WHE

WHAT THE STARS ARE MADE OF.

HEN I was quite small, I lived a good deal in the country. On bright summer mornings the first thing that caught my waking eyes was the sunlight streaming in through the vines around my window. Sunrise gave to me, as well as to the birds and flowers, the signal for awakening. Such was the eagerness with which I welcomed the early sunbeams, that I think if I had been brought up among pagans I should have learned without much trouble to worship the great luminary. I believe that all children, certainly most little ones, are alike in that respect. The heavens are full of marvels for them, and suggest questions innumerable. As for myself, I wanted to know what the sun was, and how the stars were lighted, and what made them fall sometimes; and my curiosity extended even to the rainbow. Ever since I have been full of interest in everything relating to the sky. I shall take it for granted that my readers are equally anxious for knowledge, and try to give them an inkling as to the nature of these heavenly bodies, with some idea of the manner in which their composition was discovered.

I shall confine myself chiefly to the sun and fixed stars. The moon, examined with a telescope, is found to be a dreary volcanic waste of huge mountains, vast craters, and forlorn deserts. There is no water visible upon its surface, no clouds can be seen around it, and most of its volcanoes seem to have quenched their fires. It is a dead world. The other planets, which move with our own earth around the sun, are supposed to be very much alike, and some of them, perhaps, may be inhabited. But we do not know so much about them as we might justly wish to know. About the sun and stars, however, and also about the nebulæ, we have learned a great deal. The nebulæ are the dim clouds of light seen in various parts of the heavens, the Milky-Way being the most conspicuous. We can even tell of what substances these various bodies are composed, and find out much about what is going on in them to-day. The light which they send to us is the telltale, and a most reliable one. It tells us that the stars are really suns, or rather that our sun, which is one million two hundred and seventythree thousand times as large as the world we live in, is merely a small star, possibly so small as to be insignificant in comparison with some of its more distant kindred.

Let us begin with the sun, and see what that is made of. First, we can point a telescope at it, and find out how it looks; then we may examine its light with a proper instrument, and see what that has to say. But our telescope must be provided with suitable apparatus for lessening the brilliancy of the sun's rays, and protecting our eyes. Now, upon looking at the sun, it is seen to be an immense luminous globe, with a strangely mottled surface. Here and there black spots appear upon it, of various shapes and sizes, some of them being more than one hundred and eighty thousand

miles long. These spots are constantly changing in appearance, varying both in form and dimensions, and are found to be cavities in the outer surface of the sun. Some of them are like whirlpools, revealing to us the fact that the centre of the vast globe is less brilliant than its exterior. And yet the blackest of the spots is probably brighter than the brightest red-hot iron. They seem black to the eye because the surrounding portions of the sun's surface are so much more brilliant than they. In short, the immense globe presents the appearance of a huge fiery ball, surrounded by a tumultuous atmosphere of heated vapor, which is constantly in commotion.

If we look at the sun during an eclipse, this opinion will be confirmed. For, just at the moment that the moon completely hides the mighty centre of light from our view, long rose-colored flames and streamers of light will shoot out all around it. These flames, or protuberances, as they are called, are sometimes ninety thousand miles in length, and change their shapes most strangely. Now they resemble trees in form, now outlandish animals, and now delicate moss. They have almost the variety of frost-pictures. The ac

companying cuts will serve to illustrate these prominences. The first is a view of an eclipse. The black centre represents the moon, and the shaded mountainous projections close to it the protuberances. The other pictures are photographic views of prominences.

Now let us pay our respects to the sun's light. Of course you all know that there are different kinds of light. You have seen the gorgeous colors so often displayed in fireworks, and you have heard that they are produced by burning certain chemical substances. Now the sun's light has its peculiarities, just as much as these. Let us see what the rainbow has to say. about it. At certain times in the day, when the solar rays slant through the scattered drops of a retreating shower, the sky is arched by a bow of

If we

seven colors, red, orange, yellow, green, blue, indigo, and violet. allow common white light to pass through a triangular fragment of glass, a prism as it is called, —we shall get the same colors in the form of a confused spot. The cut-glass pendants of a chandelier often produce such spots upon the wall. But if the light be admitted to the prism through a narrow slit in the window-shutter, and after passing through the glass be made to fall upon a proper screen, we shall have, instead of a mere spot, a lengthened band of color, in which each hue may be studied by itself.

These colors were first explained by Sir Isaac Newton, about two hundred years ago. He proved that white light, such as we get from the sun, is a mixture of different kinds of light; or, in other words, that one of its rays is really a bundle of rays of the seven primary colors. The glass prism separates these rays, so that they no longer seem blended.

But if white light is a mixture of various tints, what shall we say of red, or green, or blue light? Are these, as we commonly have them, simple colors, or will they give us mixtures when analyzed by a prism? Let us see. For the purpose of analyzing light, an instrument called the spectroscope has been devised. It was invented about a dozen years ago by two German philosophers, named Kirchoff and Bunsen. And, young as the instrument is, it already ranks with the telescope and microscope in importance, and may safely be called one of the most astonishing inventions of our century. But as there are several different forms of the spectroscope, I will speak only of the simplest and most common. It consists mainly of two short telescopes, and one or more prisms. Sometimes several prisms are used, and a longer band of light is obtained than would be given by one alone.

One of the telescopes is closed at the outer end by a plate of metal having in it a very narrow slit. The light to be examined is admitted through this slit, passes through the telescope, is analyzed by the prism which is enclosed in a dark chamber, and is examined through the second telescope. The whole apparatus, telescopes and all, need not be more than a foot and a half across. In fact, there are simpler spectroscopes, which may be carried in the vest pocket. Suppose now a ray of sunlight be examined with the instrument. We shall see a long band of color, beginning with red and ending with violet, in which each tint may easily be studied apart from the others. The band, moreover, is crossed by a multitude of fine black lines, each of which always has a definite position. The heaviest of these lines, of which there are myriads, is in the yellow part of the band, and is known as "Frauenhofer's line D." For, having been first carefully studied by a German named Frauenhofer, these lines are named in his honor, and for convenience the more prominent ones are known by a few of the letters of our alphabet. The whole band, lines and all, is called the solar spec

trum.

Now let us look at the red light of the firework-makers. The color of the flame is due to certain compounds of two metals,― strontium and potassium. But now we have no long band of color, only scattered bright lines