Sirius, the brightest star of the sky, exerted a real fascination on astronomers of all times. More than 5000 years ago, the Egyptian priests used the heliacal rising of this bright star to calculate their calendar and for centuries the Chinese astronomers carefully observed it. In modern astronomy, its fast apparent displacement on the sky, of approximately one degree in only 2700 years, was the first proper motion noted by Edmund Halley and it is the study of its sinusoidal displacement which allowed Friedrich Bessel to deduce in 1844 the existence of an obscure companion around the star, the first dark matter prediction.

Sirius-B, the brightest and nearest white dwarf is the only known companion of Sirius. A compact star of mass 1.03 Mo for a radius of only 5800km (0.007Ro), it rotates around Sirius-A in 50.09 years according to an eccentric orbit with a distance varying between 8 and 32 UA (UA = distance Earth-Sun).

This companion, Sirius-B, was discovered visually in 1862 and identified as a white dwarf, by a spectrum obtained in 1915, with the 1.5m telescope of the MT Wilson (USA).

Double Sirius or triple?

Since the discovery of Sirius-B, several observations came to suggest the existence of another companion. The system was observed first visually then photographically to determine the characteristics of the orbit. Several times, a visible companion was noted during the years 1920-1930. More recently, a detailed study of the residual orbital variations has concluded to the possible existence of a low-mass stellar companion. (Benest et Duvent 1995, A&A 299, 621). Lastly, independent and concordant historical observations indicate that the star could have undergone a temporary change of color, approximately 2000 years ago. This transient event could be the consequence of an interaction with a companion on a very eccentric orbit (Bonnet-Bidaud & Gry 1991, A&A 252, 193 ). Among the old texts which attest of a change of color of Sirius, most famous is the catalogue of Ptolémée (Almageste) in which the Greek astronomer of IInd century of modern era, famous for his observations, classifies Sirius as a red star. A concordant and independent text is this Chinese manuscript of Ist century before our era in which the case is made very clearly of a change of color. (translation in Bonnet-Bidaud & Gry (1991)).