TILE STARS. Far beyond the confines of the solar system, and, indeed, at a distance almost inconceivably great, is situateol the universe of stars ( q.v.) . According to the accepted theory these stars, which to us appear merely as points of light. are really great blazing suns, in many cases, doubtless, attended by systems of planets analogous to our own. From the earliest ages it has been the custom, for purposes of classi fication, to divide the stars into the so-called constellations (q.v.). These are purely arbitrary subdivisions of the surface of the celestial vault, and are supposed to represent usually some mythological personage or fantastic animal. Exact science has substituted for the constella tions. carefully made catalogues of the stars, in which their precise positions are set down much in the same manner as. on the earth, latitude and longitude are used to define the positions of various places. The stars are usually called 'fixed stars.' It is known, however, that they are not really immovable, but are in continuous motion through space in orbits so vast that we have as yet been unable to do more than guess at their dimensions. Even the sun, regarded as a star, is known to be moving through space toward a point in the constellation Hercules with a velocity of about 16 miles per second. The whole solar system of course partakes of this motion.
Many attempts have been made to measure the distances of the stars. But as we can actually observe only their directions in space, the sole method open to us is to note whether such direc tions stiffer any change when the earth is situ atcd at opposite sides of its orbit around the sun. In other words, we employ the diameter of the earth's orbit as a sort of base-line. And though this baseline is 185,000,000 miles long, so great are the stellar distances that no star has yet been found for which this great orbit diameter subtends- an angle greater than about one second of are. (See PARALLAX.) With refer ence to their brilliancy, the stars have been classified in groups, and the term magnitude has been used to designate brilliancy. Thus a star
of the first magnitude means one whose light is so brilliant as to place it among, say, the twenty brightest stars. Of course, the num ber of stars of each magnitude increases very rapidly as we come to the classes of lower bril liancy. Strangely enough, the magnitudes of the stars are not constant. There is, for instance, one star in the constellation Argo that has diminished so much in brightness that it is at present no longer visible to the naked eye, though about the middle of the past century it was at least the equal of any star in the sky. To this class of variable stars belong also the `temporary stars,' which blaze up now and then like a great conflagration, only again to sub side into invisibility after a short time. The chemical composition of all these stars admits, like that of the sun, of being studied to a cer tain extent by means of a spectroscope. It is found that the stars are probably composed of the same elements that go to make up the sun and the earth.
Frequently we find a couple of stars very close together on the sky. Indeed, often they are so near that the eye fails to distinguish them, and it requires a powerful telescope to separate their light. It was thought at first that these double stars (see DOUBLE STARS) were accidental merely, and that while the two stars had nearly the same direction in space, they might never theless be separated by an immense linear dis tance. But it has been found that some such doubles are true binaries, showing an orbital revolution about the common centre of gravity of the two component stars. Sometimes such stellar Systems have three or more components. Even entire clusters may have a physical connec tion within themselves, and it is not improbable that the forces of gravitational attraction com pel and control the complex movements of such aggregations of stars, just as they do the lesser intricacies of our own solar system. A magnifi cent field for speculation is opened by a consid eration of the vast possibilities of such mighty stellar groups—true universes within the uni verse.