Many thousands of double stars have been discovered; and a large number of these are now known to be, not merely two distinct suns lying in the same line of sight, but two brother-suns, each probably the center of his own system of planets. We have not only to consider the number of suns, though of simple numbers more yet remains to be said. Attention must also be given to the varying colors of different stars; for all suns in the universe are not made after the model of our sun. All suns are not yellow. So far as single stars are concerned, colors seem rather limited. White stars, golden or orange stars, ruby-red stars, placed alone, are often seen; but blue stars, green stars, gray stars, silver stars, purple stars, are seldom if ever visible to the naked eye, or known to exist as single stars. Take a powerful telescope and examine star-couples, and a very different result you will find. Not white, yellow, and red alone, but blue, purple, gray, green, fawn, buff, silvery white, and coppery hues, will delight you in turn. As a rule, when the two stars of a couple are alike in color, they are either white, or yellow, or red. Also in the case of double-stars of different colors, the larger of the two is almost invariably white or some shade of yellow or red. There are, however, exceptions to all such rules. Blue stars are almost never seen alone, and as one of a pair the blue star is generally, if not invariably, the smaller. But instances are known of double-stars, both of which are blue; and one group in the southern heavens is entirely made up of a multitude of bluish suns. It is when we come to consider double-stars of two colors that the most striking effects are found. Now and then the two suns are nearly the same in size; but more commonly one is a good deal larger than the other. This is known by the brighter light of the largest, and the more rapid movements of the smallest. The lesser star is often only small by comparison, and may be in reality a very goodly and brilliant sun. Among nearly six hundred “doubles” examined by one astronomer, there were three hundred and seventy-five in which the two stars were of one color, generally white, yellow, orange, or red. The rest were different in tint, the difference between the two suns in about one hundred and twenty cases being very marked. For instance, a red “primary,” as the larger star is called, will be seen with a small green satellite; or a white primary will have a little brother-sun of purple, or of dark ruby, or of light red. Sometimes the larger sun is orange, the companion being purple or dark blue. Again, the chief star will be red with a blue satellite, or yellow with a green satellite, or orange with an emerald satellite, or golden with a reddish-green satellite. We hear of golden and lilac couples, of cream and violet pairs, of white and green companions. But, indeed, the variety is almost endless. [Illustration: CONSTELLATIONS IN THE NORTHERN HEMISPHERE.]