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points, when carefully examined are found to be in reality systems of two or more suns, some revolving about a common centre. These binary and multiple stars are extremely remote, requiring the most powerful telescopes to show them separately. The first catalogue of double stars, in which their places and relative positions are determined, was accomplished by the talents and industry of Sir William Herschel, to whom astronomy is indebted for so many brilliant discoveries, and with whom the idea of their combination in binary and multiple systems originated—an idea completely established by his own observations, recently confirmed by those of his son. The motions of revolution of many round a common centre have been ascertained, and their periods determined with considerable accuracy. Some have, since their first discovery, already accomplished nearly a whole revolution, and one, Coronæ, is actually considerably advanced in its second period. These interesting systems thus present a species of sidereal chronometer, by which the chronology of the heavens will be marked out to future ages by epochs of their own, liable to no fluctuations from planetary disturbances, such as obtain in our system.
In observing the relative position of the stars of a binary system, the distance between them, and also the angle of position, that is, the angle which the meridian or a parallel to the equator makes with the line joining the two stars are measured. The accuracy of each result depends upon taking the mean of a great number of the best observations, and eliminating error by mutual comparison. The distances between the stars are so minute that they cannot be measured with the same accuracy as the angles of position; therefore to determine the orbit of a star in
dependently of the distance, it is necessary to assume, as the most probable hypothesis, that the stars are subject to the law of gravitation, and consequently, that one of the two stars revolves in an ellipse about the other, supposed to be at rest, though not necessarily in the focus. A curve is thus constructed graphically by means of the angles of position and the corresponding times of observation. The angular velocities of the stars are obtained by drawing tangents to this curve at stated intervals, whence the apparent distances, or radii vectores, of the revolving star become known for each angle of position; because, by the laws of elliptical motion, they are equal to the square roots of the apparent angular velocities. Now that the angles of position estimated from a given line, and the corresponding distances of the two stars, are known, another curve may be drawn, which will represent on paper the actual orbit of the star projected on the visible surface of the heavens so that the elliptical elements of the true orbit and its position in space may be determined by a combined system of measurements and computation. But as this orbit has been obtained on the hypothesis that gravitation prevails in these distant regions, which could not be known à priori, it must be compared with as many observations as can be obtained, to ascertain how far the computed ellipse agrees with the curve actually described by
By this process Sir John Herschel has discovered that several of these systems of stars are subject to the same laws of motion with our system of planets: he has determined the elements of their elliptical orbits, and computed the periods of their revolution. One of the stars of y Virginis revolves about the other in 629 years; the periodic
time of σ Coronæ is 287 years; that of Castor is 253 years; that of & Bootes is 1600; that of 70 Ophiuci is ascertained by M. Savary to be 80 years; and Professor Encke has shown that the revolution of Ursæ is com
pleted in 58 years. The two first of these stars are approaching their perihelia,-y Virginis will arrive at it on the 18th of August, 1834, and Castor some time in 1855. The actual proximity of the two component stars in each case will then be extreme, and the apparent angular velocity so great, that, in the case of y Virginis, an angle of 68° may be described in a single year. σ Coronæ will also attain its perihelion about 1835. Sir John Herschel, Sir James South, and Professor Struve of Dorpat, have increased Sir William Herschel's original catalogue to more than 3000, of which thirty or forty are known to form revolving or binary systems, and Mr. Dunlop has formed a catalogue of 253 double stars in the southern hemisphere. The motion of Mercury is more rapid than that of any other planet, being at the rate of 107000 miles in an hour; the perihelion velocity of the comet of 1680 was no less. than 880000 miles an hour; but if the two stars of § Ursæ be as remote from one another as the nearest fixed star is from the sun, the velocity of the revolving stars must exceed imagination. The discovery of the elliptical motion of the double stars excites the highest interest, since it shows that gravitation is not peculiar to our system of planets, but that systems of suns in the far distant regions of the universe are also obedient to its laws.
Possibly, among the multitudes of small stars, whether double or insulated, some may be found near enough to exhibit distinct parallactic motions, arising from the revolution of the earth in its orbit. Of two stars apparently
in close approximation, one may be far behind the other These may seem near to one another when viewed from the earth in one part of its orbit, but may separate widely when seen from the earth in another position, just as two terrestrial objects appear to be one when viewed in the same straight line, but separate as the observer changes his position. In this case the stars would not have real, but only apparent motion. One of them would seem to oscillate annually to and fro in a straight line on each side of the other-a motion which could not be mistaken for that of a binary system, where one star describes an ellipse about the other. Such parallax does not yet appear to have been made out, so that the actual distance of the stars is still a matter of conjecture.
The double stars are of various hues, but most frequently exhibit the contrasted colors. The large star is generally yellow, orange, or red; and the small star blue, purple, or green. Sometimes a white star is combined with a blue or purple, and more rarely a red and white are united. In many cases, these appearances are due to the influences of contrast on our judgment of colors. For example, in observing a double star, where the large one is a full ruby-red or almost blood-color, and the small one fine green, the latter loses its color when the former is hid by the cross wires of the telescope. But there is a vast number of instances where the colors are too strongly marked to be merely imaginary. Sir John Herschel observes in one of his papers in the Philosophical Transactions, as a very remarkable fact, that, although red stars are common enough, no example of an insulated blue, green, or purple one has yet been produced.
Besides the revolutions about one another, some of the
binary systems are carried forward in space by a motion common to both stars, towards some unknown point in the firmament. The two stars of 61 Cygni, which are nearly equal, and have remained at the distance of about 15" from each other for fifty years, have changed their place in the heavens during that period, by a motion which for ages must appear uniform and rectilinear: because, even if the path be curved, so small a portion of it must be sensibly a straight line to us. Multitudes of the single stars also have proper motions, yet so minute that that of μ Cassiopeia, which is only 3" 74 annually, is the greatest yet observed; but the enormous distances of the stars make motions appear small to us which are in reality very great. Sir William Herschel conceived that, among many irregularities, the motions of the stars have a general tendency towards a point diametrically opposite to that occupied by the star Herculis, which he attributed to a motion of the solar system in the contrary direction. Should this really be the case, the stars, from the effects of perspective alone, would seem to diverge in the direction to which we are tending, and would apparently converge in the space we leave, and there would be a regularity in these apparent motions which would in time be detected; but if the solar system and the whole of the stars visible to us be carried forward in space by a motion common to all, like ships drifting in a current, it would be impossible for us, who move with the rest, to ascertain its direction. There can be no doubt of the progressive motion of the sun and many of the stars, but sidereal astronomy is not far enough advanced to determine what relations these bear to one another.
The stars are scattered very irregularly over the firma