Popular Science Monthly/Volume 39/September 1891/Can We Always Count Upon the Sun?
|CAN WE ALWAYS COUNT UPON THE SUN?|
THE study of the origin and development of species may be pursued with reference to the starry hosts, for there are different species of suns as well as of animals. The wide-ranging eye of the astronomer perceives in the dazzling orb whose rising turns night into day and whose beams vivify the face of the earth, only a minor representative of a great order of radiating bodies peopling the profundities of space. But, besides placing the sun in the comparatively humble rank to which he belongs by virtue of his inferiority in magnitude to many of his brilliant comrades, we are able to distinguish his particular breed, so to speak. He is not of the same kidney with such a sun as the dazzling Sirius, while the diamond radiance of Rigel and the sparkling blue beams of Vega proclaim that those stars are in some important respects different and more splendid organisms than our sun.
While there can be no question that suns have a life-history, a beginning and an end marking the termini of a regular process of development, and that consequently the stars that we see differ in age, still it is not yet possible to say with absolute certainty at just what point in the scale of solar development our sun, or any other particular star, has arrived. According to some views, the sun is an older star than Sirius; according to others, it is younger. The question whether a sun is still approaching the climax of its radiative energy, or has passed that point and is descending the scale, is an important one as regards the ultimate destiny of inhabited worlds revolving around it. It is upon the revelations of the spectroscope, assisted by photography, that we must depend for the advance of our knowledge of the condition of the various orders of suns. Upon spectroscopic evidence the stars are ranged into four principal classes distinguished by the kind and extent of the absorption which their light undergoes in passing outward from their photospheres through their gaseous envelopes or atmospheres. Sirius stands as the most brilliant representative of the first class, sometimes called the white stars, in whose spectra the lines of hydrogen are very conspicuous while other lines are few and faint. Some of the stars of this class have a splendid blue tint in their light. More than half of all the stars whose spectra have been studied belong to the first class. Vogel thinks they are the youngest stars, and that their youthful fires represent the most intense development of solar heat. If they are the youngest, then, on account of their great number as compared with the other stellar classes, it is evident that the universe has not yet passed the high noon of life; in other words, that the starry system, taken as a whole, is still in its prime; if indeed it has as yet attained the summit of its development as a community of suns. But the correctness of Vogel's assumption that the stars of the first class are all younger than those of the other classes is seriously questioned, and, as Prof. Young says, "it is very far from certain that a red star is not just as likely to be younger than a white one as to be older."
The second class is represented by Capella and our own sun. These stars are evidently surrounded by a far more complicated envelope than is the case with the white stars. Metallic vapors suspended above their photospheres in great variety serve to absorb a large part of their radiation, so that the spectrum of their light as it comes to us presents an enormous number of black lines, showing that it has been sifted through the vapors of iron, calcium, platinum, and many other metals, each of which, existing at a lower temperature than the fiercely glowing surface beneath, has arrested from among the passing rays those peculiar to itself. Some of the hydrogen lines also exist in the spectra of stars of the second class, but they are no longer conspicuous above the others. The second-class stars often called the solar and sometimes the yellow stars, are far less numerous than the white stars, the proportion, so far as known, being about one to six.
In the third class we come to the red stars, the majority of the stars of that color, as well as most of the variable stars, falling within this category. In these stars the absorption of their vaporous envelopes is so pronounced that their spectra are darkened by bands as well as lines. Sometimes bright lines of hydrogen appear in the spectra of stars of this class, indicating that an envelope of that element surrounding them has blazed out with an intensity of heat exceeding that of the photosphere itself. Betelgeuse, the great orange-colored star in the shoulder of Orion, is a representative of the third class. The wonderful variable Mira, in Cetus, also belongs in the third class of stars.
The fourth class is small in number, and its members are inconspicuous in brightness and shine with a deep red light. Their spectra are also filled with bands of absorption which are peculiar in that they shade off gradually toward the blue end of the spectrum, while the bands in the third-class spectra shade off toward the red end. This peculiar spectrum appears to arise from a compound of carbon filling the atmosphere of the star. Variable stars also abound in the fourth class and bright lines are sometimes seen in their spectra. Even if we grant that the progress of stellar evolution is from the white through the yellow to the red stars, and so on to complete extinction, it does not appear possible to say with certainty that the stars of the fourth class are any closer to final extinguishment than those of the third. It would be a very beautiful thing if one variety of red stars could be recognized as representing a class younger than Sirius, while all other red stars were known to be older than the sun, but that can not be affirmed. So far as our present knowledge guides us, the most that we can assert is that red stars may be either the youngest or the oldest of suns, or some may be young and some old; but that, at any rate, they probably stand near one or the other end of the progression, since they are clearly inferior in efficiency of radiation to the other stellar varieties.
Now, as regards the existence of planets circling around the various classes of suns, we can only reason from analogy; and opinions upon the subject range all the way from Dr. Whewell's conclusion that the earth is probably the only inhabited world in the universe, to Dr. Chalmers's delightful picture of the starry heavens filled everywhere with intelligent beings worshiping their Creator. Suppose we examine the probable conditions prevailing around the stars of each of the four great classes. The white stars, like Sirius, possess an extraordinary potency of radiation. Their atmospheres are not strongly absorbent, and probably not extensive, and consequently nearly the full vigor of their beams is poured upon the satellites that surround them, if any such there be. According to recent estimates, Sirius, while shining with perhaps seventy times the light of our sun, is only between two and three times as massive, so that the intensity of its radiation is enormously greater than the sun's. Planets situated as close to Sirius as the earth and the other inner planets of our system are to the sun, would be unable to endure, so far as their life-bearing functions are concerned, the gush of heat and blaze of light poured upon them—unless, indeed, the organization of living beings there were entirely different from that prevailing here. "We should then expect such stars as Sirius, if they are the centers of planetary systems at all, to be surrounded by globes revolving at comparatively great distances and in long periods of time.
Coming to the second class, or solar stars, we find that the more extensive atmospheres which surround them, and absorb no inconsiderable portion of their rays, serve as a sort of protective curtain for their planets. There can hardly be a doubt that if the envelope of metallic vapors that incloses the photosphere of the sun were suddenly removed, life, at least in many of its more complex forms, would be banished from the earth, and perhaps be rendered impossible upon any planet nearer than Jupiter.
But it is the red stars and variable stars of the third and fourth classes that present the most unfavorable features from the planetary point of view. Probably no star belonging to these varieties is free from extensive and more or less spasmodic alterations in the amount and intensity of its radiation. Take such a star as Mira, for instance, alternately dying down almost to extinction and then blazing out with more than a thousand times its former brilliancy, these tremendous changes occupying, for a complete cycle, only eleven months! Is it possible to suppose that inhabited planets exist within the domain of an orb like that? When a sun is half smothered in absorbing vapors, and subjected to paroxysms such as those which are occasionally beheld when the atmosphere of a star appears to catch fire, as it were, and the lines of hydrogen and other elements flame bright like signals of conflagration, it can no longer be the center of a system of life-bearing worlds, no matter what its past history may have been in that respect.
It is apparent, from what we have just said, that progress by our sun in either direction toward the white stars or toward the red stars—would, in the end, prove exceedingly uncomfortable if not fatal to the inhabitants of the earth. By the subsidence of the vapors of metals that now stripe the solar spectrum with their absorption we should be, in effect, removed into the presence of a Sirius whose fierce beams would smite the living world with death. On the other hand, let the sun sink into the condition of a red star, and become variable in its outpourings, and our condition would be even worse. If it be thought that a planet whose orbit is as eccentric as that of Mercury is hardly habitable because it receives twice as much solar heat at perihelion as at aphelion, what is to be said of the condition of a planet subjected to the terrific mutations of Eta Argus, a star that in 1843 rivaled Sirius itself in brilliancy, and that since 1868 has been invisible to the naked eye, and has sunk as low as the eighth magnitude? Some of the comets undergo far less severe alternations than such a world must endure. In either direction, then, the prospect of solar evolution seems unfavorable, considered from the planetary standpoint. What the planet most wants is an unchanging and unchangeable sun. But this is impossible. In the presence of eternity a sun, whether it grows hot or grows cold, white or red, with age, is a thing as essentially evanescent as a zephyr.
But we can not rest with the assumption that, since the sun is evidently no Mira and no Sirius, therefore it is practically an unchanging radiator which for an indefinite period will continue to cause the earth to bloom in the beneficent effulgence of its life-inspiring rays. A sun may affect the welfare of its planets either through the gradual mutations which it undergoes in the course of its evolution, or through the more rapid and violent changes that characterize the stars that are ranked as variable. We have seen that most of these latter belong to the third and fourth classes, but there is reason to suspect that the majority of all the stars are variable to a slight degree, and evidence of variability in the case of the sun is furnished by the phenomena of sun-spots. A spectator, viewing the sun from a distant point in space, would perceive that its brilliancy was slightly increased once in about every eleven years. These accessions of light should correspond, not with the periods of fewest spots, but with those of most spots, because the energy of the sun's radiation is greatest during the spot maxima. At present a sun-spot maximum is approaching, and since last winter the face of the sun has frequently exhibited startling indications of the tremendous disturbances now affecting the solar globe. Our imaginary observer in space would probably behold at the present time a very slight increase in the sun's brilliancy, and this increase may go on for three or four years to come. While we, dwelling upon a globe that is bathed in the sun's rays, may be unable to perceive these variations directly, yet their effects have long been recognized by the changes that they produce in terrestrial magnetism. It is also highly probable that a perceptible influence upon the weather is exercised by variations in solar radiation corresponding with the presence or absence of sun-spots. So far as trustworthy observations have gone, it appears that the temperature upon the earth is slightly lower when sun-spots are most numerous. This is exactly the opposite to the effect that might have been anticipated; but as the observations from which the inference is derived are confined to India, it seems probable that the lowering of temperature, while primarily brought about by the condition of the sun, is directly due to the action of local causes, and that in other parts of the earth a simultaneous increase of heat may be experienced. A very great increase of solar radiation, however, could not be thus masked in its effects upon the earth.
Although during the historical period there has probably been no sufficient variation in the activity of the sun to produce very serious terrestrial results, yet it is known that the sun-spot cycle is subject to considerable variations, both as regards the length of the periods and the intensity of the forces concerned in the disturbance. The latest maximum of sun-spots in 1883-'84 was a couple of years overdue. What peculiarities may mark the maximum now approaching time alone can reveal. But, at any rate, the known irregularities of the sun suggest a striking resemblance to what we see in some of the variable stars; and it is highly probable that the changes of the latter, except in certain cases where other more satisfactory causes have been inferred, are due to phenomena resembling sun-spots, if not in fact directly analogous to them. Is sun-spottedness, then, a progressive condition; and will our sun in time become, through this cause, variable to the extent shown by many of its compeers in the heavens?
It is true that on account of the remoteness of any calamitous effects resulting from such gradual changes in the sun's condition we can afford to regard them with indifference, so far as the welfare of our race for many thousands of years is concerned; but when we rise to a higher point of view, and put aside merely human measures of time, the question becomes one of deep interest, since it involves the probable ultimate fate of our planet as the scene of the development and achievements of intelligent creatures. Will the earth become a desert like its companion the moon through the exhaustion of its vital forces and the disappearance of its air and water, while the sun yet shines upon it with unfailing splendor; or will the end of terrestrial life be brought about by the agency of the sun itself, either through the failure of the solar energies, or through an overwhelming outburst of them? These questions are not the less interesting, and not the less certain to obtrude themselves, because it is at present impossible for us to answer them. They have also a bearing upon the geological life record of the globe. Already, under the enormous demands for time made by the evolutionary doctrine, geology is asking for far longer periods of stability in the light and heat supplies of the sun than astronomy, also supporting itself upon the principles of evolution, is able to grant. But if the sun has emerged from the stage of a third or fourth type star, and by the gradual elimination of its obstructive envelope has arrived at that point of comparative regularity of radiation in which we behold it, the time during which it can have maintained the earth in a habitable condition is proportionally shortened, for we can not suppose that animal and vegetable life could be developed under the dominion of a distinctively variable star. The assumption is here made, of course, that a variable star is really a sun and not a cloud of meteorites in collision, or a partially condensed nebula, and that its planets, if it is ever to have any, have already been formed. Progress in the other direction—that is, from the white star toward the red star and variable star stage—would seem to supply longer periods of unvarying solar radiation for the evolution of planetary life, since a sun developing in that way would become a stable radiator sooner than if it had first to free itself from a sheathing of absorbing vapors created, it may be, by its own action at a certain stage of its career rather than left behind as a subsiding remnant of the original nebula.
We have remarked that, so far as the records of human history inform us, the emission of light and heat by the sun has never seriously varied. Yet it has been thought, though the evidence is not clear, that there are geological indications of considerable variations in the amount of solar radiation in past time, and the famous myth of Phaeton driving the chariot of the sun and getting so far out of his road that he endangered the earth and made it smoke with unwonted heat, has often been referred to as a possible tradition of some extraordinary outburst of solar heat within the period of man's existence. The variable character of sun-spot phenomena certainly does not contradict that supposition. The margin of existence is so narrow for many forms of life that no very great change would be required to cause a disaster. Still, notwithstanding the vagaries of sun-spots, and the apparent analogy between the sun and the variable stars, it would not do to assume that the earth is at present in any danger from a changing mood in its great governor and benefactor. So far as positive records serve as an indication of the future, there is every reason to believe that the sun will long continue in its present condition, and that astronomers a million years hence, if some cataclysm arising from ulterior causes does not intervene, may still be found studying the sun, having probably by that time ascertained whether it is getting hotter or colder. But it would not be safe to assume that any astronomers will be left upon the earth five million years hence.