Part 19

During a shooting-star shower it is interesting to notice that all the meteors seem to diverge from a single point. In the adjoining figure (Fig. 81), which shows the directions of a number of meteors’ tracks, you will notice that every one seems to radiate from a certain point of the sky. In the case of the shower of the 13th–15th of November this point lies in the constellation Leo. I must refer you to the Appendix for a description of the way to find Leo or the Lion. The radiant point, as we term it, of this system of meteors is there situated. It is true that the meteors themselves do not generally seem to come all the way from this place. It is the direction of their luminous trails produced backward that carries the eye to the radiant (Fig. 81). If a meteor were actually seen there, it would be certainly coming straight towards us; it would not then appear as a streak of light at all; it would merely seem like a star which suddenly blazed into splendor and then again sank down into invisibility. Every meteor which appeared near this point would be directed very nearly at the observer, and its path would therefore seem very much foreshortened. I can illustrate this with a long straight rod. If I point it directly at you, you can only see the end. If I point it nearly at you, it will seem very much shortened. During the great shower in 1866 many of the meteors could be observed so close to the radiant in Leo that they seemed merely like very short marks in the sky; some of them, indeed, seemed to be merely starlike points swelling up into brilliance and then going out. Hence it is that we call this system of shooting stars the “Leonids.” They bear this name because their radiant lies in the constellation Leo, and unless the direction of a shooting star emanates from this point it does not belong to the Leonids. Even if it did so, the meteor would not be a Leonid unless the date was right, namely, on the 13th of November, or within a day thereof. We thus have two characteristics which belong to a system of shooting stars; there is the date on which they occur and the point from which they radiate.

OTHER GREAT SHOWERS.

To illustrate what I have said, we will speak about another system of shooting stars; they are due every August, from the 9th to the 11th, and their directions diverge from a point in the constellation of Perseus. I may remind you of the dates of the recurrence of this shower as well as of the November meteors of which we have just spoken, by quoting the following production:--

If you November’s stars would see, About the fourteenth watching be. In August, too, stars shine through heaven, On nights between nine and eleven.

It may be worth your while to remember these lines, and always to keep a look-out on the days named. The August meteors, the Perseids we often call them, do not make gorgeous displays, in particular years, with the regularity of the Leonids. There have been, no doubt, some exceptionally grand showers between the 9th and the 11th of August, but we cannot predict when the next splendid one is due. There are vast numbers of stragglers all round the track of the Perseids. In fact, it would seem as if the great race had gone on for such a long period that the cluster had to a great extent broken up, and that a large proportion of the meteors were now scattered the whole way around the course with tolerable uniformity. This being so, it follows that every time we cross the track we are nearly certain to fall in with a few of the stragglers, though we may never enjoy the tremendous spectacle of a plunge through a dense host of meteoroids.

There are many other showers besides the two I have mentioned. Some shooting stars are to be seen almost every fine night, and those astronomers who pay particular attention to this subject are able to make out scores of small showers which might not interest you. Each of these is fully defined by the night of the year on which it occurs and the position of the point in the heavens from which the meteors radiate. Of these I must mention one. It is not usually very attractive, but it has a particular interest, as I shall now explain.

On the 27th of November, 1872, a beautiful meteoric shower took place. You will notice that though the month is the same, the day is entirely different from that on which the Leonids appear. This shower of the 27th is called the Andromedes, because the lines of direction of the shooting stars of which it is composed seem to diverge from a point in the constellation Andromeda. Ordinarily speaking, there is no special display of meteors connected with the annual return of this day; but in 1872 astronomers were astonished by an exhibition of shooting stars belonging to this system. They were not at all bright when compared with the Leonid meteors. They were, however, sufficiently numerous to arrest the attention of very many, even among those who do not usually pay much attention to the heavens.

The chief interest of the shower of Andromedes centers in a remarkable discovery connecting meteors and comets. There is a comet which was discovered by the astronomer Biela. It is a small object, requiring a telescope to show it. This comet completes each revolution in a period of about seven years; or rather, I should say, that was the time which the comet used to spend on its journey, for a life of trouble and disaster seems of late to have nearly extinguished the unfortunate object. In 1872 the comet was due in our neighborhood, and on the night of the 27th of November, in the same year, the earth crossed the track, and, in doing so, the shower of shooting stars was seen. This was a remarkable coincidence. We crossed the path of the comet at the time when we knew the comet ought to be there; and though we did not then see the comet, we saw a shower of shooting stars, and a wonderful shower too. A circumstance so peculiar suggested at once that the comet and the shooting stars must in some way or other be connected together. This is a suggestion we can test in another manner. We know the history of the comet, and we are aware that at the very time of the shower, the comet was approaching from the direction of the constellation of Andromeda. It was coming, in fact, from the very quarter whence the shooting stars have themselves travelled. Taking all these things together, it seems impossible to doubt that the shoal of shooting stars was, if not actually the comet itself, something closely connected with that famous body.

METEORITES.

Some years ago, a farmer living near Rowton, in Shropshire, noticed on a path in a field a hole which had been suddenly made by some mysterious and unknown agent. The laborers who were near told him they had just heard a remarkable noise; and when the farmer put his hand down into the hole, he felt something hot at the bottom of it. He took a spade and dug up the strange body, and found it to be a piece of iron, weighing about seven pounds. He was naturally amazed at such an occurrence, and brought the body home with him.

Where did that piece of iron come from? It is plain that it could not have been always in the ground. The noise and the recently made hole showed that was not the case, and that is confirmed by the fact that the iron was hot. A piece of iron within a few feet of the earth’s surface cannot have remained warm for any length of time. It is therefore clear that the iron must have tumbled from the sky. This is a marvellous notion; in fact, it seems so incredible that at first people refused to believe that such things as stones or solid lumps of iron could have fallen from the heavens to the earth. But they had to believe it; the evidence was too conclusive. Fortunately, however, the occurrence is a comparatively rare one; indeed, our life on this globe would have an intolerable anxiety added to it if showers of iron hailstones like that at Rowton were at all of frequent occurrence. We should want umbrellas of a more substantial description than those which suffice for the rains we actually experience. There are, indeed, instances on record of persons having been killed by the fearful blows given by these bodies in falling.

The Rowton siderite is a comparatively small one; pieces weighing hundredweights, and even tons, have been collected together in our museums. I would recommend you to pay a visit to that interesting room in our great British Museum in which these meteorites are exhibited. There we see actual specimens of celestial bodies which we can feel or weigh, and which our chemists can analyze. It may be noticed that they only contain substances that we already know on this earth. This celestial iron has often been made use of in primitive times before man understood how to smelt iron from its ore and how to transform it from cast iron to wrought iron. Nature seems to have taken heed of their wants, and occasionally to have thrown down a lump or two for the benefit of those who were so fortunate as to secure them.

That these stones or irons drop from the sky is absolutely certain, but when we try to find out their earlier history we become involved in not a few difficulties. Nobody really knows the true history of these objects, but the view of their origin which seems to me to possess fewer difficulties than any other view is that which we may call the Columbiad Theory. I use this expression because every boy or girl listening to me ought to have read Jules Verne’s wonderful book, “From the Earth to the Moon,” and if any of you have not read it, the sooner you do so the better. It is there narrated how the gun club of Baltimore designed a magnificent cannon which was sunk deep into the ground, and then received a terrific charge of guncotton, on which a great hollow projectile was carefully lowered, containing inside the three adventurous explorers who desired to visit the moon. Calculations were produced with a view of showing that by firing on a particular day the explosion would drive the projectile up to the moon. There was, however, the necessary condition that the speed of projection should be great enough. The gun club were accurate in saying that if the cannon were able to discharge the projectile with a speed twenty or thirty times as great as that which had ever been obtained with any other cannon, then the missile would ascend up and up forever if no further influence were exerted on it. No doubt we have to overlook the resistance on the air and a few other little difficulties, but to this extent, at all events, the gun club were right: that a velocity of about six or seven miles a second would suffice to carry a body away from the gravitation of the earth.

No one supposes that there were ever Columbiad cannons on our globe by which projectiles were shot up into space; but it seems possible that there may have been in very ancient days volcanoes on the earth with a shooting power as great as that which President Barbicane designed for the big cannon.

Even now we have some active volcanoes of great energy on our earth, and we know that in former days the volcanoes must have been still more powerful; that, in fact, the Vesuvius of the present must be merely a popgun in comparison with volcanoes which have shaken the earth in those primitive days when it had just cooled down from its original fiery condition. Some of these early volcanoes, in the throes of their mighty eruptions, appear to have shot forth pieces of iron and volcanic substances with a violence great enough to carry them off into space.

Suppose that a missile were projected upwards, it would ascend higher and higher, and gravity would, of course, tend to drag it back again down to earth. It can be shown that with an initial speed of six or seven miles a second the missiles would never return to the earth if only influenced by its attraction. The subsequent history of such a projectile would be guided by the laws according to which a planet moves. The body is understood to escape the destination which was aimed at by the Columbiad. I mean, of course, that it is not supposed to hit the moon. Of course, this might conceivably happen; but most of the projectiles would go quite wide of the mark, and would travel off into space.

Though the earth would be unable to recall the projectile, the attraction of the sun would still guide it, whether it was as big as a paving-stone or ever so much larger or smaller. The body would be constrained to follow a path like a little planet around the sun. This track it would steadily pursue for ages. The wanderer would, however, cross the earth’s track once during each of its revolutions at the point from which it was projected. Of course, it will generally happen that the earth will not be there at the time the meteorite is crossing, and the meteorite will not be there at the time the earth is crossing. Nothing will therefore happen, and the object goes again on its long rounds. But sometimes it must occur that a meteor does not get past the junction without coming so close to the earth that it plunges into the air, often producing a noise and generating a streak of light like a shooting star. Then it tumbles down, and is restored to that earth whence it originally came.

If this be the true view--and I think there are less weighty objections to it than to any other I know of--then the history of the piece of iron that was found in Shropshire would be somewhat as follows. Many millions of years ago, when the fires of our earth were much more vigorous than they are in these dull times, a terrific volcanic outbreak took place, and vast quantities of material were shot into space, of which this is one of the fragments. During all the ages that have since elapsed this piece of iron has followed its lonely track. In a thousandth part of the time rust and decay would have destroyed it had it lain on the earth, but in the solitudes of space there was found no air or damp to produce corrosion. At last, after the completion of its long travels, it again crashed down on the earth.

We have now briefly surveyed the extent of the solar system. We began with the sun, which presides over all, and then we discussed the various planets with their satellites, next we considered the eccentric comets, and finally the minute bodies which, as shooting stars or meteorites, must be regarded as forming part of the Sun’s system. In our closing lecture we shall have to deal with objects of a far more magnificent character.

LECTURE VI.

STARS.

We try to make a Map--The Stars are Suns--The Numbers of the Stars--The Clusters of Stars--The Rank of the Earth as a Globe in Space--The Distances of the Stars--The Brightness and Color of Stars--Double Stars--How we find what the Stars are made of--The Nebulæ--What the Nebulæ are made of--Photographing the Nebulæ--Conclusion.

WE TRY TO MAKE A MAP.

The group of bodies which cluster around our sun forms a little island, so to speak, in the extent of infinite space. We may illustrate this by a map in which we shall endeavor to show the stars placed at their proper relative distances. We first open the compasses one inch, and thus draw a little circle to represent the path of the earth. We are not going to put in all the planets. We take Neptune, the outermost, at once. To draw its path I open the compasses to thirty inches, and draw a circle with that radius. That will do for our solar system, though the comets no doubt will roam beyond these limits. To complete our map we ought of course to put in some stars. There are a hundred million to choose from, and we shall begin with the brightest. It is often called the Dog Star, but astronomers know it better as Sirius. Let us see where it is to be placed on our map. Sirius is beyond Neptune, so it must be outside somewhere. Indeed, it is a good deal further off than Neptune; so I try at the edge of the drawing-board; I have got a method of making a little calculation that I do not intend to trouble you with, but I can assure you that the results it leads me to are quite correct; they show me that this board is not big enough. But could a board which was big enough fit into this lecture theatre? Here, again, I make my little calculations, and I find that there would not be room for a board sufficiently great; in fact, if I put the sun here at one end, with its planets around it, Sirius would be too near on the same scale if it were at the further corner. The board would have to go out through the wall of the theatre, out through London. Indeed, big as London is, it would not be large enough to contain the drawing-board that I should require. It would have to stretch about twenty miles from where we are now assembled. We may therefore dismiss any hope of making a practical map of our system on this scale if Sirius is to have its proper place. Let us, then, take some other star. We shall naturally try with the nearest of all. It is one that we do not know in this part of the world, but those that live in the southern hemisphere are well acquainted with it. The name of this star is Alpha Centauri. Even for this star, we should require a drawing three or four miles long if the distance from the earth to the sun is to be taken as one inch. You see what an isolated position our sun and his planets occupy. The members of the family are all close together, and the nearest neighbors are situated at enormous distances. There is a good reason for this separation. The stars are very pretty and perfectly harmless to us where they are at present situated. They might be very troublesome neighbors if they were very much closer to our system. It is therefore well they are so far off; they would be constantly making disturbance in the sun’s family if they were near at hand. Sometimes they would be dragging us into unpleasantly great heat by bringing us too close to the sun, or producing a coolness by pulling us away from the sun, which would be quite as disagreeable.

THE STARS ARE SUNS.

We are about to discuss one of the grandest truths in the whole of nature. We have had occasion to see that this sun of ours is a magnificent globe immensely larger than the greatest of his planets, while the greatest of these planets is immensely larger than this earth; but now we are to learn that our sun is, indeed, only a star not nearly so bright as many of those which shine over our heads every night. We are comparatively close to the sun, so that we are able to enjoy his beautiful light and cheering heat. Each of those other myriads of stars is a sun, and the splendor of those distant suns is often far greater than that of our own. We are, however, so enormously far from them that they appear dwindled down to insignificance. To judge impartially between our sun or star and such a sun or star as Sirius we should stand halfway between the two; it is impossible to make a fair estimate when we find ourselves situated close to one star and a million times as far from the other. After allowance is made for the imperfections of our point of view, we are enabled to realize the majestic truth that the sun is no more than a star, and that the other stars are no less than suns. This gives us an imposing idea of the extent and the magnificence of the universe in which we are situated. Look up at the sky at night--you will see a host of stars; try to think that every one of them is itself a sun. It may probably be that those suns have planets circulating round them, but it is hopeless for us to expect to see such planets. Were you standing on one of those stars and looking towards our system, you would not perceive the sun to be the brilliant and gorgeous object that we knew so well. If you could see him at all, he would merely seem like a star, not nearly so bright as many of those you can see at night. Even if you had the biggest of telescopes to aid your vision, you could never discern from one of these bodies the planets which surround the sun. No astronomer in the stars could see Jupiter even if his sight were a thousand times as good or his telescopes a thousand times as powerful as any sight or telescope that we know. So minute an object as our earth would, of course, be still more hopelessly beyond the possibility of vision.

THE NUMBERS OF THE STARS.

To count the stars involves a task which lies beyond the power of man to accomplish. Even without the aid of any telescope, we can see a great multitude of stars from this part of the world. There are also many constellations in the southern hemisphere which never appear above our horizon. If, however, we were to go to the equator, then, by waiting there for a twelve-month, all the stars in the heavens would have been successively exposed to view. An astronomer, Houzeau, with the patience to count them, enumerated about 6000. This is the naked-eye estimate of the star-population of the heavens; but if, instead of relying on unaided vision, you get the assistance of a little telescope, you will be astounded at the enormous multitude of stars which are disclosed.