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��Popular Science Monthly
��mer. Such an arrangement is shown in connection with the telescope of twelve inches aperture at the University of Illi- nois Observatory. Wires are lead from the telescope to a galvanometer in an adjacent room. Two observers are neces- sary, one to point the telescope and ex pose the selenium cell to the stars, while the other reads the galvanometer and records the measures.
With this short description of the de- vice, let us see how results are obtained on the stars. Nearly every one has heard of the wonders of spectrum analy- sis; how, by studying the light of a star, split up into the different colors, the astronomer has been able to draw certain conclusions about the constitution of the body. For example, it is easily demon- strated that metals, such as iron and calcium, exist as hot vapors above the surface of the sun. It is not so well known, however, that by means of the spectroscope we can study the motions of the stars as well as their chemical constitutions.
It would lead us too far afield to dis- cuss this phase of the subject, but let us state that peculiarities in the spectra of certain stars lead us to conclude that they are attended by large companions or planets which move about them. Such stars are called "spectroscopic binaries," since they are revealed by the spectroscope. The North Star is an object of this class, being in fact a triple system, as there is one body which re- volves about the main star in only four days, while a second and more distant companion has a period of a dozen years. In some cases the planes of the orbits of these companions are at such angles that when they pass in front of the main stars there are eclipses as seen from the earth. About one hundred such cases are known, but more are being'continually found. The study of these eclipsing binaries is espe- cially important, since they give us the most direct measure of the diameters of the stars. Spectroscopic measures de- termine the size of the orbit in which the second body moves, while with the photometer is found the duration of the eclipse, which is simply the time neces- sary for the companion to pass in front of the main star, and hence gives at once the sum of the diameters of the two bodies.
��The Stars in Orion
Any one who is familiar with a few of the constellations knows Orion, which is in the south in the winter sky. The striking feature of this group consists of three stars in a row, known as the Belt of Orion. The right hand star of the three is Delta Orionis, the Greek letter. Delta, meaning the fourth star in the order of lettering. This object is a spectroscopic binary, the period of the companion being six days. The star was one of the first observed with the selenium photometer, and by comparing it with other stars in the vicinity it was soon found that at intervals of six days there is always a loss of eight per cent of the light, an amount imperceptible to the eye. The eclipse lasts slightly less than one day. After an exhaustive study, the main facts of the system have been brought out, and the appearance of the two bodies as viewed from the direction of the earth is shown to scale in the oval diagram . From simple considera- tions it is established that the companion is about six tenths the diameter of the main body, and the four small circles show the successive positions of the com- panion in its orbit, which is not circular but slightly elliptical, and of course viewed at an angle. The dotted circles show the position for eclipses, and we find as expected that when the smaller body is behind the primary there is also an eclipse, but in this case only seven per cent of the light of the system is cut off, as compared with eight per cent when the companion is in front. This demonstrates that the smaller body is seven-eighths as intense for the same surface as the main body, and is hence far from being a dark planet.
The figure shows how close together the bodies are as compared with their diameters, and we also find that we are dealing with a giant system. It is very interesting to note the comparative size of the sun, eight hundred and sixty thousand miles in diameter. The larger star of Delta Orionis has fifteen times and the small star nine times the sun's diameter. The system, brought up and placed beside the sun, would not only appear large, but would be extraordinarily intense in comparison, the surface brill-
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