Division of systems engineering

Sirius is the brightest star of the sky, twice as bright as the second star Canopus. At a distance of 8,6 light years from the Sun, it is also one of the closest stars. With a temperature of 10 000 degrees, it is a very blue star and its luminosity is 25 times that of the Sun. It has been observed in all historical times and was a particular landmark in the ancient Egyptian calendar. One major enigma with the star is the report by the ancient Greek astronomer Ptolemy in his famous catalogue “Almagest” that the star was red at his epoch (year +150), a fact also substantiated by various Latin authors. Though it has been sometimes suggested that the report may be a mistranslation or misidentification of the star, a report from independent cultural sources seems to corroborate the observation: in a text dating from around year -100, the Chinese astronomer Sima Qian was also referring to a change of colour of the star. 
“When we found the Chinese text a few years ago, we realized that something might really have happened to Sirius quite recently” recalls J.M. Bonnet-Bidaud, “the only problem is that this colour change is a real enigma, it is too recent to be explained by the stellar evolution which takes place on a much longer timescale. This led us to search for a third star in the system”.

Sirius is already a double star with a heavy dense star, a white dwarf called Sirius-B, orbiting the bright star, Sirius-A, in about 50 years [1].  The companion Sirius-B was discovered in 1862 but it cannot be responsible for the change of colour because the white dwarf is too far away from Sirius-A to influence the bright star. The white dwarf itself went through a red giant phase before its formation but this stage happened in a much earlier time. This has motivated the hypothesis of a third member in the Sirius system, a small but yet undetected companion orbiting Sirius-A in a very long (> 2500 year) and elongated orbit. 

The animation below illustrates the possible effect of a third body with a long period (2500 yr) orbit. The tidal interaction during the close encounter at the passage of the object in the inner part of its orbit could cause a temporary cloud expansion that will result in a temporary reddening during a few centuries as observed for Sirius. 

- see also "The deepest infrared image around the brightest star"  26 septembre 2008