826 



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- 



