BINARY

(OR DOUBLE) STARS, which to the naked eye may appear single, but through the telescope reveal themselves as double. Over 20,000 have been located and tabulated. The majority of these are what are known as optical pairs;’ that is, though apparently close together, one is perhaps millions ef miles nearer or further in space. Some hundreds of them, however, are now definitely known to be true double stars, revolving around a common center of gravitation, as the moon revolves around the earth. The orbits ef about 60 have been closely studied, some making their revolutions in a little ever five years, others in a period estimated as high as ten centuries. Some are so distant that their motions can only be studied through their spectras, the duplication of lines in the spectra indicating the rate of their mutual revolutions,