Through Magic Glasses and Other Lectures Part 9

[Ill.u.s.tration: Fig. 49.

The spectroscope attached to the telescope for the examination of the sun. (Lockyer.)

P, Pillar of Telescope. T, Telescope. S, Finder or small telescope for pointing the telescope in position. _a_, _a_, _b_, Supports fastening the spectroscope to the telescope. _d_, Collimator or tube carrying the slit at the end nearest the telescope, and a lens at the other end to render the rays parallel. _c_, Plate on which the prisms are fixed. _e_, Small telescope through which the observer examines the spectrum after the ray has been dispersed in the prisms. _h_, Micrometer for measuring the relative distance of the lines.]

When an astronomer wishes to examine the spectrum of any special part of the sun, he takes off the eye-piece of his telescope and screws the spectroscope upon the draw-tube. The spectroscope is made exactly like the large one for ordinary work. The tube _d_ (Fig. 49) carries the slit at the end nearest the telescope, and this slit must be so placed as to stand precisely at the princ.i.p.al focus of the lens where the sun's image is formed (see _i_, _i_, p. 44). This comes to exactly the same thing as if we could put the slit close against the face of the sun, so as to show only the small strip which it covers, and by moving it to one part or another of the image we can see any point that we wish and no other.

The light then pa.s.ses through the tube _d_ into the round of prisms standing on the tray _c_, and the observer looking through the small telescope _e_ sees the spectrum as it emerges from the last prism. In this way astronomers can examine the spectrum of a spot, or part of a spot, or of a bright streak, or any other mark on the sun's face.

Now in looking at the prominences we have seen that the difficulty is caused by the sunlight, between us and them, overpowering the bright lines of the gas, nor could we overcome this if it were not for a difference which exists between the two kinds of light. The more you disperse or spread out the continuous sun-spectrum the fainter it becomes, but in spreading out the bright lines of the gas you only send them farther and farther apart; they themselves remain almost as bright as ever. So, when the telescope forms an image of the red flame in front of the slit, though the glowing gas and the sunlight both send rays into the spectroscope, you have only to use enough prisms and arrange them in such a way that the sunlight is dispersed into a very long faint spectrum, and then the bright lines of the flames will stand out bright and clear. Of course only a small part of the long spectrum can be seen at once, and the lines must be studied separately. On the other hand, if you want to compare the strong light of the sun with the bright lines of the prominences, you place the slit just at the edge of the sun's image in the telescope, so that half the slit is on the sun's face and half on the prominence. The prisms then disperse the sunlight between you and the prominences, while they only lessen the strong light of the sun itself, which still shows clearly. In this way the two spectra are seen side by side and the dark and bright lines can be compared accurately together (see Fig. 50).

[Ill.u.s.tration: Fig. 50.

Bright lines of prominences.

Sun-spectrum with dark lines.]

Wherever the telescope is turned all round the sun the lines of luminous gas are seen, showing that they form a complete layer outside the photosphere, or light-giving ma.s.s, of the sun. This layer of luminous gases is called the _chromosphere_, or coloured sphere. It lies between the photosphere and the corona, and is supposed to be at least 5000 miles deep, while, as we have seen, the flames shoot up from it to fabulous heights.

The quiet red flames are found to be composed of hydrogen and another new metal called helium; but lower down, near the sun's edge, other bright lines are seen, showing that sodium, magnesium, and other metals are there, and when violent eruptions occur these often surge up and mingle with the purer gas above. At other times the eruptions below fling the red flames aloft with marvellous force, as when Professor Young saw a long low-lying cloud of hydrogen, 100,000 miles long, blown into shreds and flung up to a height of 200,000 miles, when the fragments streamed away and vanished in two hours. Yet all these violent commotions and storms are unseen by us on earth unless we look through our magic gla.s.ses.

You will wonder no doubt how the spectroscope can show the height and the shape of the flames. I will explain to you, and I hope to show them you one day. You must remember that the telescope makes a small real image of the flame at its focus, just as in one of our earlier experiments you saw the exact image of the candle-flame upside down on the paper (see p. 33). The reason why we only see a strip of the flame in the spectroscope is because the slit is so narrow. But when once the sunlight was dispersed so as no longer to interfere, Dr. Huggins found that it is possible to open the slit wide enough to take in the image of the whole flame, and then, by turning the spectroscope so as to bring one of the bright hydrogen lines into view, the actual shape of the prominence is seen, only it will look a different colour, either red, greenish-blue, or indigo-blue, according to the line chosen. As the image of the whole sun and its appendages in the telescope is so very small, you will understand that even a very narrow slit will really take in a very large prominence several thousand miles in length. Fig 51 shows a drawing by Mr. Lockyer of a group of flames he observed very soon after Dr. Huggins suggested the open slit, and these shapes did not last long, for in another picture he drew ten minutes later their appearance had already changed.

[Ill.u.s.tration: Fig. 51.

Red prominences, as drawn by Mr. Lockyer during the total eclipse of March 14, 1869.]

These then are some of the facts revealed to us by our magic gla.s.ses. I scarcely expect you to remember all the details I have given you, but you will at least understand now how astronomers actually penetrate into the secrets of the sun by bringing its image into their observatory, as we brought it to-day on the card-board, and then making it tell its own tale through the prisms of the spectroscope; and you will retain some idea of the central light of the sun with its surrounding atmosphere of cooler gases and its layer of luminous lambent gases playing round it beyond.

Of the corona I cannot tell you much, except that it is far more subtle than anything we have spoken of yet; that it is always strongest when the sun is most spotted; that it is partly made up of self-luminous gases whose bright lines we can see, especially an unknown green ray; while it also shines partly by reflected light from the sun, for we can trace in it faint dark lines; lastly it fades away into the mysterious zodiacal light, and so the sun ends in mystery at its outer fringe as it began at its centre.

And now at last, having learnt something of the material of the sun, we can come back to the spots and ask what is known about them. As I have said, they are not always the same on the sun's face. On the contrary, they vary very much both in number and size. In some years the sun's face is quite free from them, at others there are so many that they form two wide belts on each side of the sun's equator, with a clear s.p.a.ce of about six degrees between. No spots ever appear near the poles. Herr Schwabe, who watched the sun's face patiently for more than thirty years, has shown that it is most spotted about every eleven years, then the spots disappear very quickly and reappear slowly till the full-spot time comes round again.

Some spots remain a very short time and then break up and disappear, but others last for days, weeks, and even months, and when we watch these, we find that a spot appears to travel slowly across the face of the sun from east to west and then round the western edge so that it disappears.

It is when it reaches the edge that we can convince ourselves that the spot is really part of the sun, for there is no s.p.a.ce to be seen between them, the edge and the spot are one, as the last trace of the dark blotch pa.s.ses out of sight. In fact, it is not the spot which has crossed the sun's face, but the sun itself which has turned, like our earth, upon its axis, carrying the spot round with it. As some spots remain long enough to reappear, after about twelve or thirteen days, on the opposite edge, and even pa.s.s round two or three times, astronomers can reckon that the sun takes about twenty-five days and five hours in performing one revolution. You will wonder why I say only _about_ twenty-five, but I do so because all spots do not come round in exactly the same time, those farthest from the equator lag rather more than a day behind those nearer to it, and this is explained by the layer of gases in which they are formed, drifting back in higher lat.i.tudes as the sun turns.

It is by watching a spot as it travels across the sun, that we are able to observe that the centre partlies deeper in the sun's face than the outer rim. There are many ways of testing this, and you can try one yourselves with a telescope if you watch day after day. I will explain it by a simple experiment. I have here a round lump of stiff dough, in which I have made a small hollow and blackened the bottom with a drop of ink. As I turn this round, so that the hollow facing you moves from right to left, you will see that after it pa.s.ses the middle of the face, the hole appears narrower and narrower till it disappears, and if you observe carefully you will note that the dark centre is the first thing you lose sight of, while the edges of the cup are still seen, till just before the spot disappears altogether. But now I will stick a wafer on, and a pea half into, the dough, marking the centre of each with ink.

Then I turn the ball again. This time you lose sight of the foremost edge first, and the dark centre is seen almost to the last moment. This shows that if the spots were either flat marks, or hillocks, on the sun's face, the dark centre would remain to the last, but as a fact it disappears before the rim. Father Secchi has tried to measure the depth of a spot-cavity, and thinks they vary from 1000 to 3000 miles deep. But there are many difficulties in interpreting the effects of light and shadow at such an enormous distance, and some astronomers still doubt whether spots are really depressions.

For many centuries now the spots have been watched forming and dispersing, and this is roughly speaking what is seen to happen. When the sun is fairly clear and there are few spots, these generally form quietly, several black dots appearing and disappearing with bright streaks or _faculae_ round their edge, till one grows bigger than the rest, and forms a large dark nucleus, round which, after a time, a half-shadow or _penumbra_ is seen and we have a sun-spot complete, with bright edges, dark shadow, and deep black centre (Fig. 52). This lasts for a certain time and then it becomes bridged over with light streaks, the dark spot breaks up and disappears, and last of all the half-shadow dies away.

[Ill.u.s.tration: Fig. 52.

A quiet sun-spot. (Secchi.)]

But things do not always take place so quietly. When the sun's face is very troubled and full of spots, the bright _faculae_, which appear with a spot, seem to heave and wave, and generally several dark centres form with whirling ma.s.ses of light round them, while in some of them tongues of fire appear to leap up from below (Fig. 53). Such spots change quickly from day to day, even if they remain for a long time, until at last by degrees the dark centres become less distinct, the half-shadows disappear, leaving only the bright streaks, which gradually settle down into luminous points or _light granules_. These light granules are in fact supposed by astronomers to be the tips of glowing clouds heaving up everywhere, while the dark s.p.a.ces between them are cooler currents pa.s.sing downwards.

[Ill.u.s.tration: Fig. 53.

A tumultuous sun-spot. (Langley.)]

Below these clouds, no doubt, the great ma.s.s of the sun is in a violent state of heat and commotion, and when from time to time, whether suddenly or steadily, great upheavals and eruptions take place, bright flames dart up and luminous clouds gather and swell, so that long streaks or _faculae_ surge upon the face of the sun.

Now these hot gases rising up thus on all sides would leave room below for cooler gases to pour down from above, and these, as we know, would cut off, or absorb, much of the light coming from the body of the sun, so that the centre, where the down current was the strongest, would appear black even though some light would pa.s.s through. This is the best explanation we have as yet of the formation of a sun-spot, and many facts shown in the spectroscope help to confirm it, as for example the thickening of the dark lines of the spectrum when the slit is placed over the centre of a spot, and the flashing out of bright lines when an uprush of streaks occurs either across the spots or round it.

And now, before you go, I must tell you of one of these wonderful uprushes, which sent such a thrill through our own atmosphere, as to tell us very plainly the power which the sun has over our globe. The year 1859 was remarkable for sun-spots, and on September 1, when two astronomers many miles apart were examining them, they both saw, all at once, a sudden cloud of light far brighter than the general surface of the sun burst out in the midst of a group of spots. The outburst began at eight minutes past eleven in the forenoon, and in five minutes it was gone again, but in that time it had swept across a s.p.a.ce of 35,000 miles on the sun! Now both before and after this violent outburst took place a magnetic storm raged all round the earth, brilliant auroras were seen in all parts of the world, sparks flashed from the telegraph wires, and the telegraphic signalmen at Washington and Philadelphia received severe electric shocks. Messages were interrupted, for the storm took possession of the wires and sent messages of its own, the magnetic needles darting to and fro as though seized with madness. At the very instant when the bright outburst was seen in the sun, the self-registering instruments at Kew marked how three needles jerked all at once wildly aside; and the following night the skies were lit up with wondrous lights as the storm of electric agitation played round the earth.

We are so accustomed to the steady glow of sunshine pouring down upon us that we pay very little heed to daylight, though I hope none of us are quite so ignorant as the man who praised the moon above the sun, because it shone in the dark night, whereas the sun came in the daytime when there was light enough already! Yet probably many of us do not actually realise how close are the links which bind us to our brilliant star as he carries us along with him through s.p.a.ce. It is only when an unusual outburst occurs, such as I have just described, that we feel how every thrill which pa.s.ses through our atmosphere, through the life-current of every plant, and through the fibre and nerve of every animal has some relation to the huge source of light, heat, electricity, and magnetism at which we are now gazing across a s.p.a.ce of more than 93,000,000 miles.

Yet it is well to remember that the sudden storm and the violent eruption are the exceptional occurrences, and that their use to us as students is chiefly to lead us to understand the steady and constant thrill which, never ceasing, never faltering, fulfils the great purpose of the unseen Lawgiver in sustaining all movement and life in our little world.

CHAPTER VII

AN EVENING AMONG THE STARS

[Ill.u.s.tration]

"Do you love the stars?" asked the magician of his lads, as they crowded round him on the college green, one evening in March, to look through his portable telescope.

"Have you ever sat at the window on a clear frosty night, or in the garden in summer, and looked up at those wondrous lights in the sky, pondering what they are, and what purpose they serve?"

I will confess to you that when I lived in London I did not think much about the stars, for in the streets very few can be seen at a time even on a clear night; and during the long evenings in summer, when town people visit the country, you must stay up late to see a brilliant display of starlight. It is when driving or walking across country on a winter's evening week after week, and looking all round the sky, that the glorious suns of heaven force you to take notice of them; and Orion becomes a companion with his seven brilliant stars and his magnificent nebula, which appears as a small pale blue patch, to eyes accustomed to look for it, when the night is very bright and clear. It is then that Charles's Wain becomes quite a study in all its different positions, its horses now careering upwards, now plunging downwards, while the waggon, whether upwards or downwards, points ever true, by the two stars of its tail-board, to the steadfast pole-star.

It is on such nights as these that, looking southward from Orion, we recognise the dog-star Sirius, bright long before other stars have conquered the twilight, and feast our eye upon his glorious white beams; and then, turning northwards, are startled by the soft l.u.s.trous sheen of Vega just appearing above the horizon.

But stop, I must remember that I have not yet introduced you to these groups of stars; and moreover that, though we shall find them now in the positions I mention, yet if you look for them a few hours later to-night, or at the same hour later in the year, you will not find them in the same places in the sky. For as our earth turns daily on its axis, the stars _appear_ to alter their position hour by hour, and in the same way as we travel yearly on our journey round the sun, they _appear_ to move in the sky month by month. Yet with a little practice it is easy to recognise the princ.i.p.al stars, for, as it is our movement and not theirs which makes us see them in different parts of the sky, they always remain in the same position with regard to each other. In a very short time, with the help of such a book as Proctor's _Star Atlas_; you could pick out all the chief constellations and most conspicuous stars for yourselves.

One of the best ways is to take note of the stars each night as they creep out one by one after sunset. If you take your place at the window to-morrow night as the twilight fades away, you will see them gradually appear, now in one part, now in another of the sky, as

"One by one each little star Sits on its golden throne."

The first to appear will be Sirius or the dog-star (see Fig. 54), that pure white star which you can observe now rather low down to the south, and which belongs to the constellation _Canis Major_. As Sirius is one of the most brilliant stars in the sky, he can be seen very soon after the sun is gone at this time of year. If, however, you had any doubt as to what star he was, you would not doubt long, for in a little while two beautiful stars start into view above him more to the west, and between them three smaller ones in a close row, forming the cross in the constellation of Orion, which is always very easy to recognise. Now the three stars of Orion's belt which make the short piece of the cross always point to Sirius, while Betelgeux in his right shoulder, and Rigel in his left foot (see Figs. 54 and 55), complete the long piece, and these all show very early in the twilight. You would have to wait longer for the other two leading stars, Bellatrix in the right shoulder and [Greek: k] Orionis in the right leg, for these stars are feebler and only seen when the light has faded quite away.

[Ill.u.s.tration: Fig. 54.

Some of the constellations seen when looking south in March from six to nine o'clock.]

By that time you would see that there are an immense number of stars in Orion visible even to the naked eye, besides the veil of misty, tiny stars called the "Milky Way" which pa.s.ses over his arm and club. Yet the figure of the huntsman is very difficult to trace, and the seven bright stars, the five of the cross and those in the left arm and knee, are all you need remember.

No! not altogether all, for on a bright clear night like this you can detect a faint greenish blue patch (N, Fig. 54) just below the belt, and having a bright star in the centre. This is called the "Great Nebula" or mist of Orion (see Frontispiece). With your telescopes it looks very small indeed, for only the central and brightest part is seen. Really, however, it is so widespread that our whole solar system is as nothing compared to it. But even your telescopes will show, somewhere near the centre, what appears to be a bright and very beautiful star (see Fig.

55) surrounded by a darker s.p.a.ce than the rest of the nebula, while in my telescope you will see many stars scattered over the mist.

[Ill.u.s.tration: Fig. 55.

Chief stars of Orion, with Aldebaran. (After Proctor.)]