Page:Popular Science Monthly Volume 37.djvu/519

Rh hopes. With, this explanation we are prepared to understand the important discoveries that have been made at Potsdam and Cambridge.

The bright star Mizar in the Great Bear is known to all. It is resolved in the telescope into two stars, the bright star being accompanied by a dimmer one, which is evidently a satellite, but possesses a period of revolution of about two thousand years. The spectrum of the principal star has been photographed several times since 1887 at the Cambridge Observatory, Mass., and the photographs have been carefully studied by Miss A. C. Maury, a niece of the celebrated Dr. Draper. The curious fact has been brought out that one of the photographed dark lines appears, at times, as if it was split into two fine lines. The doubling appears in the photographs of May 29, 1887, and of May 17 and 27, and August 28, 1889. In other photographs the lines appear washed out, as if they consisted of two lines, yet not quite separated; while on still others they appear clearly defined. On making up the registers of the times when the lines presented their different appearances, it was found that they appeared double at intervals of fifty-two days, washed out a few days before and afterward, and at other times single and sharp. By way of test the time was predicted when they should appear double again, and they came so, true to the forecast. The other lines in the spectrum of Mizar are not very sharp, and some of them are very faint. Careful examinations have shown that those few sharp lines also appear somewhat washed and broader when the first line is doubled, while the faint lines are at the same time very hard to see. The explanation of these variations, according to Prof. Pickering, Director of the Cambridge Observatory, lies in the supposition that the chief star Mizar is itself a double star, whose components revolve around one another in one hundred and four days, but are still so close together that no telescope can separate them. They appear even in the most powerful telescope only as a single round star. When one of the two stars is moving toward the earth, all the lines in its spectrum are pushed toward the blue end; at the same time the second star, since both participate in the revolution, must be receding from the earth, and the lines of its spectrum are pushed toward the red end. As soon, again, as the motion of the stars is perpendicular to a line drawn to the earth, all the lines will have their normal position, and mutually cover one another; they will appear single and distinct. The amount of the motion is calculated, from the extent of the doubling, at a hundred English miles in a second; from the period of revolution of one hundred and four days, the circumference of the orbit is deduced to be 900,000,000 English miles, and the distance of the two stars apart 143,000,000 miles, or about the distaune of the planet Mars from the sun. The