Who would ever have thought a pretty plaything like this could have told us what we so much wanted to know—namely, what the sun and the stars are made of? It seems too marvellous to be true, yet true it is that for ages and ages light has been carrying its silent messages to our eyes, and only recently men have learnt to interpret them. It is as if some telegraph operator had been going steadily on, click, click, click, for years and years, and no one had noticed him until someone learnt the code of dot and dash in which he worked, and then all at once what he was saying became clear. The chief instrument in translating the message that the light brings is simply a prism, a three-cornered wedge of glass, just the same as those hanging lustres belonging to the chandeliers. When a piece of glass like this is fixed in a telescope in such a way that the sun's rays fall on it, then there is thrown on to a piece of paper or any other suitable background a broad coloured band of lovely light like a little rainbow, and this is called the sun's spectrum, and the instrument by which it is seen is called a spectroscope. But this in itself could tell us little; the message it brings lies in the fact that when it has passed through the telescope, so that it is magnified, it is crossed by hundreds of minute black lines, not placed evenly at all, but scattered up and down. There may be two so close together that they look like one, and then three far apart, and then some more at different distances. When this remarkable appearance was examined carefully it was found that in sunlight the lines that appeared were always exactly the same, in the same places, and this seemed so curious that men began to seek for an explanation.
Someone thought of an experiment which might teach us something about the matter, and instead of letting sunlight fall on the prism, he made an artificial light by burning some stuff called sodium, and then allowed the band of coloured light to pass through the telescope; when he examined the spectrum that resulted, he found that, though numbers of lines to be found in the sun's spectrum were missing, there were a few lines here exactly matching a few of the lines in the sun's spectrum; and this could not be the result of chance only, for the lines are so mathematically exact, and are in themselves so peculiarly distributed, that it could only mean that they were due to the same cause. What could this signify, then, but that away up there in the sun, among other things, stuff called sodium, very well known to chemists on earth, is burning? After this many other substances were heated white-hot so as to give out light, in order to discover if the lines to be seen in their spectra were also to be found in the sun's spectrum. One of these was iron, and, astonishing to say, all the many little thread-like lines that appeared in its spectrum were reproduced to a hair's-breadth, among others, in the sun's spectrum. So we have found out beyond all possibility of doubt some of the materials of which the sun is made. We know that iron, sodium, hydrogen, and numerous other substances and elements, are all burning away there in a terrific furnace, to which any furnace we have on earth is but as the flicker of a match.
THE SPECTRUM OF THE SUN AND SIRIUS.
It was not, of course, much use applying this method to the planets, for we know that the light which comes from them to us is only reflected sunlight, and this, indeed, was proved by means of the spectroscope. But the stars shine by their own light, and this opened up a wide field for inquiry. The difficulty was, of course, to get the light of one star separated from all the rest, because the light of one star is very faint and feeble to cast a spectrum at all. Yet by infinite patience difficulties were overcome. One star alone was allowed to throw its light into the telescope; the light passed through a prism, and showed a faint band of many colours, with the expected little black lines cutting across it more or less thickly. Examinations have thus been made of hundreds of stars. In the course of them some substances as yet unknown to us on earth have been encountered, and in some stars one element—hydrogen—is much stronger than in others; but, on the whole, speaking broadly, it has been satisfactorily shown that the stars are made on the same principles as our own sun, so that the reasoning of astronomers which had argued them to be suns was proved.
We have here in the picture the spectrum of the sun and the spectrum of Arcturus. You can see that the lines which appear in the band of light belonging to Sirius are also in the band of light belonging to the sun, together with many others. This means that the substances flaming out and sending us light from the far away star are also giving out light from our own sun, and that the sun and Sirius both contain the same elements in their compositions.
This, indeed, seems enough for the spectroscope to have accomplished; it has interpreted for us the message light brings from the stars, so that we know beyond all possibility of mistake that these glowing, twinkling points of light are brilliant suns in a state of intense heat, and that in them are burning elements with which we ourselves are quite familiar. But when the spectroscope had done that, its work was not finished, for it has not only told us what the stars are made of, but another thing which we could never have known without it—namely, if they are moving toward us or going away from us.