Through Magic Glasses and Other Lectures Part 10

Now first let me tell you that these last stars do not, so far as we know, lie _in_ the nebula, but are scattered about in the heavens between us and it, perhaps millions of miles nearer our earth. But with the bright star in the centre it is different, for the spectroscope tells us that the mist pa.s.ses _over_ it, so that it is either behind or in the nebula. Moreover, this star is very interesting, for it is not really one star, but six arranged in a group (see Fig. 56). You can see four distinctly through my telescope, forming a trapezium or four-sided figure, and more powerful instruments show two smaller ones. So [Greek: th] Orionis, or the Trapezium of Orion, is a multiple star, probably lying in the midst of the nebula.

[Ill.u.s.tration: Fig. 56.

The trapezium, [Greek: th] Orionis, in the nebula of Orion. (Herschel.)]

The next question is, What is the mist itself composed of? For a long time telescopes could give us no answer. At last one night Lord Rosse, looking through his giant telescope at the densest part of the nebula, saw myriads of minute stars which had never been seen before. "Then,"

you will say, "it is after all only a cl.u.s.ter of stars too small for our telescopes to distinguish." Wait a bit; it is always dangerous to draw hasty conclusions from single observations. What Lord Rosse said was true as to that particular part of the nebula, but not the whole truth even there, and not at all true of other parts, as the spectroscope tells us.

For though the light of nebulae, or luminous mists, is so faint that a spectrum can only be got by most delicate operations, yet Dr. Huggins has succeeded in examining several. Among these is the nebula of Orion, and we now know that when the light of the mist is spread out it gives, not a continuous band of colour such as would be given by stars, but _faint coloured lines_ on a dark ground (see Fig. 57). Such lines as these we have already learnt are always given by gases, and the particular bright lines thrown by Orion's nebula answer to those given by nitrogen and hydrogen, and some other unknown gases. So we learn at last that the true mist of the nebula is formed of glowing gas, while parts have probably a great number of minute stars in them.

[Ill.u.s.tration: Fig. 57.

Nebula-spectrum.

Sun-spectrum.

Spectrum of Orion's Nebula, showing bright lines, with sun-spectrum below for comparison.]

Till within a very short time ago only those people who had access to very powerful telescopes could see the real appearance of Orion, for drawings made of it were necessarily very imperfect; but now that telescopes have been made expressly for carrying photographic appliances, even these faint mists print their own image for us. In 1880 Professor Draper of America photographed the nebula of Orion, in March 1881 Mr. Common got a still better effect, and last year Mr. Isaac Roberts succeeded in taking the most perfect and beautiful photograph[1]

yet obtained, in which the true beauty of this wonderful mist stands out clearly. I have marked on the edge of our copy two points [Greek: th]

and [Greek: th]', and if you follow out straight lines from these points till they meet, you will arrive at the spot where the multiple star lies. It cannot, however, be seen here, because the plate was exposed for three hours and a half, and after a time the mist prints itself so densely as to smother the light of the stars. Look well at this photograph when you go indoors and fix it on your memory, and then on clear nights accustom your eye to find the nebula below the three stars of the belt, for it tells a wonderful story.

[1] Reproduced in the Frontispiece with Mr. Roberts's kind permission.

The star-halo at the top of the plate is caused by diffraction of light in the telescope, and comes only from an ordinary star.

More than a hundred years ago the great German philosopher Kant suggested that our sun, our earth, and all the heavenly bodies might have begun as gases, and the astronomer Laplace taught this as the most likely history of their formation. After a few years, however, when powerful telescopes showed that many of the nebulae were only cl.u.s.ters of very minute stars, astronomers thought that Laplace's teaching had been wrong. But now the spectroscope has revealed to us glowing gas actually filling large s.p.a.ces in the sky, and every year accurate observations and experiments tell us more and more about these marvellous distant mists. Some day, though perhaps not while you or I are here to know it, Orion's nebula, with its glowing gas and minute star-dust, may give some clue to the early history of the heavenly bodies; and for this reason I wish you to recognise and ponder over it, as I have often done, when it s.h.i.+nes down on the rugged moor in the stillness of a clear frosty winter's night.

But we must pa.s.s on for, while I have been talking, the whole sky has become bespangled with hundreds of stars. That glorious one to the west, which you can find by following (Fig. 54) a curved line upwards from Betelgeux, is the beautiful red star Aldebaran or the hindmost; so called by the Arabs, because he drives before him that well-known cl.u.s.ter, the Pleiades, which we reach by continuing the curve westwards and upwards. Stop to look at this cl.u.s.ter through your telescopes, for it will delight you; even with the naked eye you can count from six to ten stars in it, and an opera-gla.s.s will show about thirty, though they are so scattered you will have to move the gla.s.s about to find them. Yet though my telescope shows a great many more, you cannot even count all the chief ones through it, for in powerful telescopes more than 600 stars have been seen in the single cl.u.s.ter! while a photograph taken by Mr. Roberts shows also four lovely patches of nebula.

And now from the Pleiades let us pa.s.s on directly overhead to the beautiful star Capella, which once was red but now is blue, and drop down gently to the south-east, where Castor and Pollux, the two most prominent stars in the constellation "Gemini" or the twins, show brilliantly against the black sky. Pause here a moment, for I want to tell you something about Castor, the one nearest to Capella. If you look at Castor through your telescopes, some of you may possibly guess that it is really two stars, but you will have to look through mine to see it clearly. These two stars have been watched carefully for many years, and there is now no doubt that one of them is moving slowly round the other.

Such stars as these are called "binary," to distinguish them from stars that merely _appear_ double because they stand nearly in a line one behind the other in the heavens, although they may be millions of miles apart. But "binary" stars are actually moving in one system, and revolve round each other as our earth moves round the sun.

I wonder if it strikes you what a grand discovery this is? You will remember that it is gravitation which keeps the moon held to the earth so that it moves round in a circle, and which keeps the earth and other planets moving round the sun. But till these binary stars were discovered we had no means of guessing that this law had any force beyond our own solar system. Now, however, we learn that the same law and order which reigns in our small group of planets is in action billions of miles away among distant suns, so that they are held together and move round each other as our earth moves round our sun. I will repeat to you what Sir R. Ball, the Astronomer-Royal of Ireland, says about this, for his words have remained in my mind ever since I read them, and I should like them to linger in yours till you are old enough to feel their force and grandeur. "This discovery," he writes, "gave us knowledge we could have gained from no other source. From the binary stars came a whisper across the vast abyss of s.p.a.ce. That whisper told us that the law of gravitation is not peculiar to the solar system.

It gives us grounds for believing that it is obeyed throughout the length, the breadth, the depth, and the height of the entire universe."[1]

[1] _The Story of the Heavens._

And now, leaving Castor and going round to the east, we pa.s.s through the constellation Leo or the Lion, and I want you particularly to notice six stars in the shape of a sickle, which form the front part of the lion, the brightest, called Regulus, being the end of the handle.[1] This sickle is very interesting, because it marks the part of the heavens from which the brilliant shower of November meteors radiates once in thirty-three years. This is, however, too long a story to be told to-night, so we will pa.s.s through Leo, and turning northwards, look high up in the north-east (Fig. 58), where "Charles's Wain" stretches far across the sky. I need not point this out to you, for every country lad knows and delights in it. You could not have seen it in the twilight when Sirius first shone out, for these stars are not so powerful as he is. But they come out very soon after him, and when once fairly bright, the four stars which form the waggon, wider at the top than at the bottom, can never be mistaken, and the three stars in front, the last bending below the others, are just in the right position for the horses.

For this reason I prefer the country people's name of Charles's Wain or Waggon to that of the "Plough," which astronomers generally give to these seven stars. They really form part of an enormous constellation called the "Great Bear" (Fig. 59), but, as in the case of Orion, it is very difficult to make out the whole of Bruin in the sky.

[1] In Fig. 54 the sickle alone comes within the picture.

[Ill.u.s.tration: Fig. 58.

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

[Ill.u.s.tration: Fig. 59.

The Great Bear, showing the position of Charles's Wain, and also the small binary star [Greek: x] in the hind foot, whose period has been determined.]

Now, although most people know Charles's Wain when they see it, we may still learn a good deal about it. Look carefully at the second star from the waggon and you will see another star close to it, called by country people "Jack by the second horse," and by astronomers "Alcor." Even in your small telescopes you can see that Jack or Alcor is not so close as he appears to the naked eye, but a long way off from the horse, while in my telescope you will find this second horse (called Mizar) split up into two stars, one a brilliant white and the other a pale emerald green. We do not know whether these two form a binary, for they have not yet been observed to move round each other.

Take care in looking that you do not confuse the stars one with another, for you must remember that your telescope makes objects appear upside down, and Alcor will therefore appear in it _below_ the two stars forming the horse.

But though we do not know whether Mizar is binary, there is a little star a long way below the waggon, in the left hind paw of the Great Bear ([Greek: x], Figs. 58 and 59), which has taught us a great deal, for it is composed of two stars, one white and the other grey, which move right round each other once in sixty years, so that astronomers have observed more than one revolution since powerful telescopes were invented. You will have to look in my telescope to see the two stars divided, but you can make an interesting observation for yourselves by comparing the light of this binary star with the light of Castor, for Castor is such an immense distance from us that his light takes more than a hundred years to reach us, while the light of this smaller star comes in sixty-one years, yet see how incomparably brighter Castor is of the two.

This proves that brilliant stars are not always the nearest, but that a near star may be small and faint and a far-off one large and bright.

[Ill.u.s.tration: Fig. 60.

The seven stars of Charles's Wain, showing the directions in which they are travelling. (After Proctor.)]

There is another very interesting fact known to us about Charles's Wain which I should like you to remember when you look at it. This is that the seven stars are travelling onwards in the sky, and not all in the same direction. It was already suspected centuries ago that, besides the _apparent_ motion of all the stars in the heavens caused by our own movements, they have each also a _real_ motion and are travelling in s.p.a.ce, though they are so inconceivably far off that we do not notice it. It has now been proved, by very accurate observations with powerful instruments, that three of the stars forming the waggon and the two horses nearest to it, together with Jack, are drifting forwards (see Fig. 60), while the top star of the tailboard of the waggon and the leader of the horses are drifting the other way. Thus, thousands of years hence Charles's Wain will most likely have quite altered its shape, though so very slowly that each generation will think it is unchanged.

One more experiment with Charles's Wain, before we leave it, will help you to imagine the endless millions of stars which fill the universe.

Look up at the waggon and try to count how many stars you can see inside it with the naked eye. You may, if your eye is keen, be able to count twelve. Now take an opera-gla.s.s and the twelve become two hundred. With your telescopes they will increase again in number. In my telescope upstairs the two hundred become hundreds, while in one of the giant telescopes, such as Lord Rosse's in Ireland, or the great telescope at Was.h.i.+ngton in the United States, thousands of stars are brought into view within that four-sided s.p.a.ce!

Now this part of the sky is not fuller of stars than many others; yet at first, looking up as any one might on a clear evening, we thought only twelve were there. Cast your eyes all round the heavens. On a clear night like this you may perhaps, with the naked eye, have in view about 3000 stars; then consider that a powerful telescope can multiply these by thousands upon thousands, so that we can reckon about 20,000,000 where you see only 3000. If you add to these the stars that rise later at night, and those of the southern hemisphere which never rise in our lat.i.tude, you would have in all about 50,000,000 stars, which we are able to see from our tiny world through our most powerful telescopes.

But we can go farther yet. When our telescopes fail, we turn to our other magic seer, the photographic camera, and trapping rays of light from stars invisible in the most powerful telescope, make them print their image on the photographic plate, and at once our numbers are so enormously increased that if we could photograph the whole of the heavens as visible from our earth, we should have impressions of at least 170,000,000 stars!

These numbers are so difficult to grasp that we had better pa.s.s on to something easier, and our next step brings us to the one star in the heavens which never appears to move, as our world turns. To find it we have only to draw a line upwards through the two stars in the tailboard of the waggon and on into s.p.a.ce. Indeed these two stars are called "the Pointers," because a line prolonged onwards from them will, with a very slight curve, bring us to the "Pole-star" (see Fig. 58). This star, though not one of the largest, is important, because it is very near that spot in the sky towards which the North Pole of our earth points.

The consequence is, that though all the other stars appear to move in a circle round the heavens, and to be in different places at different seasons, this star remains always in the same place, only appearing to describe a very tiny circle in the sky round the exact spot to which our North Pole points.

Month after month and year after year it s.h.i.+nes exactly over that thatched cottage yonder, which you see now immediately below it; and wherever you are in the northern hemisphere, if you once note a certain tree, or chimney, or steeple which points upwards to the Pole-star, it will guide you to it at any hour on any night of the year, though the other constellations will be now on one side, now on the other side of it.

The Pole-star is really the front horse of a small imitation of Charles's Wain, which, however, has never been called by any special name, but only part of the "Little Bear." Those two hind stars of the tiny waggon, which are so much the brightest, are called the "Guards,"

because they appear to move in a circle round the Pole-star night after night and year after year like sentries.

[Ill.u.s.tration: Fig. 61.

The constellation of Ca.s.siopeia, and the heavenly bodies which can be found by means of it.[1]]

[1] For Almach see Fig. 58, it has been accidentally omitted from this figure.

Opposite to them, on the farther side of the Pole-star, is a well-marked constellation, a widespread W written in the sky by five large stars; the second V of the W has rather a longer point than the first, and as we see it now the letter is almost upside down (see Fig. 58).

These are the five brightest stars in the constellation Ca.s.siopeia, with a sixth not quite so bright in the third stroke of the W. You can never miss them when you have once seen them, even though they lie in the midst of a dense layer of the stars of the Milky Way, and if you have any difficulty at first, you have only to look as far on the one side of the Pole-star as the top hind star of Charles's Wain is on the other, and you must find them. I want to use them to-night chiefly as guides to find two remarkable objects which I hope you will look at again and again. The first is a small round misty patch not easy to see, but which you will find by following out the _second_ stroke of the first V of the W. Beginning at the top, and following the line to the point of the V, continue on across the sky, and then search with your telescope till you catch a glimpse of this faint mist (_c_, Fig. 58; star-cl.u.s.ter, Fig.

61). You will see at once that it is sparkling all over with stars, for in fact you have actually before you in that tiny cl.u.s.ter more stars than you can see with the naked eye all over the heavens! Think for a moment what this means. One faint misty spot in the constellation Perseus, which we should have pa.s.sed over unheeded without a telescope, proves to be a group of more than 3000 suns!

The second object you will find more easily, for it is larger and brighter, and appears as a faint dull spot to the naked eye. Going back to Ca.s.siopeia, follow out the _second_ V in the W from the top to the point of the V and onwards till your eye rests upon this misty cloud, which is called the Great Nebula of Andromeda, and has sometimes been mistaken for a comet (Figs. 58 and 61). You will, however, be disappointed when you look through the telescope, for it will still only appear a mist, and you will be able to make nothing of it, except that instead of being of an irregular shape like Orion, it is elliptical; and in a powerful telescope two dark rifts can be seen separating the streams of nebulous matter. These rifts are now shown in a photograph taken by Mr. Roberts, 1st October 1888, to be two vast dusky rings lying between the spiral stream of light, which winds in an ellipse till it ends in a small nucleus at the centre.

Ah! you will say, this must be a cloud of gas like Orion's nebula, only winding round and round. No! the spectroscope steps in here and tells us that the light shows something very much like a continuous spectrum, but not as long as it ought to be at the red end. Now, since gases give only bright lines, this nebula cannot be entirely gaseous. Then it must be made of stars too far off to see? If so, it is very strange that though it is so dense and bright in some parts, and so spread out and clear in others, the most powerful telescopes cannot break it up into stars. In fact, the composition of the great nebula of Andromeda is still a mystery, and remains for one of you boys to study when he has become a great astronomer.

Still one more strange star we will notice before we leave this part of the heavens. You will find it, or at least go very near it, by continuing northwards the line you drew from Ca.s.siopeia to the Star Cl.u.s.ter (_c_, Fig. 58), and as it is a bright star, you will not miss it. That is to say, it is bright to-night and will remain so till to-morrow night, but if you come to me about nine o'clock to-morrow evening I will show you that it is growing dim, and if we had patience to watch through the night we should find, three or four hours later still, that it looks like one of the smaller stars. Then it will begin to brighten again, and in four hours more will be as bright as at first.

It will remain so for nearly three days, or, to speak accurately, 2 days, 20 hours, 48 minutes, and 55 seconds, and then will begin to grow dull again. This star is called Algol the Variable. There are several such stars in the heavens, and we do not know why they vary, unless perhaps some dark globe pa.s.ses round them, cutting off part of their light for a time.

And now, if your eyes are not weary, let us go back to the Pole-star and draw a line from it straight down the horizon due north. Shortly before we arrive there you will see a very brilliant bluish-white star a little to the east of this line. This is Vega, one of the brightest stars in the heavens except Sirius. It had not risen in the earlier part of the evening, but now it is well up and will appear to go on, steadily mounting as it circles round the Pole-star, till at four o'clock to-morrow morning it will be right overhead towards the south.

But beautiful as Vega is, a still more interesting star lies close to it (see Fig. 58). This small star, called [Greek: e] Lyrae by astronomers, looks a little longer in one direction than in the other, and even with the naked eye some people can see a division in the middle dividing it into two stars. Your telescopes will show them easily, and a powerful telescope tells a wonderful story, for it reveals that each of these two stars is again composed of two stars, so that [Greek: e] Lyrae (Fig. 62) is really a double-double star. There is no doubt that each pair is a binary star, that is, the two stars move round each other very slowly, and possibly both pairs may also revolve round a common centre. There are at least 10,000 double stars in the heavens; though, as we have seen, they are not all binary. The list of binary stars, however, increases every year as they are carefully examined, and probably about one star in three over the whole sky is made up of more than one sun.