Dog Star, Or Sirius, the brightest star in the heavens, belonging to the constellation Cams Major. Sir J. Herschel estimates its light as exceeding more than twofold that of Canopus, the second star in brilliancy, and more than fourfold that of Alpha Centauri, the third, whose light the same astronomer takes as the standard for first magnitude stars. But the variable Eta Argus, shining now as a sixth magnitude star, nearly equalled Sirius in lustre 30 years ago. Sirius was known to the ancients as a red star, Ptolemy describing it as reddish yellow, and Seneca speaking of it as ruddier even than Mars. Its light at present is perfectly white. Examined with the spectroscope by Rutherfurd and Huggins, it shows a spectrum crossed by a multitude of fine lines, and by four very strong lines. The four lines are identified with the hydrogen lines. Their strength and breadth seem to imply that the star is surrounded by an extensive atmosphere of hydrogen, existing close to the photosphere, at a very great pressure. From a measurement of the position of the F line of hydrogen, which appears slightly displaced toward the red end of the spectrum, Huggins has inferred that the star is receding from us at the rate of about 29 miles a second, or, taking into account the sun's own motion, at the rate of 2G miles a second in space. Lately, however, he has seen reason to believe that this estimate of the velocity is too high. The annual parallax of Sirius has been estimated at 0.150" by Henderson and Maclear, and at 0.250" by Cleveland Abbe. If we assume 0'2" as the value, the distance of the star exceeds about a million times the distance of the sun.
From the observed apparent motion of the star upon the celestial sphere, it follows, assuming this estimate, that it is travelling athwart the direction of the line of sight at the rate of about 15 miles a second. Combining this motion with Huggins's estimate of the motion of recession, it follows that it is actually moving through space at the rate of 33 miles a second. Certain peculiarities of the proper motion of Sirius have led astronomers to the belief that the star has a companion, not equally bright, but large enough to give Sirius an appreciable motion of circulation around their common centre of gravity.