There are two binary systems of special in terest because the companion was first discov ered through its attraction upon the larger star. One of these cases is the brightest star in the heavens, Sirius. About 1840 it was discovered by comparing observations made since the time of Bradley (1750) that Sirius did not move forward on the celestial sphere with a uniform and even motion, as the other stars did, but that there was a slight irregularity. A few years later a careful investigation of the obser vations showed that Sirius was moving in a very small orbit with a period of about 50 years. This made it possible to calculate the orbit of an unseen companion which, moving round the star, would produce the apparent orbital mo tion. In 1860 Alvan Clark, the son and suc cessor of the celebrated maker of telescopes in • Cambridgeport, Mass., on pointing one of the newly made great instruments at Sirius, saw a companion star. He knew nothing about the computations that had been made on this ob ject, but when his observations were compared with them, this companion was found to lie in the right direction to produce the observed ine quality. The observations since made show that it is going round in the orbit prescribed for it before its discovery.
The other case is that of Procyon, the brightest star in the constellation Canis Minor. Its motion was found to be affected by an in equality somewhat smaller than that of Sirius; but the companion which produced it evaded the scrutiny of the most careful observers until it was discovered by Schaeberle with the great 36-inch telescope of the Lick Observatory. It also was found to be moving in the orbit com puted for it before its discovery.
As the power of the telescope has been in creased closer binary systems having shorter periods have been continually brought to light. This raises the question whether there may not be vast numbers of such systems in which the two stars are so close together that no telescope will separate them. This question has been answered in recent times by the aid of the spec troscope. We have already described how the radial motions of stars or the motion in the di rection to or from our system is measured by this instrument. The determinations have been going on since 1880. As the number of meas ures increases, it is found that the radial mo tion of many of these stars varies in a regular period, thus showing that invisible planets are moving round them. A new feature of the universe is thus brought to our knowledge, which, if we had no other instrument than the telescope, would remain forever hidden. The difference between the measures made with the telescope and the spectroscope consists in this: The telescope can show the change in the position of a star only when it is great enough to be visible at the enormous distance of the star. Commonly this change must be measured by hundreds of millions of miles, nay, thou sands of millions, else it would never be seen in the telescope. But the spectroscope shows, at any moment, how fast the star is moving to or from us, though it gives us no information as to its actual distance. It is not, therefore, necessary that the star should have moved through a definite space in order to have its motion detected by the change in its spectrum.
Having given the speed of the revolution and the period, it becomes possible to determine the size of the orbit.
Pairs of stars discovered with the spectro scope are called spectroscopic binary systems. The comparisons in such systems are not, how ever, necessarily dark stars; the revolutions of a pair of stars, both of which are bright, may be determined with the spectroscope as if one of the bodies were dark. In this case the lines of the spectrum are sometimes double and sometimes single. They seem double when one star is moving from us and the other toward us, and single when the stars are moving later ally to the line of sight from us to them. Thus bright double stars are shown by the use of the spectroscope which no telescope, however powerful, can distinguish to be more than a single star.
That this doubling of the lines is a true indication of a double star system is shown by the fact that at least one spectroscopic pair can also be seen as a double in the telescope. The orbits derived from the visual measures and from a study of the light, respectively, are in remarkably close agreement.
The most successful discoveries in this line have been made in recent times by or under the direction of Professor Campbell, in the Lick Observatory, where it may almost be said that a new branch of astronomy has been cre ated. The discoveries of these objects are now going on so rapidly that it is impossible to set any limit to their number. The general rule is found to be that the orbits described do not differ greatly in magnitude from those de scribed by our planets in moving round the sun. That is to say, there are few or no orbits yet known which are as large as that of Neptune. There is, however, no limit to the minuteness of the orbits, some stars revolving almost in contact with each other.
Besides double stars, there are triple and quadruple stars of such variety that it is im possible to give any general description of them. The most remarkable quadruple system is found in the great nebula of Orion, where Theta Orionis, which looks to the naked eye like a single star, is shown by the telescope to con sist of four stars forming a trapezium; this is, therefore, known as the trapezium of Orion. Altogether, these systems show the infinite vari ety of the great bodies which make up the universe. The pairs of stars which form a binary system may be of every degree of bright ness and of very different masses. Their times of revolution range from two or three days to unknown thousands of years. In a few cases the masses have actually been determined. It is a curious fact that they do not vary so much as does the brightness of the stars. When a bright and a faint star form a binary system, the mass of the faint star is generally out of proportion to the amount of light it emits. We have already remarked that many stars are thousands of times as bright as the sun; but it is a curious fact that none have been found to have even hundreds of times the mass of the sun.