Popular Science Monthly/Volume 37/August 1890/The Discovery of Invisible Worlds

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SOME discoveries have very recently been made in the starry heavens which must be regarded, not only in what they are of themselves, but also on account of the way in which they were made, as among the most interesting of scientific events. It seems, in fact, like a contradiction to say that astronomers in Europe and America have been able to determine the velocity of motion, size, and weight of stars that are not visible in any telescope, and which no telescope to be made in the future, no matter how great its power may be, will be able to show. The new science also has the peculiar property that it recognizes mutual relations between objects apparently lying far from one another, connects with one another phenomena which appear to have no common measure, and draws fine lines of connection between earth and sky. One of these lines was drawn some thirty years ago in spectrum analysis; and by its aid man has risen in mind to remote worlds, and has sounded their physical and chemical constitution. The same spectrum analysis has now again celebrated a great triumph—a victory which might have been predicted, but the time for which did not seem yet to have come.

Every one is acquainted with the spectrum which we see when a ray of sunlight coming through a narrow opening passes through a prism. With the aid of suitable instruments there can also be seen in this spectrum a considerable number of dark cross-lines; and science has shown that these lines are caused by the presence of certain simple bodies or elements, including iron, hydrogen, sodium, etc. When we examine the light of the stars through those instruments, we shall perceive that in their spectrums too the dark lines denoting these elements are present. On this is founded the chemistry of the stars, for which we are wholly indebted to spectrum analysis. The situation of the dark lines in the spectrum is unchangeable, or else we would not be able to conclude from it respecting the elements represented there. The unchangeable character persists, however, only when the source of light is at rest as to the observer. If the shining body we are regarding is going away from us very rapidly, the dark lines incline to shift themselves slightly toward the red end of the spectrum; while, if it is approaching us with great rapidity, they slide over toward the violet. Without stopping to explain the causes of the shifting, we may remark that it is very small even with the greatest velocities. Former observers could hardly recognize it with certainty, because their instruments were not delicate enough to reveal such slight changes. Gradually makers have succeeded in constructing instruments that will show the changes with certainty. At the Greenwich Observatory, where observations of this kind have been carried on for several years, the motions in space of several stars have been ascertained with the help of the spectroscope. It has thus been found that the clear-shining Capella is receding from the earth at the rate of twenty-seven English miles a second, and that the brilliant star Vega in Lyra is approaching us at the rate of thirty-four miles a second. As such observations deal with infinitesimally small magnitudes, they are necessarily very difficult and precarious. It has been found, by investigations at the Astrophysical Observatory in Potsdam, that much more certain results are obtained if the spectrums of the stars are photographed and the measurements of the lines are made afterward on the pictures. These results have been confirmed by spectro-photographic researches at the Cambridge Observatory in North America; and thus the spectrographic method justifies the greatest 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 period of revolution of Mars is six hundred and eighty-seven days, and would be less if the mass or weight of our sun was greater. We can hence calculate how many times greater than the mass of the sun must be the mass of the two stars of Mizar for the revolution to be accomplished in one hundred and four days. The result is forty times the mass of the sun. So this little point of light which Mizar in the Great Bear appears to the eye is the equivalent of forty of our suns.

Before the news of this astonishing discovery made at Cambridge had reached Europe, a similar investigation made at the Potsdam Observatory was published. It was directed to the star Algol in the head of Medusa. This star has been known for more than two hundred years to be variable in brightness. It shines for two days and a half with a steady white light, then loses brightness for about four hours and a half, recovers during about four hours and a half, and then continues steady again for two days and a half. The changes go on with great regularity, and it has been believed for the last hundred years that Algol is attended by a double star revolving around it, by which it is concealed from the earth at regular intervals, depending on the period of its revolution. The periodical decrease of brilliancy is similar in its nature and cause to an eclipse of the sun, when the dark moon is interposed between it and the earth. But probable as this belief was, the fact had not been demonstrated. A complete solution has been obtained by spectrum analysis. Prof. Vogel, of the Astrophysical Observatory in Potsdam, and his fellow-worker, Dr. Scheiner, have taken photographs of the spectrum of Algol and carefully measured the dark lines. It has thus been ascertained that these lines move toward the red before the star appears at its weakest, toward the violet after that moment; or, in other words, that Algol is receding from the sun in the first half of its change, approaching it in the second half. This would necessarily occur if the star was describing an orbit around a dark body which should periodically conceal it for a time from our view. The rate of motion of Algol is twenty-three English miles in a second, and its period of revolution is two days, twenty hours, and forty-nine minutes; whence the circumference of its orbit and the distance apart of the centers of the two stars may be computed as was done in the case of Mizar. The latter is found to be less than 3,000,000 English miles, a small enough distance for two so large bodies. From the period of the light-changes and the velocity of the motion we calculate the diameter of the principal star to be 920,000 and of its dark companion 750,000 English miles. The two bodies which form the Algol system are each nearly as large as our sun, the diameter of the sun being taken at 750,000 miles, but their total mass is only about two thirds the mass of the sun. We have, says Prof. Vogel, to think of these two bodies as surrounded by extensive atmospheres, and that that of the principal body, or Algol itself especially, must possess considerable illuminating power. Under certain presuppositions, the height of this atmosphere is estimated at 216,000 English miles, and that of the atmosphere of its dark companion at 168,000 miles. The smallest interval between the atmospheres of the two bodies will thus be 1,600,000 English miles, or less than can be found in our solar system. It is not easy, as Prof. Vogel suggests, to conceive two bodies so near of nearly equal size, one of which is in the highest glow of heat, and the other in a condition of far-advanced cooling. But the facts of observation lead to this conclusion, and in science facts constitute the highest and ultimate authority, before which everything must yield. Thus, we learn from the remarkable discoveries in Potsdam and Cambridge that the world-order we meet in our solar system does not reign throughout the kingdom of the fixed stars, but that other relations come in which are quite different from those under which we live.—Translated for the Popular Science Monthly from Daheim.