In the case of the binary star, Eta Coronæ Borealis, it was, some forty years ago, uncertain whether its period was forty-three or sixty-six years, but now that two complete revolutions have been performed since its discovery by Sir William Herschel in 1781, the question has been finally decided in favor of the shorter period. The brilliant star Sirius is also an interesting binary star. The companion, which is relatively very faint—about tenth magnitude—was discovered by Alvan Clark in 1862. The existence of some such disturbing body was previously suspected by astronomers, owing to observed irregularities in the proper motion of Sirius. Several orbits, giving periods of about fifty years, have been computed. The great brilliancy of Sirius, the brightest star in the heavens, naturally suggests a sun of great size. Recent investigations do not favor this idea. Its spectrum is, however, of the first type, and the star is therefore not comparable with the sun in brilliancy. The above result would indicate that stars of the first, or Sirian type, are intrinsically brighter than our sun. Sirius is about eleven magnitudes brighter than its faint companion. This makes the light of Sirius about 25,000 times the light of the small star. The two bodies must, therefore, be differently constituted, and, indeed, the companion must be nearly a dark body. If Sirius has any planets revolving round it—like those of our solar system—they must forever remain invisible in our largest telescopes. This remark, of course, applies to all the fixed stars, single and double. They may possibly have attendant families of planets, like our sun, but if so, the fact can never be ascertained by direct observation. The star Zeta Cancri is a well-known triple star, the close pair revolving in a period of about sixty years. Nearly two revolutions have now been completed since its discovery by Sir William Herschel in 1781. All three stars probably form a connected system, but the motion of the third star round the binary pair is very slow and irregular. [Illustration: Fig. 18.—System of the Double Sun Alpha Centauri] Another interesting binary star is Xi Ursæ Majoris. As already stated, this was the first pair for which an orbit was computed. More than a complete revolution has now been performed since its discovery by Sir William Herschel in 1780. The period has, therefore, been well determined, and seems to be about sixty years. The bright southern star, Alpha Centauri, the nearest of all the fixed stars to the earth, so far as is known at present, is also a remarkable binary star. It seems to have been first noticed as a double star by Richaud in 1690.