THE SPECTROSCOPE: SPECTRUM ANALYSIS

For more than two thousand years, astronomy remained a purely mechanical and mathematical science, being limited to observations and deductions therefrom; but in 1860 the method of spectrum-analysis was discovered. This was a most revolutionary discovery, inaugurating, as it did, the whole science of astro-physics; and enabling us to know as much of the physics and chemistry of distant stars and nebulæ—their nature, constitution, and temperature—as we know of the planets of our own system! Even the existence of otherwise invisible stars has been demonstrated in this manner—their orbits, rate of motion, and mass. The science of astro-physics is now one of the most exact in the whole realm of science; and has only been rendered possible by the invention of the spectroscope. As this instrument plays such an important part in all astronomical research, a brief explanation of the instrument becomes necessary.

If a ray of sunlight be passed through a glass prism, the ray is split up into its primary colors; so that, instead of a single spot of white light being visible a narrow band of brilliant colors is seen—ranging from red to violet. But this is not the most important part of the discovery. When this spectrum was closely examined, it was found to be crossed by numerous black bands of various thicknesses. Sometimes these occurred in groups, sometimes singly. By enlarging the spectrum by passing it through several prisms, as many as 3,000 of these bands could be counted. The nature and explanation of these strange bands of blackness remained long uninterpreted, however. It remained for Kirchoff, in 1860, to discover their uses and significance.

Briefly, it is this. The chemical elements, when heated to a state of incandescence, present each one its own characteristic spectrum; each one has its own peculiar markings, or band of lines. No two elements are exactly like in their bands, as shown in the spectrum. Hence, whenever that particular marking is observed, it becomes certain that that element, and none other, is present. These spectra are very varied; iron, for example, has more than 2,000 such bands, while lead and potassium have but one each.

In this way—all the chemical elements having been studied, and their characteristic bands known—it became possible to explore the stars, planets and suns, and discover their chemical composition. For, no matter where an element was discovered—on this earth or on the remotest star—it would always cast its particular spectrum, when thus examined. The effect of all this upon astronomy can be perceived at once. Not only the heavenly bodies known to us, but those which have never been seen by human eye—even when aided by the most powerful telescopes—can be studied and their chemical composition and structure accurately determined. Here is progress indeed!

All this becomes the more remarkable when we stop to consider the immense distances of space, and how widely separated the heavenly bodies are from one another. This may, perhaps, be shown by one or two illustrations. We are, roughly, about 93,000,000 miles from our own sun. Now, the majority of the stars we see are suns, like ours. The sun next removed from us in space is about 275,000 times as far from us as we are from our sun. The orbit of Halley’s comet, of which so much has been written lately, is some 3,280,000,000 miles in length; and this sporadic body, coursing through space at a speed 50 times greater than a rifle bullet, takes 75 years to complete its circuit. The nearest star has been calculated to be nearly 25 trillion miles away; while some of the stars are 40 times as far from us as that!