was produced by the attraction of an invisible companion, revolving around the gigantic star. On Jan. 31, 1862, Alvan G. Clark, at Cambridge, Mass., while testing the l8j½" glass for the Dearborn Observatory at Chicago, pointed the glass at Sirius, when the disturbing companion came suddenly into view at a distance of about 10 seconds from Sirius, and exactly in the direction predicted for that time.
The period of revolution of the companion around Sirius was found to be nearly fifty years, and within a few months of the time calculated by Bessel, long before the telescope had revealed its presence. The mass of Sirius is about twice the mass of its companion, yet its light is 40,000 times greater.
The following facts concerning Sirius may be of interest:
We know now that the brightness of a star is no indication of its distance from us, but Sirius which is 9½ times brighter than a standard first magnitude star is only 8½ light years away, and only four other stars are known to be nearer.
If our sun occupied the place of Sirius in the sky, it would appear as a third magnitude star.
There is considerable discrepancy among the authorities as to the size and brilliance of Sirius as compared with the sun. Its diameter is given as fourteen or eighteen times that of the sun. As regards its brightness, Newcomb states that Sirius is thirty times brighter than the sun, a modest estimate, as other authorities claim for Sirius a brilliance of forty, sixty-three, two hundred, and even three hundred times that of the day-star.
The spectroscope reveals that Sirius is completely enveloped in a dense atmosphere of hydrogen gas. It is the brightest of the so-called Sirian stars, the spectroscopic type I., which includes more than half of all the stars yet studied.
Sirius has a large proper motion—that is the angular change in the position of a star athwart the line of vision— as compared with the average proper motion of stars of the
