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Instead of a single measure for each star, in the case of the so-called spectroscopic binaries, we must make enough measures to determine the dimensions of the orbit, its form and the period of revolution. What has been said of the motions of the stars applies also, in general, to the determination of their distances. A vast amount of labor has been expended on this problem.
The logical and necessary explanation given of the cause of ordinary spectroscopic binaries, and of irregular proper motions of Sirius and Procyon, leads to the inference that if ever the plane of such a binary orbit were edge-on to us there ought to be an eclipse of the luminous partner whenever the non-luminous one is interposed between us.
In the case of some of these binaries the parallax has been measured, from which it appears that in four of the surest cases the orbits are about the size of the orbit of Uranus, these being probably among the smallest stellar orbits. In 1834 and 1840 he began to suspect a want of uniformity in the proper motion of Sirius and Procyon respectively.
It should also show itself in the measures of all spectroscopic binaries. A third method of great promise depends on a remarkable investigation carried on in the physical laboratory of the Case School of Applied Science.
Here the magnitudes are both six, distance 0.65", p. 3°. Hall assigns a period of forty years to this star. The assemblage of close binaries in this neighborhood is very curious. Only a few degrees away we find one that is still more remarkable, the star gamma. What has previously been said about 42 Comæ Berenicis applies in a measure to this star also.
The components are approximately equal, both in mass and light, and the system formed by them is transported towards us with a speed of some sixteen miles a second. The line-shiftings so singularly communicative proceed, in this star, with perfect regularity. This new class of "spectroscopic binaries" could never have been visually disclosed.
Through his systematic investigations of stellar radial velocities with the Mills spectrograph, knowledge in this department has, since 1897, progressed so rapidly that the spectroscopic binaries of our acquaintance already number half a hundred, and ten times as many more doubtless lie within easy range of detection.
It was first pointed out by Bessel that the fact of stars exhibiting a common proper motion might serve as an unfailing test of their real association into systems. This was, accordingly, one of the chief criteria employed by Struve to distinguish true binaries from merely optical couples.
On measuring the velocity of a star in the line of sight at frequent intervals, periodic variations have been found, leading to a belief in motion round an invisible companion. Great numbers of binaries of this type have since then been discovered, all of short period.
While the group of double stars in the southern part of Corona Borealis consists, as we have seen, of remarkably close binaries, another group in the northern part of the same constellation comprises stars that are easily separated. Let us first try zeta. The powers of the three-inch are amply sufficient in this case.
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