The constellation of Cassiopeia, and the heavenly bodies which can be found by means of it.[4]
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 Cassiopeia, 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-cluster, 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 cluster 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 passed 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 Cassiopeia, 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 Cassiopeia to the Star Cluster (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 passes 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 η Lyræ 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 η Lyræ (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.
Fig. 62.
