that light travels one hundred and eighty-six thousand miles in a second, In the case of the nearest stars the time thus required is about three and a half years. With regard to the twelfth-magnitude stars, we find that in all probability the distance in their case is so great that light, instead of taking three and a half years, takes three thousand five hundred years to reach us.
The space included in a sphere with this radius will be sufficient for our purpose. The stars that we shall have to abolish for the purpose of this preliminary inquiry number something like six millions; the probability being that, if we consider the stars visible, not in the largest telescopes, but in those which are now considered of moderate dimensions, their numbers may be reckoned at something between thirty and fifty millions.
Imagine, then, this part of space cleared of all matter. We shall have a dark void, and the probability is that all that dark void will sooner or later, in consequence of conditions existing in other parts of space into which we have not inquired, be filled with some form of matter so fine that it is impossible to give it a chemical name.
Next we may imagine that this something without a chemical name may curdle into something which is more allied with our terrestrial chemistry, and the chances are, so far as we know, that that first substance will be either hydrogen itself or some substance seen in the spectrum of hydrogen or closely associated spectra.
It is just possible that at this point we enter the region of ​observation. In the nebulæ we are brought face to face with a substance (or substances) which, as far as our observations go, exists nowhere else except in the very hottest region of the sun that we can get at with our instruments. It is unknown here, and all attempts to match the spectrum by exposing terrestrial substances to the highest temperatures available in our laboratories have so far been unavailing. Both in sun and nebulæ this substance (or substances) is associated with hydrogen. This curdling process will go on until at length further condensation will take place, and instead of having simply the substance (or substances) to which I have referred, and hydrogen, we shall have an excess of hydrogen with an infinitely fine dust interspersed in it, which will go on condensing and condensing until at last we get dust of substances the existence of which is revealed to us in the spectra of bodies known to terrestrial chemistry; among these are magnesium, carbon, oxygen, iron, silicon, and sulphur.
This dust, fortunately for those interested in such inquiries as this, comes down to us in more condensed forms still, and it is in consequence of the messages which they bring from the heavens that I am engaged in writing this article. Not only have we dust falling, but large masses; magnificent specimens of meteorites which have fallen from the heavens at different times, some of them weighing tons, are open to our inquiries. Although, therefore, it is very difficult for us to collect the dust, it is perfectly easy to produce it by pulverizing any specimens of these meteorites that we choose into the finest powder. If we examine this dust spectroscopically, we find that, in addition to hydrogen, its chief constituents are magnesium, iron, carbon, silicon, oxygen, and sulphur.
I have, therefore, in this first sketch of a possible result of a process going on in our space-clearing at an early stage, not arrived at something that is unreal and merely the creation of the imagination, but something very definite indeed, which we can analyze and work with in our laboratories.
How it comes that this infinitely fine dust, finer probably than anything we can imagine, becomes at last, in the celestial spaces, agglomerated into meteoric irons and stones with which the earth is being continually bombarded, is one of the most interesting questions in the domain of science. Space is no niggard of this dust, for if we deal with agglomerations of it sufficient in quantity to give rise to the appearance of a "falling star" to the unaided eye, we know that the number of such masses which fall upon the earth every day exceeds twenty millions.
We have, then, the idea before us that, here and there in-this space that we have cleared, we have initial curdling, as I have called it; we need not assume that these curdlings are uniform.
​It is impossible with our present knowledge to suppose that at any prior stage of the history of the heavens gravitation did not exist. It is impossible, from what we know now, to suppose that even the finest form of matter which entered our clearing in space was not endowed with motion. Given this matter, its motion and gravitation, let us next see what must very quickly follow.
Gravitation will give us a formation of centers; we shall get a rotation (moment of momentum) due to the prior existence of motion and to this formation of centers; we shall eventually in that way get condensing masses of this curdled substance.
The moment we have these centers formed, gravitation again will give us the motion of exterior particles toward these centers, and the condensation in one part of space will necessarily be counterbalanced by a clearing in another, so that, if we suppose that the curdling was not uniform to begin with, the uniformity will be less and less as time and this action go on.
Let us imagine that here and there we have isolated eddies, and here and there in the larger aggregations of the dust—in the most enormous swarms we can imagine—we have also eddies; these eddies involved in the larger curdlings will be associated with the phenomena of the general system of which they form an insignificant part. These cosmical molecules aggregating in this way will be, to compare great things with small, like the invisible molecules of a gas. It is not too much to say, as Prof. George Darwin has recently shown, that we shall have in effect the whole mechanism of the kinetic theory of gases before us; but, instead of dealing with invisible gaseous particles, we shall have particles, large or small, of meteoric dust. The kinetic theory tells us that if we have encounters we must have a production of heat; if we have production of heat we must have the production of radiation, although, if the heat be insufficient, the radiation may not produce light enough to be visible to the human eye.
It is a remarkable thought that all these changes to which I have so far drawn attention may have been going on in different parts of space for æons without any visible trace of the action being possible to any kind of visual organs. I refer to this because it is right that I should point out here that Halley, who was one of the first to discuss the possible luminosity of sparse masses of matter in space, and Maupertuis, who followed him, both laid great stress upon it. When, then, these encounters, which we may call collisions, take place, and when the heat due ​to the arrested motion of the particles coming together, and the accompanying light are produced, we must expect that that light will at first be very dim, and will require very considerable optical power to render it visible.
We may now consider some early results obtained in connection with this matter. Sir William Herschel, although not the first to examine into it, was the first to bring before us an idea of the magnificent spectacle which the heavens present to mankind, and he, without any difficulty, with his large instruments, began by dividing these dim bodies into nebulosities and nebulæ; the nebulosities extending over large spaces of the heavens, and being of very, very feeble luminosity.
When we pass from these we become acquainted with bodies which may be truly termed nebulæ, as opposed to nebulosities, and the most magnificent of these is that in Orion, which has recently been so grandly photographed by Mr. Common and Mr. Roberts, the latter using the intensifying action of four hours' exposure of the photographic plate, hereby revealing details that no human eye will ever see, thus demonstrating how true it is that these changes may go on for æons and æons, though the eye may never become acquainted with them.
There is a magnificent arrangement in the human eye which, though it invalidates it for some astronomical purposes, is convenient, because it enables us to go on using our eyes all our lives, whereas a prepared photographic plate can only be used once. By this arrangement, however long we look at an object, it does not appear brighter, but in the case of the photographic plate all the action upon it is totaled, so to speak, so that if the plate be exposed, say for two hours or sixty hours, we shall go on getting impressed upon it more and more of the unseen. Thus the nebula of Orion, as seen, is almost insignificant compared with the glorious object which the photographic plate portrays if the integrating power be allowed to go on for hours.
It seemed pretty obvious, since the light of such bodies is so dim that a large portion of it beats upon the earth and upon our eyes without having any effect upon either, that the temperature was low; and it seemed also that to test the idea that this luminosity might be produced, as I have suggested, by collisions of meteoric dust, the way was open for laboratory work.
​Smash, a meteorite, collect the dust, expose it to a low temperature; compare its spectrum with the spectrum of such, a body as those we have been considering, and see by actual experiment if there is any similarity. This was done.
The result was almost identical. It seemed, therefore, that one had at last got to solid ground, and could go ahead. But how to go ahead in a scientific way? Naturally by developing the argument which had led us so far. Let us agree that the nebulæ are condensations of meteoritic dust, and see whether we are led to the true or the false by such a concession. Let us further grant that the condensations go on. What will happen next?
In certain regions of space the encounters—the collisions—will increase in number in consequence of the accumulation of meteoric dust in these regions; the temperature will, therefore, be higher and the light more intense.
Is there only one process by which, the temperature can be increased? It did not take very long to recognize that there might possibly be three lines of action, each one of which would result in the production of a higher temperature. In the first place, moment of momentum—rotation—being at our disposal to start with, it was obvious, in virtue of mechanical laws, that as the condensation went on the rotation would be accelerated; the motions of the particles of dust in the reaction, so to speak, would be more violent; the collisions, therefore, would produce more smashes, and more heat, and therefore more light.
We should get a central system and surroundings, such as Mr. Roberts has recently photographed in the great nebula of Andromeda. The exposure he gave was four hours, and again this photograph brings us face to face with phenomena which will probably never be seen by the eye alone.
A central condensation, here and there fragments of spirals, and here and there dark gaps, are seen. These gaps were observed by Bond and others years ago, but it remained for Mr. Roberts to demonstrate to us that they are produced by the wonderful indraught action which we can now, by means of the photograph, see going on. We have a concentration toward the center, the dark gaps representing to us either the absence of matter or the presence of meteoritic dust in a region where it is all going the same way, and in which, therefore, there are no collisions. Here and there we get regions of great luminosity, and associated with the spirals we get obvious loci of encounters. External swarms are also seen which have been thought, with great probability, to belong to the system—smaller condensations partaking in the general motion of the whole. Here, then, we are in presence of one possible cause of increased temperature.
There is another. One of the early results obtained by Sir ​William Herschel was, that it was a very common thing for double nebulæ to make their appearance in his gigantic telescope. Now, it is difficult for us to imagine that these double nebulæ, like their allied systems of stars, should not be in motion; and if we imagine a condition of things in which one swarm is going around a larger one in an elliptic orbit, and occasionally approaching it and mingling with it, we shall have at one part of the orbit the centers nearest together; so that a greater number of particles of meteoritic dust will be liable to encounters at this time than at others. Hence we shall get a cause of increased temperature of a periodic kind; there must be variable stars in the heavens—and there are.
As a third possible condition we have the known movement of these swarms of dust through space. If we take note of the known movements of the star which forms the center of our own system, we can learn that these movements may be gigantic. We know that the sun is traveling nearly half a million of miles every twenty-four hours toward a certain region; we know that other stars are moving so quickly that Sir Robert Ball has calculated that one among them would travel from London to Pekin in something like two minutes. We have, therefore, any amount of velocity. Now suppose that without the formation of either a single or a double system, such as we have considered—by the ordinary condensation of an initial single or initial double swarm—we have what we may call a "level crossing" at which two or more streams of meteoritic dust meet. There, of course, we shall have a tremendous cause of collisions. Have we such instances in the heavens? Again I appeal to Mr. Roberts's photographs of the Pleiades; we see in them four nebulæ which have been stated to surround four of the stars. But if we look at the nebulæ more carefully, we find that distinct stream-lines are seen in each in certain directions; we have interlacing, the meeting of these streams at some angle or other, and in each such region we have the locus of one of the chief stars.
This may be considered to be an irregular cause of a production of high temperature; but so long as such an action as that continues, an apparent star will be seen, distinct, of constant light, and not to be discriminated, without such photographs as these, from those stars which have been produced by more ordinary sequences connected with the more ordinary processes of condensation.
If, however, the above explanation be the true one, we should expect to find cases in which we may see such an action beginning or ending suddenly; the action will be less constant and durable—that is to say, the supply of these streams of meteoritic dust may not be continuous; it may be smaller, and then the effect ​will be produced during a much shorter period of time. In that case the light of the star will not last long. If the onrush of one stream upon another or a more regular swarm is sudden, we shall have a sudden blaze out of light; if the onrushing stream is short, the light will soon die; if it continues for some time, and reduces its quantity, the light will die out gradually. Or again, such a source of supply may fail by the complete passage of one stream through the other. In these ways we shall have various bodies in the heavens, suddenly or gradually increasing or decreasing their light quite irregularly, unlike those other bodies where we get a periodical variation in consequence of the revolution of one round the other. We shall have "new stars" appearing from time to time in the heavens, and they do.
Unfortunately, no photographs of these bodies to which I refer have been taken. Observations have been recorded, however, of their changing light. The changes can be easily explained upon this hypothesis, but, so far as I know, can not be explained upon any other.
In one case we had a known star (in Corona) suddenly blazing out from the ninth magnitude to the second, and almost as suddenly going down again. In another star (Nova Cygni) we had an outburst in a region which observation showed to be without a star, although I do not know whether any special observation of that region had been made for the existence of nebulæ. Suddenly in that part of the heavens a third-magnitude star blazed out; this took a very considerable time to die down, as compared to the first star, in Corona, and ultimately it got down to the tenth magnitude, and now telescopically it appears as a nebula.
As in condensing these swarms get hotter, they will get brighter as their volume decreases, and we shall pass from what we term nebulæ to what we term stars. It can not be too strongly insisted upon that chief among the new ideas introduced by the recent work is that a great many stars are not stars like the sun, but simply collections of meteorites, the particles of which may be probably thirty, forty, or fifty miles apart. Such eddies and systems, which are not simple, will vary in brightness. In the case of double nebulæ condensing we shall get, as I have already stated, a periodic variation in light; and here we have a simple explanation of the facts observed, and hitherto held to be mysterious, in a large number of variable stars. The "new" stars I have already referred to are also easily accounted for on the hypothesis of meteoric streams.
It may be asked, Why, considering the millions of bodies in motion capable by this hypothesis of producing them, are not "new stars" seen more frequently? The reply is simple: We, as a rule, deal with the clashing of small streams; the temperature does ​not generally exceed that of a comet, probably; and hence the action takes place invisibly to us. Photographic surveys of the heavens often repeated will doubtless give us more numerous records.
We now return to the regularly condensing swarms. In these the condensation will go on, and the temperature will rise until the loss by radiation equals the increase of temperature due to the fall of meteorites upon the continually condensing center. If we imagine a star to be condensed more and more by the fall of meteoritic material upon it, we shall arrive at a time in which, provided that the supply of material ceases, the increase of temperature of the star from that reason will also cease, and then will arise a condition of things in which the heat radiated from the star will be greater than the heat produced in the body of gas which is ultimately formed in consequence of the tremendous temperature caused by the continual fall of meteoritic matter toward the center.
If it be true that in the nebulæ we begin with meteoritic dust-particles far separate from each other, we must gradually get an increase of temperature so long as they approach nearer the center of the swarm by condensation; and so long as the heat produced by bombardment is in excess of the loss by radiation, the temperature will increase; but when the loss by radiation exceeds the gain by the bombardment we must get a reduction of temperature. A temperature curve like one of the arches of Westminster Bridge flattened at the top will illustrate this idea. We have on the left-hand arm of the curve those bodies in which we get a rise of temperature due to collisions and to condensation; along the top of the curve we have the gradual formation of a globe of gas; the gas begins to cool and gradually condenses, until at the lower end of the right-hand arm of the curve, as a result of the total action, we get the formation of a body like the earth.
Such a temperature curve has been provisionally divided into seven parts, and what has been done so far is to show that there are seven well-defined groups of bodies in space, which may be located, three on the rising part of the curve, one at the top, and three on the descending part; representatives of each of these groups have been classified and their spectra have been carefully studied. There is absolutely no difficulty whatever about placing all the celestial bodies which have been so observed by means of the spectroscope in one group or the other; and further, where the spectroscopic evidence is complete, there is again no difficulty in dividing these groups into species, just in the same way that the biologist deals with organic forms. This has already been done for one group, and in a very few years it will no doubt be done for more, so that here again we are definitely in the region of hard, detailed facts.
​There are two or three points to consider with regard to the history of a system, so long as it is on the rising part of the curve. If we begin with globular condensations, such as those first described by Sir William Herschel, we shall get, soon after the initial stage, spiral and irregular intakes, and then these may in time give place to rings such as we are already familiar with in a member of our own system; I refer to the rings of Saturn. Other dust-swarms near which such a system passes will be attracted to it, and in addition to the initial revolving swarm and its intakes and rings, we shall have a new order of things introduced which we may term comets.
Now the whole history of cometic astronomy goes to show that no comet can enter such a system as ours without feeling the influence of the central system in a very remarkable way. We know from other considerations that the nucleus of such a body is simply a swarm of meteoritic dust-particles, large or small.
The tail is always produced in a direction opposite to that of the sun, and by some electrical energy, thermal energy, or what not; the result being that something is driven from the swarm of meteorites in a direction away from the sun. Further, the stuff, whatever it may be, thus repelled, is brought by the comet from outer space; for some of the short-period comets, those that never leave our system, after they have passed round the sun a few times, throw out no tail at all.
If this can be universally proved for all comets, this is what must happen: each central body will, by means of this energy, place, as it were, a cordon round itself, inside of which no such matter can remain as is thus driven off from comets and produces the phenomena of a tail; and if it be ever possible to state the chemical nature of a comet's tail, the particular substances repelled by this central energy will be known. It looks as if the tails may consist, to a large extent, of the gases which exist in meteorites, and which can be driven out of them at not very high temperatures. Seeing that these are thrown off with great velocity and shine through millions of miles in the depths of space, it is not likely that we are dealing with any such condensable substances as the vapors of iron, magnesium, or any other metal. This consideration may help us eventually in the chemistry of the repelling body.
These revolving dust-swarms, as they increase their temperature, will go through the same temperature changes as other non-revolving ones. The existence of comets drawn into our system from without, composed, like the nebulæ, of meteoritic dust, enables us to subject the view we are now considering to a very crucial test.
We know that the temperature of comets is increased, chiefly, ​it has been supposed, by tidal action, as they approach the sun; because such an action must make a considerable difference in the movements of the particles of the swarm nearer the sun, as compared to those farther away from it; we know, in any case, by their increased light, that the temperature of comets does increase considerably as the sun is approached. It has been shown that many of the phenomena presented by comets, which are acknowledged to be clouds of meteoritic particles in the solar system, are identical with those presented by nebulæ and stars in space; hence the hypothesis now under consideration, which affirms the nebulæ to be also clouds of meteoritic dust, is greatly strengthened. Indeed, if the facts had not been found to be as I have stated them, the hypothesis would have been worth nothing.
I should here add that the recent work has shown how right Schiaparelli was, when, in 1866, he stated that comets were nebulous masses drawn into the solar system.
The top of what we agreed to call the temperature curve may now be considered. We have dealt with the ascending arm of it, and referred to the groups I, II, and III. In these groups there was evidence to show that, under normal conditions, we were dealing with orders of celestial bodies in which the temperature was gradually increasing, in consequence of the continual nearing of the constituent meteorites in the swarm due to collisions and gravitation.
It may be convenient that I should very briefly give, even at the risk of being charged with repetition, a normal case carrying us up to the top of the curve. For that purpose we may content ourselves by considering those globular and elliptic nebulæ first recorded by Sir William Herschel in the last century. In these there is evidence of different stages of condensation; in one series first of all something which is hardly visible is noted, and the end of that series consists of a dim, diffused, globular mass. In another we pass from the minimum gradually into another form of condensation, in which the luminosity increases toward the center. In still another series the condensation toward the center goes as it were by jumps, so that finally what appears to be a nebulous star with a surrounding of very nearly equal density is seen. Passing from these forms we come to elliptic nebulæ, which doubtless indicate a further condensation of those forms which, in the first instance, are globular. We have already become familiar with a representative of these elliptic nebulæ in that of Andromeda, as it has been revealed to us by the magnificent photograph taken by Mr. Roberts. In connection with such an elliptic figure we often get clear indications of spirals.
A further condensation then will no doubt land us among stars having a peculiar and special spectrum; indeed, though they ​appear as stars in our telescopes, their spectrum closely resembles that of the nebula. Going still further—still increasing the condensation, still increasing the temperature—the region of stars properly so called is reached, until at last we find those which are represented at the top of the curve. These results have been arrived at by spectroscopic work, and the facts recorded have been the chemical changes which take place in these swarms as their temperature increases, from the most sparse condition at the bottom of the curve to the most condensed one at the top.
In the sparsest swarms, in the so-called nebulæ, and those which are so dim as to be with difficulty visible, indications are found of the so far unknown substance or substances to which I have referred at the beginning of this article, together with carbon and hydrogen, and, in all probability, magnesium, one of the most common metals in meteorites, which has a bright spectrum visible at a low temperature; though I should add that the visible presence of magnesium has recently been contested. Its visible presence or absence, however, is not of fundamental importance. As the temperature increases, we find carbon more abundant, and traces of manganese and lead, metals which volatilize at a low temperature.
The next greatest change that supervenes is the addition of more familiar indications of the metals magnesium, manganese, and sodium, while the spaces between the meteorites glow more intensely with the light of hydrogen and carbon, probably brought about by some electrical action. Here the sparseness is still so great that we have little to do with the absorption of light; we simply deal with incandescent vapors due to the high temperature brought about by collisions among the meteorites and to the glow of the gases between the meteorites. But although the particles of meteoritic dust are so far apart that there is no possibility of any obvious absorption of their light occurring at this stage, to any large extent, the story is soon changed, for, when real condensation begins, the light of the meteoritic dust itself is absorbed by the vapors produced at low temperatures which lie between each particle of dust and our eyes. The whole theory of absorption is dependent upon the fact that light must come from the light-source through a vapor which is cooler than the light-source itself.
Thus we get a clear indication that, when this stage is reached, the meteoritic dust is very much closer together, and is on this account capable of forming a background enabling us to see these light-absorption phenomena. Absorption of light by the vapors of substances known to exist in meteorites, such as manganese and lead, is the first to occur, and these absorption phenomena gradually preponderate, and indicate change from low to high ​temperature, till finally the main absorption of light is caused by hydrogen and iron. Toward the top of the curve we get hydrogen enormously developed. It seems that we deal with a greater and greater quantity of hydrogen as the temperature gets higher.
Side by side with this sequence in the case of stars, a similar one up to a certain point is noted in the comets. As a rule the temperature of comets is, as we should expect, very much below that reached by stars. There is, therefore, no overwhelming indication of light-absorption, and it is only in those which closely approach the sun that any indication of the absorption of light caused by the presence of iron vapor is to be seen. A comparison of the spectra observed gives a clear indication that the nature of comets and nebulæ, so far as the spectroscope can seize them, is very similar: the phenomena present themselves in the same order; a line common to both begins the story, and then bright carbon is found among the first substances indicated, and afterward absorption phenomena, produced by manganese and lead chiefly, it is supposed, are superadded.
After this cometary parenthesis I now return to consider the top of the temperature curve. I repeat that we have this sort of condition. The swarms, whether single or multiple in origin, have by collisions and gravity brought about the highest point of temperature which they can reach in consequence of these actions. Swarms of separate meteorites now give place to a globular mass of gas produced by their volatilization. It may be that this very high temperature may be produced, and this enormous globular mass of gas formed, long before all the meteorites and meteoritic dust in the parent swarm, or in that particular region of space, shall be absolutely condensed to the center; so that we see it is quite possible that this high temperature condition may last for a very long time. Hence the curve should be flat-topped—in all probability very flat—for, so far as the spectrum analysis of stars has gone at present, more than half of those which have been examined give us evidence of extremely high temperature. However that may be, it is easily to be understood that such a mass as that we are considering must be radiating with tremendous energy; for a time probably the heat which it receives by the collisions and condensation of the outer members of the parent swarm may be as great as the heat which it radiates, and under these conditions the average temperature of the gas will remain constant; but the moment the input is less than the output the mass of gas must cool, so that we have next to consider what will happen to a mass of gas cooling under these circumstances.
What will cool first? The outside. We know pretty well the chemical nature of the outside of the mass of gas we are dealing ​with; we are practically dealing with a cooling globe of which the exterior absorbing layers consist of hydrogen, iron, magnesium, and sodium. And now perhaps it will be obvious why I was anxious in this general statement to begin as near as I could at the beginning of things. It is only by going back in that way that it is possible to explain this enormous development of hydrogen in the hottest stars. We saw that first one or perhaps two unknown substances—together with hydrogen, carbon, magnesium, manganese, lead, and iron—wrote their record in the spectrum, and that finally hydrogen was present in excess in the hottest stars. By the phenomena of comets it has been demonstrated that the radiant energy of our sun, and therefore the radiant energy of all other masses of equal temperature to our sun, drives, in all probability, everything of the nature of a permanent gas, like hydrogen or carbon compounds, away from the center of the system. Thus we may possibly explain the absence of oxygen and carbon from the sun; but hydrogen is present. The unknown substance or substances are concerned in most of the actions which take place in the hottest parts of the sun, and they are always associated with hydrogen. In the atmospheres of the hottest stars, again, hydrogen is enormously developed. Now that hydrogen, we have reason to believe, can not have passed the cordon to which I referred. The only supposition is that it and the unknown substances have as such been produced by the dissociation of the chemical elements of which the meteoritic particles which have formed the star in the manner I have indicated are composed. Here, then, we have a series of facts which add very great probability to the idea which has been arrived at on other grounds, that the chemical elements themselves are forms of hydrogen, or have a common origin.
On the right-hand part of the temperature curve the hottest state of things is represented at the top and the coolest at the bottom, and we pass through groups IV, V, and VI. As the temperature runs down, the hydrogen gradually disappears; as this happens in a mass of gas, the temperature of which is gradually but constantly reduced, we can only suppose that it is used to form something else. We get association due to reduced temperature in the same way that we get dissociation due to increasing temperature. The sun is a star just about half-way down the descending side of the curve; we know on other grounds that the sun is cooling.
The next part of the story is this: with decreasing hydrogen we get gradually associated an increasing quantity of the metallic elements (group V), and subsequently of carbon; but now the carbon vapors are absorbing, they are not radiating—in other words, the spectrum includes dark bands instead of bright ones, as they ​were on the other side of the curve. The light of the star is gradually blotted out by an enormous quantity of carbon compounds in some form or other, till at last the star gets blood-red (group VI), and finally is lost to human ken. The solar atmosphere at present contains chiefly iron, calcium, and other similar metals, but the hydrogen is disappearing, and there is possibly the slightest trace of carbon, but that trace is so small as to be somewhat doubtful. The composition of the sun's atmosphere at present is, moreover, almost identical with that of a mixture of meteorites driven into vapor by a strong electric current, and, if we except hydrogen, there is scarcely a line of any importance in the spectrum of the one which is not represented in the spectrum of the other. Calcium, aluminium, iron, manganese, and certain lines of nickel and other substances, are present. By means of such experiments as this, the wonderfully close connection between the gases at present existing in the atmosphere of the sun and the gases obtained from the volatilization of meteorites is put before us in the clearest and most convincing manner.
With regard to the fact that carbon comes in and takes the place of highest importance in the atmospheres of these cooling bodies, it is worth while to remark that if, as seems possible, these permanent gaseous compounds of carbon with different substances like oxygen, nitrogen, and hydrogen, and probably hydrogen itself, are kept away from the swarm during its condensation by that form of radiant energy of the center which is evidenced in the case of the sun by its tail-producing action on comets, it is easy to imagine that when that radiant energy is reduced, the carbon compounds will gradually approach the central body, until at length the flickering energy is no longer able to keep these permanent gases away, and then the surroundings of the central body are invaded by these gases in such tremendous quantity that an absorption is produced which first turns the cooler star blood-red, and finally blots it out.
There are several very interesting questions connected with this. Suppose, for instance, that we attempt to discuss the future of that magnificent nebula in Andromeda, the true structure of which Mr. Roberts has recently revealed to us. It is already suspected that the two subsidiary swarms partake of the motion and form a part of the system. Those smaller swarms will naturally condense before the larger ones. Let us imagine ourselves no longer dealing with anything so far away, but with the solar system when it was in that stage. The central sun having this cordon round it can only be formed of those substances which are not repelled by its radiant energy; it will, therefore, be chiefly a mass of metallic vapor. The masses near it for the same reason will be also chiefly of metallic vapors, and their density will be high; ​those farther away will be less metallic. Bit by bit, in the case of the interior bodies, we shall have these permanent gases coming back again, and more carbon will be added to their superficial layers; those bodies also must condense before the central one.
If we consider the conditions of the outer condensations, they must be particularly rich in permanent gases. We shall, therefore, get in the case the outer bodies excessively small density, and probably associated with that only the very sparse presence of these metals which have been alone allowed to penetrate toward the center, because their vapors can condense.
Our sun must ultimately go through the stage in which its absorption will be due no longer to hydrogen, or to iron, but to carbon, chiefly by virtue of the process which has been referred to; and eventually, as its radiant energy gets less and less, as it gets cooler and dimmer, the last speck of blood-red sunlight will be put out by an excess of carbon vapors in its atmosphere.
That is what must have happened to our own earth. It is a very interesting question indeed to attempt to determine at what period of the sun's history a solid crust was formed on the planet on which we dwell. It looks very much as if the consolidation of the earth may have preceded the highest point of temperature of the sun—that is to say, that the earth may have reached a condition closely resembling its present one at the time the sun occupied the apex of the temperature curve to which reference has been made.
In any case the high density of the earth, compared with the density of its crust (the enormous quantity of silicon and oxygen and carbon near the crust having an entirely different specific gravity from the specific gravity of the earth taken as a whole), seems to follow as a matter of course from these considerations.
I trust it will be seen that the hypothesis we have been considering supplies us with an orderly progression of meteoritic dust through heat conditions produced by collisions till finally a cool mass is produced; that this orderly progression brings about all the known phenomena of the heavens on its way, and simply and sufficiently explains them. But, though much of the mystery is gone, all the majesty is left—indeed, to my mind it is vastly increased. It seems as if the working out of the meteoritic idea will entirely justify Kant's conviction that the physical side of the science of the universe would in the future reach the same degree of perfection to which Newton had in his time brought the mathematical side.—Nineteenth Century.