Popular Science Monthly/Volume 40/February 1892/Universe of Stars
IT is only, curiously enough, within the last decade or two that the science of astronomy has answered to its name. Until the methods of spectrum analysis and of photography were applied to the stars, astronomers were scarcely justified in their title, for they knew little about the stars, and, hardly hoping to know more, almost confined their attention to the solar system. Now, although sidereal astronomy, the science of astronomy par excellence, is still in its infancy, we may discern pretty clearly what will be the nature of its achievements. Surpassing the wildest dreams of the older astronomers in range and penetration, modern astronomy yet brings the whole cosmos within the grasp of human intelligence. Not only are the stars in process of being numbered, their motions, proper and relative, in course of measurement, their physical constitution subjected to analysis, and their distances brought within computation; but the entire sidereal system is recognized as limited in extent, and the form and magnitude of the vast group in space will at no distant date become susceptible of approximate delineation and calculation.
Of the methods referred to, photography has had, perhaps, the largest share in the recent advancement of sidereal science. The chemistry of the stars, it is true, is founded wholly on spectrum analysis, that profound and searching means of testing the composition of bodies by the action of elementary substances, under proper conditions, upon the infinitesimal undulations which give rise to the phenomena of light; but without the aid of photography, the mapping of star-spectra must have remained a slow and inaccurate process. The camera, on the other hand, has revealed almost all that is known concerning the number, distances, masses, and motions of the stars; the lens has no "personal equation," and never gets tired; sensitized gelatin responds with infinite celerity to the undulations which make no impression whatever upon the eye; and star-pictures of the heavens are not only permanent records, but, with the proper instruments and skill, can be so readily taken that before very long it is probable that some seven hundred thousand out of the whole sixty millions of stars will be accurately charted and indexed.
For such is the least number of the heavenly host—which a French astronomer somewhat extravagantly estimates to contain nearly seventy thousand millions of suns; for each star we see is a sun shining with its own light, and governing probably, like our own, the motions of a system of planets. Nor is the light they send us inconsiderable, for the total effulgence of the stars down to the 9½ light-magnitude is equal to one eightieth part of the effulgence of a full moon in a clear sky. What light we get from the stars of lower magnitude it is difficult to say, but it is clear that the stellar world is not boundless, for were it so the light from the infinite hosts of more and more remote suns would, as Miss Clerke says, fill the sky with an indefinitely intense radiance. It must, however, be remembered that it is not known whether the undulations which cause light are capable of infinite propagation. Nor, it may be added, can one be certain that the mass of ether in which our cosmos swims is the only one in space; or, if space and ether be taken as convertible terms, that it is the only mass differentiated—coarsened, so to speak, into a condition fit for the evolution of matter and energy, and of the suns and solar systems thus brought into being. The stars are arranged according to their light-magnitudes, to each magnitude the numerical value 2½ being assigned, for mathematical reasons that can not be here explained. Altair and Aldebaran are, strictly speaking, the only stars of the first magnitude, and the light of either of them would equal that of one hundred stars of the sixth and one million stars of the sixteenth magnitude. Sirius, however, is nine times as bright as Aldebaran, and its magnitude accordingly is expressed by the value -1'4. Among the suns visible to us, it comes next to our own sun, whose magnitude is reckoned at -25'4; in other words, the sun is (to our earth) between three and four million times as luminous as the Dog-star. The most accurate photometric measures of the stars are now made by the aid of photography, and the astronomers of a thousand years hence will have before them exact light-histories of nearly all the millions of stars of which the delicate and tireless gelatin films can seize and retain the faintest light-impressions. To what undreamed-of knowledge of our cosmos this wealth of accurate records will lead!
One of the most important results of stellar photometry is the aid it affords toward determining the distances of the stars. The mean distance of stars of the same magnitude is approximately the same; and if, therefore, the distances of some of the nearer stars are obtained, the approximate remoteness of any given category is easily calculated. But to find independently the distance of any individual star, its parallax must be known—the angle, that is, between two lines drawn from the ends of a base-line of known length to the star in question. Now, if the mean distance between earth and sun be taken as such base-line, 93,000,000 miles in length, to include an angle of one second (one 324,000th of a right angle), the line must be drawn to an object 206,205 × 93,000,000 miles distant. Well, no star is so near as this. The nearest star, α Centauri, has a parallax of three fourths of a second. To bring within easier comprehension the enormous distance this parallax involves, let the rapidity of light be considered. Light travels at the rate of over one hundred and eighty thousand miles a second, and a year of such travel may be taken as a unit for star-distances. Thus, the distance of a Centauri would be measured by nearly 4½ "light-years." The Polar Star is forty light-years, Sirius one hundred and twenty-one light-years, distant from our globe; while stars of the sixteenth magnitude may be so remote that it would take a wave of light thirty-six thousand years to reach the solar system. The parallax of Sirius is only about one thirty-third of a second—a striking example of the dependence of the most prodigious measurements of astronomy upon the minutest readings of apparatus, necessitating the utmost perfection of workmanship, as well as consummate skill and knowledge on the part of the observer.
Over eight thousand nebulæ have now been subjected to examination. The great nebulæ in Andromeda and Orion are, of course, familiar to every one. The telescopic nebulæ are of all sizes and shapes, and scattered over the whole heavens. Many stars have nebulous wisps and whirls, tails and helices, attached to them. The nature of nebulæ is still more or less of a mystery. But it is certain that they are initial, or at least early, phases of the life-history of stars. That life-history may be shortly stated in Miss Clerke's own words:
By the ceaseless advance of condensation nebulæ are transformed first into gaseous stars (showing bright lines in the spectrum, and therefore shrouded in glowing atmospheres, chiefly of hydrogen and helium), then into stars with banded spectra (showing outer atmospheric strata below incandescence over inner strata at glowing heat), from which (by further condensation and increase of inner heat below irregular outer clouds of metallic vapor) solar stare, and from these again Sirian stars, gradually emerge. Here the ascent ends; the maximum of temperature is reached, and a descent begins, the initial stage of which is marked by a second group of objects like our sun and Capella, distinguished from the first by the circumstance that they are losing instead of gaining heat; while, lower still, the condition immediately antecedent to solidification and obscurity (dark stars) is represented by Father Secchi's "carbon stars."
The nebula in Orion is of a very irregular shape; imbedded in it lies the stellar group?1 of the constellation, and some other stars, all of which together seem to form an enormous system whose dimensions can scarcely even be guessed at. Examined by the spectroscope, the nebula is found to consist of glowing gas, which the spectrum indicates to be a mixture of hydrogen and nitrogen. The Andromeda nebula, on the other hand, presents a well-defined oval, and gives a continuous spectrum in which no bright lines have been certainly distinguished. It may, therefore, be not a nebula at all, but a cluster of stars so enormously remote as to be unanalyzable by the most powerful of modern telescopes. In relation to nebulæ, a word may be said on Mr. Lockyer's ingenious "meteoric theory," submitted to the scientific world in 1887. Nebulæ, he asserts, "are composed of spare meteorites, the collisions of which bring about a rise of temperature sufficient to render luminous one of their chief constituents, magnesium." But the spectroscopic coincidences upon which this theory is based are by no means verified, nor has any comprehensible theory of the origin of these meteorites—very complex bodies, according to the samples that have reached our earth—been offered. If, following the indications of recent chemical and physical research, we consider the elements as molecular differentiations of the ether, the nebulæ may present stages in this differentiation in which the molecular states of some of the elements are not identical with those with which we are familiar in the laboratory, in which, indeed, certain of the elements may not yet have been evolved.—The Spectator.