Sirius (sĭrˈēəs) [key], or Dog Star, brightest star in the sky. It is located in the constellation Canis Major (1992 position R.A. 6h44.8m, Dec. - 16°42−); its Bayer designation is Alpha Canis Majoris. Sirius [Gr., = scorching], having an apparent magnitude of - 1.45, is exceeded in brightness only by the sun, the moon, and Venus and by Mars and Jupiter at their maximum brightness. A white, main-sequence star of spectral class A1 V, Sirius is about twice the size of the sun and about 20 times as luminous. It is also one of the nearest stars, lying at a distance of 8.7 light-years, so that it has been studied extensively. From an analysis of its motions, F. W. Bessel concluded (1844) that it had an unseen companion, which was later (1862) confirmed by observation. The companion, Sirius B, is a white-dwarf star and has also been the object of considerable study because it is the first white dwarf whose spectrum was found to exhibit a gravitational red shift as predicted by the general theory of relativity.