Page:Popular Science Monthly Volume 40.djvu/564

546 they send us inconsiderable, for the total effulgence of the stars down to the 9 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 2 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,