Every year, when the star rose above the horizon just before dawn, the Romans paid bizarre tribute to it by sacrificing dogs with red fur. Seneca the Younger wrote that "the redness of the dog star is deeper, that of Mars milder." Ptolemy called it "reddish," a description also used by Cicero, Horace and other classical authors. The same hue was attributed to the star in cuneiform texts of Babylonia dating as far back as 1000 B.C.
The object of all this historic attention was the dominant member of the constellation Canis Major--Sirius, the brightest[*]star in the night sky and one of the closest to earth (less than nine light-years away). There is just one problem: as any modern stargazer can testify, Sirius is not red but white. How could the ancients have been so wrong?
Some scientists have attempted to explain away the discrepancy by suggesting that the astronomers of antiquity had observed the star when it was low in the sky; like the setting sun, it appeared red because of particles in the earth's atmosphere. Now two German researchers argue that the ancients did see a red Sirius--and as recently as the 6th century A.D.
Astronomer Wolfhard Schlosser and Historian Werner Bergmann of Ruhr-University Bochum, in West Germany, were led to their conclusion by the discovery of references to Sirius in the chronicles of a Frankish bishop, Gregory of Tours. Written around A.D. 577, Gregory's tome was designed to provide monasteries with clear instructions for setting their predawn prayer schedules; thus it listed for each month the time that certain constellations would rise above the horizon. From the rise times and periods of visibility, the researchers report in the journal Nature, they were able to identify Sirius, which Gregory called Rubeola or Robeola, meaning "red" or "rusty." They point out that because Gregory did not use the classical names of the stars, he was probably unaware of Roman and Greek astronomy. Therefore, the Ruhr team concluded, Sirius looked red no more than 1,400 years ago. Yet only 400 years later, when the Arab astronomer Al Sufi categorized all the stars named by Ptolemy, he did not list Sirius among the red ones. Sometime during that interval, the Ruhr team believes, Sirius changed its hue.
A possible key to that puzzling change is the fact, discovered by 19th century astronomers, that Sirius is part of a binary, or two-star, system. It has a small companion star far too dim to be seen by the naked eye. Sirius B, as the diminutive star was named (the familiar Sirius was renamed Sirius A), is now known to be a white dwarf, the dying ember of a star.
Prior to entering the white-dwarf stage, however, an aging star cools and balloons into a red giant. And that, the Ruhr researchers speculate, is probably what Sirius B was when the Babylonians--and then the Greeks, Romans and Franks--gazed skyward. To the unaided eyes of the ancients, the two closely spaced stars looked like a single pinpoint, with a decided reddish tint imparted by the dominating giant. The combined light of the binary pair would certainly have been brighter than it is today, and indeed Babylonian cuneiforms tell of Sirius' being visible in the daytime sky.