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X Double Stars.—To the naked eye all the stars appear single. With the telescope, over 6,000 have been found to be double. Thus, Polaris consists of two stars about 18" apart, Eigel has a companion about 10" from it, and Sirius one distant 7". A good opera-glass will separate e Lyrae into two components. In case two stars happen to lie in the same straight line from us, though at immense distances from each other, their light will blend. They will be seen by the naked eye as a single star, and by the telescope as a double star. They are called optical double stars. Over 650, however, of the double stars have been found to be physically connected. Each double star of this class forms a binary system of two suns revolving in an elliptical orbit about their common centre of gravity, like the planets in the solar system, in accordance with Newton's law of gravitation. In a few instances there are combinations of triple, quadruple, and even septuple stars. Thus e Lyras is a double-double star, while 6 Orionis is a system of seven suns. The components of a double star commonly differ in brightness; so that frequently the fainter one is nearly lost in the brilliancy of its companion sun.

The periods of some of these systems have been ascertained. Thus, £ Ursae is a double star, and the two stars of which it is composed have performed an entire revolution abont each other since they were known to be connected. There are only eight binary stars whose periods are less than a century, while 325 have periods which seem to extend one thousand years.

Orbits.—It is not possible to estimate the dimensions of the orbits of the double stars, until their distances from us are known. Taking the estimated distance of 61 Cygni (550,000 times the sun's mean distance from the earth) as a basis, the companions of that system cannot cultivate a very intimate acquaintance, since they must be over A billion miles apart. From these data, astronomers have even attempted to calculate the mass of some of the double stars. 61 Cygni, although scarcely visible to the naked eye, and known to be the second nearest to us of any of the fixed stars, is yet estimated to weigh one-third as much as our sun.

Colored Stars.—We have already noticed that the stars are of various colors. Sirius is white, Antares red, and Capella yellow; while Lyra has a blue tint; and Castor a green one. In the pure transparent atmosphere of tropical regions, the colors are far more brilliant. There, oftentimes, the nocturnal sky is a blaze of jewels,—the stars glittering with the green of the emerald, the blue of the amethyst, and the red of the topaz. In our latitudes, there are no stars visible to the naked eye which are decidedly blue or green. In the double and multiple stars, every color is presented in all its richness and beauty. We find also combinations of colors complementary to each other. Here is a green star with a bloodred companion: here an orange and blue sun- there a yellow and purple one. The triple star y Andromedae, is formed of an orange-red sun and two others of an emerald green. Every tint that blooms in the flowers of summer, flames out in the stars at night "The rainbow flowers of the footstool and the starry flowers of the throne," proclaim their common Author; while rainbow, flower, and star alike evince the same Divine love of the beautiful.

As to the effects produced in a system having colored suns we can hardly conceive. Take a planet revolving about + Cassiopeiae for instance. This is illuminated by a red, a blue, and a green sun. Sometimes, by the succession of these suns, a cheerful green day would present a charming relief to a . fiery red one; and that might be still further subdued by a gentle blue one. The odd contrasts of color and the vicissitudes of extreme heat and cold which obtain on such a world, present a picture which our fancy can sketch better than words can paint. The colors of the stars change. Sirius was anciently red. It is now unmistakably white. There are two double stars which were described by Herschel as white; they are each now composed of a golden-yellow and a greenish star.

Variable Stars.—These are stars which have periodic changes of brilliancy. There are many of this class, of which the following are most conspicuous. Algol, in the head of Medusa, is a star of the second magnitude for about two and a half days, when it suddenly decreases, and in three and a half hours descends to the fourth magnitude. It then rekindles, and in three and a half hours again is as brilliant aa ever. Mira, the wonderfvl, a star in the Whale, has a period of eleven months. Its irregularities are very curious and fickle. It is ordinarily of the second magnitude for about fifteen days. It then decreases for three months, until it is reduced to the 9th magnitude. This period of darkness lasts five months; it then rebrightens for three months, until it regains its former lustre. Occasionally, however, it fails to brighten at all beyond the fourth magnitude, while on one occasion its light was almost equal to that of Aldebaran. Sometimes no perceptible change takes place for a month; then again, there is a sensible alteration in a few days.

The reason of this variability is not understood. It has been suggested, in the case of Mira, that it may be a globe revolving on its axis, and that different portions of its surface, illuminated to different degrees of intensity, are thus presented to us. Others have conceived that there may be satellites revolving about these suns, and that when their dark bodies interpose between the stars and our earth, they eclipse their light wholly or in part.

Tempobary Stabs.—These are stars which suddenly blaze out in the heavens, and then gradually fade away. The most celebrated one of this class burst forth in Cassiopeia, in the year 1572. Tycho Brahe says: "One night as I was examining the celestial vault, I saw with unspeakable astonishment a star of extraordinary brightness in Cassiopeia Struck with surprise, I could scarcely believe my eyes. To convince myself that there was no illusion, I called the workmen of my laboratory and the passers-by, and asked them if they saw the star which had so suddenly made its appearance." It was more brilliant than Sirius or Jupiter even, and could be compared only with Venus at her quadrature, except that it twinkled wonderfully. It was seen distinctly at midday. Its color was at first white, then yellow, and finally red. Its brightness decreased gradually^until the spring of 1574 when the star disappeared from view and has not since been seen. As two brilliant stars had previously appeared in Cassiopeia, at intervals of about three centuries, they have been thought, by some, to be identical, and that it is only a variable star of long period.

Since the discovery of Tycho Brahe, numerous instances are recorded of stars which have suddenly burst forth, and then either faded out entirely, or remained only as faint telescopic objects. In the latter case they are termed new stars. One of this kind appeared in Corona Boresalis, in 1866. At first it was of the second magnitude, but in a week changed to the fourth, and in a month diminished to the 9th. Strangely, too, some stars have disappeared from the heavens, and are styled lost stars. These changes which are thus constantly taking place are calculated to make the term "eternal stars" seem a very indefinite phrase.

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