It was in 1803, after 20 years' observation, that sir W. Herschel advanced the view, which has been more and more confirmed since, that double stars are connected systems of two or more stellar bodies, revolving in regular orbits around one another, or rather round their common center of gravity. Their motions are found to follow the same laws as prevail in the solar system, and the orbits are elliptical. These distant bodies are therefore subject to the Newtonian law of gravitation. The period of revolution has, in several cases, been roughly approximated; among the shortest is that of C Herm]Hs, estimated at 30 years; others are set down at hundreds. In cases where the parallax is known, the size of the orbits can be determined; and thus the astronomer is able to assert in regard to the double star 61 Cygni that the orbit described by these two stars about each other undoubtedly greatly exceeds in dimensions that described by Neptune about the sun. Even the masses of these stars have been calculated as being together 0.353, that of our sun being 1. It is a consequence of these revolutions that many stars are now seen double that formerly seemed single, and vice 'versa. If the plane of revolution have its edge presented to the earth, the stars will seem to move in a straight line, and at times to cover one another. The star llerculis, seen by Herschel double in 1781, appeared single in 1802, and was first seen double again by Struve in 1826. The period of revolution is presumed to be 182 years.
The proper motion of stars, discovered by Halley, is of another kind. It consists in a displacement in various directions of the individual stars, so that the configuration of constellations is slowly changing. "The Southern Cross." says Humboldt, will not always shine in the heavens exactly in its present form; for the four stars of which it consists move with unequal velocity in different paths. how many thousand years will elapse before its total dissolution cannot be calculated." The proper motions yet observed vary from of a second to 7.7% According to Bessel, the proper motion of the binary star 61 Cygni amounts to 5.123', so that in 360 years it would pass over a space equal to the moon's diameter. It must thus take thousands of years to alter sensi bly the aspect of 'the heavens; although, taking into account the enormous distances, the actual velocities must be great. Of 3,000 stars observed by Bessel, 425 had a per-: ceptible motion. Argelauder has recently published a list of 500 stars having a proper motion.
It was first observed by sir W. Herschel that there is a perceptible tendency in the stars generally to diverge or open up in one quarter of the heavens, and to draw together in the opposite quarter; and this he attributed to a proper motion of our sun with his planets in the direction of the former point. The apparent motion thus caused is com plicated with the real independent motions of individual stars. The point toward which this motion is directed, which is called the " solar apex," was fixed by Herschel in the constellation Hercules; and the result of subsequent and independent researches gives a nearly coincident point. The velocity has been calculated at upward of 150 millions of m. a year, or 17,600 m. an hour—i.e., rather more than one-fourth of the earth's velocity in its orbit.
The spectroscope has been applied to investigate the physical constitution of the stars, with the result of identifying many of the elements composing our sun and earth. The spectra of the stars differ greatly among one another; some consisting mainly of simple lines, others having complex bands. The simple spectra are believed to indicate great intensity of heat, keeping the molecules of matter in a state of extreme dissocia tion; while the complex spectra show the molecules to be more associated in groups or compounds, owing to the repulsive force being less. The bluish stars are the hottest; a
red tinge indicates comparative coolness. Our sun would seem to be a decaying star.
Several stars exhibit well-marked periodic alterations of a striking nature, and are hence called variable stars. A considerable number have been observed, of which the most remarkable are Mira (the " wonderful") in Cetus, and Algol in Perseus. The first attains its greatest luster every 334 days, and appears for 14 days as a star of the second and even at times of the first magnitude; it then decreases for two or three months, till it becomes of the sixth even tenth magnitude, so as to be for half a year invisible to the naked eye and usually to telescopes. After this it begins again to increase, but more rapidly than it decreased. It is visible to the naked eye for three or four months of its period. Of all the variable stars yet observed in Perseus, Algol has the shortest period, being 68 hours 49 minutes. It appears for about 60 hours a star of the second magnitude, then decreases for four hours, and appears for a quarter of an hour of the fourth magnitude, after which it increases again for four hours. Various explanations have been offered of these mysterious appearances; the stars are supposed to turn on their axes, and to have their surfaces unequally luminous in different places; or a large dark body is assumed to be revolving about the luminous one, so as to intercept more or less of its light in different positions; or the stars are lens-shaped, etc. There is nothing, however, inadmissible in the supposition that the intensity of the light itself may vary; and if in other suns, why not in our own? Allied to the variable stars are the new or temporary stars that appear suddenly in great splendor, and then disappear without leaving a trace. A number of instances are on record. It is not impossible that these also may be periodic.
Star Systems.—From the appearances connected with the Milky Way or Galaxy sir W. Herschel came to the conclusion that the stars forming our firmament do not extend indefinitely into space, but are limited in all directions, the mass having a definite shape. He conceived the shape to be something like that of a huge millstone, having one side cleft, and the two lamina; set apart at a small angle. Let the diagram (fig. 1) represent a vertical section of such a broad flat stratum, and ,suppose the solar system situated las at S, to a spectator looking on either side, in the direction of the thickness, as SB, the stars would appear comparatively sparse, but all round in the direction of the breadth (as SA) there would ap pear a dense ring, which would separate into two branches (SE, SD) in the direction. of the cleft side. This supposition accounts for the appearance of the Milky Way, and all subsequent obser vations have tended to confirm tne conjecture. Situated as we are within the system, we cannot hope ever to attain more than a rude notion regarding it; to get a definite outline, we must be placed without it.
But this star system, which we may call our own, as our sun belongs to it, is but an item in the stellar universe. The appearances known as nebulm, in many cases, at least, are believed to be similar agglomerations of suns, separated from our system and from one another by unfathomable starless intervals (see NEBUI,210. Their forms are very various, but in general pretty well detined, and not without symmetry. The of some of them is even startling.
See CENTAUREA,