In the High Heavens/Chapter 8

by Robert Stawell Ball

CHAPTER VIII.

THE NEW ASTRONOMY.

 

STRONOMERS are at present endeavouring to become fully acquainted with the resources of a new tool which has recently been placed in their hands. Perhaps it would be rather more correct to say that the tool is not exactly novel in principle, but that it is rather the development of its capabilities and its application in new directions which forms the departure now creating so much interest. We have already learned much by its aid, while the expectation of further discoveries is so well founded that it is doubtful whether at any time since the invention of the telescope the prospects of the practical astronomer have seemed so bright as they are at this moment.

In the earlier periods of astronomical research it was the movements of the heavenly bodies which specially claimed attention, and it was with reference to these movements that the great classical achievements of the science have been made. But within the last two or three decades the most striking discoveries in observational astronomy have been chiefly, though by no means exclusively, concerned with the physical constitution of the heavenly bodies. It is the application of the spectroscope by the labours of Sir W. Huggins and others that has disclosed to some extent the material elements present in the stars, as well as in comets and the distant nebulae. Now, however, it seems as if the spectroscope were for the future to be utilised not merely for that chemical examination of objects which is in the scope of no other method, but also as a means of advancing in a particular way our knowledge of the movements of the heavenly bodies. The results already obtained are of a striking and interesting description, and it is to their exposition and development that this article is devoted.

In the first place, it will be observed that the application of the spectroscope which we are now considering is not merely to be regarded as an improvement superseding the older methods of determining the movements of stars. It is, indeed, not a little remarkable that the type of information yielded by the spectroscope is wholly distinct from that which the earlier processes were adapted to give. The new method of observing movements, and that which, for convenience, we may speak of as the telescopic method, are not, in fact, competitive contrivances for obtaining the same results. They are rather to be regarded as complementary, each being just adapted to render the kind of information that the other is incompetent to afford.

It is well known that the ordinary expression, fixed star, is a misnomer, for almost every star which has been observed long enough is seen to be in motion. Indeed, it is not at all likely—nay, it is infinitely improbable, that such an object as a really fixed star actually exists. When the place of a star has been accurately determined by measurements made with the meridian circle, and when, after the lapse of a number of years the place of the same star is again determined by observation, it not unfrequently happens that the two places disagree. The explanation is, of course, that the star has moved in the interval. Thus the constellations are becoming gradually transformed by the movements of the several stars which form them. It is true that the movements are so slow that even in thousands of years the changes do not amount to much when regarded as a disturbance of the configuration. Thus, to take an example, we know the movements of the stars forming the Great Bear sufficiently well to be able to sketch the position of the stars as they were ten thousands years ago, or as they will be in ten thousand years to come, and though, no doubt, some distortion from the present lineaments of the Great Bear is shown in each of these pictures, yet the identity of the group is in each case well preserved. In Fig. 27 we have shown the amount of distortion in this constellation which would be produced in the course of 36,000 years.

It is, however, obvious that if a star should happen to be darting directly towards the observer or directly from him, the telescopic method of determining its movement becomes wholly inapplicable. No change in its position could be noticed. It is, no doubt, conceivable that if the distance of a star from the earth were determined, and if the investigation were repeated after a sufficient lapse of time, then the differences between the two distances would give an indication of the star's movement along the line of sight during the interval. But we may say at once that such a method of research is wholly impracticable. Our knowledge of the star-distances is far too imperfect for the successful application of this method. Nor is there the slightest prospect of any improvements in practical astronomy which would enable us to detect movements of stars in the line of sight in the way suggested. Certainly it offers no hope of a method which could compare for a moment in simplicity or precision with the beautiful spectroscopic process. Of course if a star were moving in the line of sight, there must be a certain change in its apparent lustre corresponding to the changes in its distance, and it might be supposed that by careful measurements of the brightness of a star conducted from time to time, conclusions could be drawn as to the speed with which it was moving.

Fig. 26.-The Great Bear as it is.

But the application of such a process is beyond the sphere of available methods. It would take at least a thousand years before even the most rapidly-moving star would experience a change that would sensibly affect its lustre; and even if we had the means of measuring with precision the light emitted, our results would still be affected by the possible fluctuations in the star's intrinsic brightness.

It is thus manifest that the resources of the older astronomy were quite incapable of meeting the demands of astronomers when it became necessary to learn the movements of the stars towards us or from us, as well as the movements perpendicular to the line of vision, which had always been the subject of much investigation. It is just here that the spectroscope comes in to fill the vacant place in the equipment of the astronomer. It tells exactly what the older methods were unable to tell, and it does so with a certainty that suggests vast possibilities for the spectroscopic process in the future. The principle of the method is a beautiful illustration of the extent to which the different branches of physical science are interwoven. But the principle has been a familiar one to astronomers for many years. It is the facility and success attending its recent application that has now aroused so much interest. Once it became certain that the undulatory theory of light expressed a great truth of nature, a certain deduction from that truth became almost obvious. It was, however, by no means certain that the practical application of this deduction to astronomical research would be feasible. That it has proved to be so in any degree is somewhat of a surprise, while it now appears susceptible of developments to an extent that could hardly have been dreamed of.

The logic of the new method is simple enough. Our eyes are so constituted that when a certain number of ethereal vibrations per second are received by the nerves of the retina, the brain interprets the effect to mean that a ray of, let us say, red light has entered the eye. A certain larger number of vibrations per second is similarly understood by the brain to imply the presence of blue light on the retina. Each particular hue of the spectrum—the red, orange, yellow, green, blue, indigo, violet—is associated with a corresponding number of vibrations per second. It will thus be seen that the interpretation we put on any ray of light depends solely, as far as its hue is concerned, on the number of vibrations per second produced on the retina. Increase that number of vibrations in any way, then the hue becomes more bluish; decrease the number of vibrations per second, and the hue shows more of the ruddy tinge.

From these considerations it is apparent that the hue of a light as interpreted by the eye will undergo modification if the source from which the light radiates is moving towards us or moving from us.

Fig. 27.—The Great Bear in 36,000 years.

In order to expound the matter simply I shall suppose a case of a rather simpler type than any which we actually find in nature. Let us suppose the existence of a star emitting light of a pure green colour corresponding to a tint near the middle of the spectrum. This star pours forth each second a certain number of vibrations appropriate to its particular colour, and if the star be at rest relatively to the eye, then, we assume, the vibrations will be received on the retina at the same intervals as those with which the star emits them. Consequently we shall perceive the star to be green. But now suppose that the star is hurrying towards us, it follows that the number of vibrations received in a second by the eye will undergo an increase. For the relative movement is the same as if the earth were rushing towards the star. In this case we advance, as it were, to meet the waves, and consequently receive them at less intervals than if we were to wait for their arrival.

Many illustrations can be given of the simple principle here involved. Suppose that a number of soldiers are walking past in single file, and that while the observer stands still twenty soldiers a minute pass him. But now let him walk in the opposite direction to the soldiers, then, if his speed be as great as theirs, he will pass forty soldiers a minute instead of twenty. If his speed were half that of the soldiers, then he would pass thirty a minute, so that in fact the speed with which the observer is moving could be determined if he counts the number of soldiers that he passes per minute, and makes a simple calculation. On the other hand, suppose that the observer walks in the same direction as the soldiers; if he maintains the same pace that they do, then it is plain that no soldiers at all will pass him while he walks. If he moves at half their rate, then ten soldiers will pass him each minute. From these considerations it will be sufficiently apparent that if the earth and the star are approaching each other, more waves of light per second will be received on the retina than if their positions are relatively stationary. But the interpretation which the brain will put on this accession to the number of waves per second is that the hue of the light is altered to some shade nearer the blue end of the spectrum. In fact, if we could conceive the velocity with which the bodies approached to be sufficiently augmented, the colour of the star would seem to change from green to blue, from blue to indigo, from indigo to violet; while, if the pace were still further increased, it is absolutely certain that the waves would be poured upon the retina with such rapidity that no nerves there present would be competent to deal with them, and the star would actually disappear from vision. It may, however, be remarked that the velocity required to produce such a condition as we have supposed is altogether in excess of any known velocities in the celestial movements.

Fig 28.—The Great Bear as it will be 100,000 years hence.

The actual changes in hue that the movements we meet with are competent to effect are much smaller than in the case given as an illustration.

On the other hand, we may consider the original green star and the earth to be moving apart from each other. The effect of this is that the number of waves poured into the eye is lessened, and accordingly the brain interprets this to imply that the hue of the star has shifted from the green to the red end of the spectrum. If the speed with which the bodies increase their distance be sufficiently large, the green may transform into a yellow, the yellow into an orange, the orange into a red; while a still greater velocity is, at all events, conceivable which would cause the undulations to be received with such slowness that the nature of the light could no longer be interpreted by any nerves that the eye contains, and from the mere fact of its rapid motion away from us the star would become invisible. Here again we must add the remark that the actual velocities animating the heavenly bodies are not large enough to allow of the extreme results now indicated.

However, in the actual circumstances of the celestial bodies it seems impossible that any change of hue recognisable by the eye could be attributed to movement in the line of sight. Nor does this merely depend on the circumstance that the velocities are too small to produce such an effect. It must be remembered that the case of a star which dispenses light of perfect simplicity of composition is one that can hardly exist among the heavenly bodies, though it may be admitted that there is a certain approach to it in one or two remarkable cases. It is, however, much more usual for the light from a star to be of a highly composite type, including rays not only from all parts of the visual spectrum, but also rays belonging to the ultra-violet region, as well as others beyond the extreme red end. The effect of the retreat of a star, so far as its colour is concerned, is that though the green is shifted a little towards the red, a bluish hue moves up to supply the place of the green, and as a similar effect takes place along the entire length of the spectrum, the total appearance is unaltered.

It is a fortunate circumstance that the lines in the spectrum afford a precise means of measuring the extent of the shift due to motion. If the movement of the star be towards us, then the whole system of lines is shifted towards the blue end, whereas it moves towards the red end when the star is hastening from us. The amount of the shift is a measure of the speed of the movement. This is the consideration which brings the process within the compass of practical astronomy. We need not here discuss the appliances, optical, mechanical, and photographic, by which an unexpected degree of precision has been given to the measurements. It seems that in the skilful hands of Vogel and Keeler it is possible in favourable cases to obtain determinations of the velocities of objects in the line of sight with a degree of precision which leaves no greater margin for doubt than about five per cent, of the total amount. It is truly astounding that such a degree of accuracy should be attainable under conditions of such difficulty. It must also be remembered that the distance of the object is here immaterial, unless in so far as the reduction in the brilliancy of the star owing to its distance involves a difficulty in making the observations.

As the first illustration of the extraordinary results that are now being obtained by the application of the new process, I take the case of the celebrated variable Algol. This star is a well-known object to all star-gazers; it lies in the constellation of Perseus, and its vagaries attracted notice in early times. In ages when the stars were worshipped as divinities it is not unreasonable to suppose that a star whose light varied in any extraordinary manner should naturally be viewed with some degree of suspicion as contrasted with stars that dispense their beams with uniformity. It was doubtless a feeling of this kind which rendered Algol a star of questionable import to the ancient students of the heavens. It was accordingly known as the Demon Star, for this is the equivalent of the name by which we now know it. As to the peculiarities of Algol which have given it notoriety, these are very simply described. For two days and ten hours the star remains of uniform lustre, being ranked about the second magnitude; then a decline of brightness sets in, and the star in a few hours parts with three-fifths of its brightness. At the lowest point it remains for about twenty minutes, and then the brilliancy commences to increase, so that in a few hours more Algol has resumed its original character. The entire period required for the decline and the rise is about ten hours, and the whole cycle of the changes has been determined with much accuracy, and is at present 2 days, 20 hours, 48 minutes, 52 seconds. The length of the period seems to undergo some trifling fluctuations of a few seconds, but on the whole the uniformity of the movement is a striking part of the phenomenon. Considering that these changes can be observed without any telescope, it is not surprising that they have been known for centuries. Indeed, it fortunately happens that there is a smaller star near Algol which serves as a convenient standard of comparison. Under ordinary circumstances Algol is much brighter than its neighbour, but when it sinks to its lowest point the two stars have almost equal lustre. It is only within the last year or two that the mystery of the variability of Algol has been at last revealed and the phenomenon of the Demon Star has received its true interpretation.

It had been suggested long ago that the loss of light might be due to an eclipse of the brilliant star by some dark companion revolving about it; indeed, this theory seemed to hold the field, inasmuch as its only rival was one which supposed Algol to be a rotating body darker on one side than the other.

Fig. 29.—Orbit of the Companion of Algol.

This, however, was easily shown to be incompatible with the observed facts as to the manner in which the light waxed and waned in a single cycle of change. It was, however, impossible to subject the eclipse theory to any decisive test until astronomers were provided with the means of measuring the velocity of approach or retreat along the line of sight. The existence of the dark companion was therefore almost destitute of support from observations until Vogel made his wonderful discovery.

Applying the improved spectrographic process to Algol, he determined on one night that Algol was retreating from the earth at a speed of twenty-six miles a second. This in itself is a striking fact, but of course the velocity is not an exceptionally large one for celestial movements. We know of one star at least which moves half a dozen times as fast. When, however, Vogel came to repeat his observations he found that Algol was again moving with the same velocity, but this time the movement was towards the earth instead of from it. Here was indeed a singular circumstance demanding the careful examination which it speedily received. It appeared that the movements of Algol to and fro were strictly periodic, that is to say, for one day and ten hours the star is moving towards us, and then for a like time it moves from us, the maximum speed in each advance or retreat being that which we have mentioned, namely twenty-six miles a second. The interest awakened by this discovery culminates when it appears that this movement to and fro is directly associated in a remarkable manner with the variation of Algol's lustre. It is invariably found that every time the movement of retreat is concluded, the star loses its brilliance, and regains it again at the commencement of the return movement. It is thus plain that the changes in brilliance of the star bear an important relation to the periodic movement. Here was an important step taken.

For the next advance in this remarkable investigation we have to depend, not on our instruments, but on the laws of mechanics. We have spoken of Algol as moving to and fro, but it is necessary to observe that it is impossible for a star to run along a straight line for a certain distance, stop, turn back, again retrace its movement, stop, and again return. Such movement is simply forbidden by the laws of motion. We can, however, easily ascertain that there is a type of motion possible for Algol which shall be compatible with the results of the spectroscopic research and also be permitted by the laws of motion. There is no objection to the supposition that Algol is moving in a path which is nearly, if not exactly, a circle. In this case it would only be moving as does the moon, or the earth, or any of the other planets. It will be only necessary to suppose that the plane of the orbit of Algol is directed nearly edgewise towards us. During the description of one semicircle Algol will be coming towards us, while, during the other semicircle it will be going from us, and thus the observed facts of the movement are reconciled with the laws of motion. Of course, this involves a certain periodic shift in the position of Algol in the heavens. It must, for instance, when moving most rapidly from us be at a distance equal to the diameter of the circle from the position which it has when moving most rapidly towards us. This is true, but the extent of the shift of place is far too small to be visible to our instruments. In fact, it can be shown that the visual size of the circle in which Algol revolves could hardly be larger than is that which the rim of a three-penny bit would appear to have if viewed from a situation five hundred miles away. It is one of the extraordinary characteristics of the spectroscopic method that it renders such an orbital movement perceptible.

The fact that Algol revolves in an orbit having been thus demonstrated, we can again call in the assistance of the laws of dynamics to carry us a step further. Such a movement is possible on one condition and only one, and that is that there is an attracting body in the neighbourhood around which Algol revolves. Of course the student of mechanics knows that each of the bodies revolves around the common centre of gravity. The essential point to be noticed is that the spectroscopic evidence admits of no other interpretation save that there must be another mighty body in the immediate vicinity of Algol. We had already seen reason to believe in the possibility of the presence of such a companion for the Demon Star, simply from the fact of its variability. There cannot be any longer a doubt that the mystery has been solved. Algol must be attended by a companion star, which, if not absolutely as devoid of intrinsic light as the earth or the moon, is nevertheless dark relatively to Algol. Once in each period of revolution this obscure body intrudes itself between the earth and Algol, cutting off a portion of the direct light from the star and thus producing the well-known effect. Here we have such a remarkable concurrence between the facts of observation and the laws of dynamics that it is impossible to doubt the explanation they provide of the variability of this famous star.

There is, however, a further point in which the facts can be made to yield information of even a more striking character, inasmuch as it is unique of its kind. It is, of course, well known that stars in general show no appreciable discs, even in our best telescopes. In fact the better the instrument the smaller does the stellar point appear. This is, of course, due to the distance at which the stars are situated. It would be easy to show that if the sun were to be viewed by an observer placed on the nearest of the stars the apparent magnitude of its disc would be no greater than an eagle would seem if soaring overhead at an altitude three times as great as the distance of New Zealand beneath our feet. Of course, no instrument whatever would render the dimensions of such an object perceptible, though such is the delicacy of the sense of perception of light that the eye may be able to detect the radiation from a self-luminous object which is itself too small to form an image of recognisable dimensions on the retina. The stars, of course, are suns often comparable with, and often far exceeding, our own sun in lustre and dimensions, but their distance is far too large to enable us to measure their diameters by the ordinary processes of the observatory. Even if the stars were brought towards the earth so that their distances were reduced to a tenth of what they are at this moment, it does not seem at all likely that any one of them would be even then seen clearly enough to enable us to perceive its diameter.

This statement becomes the more significant when it is borne in mind that there are several cases in which, though we are not able to measure the dimensions of stars, yet we are able to weigh them. If the period of revolution of a binary star has been determined, and if the distance of the pair from the sun is also known, we then have sufficient data to enable us to compare the mass of the binary system with that of the sun. It will therefore be understood that the first observations which declare the actual dimensions of a star merit the utmost attention. They constitute a distinct and important departure in our knowledge of the universe. It is surely a noteworthy epoch in the history of astronomy when, for the first time, we are able to apply the celestial callipers to gauge the diameter of a star. So far as surveying and measuring goes, this is the most significant piece of work in sidereal astronomy since the epoch, half a century ago, when the determination of a stellar distance first emerged from the mistiness of mere guess-work and took a respectable position among the solved problems of astronomy. Nor is our gratification at the result of Vogel's striking work lessened by the fact of its unexpectedness. Who would have predicted some few years ago that the spectroscope was to be the instrument to which we should be indebted for the means of putting a measuring tape round the girth of a star? The process and the results are alike full of interest and are of happy augury for the future.

To explain exactly how it is possible to deduce a presumable value for the diameter of Algol would lead into some technicalities that need not be here mentioned. But the principle of the method is so plain that it would be unfitting to leave it without some attempt at exposition. We are first to notice that Algol, at the moment of its greatest eclipse, has lost about three-fifths of its light: it therefore follows that the dark satellite must have covered three-fifths of the bright surface. It is also to be noticed that the period of maximum obscuration is about twenty minutes, and that we know the velocity of the bright star, which along with the period of revolution gives the magnitude of its orbit. These facts, added to our knowledge that ten hours is required for the brilliancy to sink from and regain its original lustre, enable the sizes of the two globes to be found. There is only one element of uncertainty in the matter. We have assumed that the densities of the two bodies are the same. Of course, this may not be the case, and if it should prove to be unfounded, then some modification will have to be made in the numerical elements now provisionally assigned. There can, however, be little doubt that so far as the substantial features of the Algol system are concerned, the elements given by Vogel may be accepted.

Let us endeavour to form a conception of what Algol and its companion are like. It is worth making the attempt, because, as we have already said, Algol is the first star among "yonder hundred million spheres" of which the dimensions are approximately known. First we are to think of Algol itself. It is indeed a vast object, a glowing globe, a veritable sun, much larger than our own. The diameter of the sun would have to be increased by almost 200,000 miles to make it as great as that of Algol. But we may exhibit the relative proportions of the two bodies in a somewhat different manner. Imagine two globes, each as large as our sun; let those two be rolled into one, and we have a globe of the splendid proportions of Algol. But now for a singular circumstance which indicates the variety of types of sun which the heavens offer to our study, Though Algol is twice as big as the sun it is not twice as heavy. It is indeed an extraordinary circumstance that, notwithstanding the vast bulk of Algol, its weight is only about half that of the sun. The sun itself has a density about a fourth that of the earth, or but little more than the density of water, yet Algol has a density which is much less than that of water; in fact, this globe is apparently not much heavier than if it were made of cork. We are, of course, speaking of the average density of the star. No doubt its central portions must be dense enough, but it is impossible to resist the conclusions that the greater part of Algol must be composed of matter in a gaseous state. Of course, such a state of things is already known to exist in many celestial bodies. The figures that have been arrived at must be regarded as subject to a possible correction, but it is difficult to repress all feelings of enthusiasm at a moment when, for the first time, so startling an extension has been given to our knowledge of the universe.

And now, as to the dark companion of Algol. Here is an object which we never have seen, and apparently never can expect to see, but yet we have been able not only to weigh it and to measure it, but also to determine its movements. It appears that the companion of Algol is about the same size as our sun, but has a mass only one-fourth as great. This indicates the existence of a globe of matter which must be largely in the gaseous state, but which, nevertheless, seems to be devoid of intrinsic luminosity. We may compare this body with the planet Saturn; of course, the latter is not nearly so large as the companion to Algol, but the two globes seem to agree fairly well as to density. As to the character of the movements of the dark companion of Algol, we can learn little, except what the laws of dynamics may teach; but the information thus acquired is founded on such well understood principles that it leaves us in no uncertainty.

It would be a natural assumption that the law of gravitation is obeyed and must be obeyed in the stellar systems. It would, indeed, be surprising if that law which regulates the movements of the bodies in the solar system should not be found to prevail in the sidereal systems also. Everything would justify us in the anticipation that this is so. Have we not learned to a large extent the actual nature of the elementary bodies which enter iuto the composition of stars? We find that the elementary bodies in these other suns are in the main identical with those which exist in our own sun and in the earth itself. If iron attracts iron by the law of gravitation in the solar system, why should not iron attract iron in the sidereal systems as well? But we are not dependent solely on this presumption for our knowledge of the important fact that the law of gravitation is not confined to the solar system. The movements of binary stars have been studied, and it has been invariably found that the phenomena observed are compatible with the supposition that the law of gravitation prevails throughout the universe.

It would not, however, be correct to assert, as has been sometimes done, that the facts of the binary systems actually prove that gravitation is the all-compelling force there as here. The circumstances do not warrant us in expressing the matter quite so forcibly. The binary stars are so remote that the observations which we are enabled to make are wanting in the almost mathematical precision which we can give to such work when applied to the bodies of our own system. It is quite possible for mathematical ingenuity to devise a wholly arbitrary and imaginary system of force, which might explain the facts of binary stars, as far as we are able to observe them, on quite another hypothesis than the simple law that the attraction between two particles varies with the inverse square of the distance. No one, however, will be likely to doubt that it is the law of gravitation, pure and simple, which prevails in the celestial spaces, and consequently we are able to make use of it to explain the circumstances attending the movement of Algol's dark companion.

This body is the smaller of the two, and the speed with which it moves is double as great as that of Algol, so that it travels over as many miles in a second as an express train can get over in an hour. It revolves with apparent uniformity in an orbit which must be approximately circular, and it completes its journey in the brief period given above, which indicates the time of variability. So far the movements of Algol and its companion are not very dissimilar to movements in the solar system with which we are already familiar; but there is one point in which the Algol system presents features wholly without parallel in the planetary movements. It is that the two bodies are so very close together. I do not, of course, mean that they seem close by ordinary standards for is not their distance always some three million miles? This is, however, an unusually short distance when compared with the dimensions of the two globes themselves. The dimensions of the system may be appreciated by the simple illustration of taking a shilling and a sixpence and placing them so that the distance from rim to rim is two inches. The smaller coin will represent the dark satellite and the larger one Algol, fairly correct as to position and dimensions. Viewed in this way it is evident that the dimensions of the globes bear a monstrous proportion to their distance apart when compared with the more familiar planets and satellites of our system. The tides in such a case must be of a magnitude and importance of which we have no conception from our experiences of such agencies here.

We have dwelt thus long on the subject of Algol because it was fitting to give due emphasis to the remarkable extension of our knowledge of the universe which took place when, for the first time, we became able to measure the size of a star.

It is well known that the most difficult test-objects on which a telescope can be directed are some of those double stars of which the components have a suitable distance. If the two stars be so close together that they subtend at our system an angle not more than a few tenths of a second, then the telescopic separation of the two components is a feat to tax the powers of the most perfect instrument, and the eye of the most accomplished observer. It may, however, happen that there are double stars of which the components are much closer than this. In such a case there is not the slightest possibility of our being able to effect a visual decomposition of the pair into its components. The spectroscopic process has, however, placed at our disposal a striking method for detecting the existence of double stars, the components of which are so close that even if they were hundreds of times farther apart than they actually are they would still fall short of the necessary distance at which they must be situated before they can be separated telescopically. Indeed, we have here obtained an accession to our power so remarkable that we have not yet been able even to feel the limits within which its application must be confined.

As an illustration of this process I shall take a star which is probably as famous as Algol itself. It is Mizar, the middle star of the three which form the tail of the Great Bear (Fig. 27). Mizar has in its vicinity the small star Alcor, which is so easily seen as to make it hard for us to realise the significance of the proverb, "He can see Alcor." It is, however, possible that the lustre of Alcor may have waxed greater since ancient times. The relationship between Mizar and Alcor is closer than might be inferred from the mere fact of their contiguity on the sky. Their proximity is not an accident of situation, as is the case in some other instances when two stars happen to lie in nearly the same line of vision. The association of Alcor and Mizar is rendered highly probable from the fact that they move together in parallel directions and the same velocity. But this is the least of the circumstances that gives Mizar its interest. The star itself is a double of the easiest type, and is at the same time of striking interest and beauty. Every possessor of a telescope, large or small, knows Mizar to be one of the most suitable objects wherewith to delight the friends that visit his observatory, by a glimpse at a double star which is both easy to discern and remarkable in character. This is the second noteworthy point about Mizar; but now for the third and last, which is by far the most interesting of all, and has only lately been ascertained by a discovery which will take its place in the history of astronomy as the inauguration of a new process in the study of things sidereal.

Professor Pickering has, as is well known, been extremely successful in obtaining photographs of the spectra of the stars. Sufficient means having been placed at his disposal by Mrs. Draper, he has applied himself with remarkable results to the compilation of the Henry Draper Memorial. The photographs of the spectra of the stars that he has thus obtained exhibit a fulness of detail that some years ago could hardly have been expected even in photographs of the solar spectrum itself. Among the stars subjected to his camera was Mizar, and the photographs of the spectrum of its principal component component exhibit, as other stellar spectra do, a profusion of dark lines. These photographs being repeated at different dates, it was natural to compare them, and it was noticed that the lines sometimes appeared double and sometimes single. So striking a circumstance, of course, demanded closer investigation, and presently it appeared that this opening and closing of the lines was a periodical phenomenon. The interval between one maximum opening of the lines and the next was fifty-two days. If the star were a single object, then this phenomenon would be inexplicable. It was plain that the object could not be a single star; it must consist of a pair extremely close together, and in rapid revolution. The doubling of the lines will then be readily intelligible. When one of the components is moving towards us while the other is moving from us, all the lines belonging to one system are shifted one way, and all those belonging to the other system are shifted the other way, the effect on the spectrum being that the lines appear doubled. When the stars are moving perpendicularly to the line of sight, then their relative velocities towards the earth are equal, and the lines close up again. We thus at once learn the period of the revolution of the two components. The lines must open out twice in each circuit, and consequently we have as the first instalment of the numerical facts of the system that the period of its revolution is a hundred and four days.

It is, however, a peculiarity of the spectroscopic process that it provides us with a wealth of information on the subject. The amount by which the lines open when they separate admits of accurate measurement, and as this depends on the velocities, it follows that we obtain a determination of these velocities. It thus appears that the speed with which each of the component stars moves is about fifty miles a second.

Fig. 30.—The System of Mizar.

As, therefore, we know the pace at which the stars are moving, and the time they need for the journey, we know how large their path is, and thus we infer that the distance of the components is, speaking roundly, about one hundred and fifty millions of miles. But now, we are enabled to draw a remarkable inference. We know the size of the orbits, and we know the time in which the revolutions are accomplished. It is the mathematician who enables the mass of the bodies to be determined, and the result is not a little astonishing. It tells us that the mass of the two component stars which form the primary of Mizar is not less than forty times as great as the mass of the sun. Here is indeed a result equally striking on account of the method by which it is obtained and of the startling character of the conception to which it leads.

Remember that in all this the distance of the star from the earth is not concerned, for the results at which we have arrived are absolutely independent of the distance at which the star may happen to be placed. We already knew the masses of some few binary stars by the application of the older process, but in all such cases it was necessary that we should have a previous knowledge of the star's distance. This is always a precarious element, and in the majority of cases it is wholly out of our power to discover it. Now, however, we are entitled to expect large additions to our knowledge of the stars, their masses, and their movements, notwithstanding the fact that the distances may be too vast to be appreciated by any means at our disposal.

The instances that have been given will suffice to show the versatility of the new method. It is the alliance of photography with spectroscopy that makes the present time so full of promise. Already the orbits of 150 Stars have been determined by the beautiful spectroscopic process.