Astronomical Discovery Part 9

Search being made in the catalogues, no entry of it was found, though it still might be one of this cla.s.s which had hitherto escaped detection. Or it might be a "new star," one of those curious bodies which blaze up quite suddenly to brightness and then die away gradually until they become practically invisible. The most famous perhaps of these is the star which appeared in 1572, and was so carefully observed by Tycho Brahe; but such apparitions are rare, and altogether we have not records as yet of a score altogether; so that in this latter case the discovery would be of much greater interest than in the former. In either event it was desirable to inform other observers as soon as possible of the existence of a strange body; already some time had elapsed since the plate had been taken, March 16th, for the examination of which I have spoken was not made until March 24th. Accordingly, a telegram was at once despatched to the Central Office at Kiel, which undertakes to distribute such information all over the world, and a few post-cards were sent to observers close at hand who might be able to observe the star the same night. Certain observations with the spectroscope soon made it clear that the object was really a "new star."

[Sidenote: The discovery accidental.]

[Sidenote: Mrs. Fleming's discoveries.]

This, therefore, is the discovery which we made at Oxford: as you will see, in an entirely accidental manner, during the course of a piece of work in which it was certainly never contemplated. Its purely accidental nature is sufficiently ill.u.s.trated by the fact that if the plates originally supplied by the makers had been of the proper quality, the plate which led to the discovery would never have been taken. If the plates exposed in February had been satisfactory, we should have been content, and should not have repeated the exposure on March 16th. Again I can testify personally how purely accidental it was that the examination was made on March 24th to see whether anything could be saved, as I have said, from the wreck. The idea came casually into my mind as I was walking through the room and saw the neat pile of rejected plates; and one may fairly call it an accidental impulse. This new star is not, however, the first of such objects to have been discovered "accidentally"; many of the others were found just as much by chance, though a notable exception must be made of those discovered at the Harvard Observatory, which are the result of a deliberate search for such bodies by the careful examination of photographic plates. Mrs. Fleming, who spends her life in such work, has had the good fortune to detect no less than six of these wonderful objects as the reward of her laborious scrutiny; and she is the _only_ person who has thus found new stars by photography until this accidental discovery at Oxford. The following is a complete list of new stars discovered to date:--

LIST OF NEW STARS.

+----------------------------------------------+ Ref. No. Constellation. Year. Discoverer. +----------------------------------------------+ 1 Ca.s.siopeia 1572 Tycho Brahe. 2 Cygnus 1600 Janson. 3 Ophiuchus 1604 Kepler. 4 Vulpecula 1670 Anthelm. 5 Ophiuchus 1848 Hind. 6 Scorpio 1860 Auwers. 7 Corona Borealis 1866 Birmingham. 8 Cygnus 1876 Schmidt. 9 Andromeda 1885 Hartwig. 10 Perseus 1887 Fleming. 11 Auriga 1891 Anderson. 12 Norma 1893 Fleming. 13 Carina 1895 Fleming. 14 Centaurus 1895 Fleming. 15 Sagittarius 1898 Fleming. 16 Aquila 1899 Fleming. 17 Perseus 1901 Anderson. 18 Gemini 1903 At Oxford. +----------------------------------------------+

[Ill.u.s.tration:

MARCH 1, 1903 MARCH 14, 1903 VIII.--THE OXFORD NEW STAR.

A PAIR OF PHOTOGRAPHS TAKEN AT THE HARVARD COLLEGE OBSERVATORY BEFORE AND AFTER ITS APPEARANCE

(_The arrow indicates the place of the new star. It will be seen that the left-hand picture though it shews fainter stars than the other, has not a trace of the new star._)]

[Sidenote: Dr. Anderson.]

[Sidenote: Nova Persei.]

Generally these stars have been noted by eye observation, as in the case of the two found by Dr. Anderson of Edinburgh. In these cases also we may say that deliberate search was rewarded; for Dr. Anderson is probably the most a.s.siduous "watcher of the skies" living, though he seldom uses a telescope; sometimes he uses an opera-gla.s.s, but usually the naked eye. He describes himself as an "Astrophil" rather than as an astronomer. "I love the stars," he says; "and whenever they are shining, I must be looking."

And so on every fine night he stands or sits at his open study window gazing at the heavens. I believe he was just about to leave them for his bed, near 3 A.M. on the night of February 21, 1901, when, throwing a last glance upward, he suddenly saw a brilliant star in the constellation Perseus. His first feeling was actually one of disappointment, for he felt sure that this object must have been there for some time past without his knowing of it, and he grudged the time lost when he might have been regarding it. More in a spirit of complaint than of inquiry, he made his way to the Royal Observatory at Edinburgh next day to hear what they had to say about it, though he found it difficult to approach the subject. He first talked about the weather, and the crops, and similar topics of general interest; and only after some time dared he venture a casual reference to the "new portent in the heavens." Seeing his interlocutor look somewhat blank, he ventured a little farther, and made a direct reference to the new star in Perseus; and then found to his astonishment, as also to his great delight, that he was the first to bring news of it.

The news was soon communicated to other observers; all the telescopes of the world were soon trained upon it; and this wonderful "new star of the new century" has taught us more of the nature of these extraordinary bodies than all we knew before.

[Sidenote: Records previous to discovery.]

[Sidenote: Was Nova Geminorum previously shining faintly?]

[Sidenote: The suspicion negatived.]

Perhaps I may add a few remarks on one or two features of these bodies.

Firstly, let us note that Professor Pickering of Harvard is now able to make a most important contribution to the _former_ history of these objects--that is to say, their history preceding their actual detection.

We remember that, after Ura.n.u.s had been discovered, it was found that several observers had long before recorded its place unknowingly; and similarly Professor Pickering and his staff have usually photographed other new objects unknowingly. There are on the shelves at Harvard vast stores of photographs, so many that they are unable to examine them when they have been taken; but once any object of interest has been discovered, it is easy to turn over the store and examine the particular plates which may possibly show it at an earlier date. In this way it was found that Dr.

Anderson's new star had been visible only for a few days before its discovery, there being no trace of it on earlier plates. Similarly, in the case of the new star found at Oxford, plates taken on March 1st and 6th, fifteen days and ten days respectively before the discovery-plate of March 16th, showed the star. But, in this particular instance, greater interest attaches to two still earlier plates taken elsewhere, and with exposures much longer than any available at Harvard. One had been obtained at Heidelberg by Dr. Max Wolf, and another at the Yerkes Observatory of Chicago University, by Mr. Parkhurst; and on both there appeared to be a faint star of about the fourteenth or fifteenth magnitude, in the place subsequently occupied by the Nova; and the question naturally arose, Was this the object which ultimately blazed up and became the new star? To settle this point, it was necessary to measure its position, with reference to neighbouring stars, with extreme precision; and here it was unfortunate that the photographs did not help us as much as they might, for they were scarcely capable of being measured with the requisite precision. The point was an important one, because if the ident.i.ty of the Nova with this faint star could be established, it would be the second instance of the kind; but so far as they went, measurements of the photographs were distinctly against the ident.i.ty. Such was the conclusion of Mr. Parkhurst from his photograph alone; and it was confirmed by measures made at Oxford on copies of both plates, which were kindly sent there for the purpose. The conclusion seemed to be that there was a faint star _very near_, but _not at_, the place of the new star; and it was therefore probable that, although this faint star was temporarily invisible from the brightness of the adjacent Nova, as the latter became fainter (in the way with which we have become familiar in the case of new stars), it might be possible to see the two stars alongside each other.

This critical observation was ultimately made by the sharp eyes of Professor Barnard, aided by the giant telescope of the Yerkes Observatory; and it seems clear therefore that the object which blazed up to become the Nova of 1903 could not have previously been so bright as a faint star of the fourteenth magnitude. Although this is merely a negative conclusion, it is an important one in the history of these bodies.

[Sidenote: Nebula round Nova Persei.]

[Sidenote: Its changes.]

[Sidenote: Due to travelling illumination.]

The second point to which I will draw your attention is from the history of the other Nova just mentioned--Dr. Anderson's New Star of 1901. In this instance it is not the history previous to discovery, but what followed many months after discovery, that was of engrossing interest; and again Yerkes Observatory, with its magnificent equipment, played an important part in the drama. When, with its giant reflecting telescope, photographs were taken of the region of Nova Persei after it had become comparatively faint, it was found that there was an extraordinarily faint nebulosity surrounding the star. Repeating the photographs at intervals, it was found that this nebulosity was rapidly changing in shape. "Rapidly" is, of course, a relative term, and a casual inspection of two of the photographs might not convey any impression of rapidity; it is only when we come to consider the enormous distance at which the movements, or apparent movements, of the nebulae must be taking place that it becomes clear how rapid the changes must be. It was not possible to determine this distance with any exactness, but limits to it could be set, and it seemed probable that the velocity of the movement was comparable with that of light. The conclusion suggested itself that the velocity might actually be identical with that of light, in which case what we saw was not the movement of actual matter, but merely that of illumination, travelling from point to point of matter already existing.

[Ill.u.s.tration:

SEPT. 20, 1901 NOV. 13, 1901 IX--NEBULOSITY ROUND NOVA PERSEI (_From photographs taken at the Yerkes Observatory by G. W. Ritchey._)]

[Sidenote: When did it all happen?]

An a.n.a.logy from the familiar case of sound may make clearer what is meant.

If a loud noise is made in a large hall, we hear echoes from the walls.

The sound travels with a velocity of about 1100 feet per second, reaches the walls, is reflected back from them, and returns to us with the same velocity. From the interval occupied in going and returning we could calculate the distance of the walls. The velocity of light is so enormous compared with that of sound that we are usually quite unable to observe any similar phenomenon in the case of light. If we strike a match in the largest hall, all parts of it are illuminated so immediately that we cannot possibly realise that there was really an interval between the striking of the match, the travelling of the light to the walls, and its return to our eyes. The scale of our terrestrial phenomenon is far too small to render this interval perceptible. But those who accept the theory above mentioned regarding the appearances round Nova Persei (although there are some who discredit it) believe that we have in this case an ill.u.s.tration of just this phenomenon of light echoes, on a scale large enough to be easily visible. They think that, surrounding the central star which blazed up so brightly in February 1901, there was a vast dark nebula, of which we had no previous knowledge, because it was not shining with any light of its own. When the star blazed up, the illumination travelled from point to point of this dark nebula and lighted it up; but the size of the nebula was so vast that, although the light was travelling with the enormous velocity of 200,000 miles per second, it was not until months afterwards that it reached different portions of this nebula; and we accordingly got news of the existence of this nebula some months after the news reached us of the central conflagration, whatever it was. Remark that all we can say is that the news of the nebula reached us _some months later_ than that of the outburst. The actual date when either of the actual things happened, we have as yet no means of knowing; it may have been hundreds or even thousands of years ago that the conflagration actually occurred of which we got news in February 1901, the light having taken all that time to reach us from that distant part of s.p.a.ce; and the light reflected from the nebula was following it on its way to us all these years at that same interval of a few months.

[Sidenote: An objection.]

Now, let me refer before leaving this point to the chief objection which has been urged against this theory. It has been maintained that the illumination would necessarily appear to travel outwards from the centre with an approach to uniformity, whereas the observed rate of travel is not uniform, and has been even towards the centre instead of away from it; which would seem as though portions of the nebula more distant from the centre were lighted up sooner than those closer to it. By a simple ill.u.s.tration from our solar system, we shall see that these curious anomalies may easily be explained. Let us consider for simplicity two planets only, say the Earth and Saturn. We know that Saturn travels round the sun in an orbit which is ten times larger than the orbit of the earth.

Suppose now that the sun were suddenly to be extinguished; light takes about eight minutes to travel from the sun to the earth, and consequently we should not get news of the extinction for some eight minutes; the sun would appear to us to still go on shining for eight minutes after he had really been extinguished. Saturn being about ten times as far away from the sun, the news would take eighty minutes to reach Saturn; and from the earth we should see Saturn shining more[3] than eighty minutes after the sun had been extinguished, although we ourselves should have lost the sun's light after eight minutes. I think we already begin to see possibilities of curious anomalies; but they can be made clearer than this. Instead of imagining an observer on the earth, let us suppose him removed to a great distance away in the plane of the two orbits; and let us suppose that the sun is now lighted up again as suddenly as the new star blazed up in February 1901. Then such an observer would first see this blaze in the centre; eight minutes afterwards the illumination would reach the earth, a little speck of light near the sun would be illuminated, just as we saw a portion of the dark nebula round Nova Persei illuminated; eighty minutes later another speck, namely, Saturn, would begin to shine. But now, would Saturn necessarily appear to the distant observer to be farther away from the sun than the earth was? Looking at the diagram, we can see that if Saturn were at S{1} then it would present this natural appearance of being farther away from the sun than the earth; but it might be at S{2} or S{3}, in which case it would seem to be nearer the sun, and the illumination would seem to travel inwards towards the central body instead of outwards. Without considering other cases in detail, it will be tolerably clear that almost any anomalous appearance might be explained by choosing a suitable arrangement of the nebulous matter which we suppose lighted up by the explosion of Nova Persei.

Another objection urged against the theory I have sketched is that the light reflected from such a nebula would be so feeble that it would not affect our photographic plates. This depends upon various a.s.sumptions which we have no time to notice here; but I think we may say that there is certainly room for the acceptance of the theory.

[Ill.u.s.tration: FIG. 6.]

[Sidenote: Did the nebula cause the outburst?]

Now, if this dark nebula was previously existing in this way all round the star which blazed up, the question naturally arises whether the nebula had anything to do with the conflagration. Was there previously a star, either so cold or so distant as not to be shining with appreciable light, which, travelling through s.p.a.ce, encountered this vast nebula, and by the friction of the encounter was suddenly rendered so luminous as to outshine a star of the first magnitude? The case of meteoric stones striking our own atmosphere seems to suggest such a possibility. These little stones are previously quite cold and invisible, and are travelling in some way through s.p.a.ce, many of them probably circling round our sun. If they happen in their journey to encounter our earth, even the extremely tenuous atmosphere, so thin that it will scarcely bend the rays of light appreciably, even this is sufficient by its friction to raise the stones to a white heat, so that they blaze up into the falling stars with which we are familiar. This a.n.a.logy is suggested, but we must be cautious in accepting it; for we know so very little of the nature of nebulae such as that of which we have been speaking. But in any case, a totally new series of phenomena have been laid open to our study by those wonderful photographs taken at the Yerkes Observatory and the Lick Observatory in the few years which the present century has as yet run.

[Sidenote: Importance of new stars]

One thing is quite certain: we must lose no opportunity of studying such stars as may appear, and no diligence spent in discovering them at the earliest possible moment is thrown away. We have only known up to the present, as already stated, less than a score of them, and of these many have told us but little; partly because they were only discovered too late (after they had become faint), and partly because the earlier ones could not be observed with the spectroscope, which had not then been invented.

It seems clear that in the future we must not allow accident to play so large a part in the discovery of these objects; more must be done in the way of deliberate search. Although we know beforehand that this will involve a vast amount of apparently useless labour, that months and years must be spent in comparing photographic plates, or portions of the sky itself, with one another without detecting anything remarkable, it will not be the first time that years have been cheerfully spent in such searches without result. We need only recall Hencke's fifteen years of fruitless search, before finding a minor planet, to realise this fact.

[Sidenote: Superposition of plates.]

[Sidenote: The stereo-comparator.]

One thing of importance may be done; we may improve our methods of making the search, so as to economise labour, and several successful attempts have already been made in this direction. The simplest plan is to superpose two photographs taken at different dates, so that the stars on one lie very close to those on the other; then if an image is seen to be unpaired we _may_ have found a new star, though of course the object may be merely a planet or a variable. The superposition of the plates may be either actual or virtual. A beautiful instrument has been devised on the principle of the stereoscope for examining two plates placed side by side, one with each eye. We know that in this way two photographs of the same object from different points of view will appear to coalesce, and at the same time to give an appearance of solidity to the object or landscape, portions of which will seem to stand out in front of the background.

Applying this principle to two photographs of stars, what happens is this: if the stars have all remained in the same positions exactly, the two pictures will seem to us to coalesce, and the images all to lie on a flat background; but if in the interval between the exposures of the two plates one of the stars has appreciably moved or disappeared, it will seem, when looked at with this instrument, to stand out in front of this background, and is accordingly detected with comparatively little trouble. This new instrument, to which the name Stereo-comparator has been given, promises to be of immense value in dredging the sky for strange bodies in the future. I am glad to say that a generous friend has kindly presented the University Observatory at Oxford with one of these beautiful instruments, which have been constructed by Messrs. Zeiss of Jena after the skilful designs of Dr. Pulfrich. Whether we shall be able to repeat by deliberate search the success which mere accident threw in our way remains to be seen.

CHAPTER V

SCHWABE AND THE SUN-SPOT PERIOD

[Sidenote: Discoveries contrary to expectation.]

In preceding chapters we have reviewed discoveries, some of which have been made as a result of a deliberate search, and others accidentally in the course of work directed to a totally different end; but so far we have not considered a case in which the discoverer entered upon an enterprise from which he was positively dissuaded.

[Sidenote: Nothing expected from spots.]

In the next chapter we shall come across a very striking instance of this type; but even in the discovery that there was a periodicity in the solar spots, with which I propose to deal now, Herr Schwabe began his work in the face of deterrent opinions from eminent men. His definite announcement was first made in 1843, though he had himself been convinced some years earlier. In 1857 the Royal Astronomical Society awarded him their gold medal for the discovery; and in the address delivered on the occasion the President commenced by drawing attention to this very fact, that astronomers who had expressed any opinions on the subject had been uniformly and decidedly against the likelihood of there being anything profitable in the study of the solar spots. I will quote the exact words of the President, Mr. Manuel Johnson, then Radcliffe Observer at Oxford.

"It was in 1826 that Heinrich Schwabe, a gentleman resident in Dessau, entered upon those researches which are now to engage our attention. I am not aware of the motive that induced him--whether any particular views had suggested themselves to his own mind--or whether it was a general desire of investigating, more thoroughly than his predecessors had done, the laws of a remarkable phenomenon, which it had long been the fashion to neglect. He could hardly have antic.i.p.ated the kind of result at which he has arrived; at the same time we cannot imagine a course of proceeding better calculated for its detection, even if his mind had been prepared for it, than that which he has pursued from the very commencement of his career.